Language Experience and the Organization of Brain Activity to Phonetically Similar Words: ERP Evidence from 14- and 20-Month-Olds Debra L. Mills 1 , Chantel Prat 2 , Renate Zangl 3 , Christine L. Stager 4 , Helen J. Neville 5 , and Janet F. Werker 4 Abstract & The ability to discriminate phonetically similar speech sounds is evident quite early in development. However, inexperienced word learners do not always use this informa- tion in processing word meanings [Stager & Werker (1997). Nature, 388, 381–382]. The present study used event-related potentials (ERPs) to examine developmental changes from 14 to 20 months in brain activity important in processing phonetic detail in the context of meaningful words. ERPs were compared to three types of words: words whose meanings were known by the child (e.g., ‘‘bear’’), nonsense words that differed by an initial phoneme (e.g., ‘‘gare’’), and nonsense words that differed from the known words by more than one phoneme (e.g., ‘‘kobe’’). These results supported the behavio- ral findings suggesting that inexperienced word learners do not use information about phonetic detail when processing word meanings. For the 14-month-olds, ERPs to known words (e.g., ‘‘bear’’) differed from ERPs to phonetically dissimilar nonsense words (e.g., ‘‘kobe’’), but did not differ from ERPs to phonetically similar nonsense words (e.g., ‘‘gare’’), suggesting that known words and similar mispronunciations were processed as the same word. In contrast, for experienced word learners (i.e., 20-month-olds), ERPs to known words (e.g., ‘‘bear’’) differed from those to both types of nonsense words (‘‘gare’’ and ‘‘kobe’’). Changes in the lateral distribution of ERP differences to known and unknown (nonce) words between 14 and 20 months replicated previous findings. The findings suggested that vocabulary development is an impor- tant factor in the organization of neural systems linked to processing phonetic detail within the context of word comprehension. & INTRODUCTION By their first birthday, children are typically able to recognize and respond appropriately to as many as 100 words (Fenson et al., 1994). Controversy remains as to just what children actually know about words at this time. To fully understand a word and to be a productive member of the language community, the child needs to have a working representation of both the meaning and the phonology (sounds) of the word that matches that of adult native speakers. There is a long and rich history of studies investigating the steps children go through in building semantic representa- tions (for a review, see Naigles, 2002). There have been many fewer empirical studies exploring developmental changes in phonological representations as children build a lexicon. Prior to mapping words on to meaning, attention to phonetic detail is evident. From the first days of life, children can discriminate well-formed sylla- bles differing in only a single phonetic feature (e.g., /ta/ vs. /da/, Eimas, Siqueland, Jusczyk, & Vigorito, 1971) or in the sequence of segments (e.g., /tap/ vs. /pat/, Ber- toncini & Mehler, 1981). By the end of the first year of life, children’s phonetic perception has become finely tuned to the properties of their native language as evident in significantly better discrimination of native over nonnative speech sound differences (e.g., Werker & Tees, 1984). It would be reasonable to expect, then, that the child would use these well-honed speech perception sensitivities when first learning meaningful words. However, recent evidence suggests this may not be the case. The young child may confuse, rather than distinguish, similar sounding syllables when first map- ping words to meanings. One of the first empirical studies investigating this question involved a word recognition study by Halle´ and de Boysson-Bardies (1996). Jusczyk and Aslin (1995) had shown that children of 8 months show a preference for listening to word forms they had been familiarized to in the lab (such as ‘‘cup’’) over phonetically similar non- sense words (such as ‘‘tup’’) in a head-turn preference procedure. Halle´ and de Boysson-Bardies (1994) extend- ed this to show that by 11 months, children prefer words that are highly frequent in the input over uncommon 1 Emory University, 2 University of California at Davis, 3 Stanford University, 4 University of British Columbia, 5 University of Oregon D 2004 Massachusetts Institute of Technology Journal of Cognitive Neuroscience 16:8, pp. 1452–1464
Transcript
Language Experience and the Organization of BrainActivity to Phonetically Similar Words ERP Evidence
from 14- and 20-Month-Olds
Debra L Mills1 Chantel Prat2 Renate Zangl3 Christine L Stager4Helen J Neville5 and Janet F Werker4
Abstract
amp The ability to discriminate phonetically similar speechsounds is evident quite early in development Howeverinexperienced word learners do not always use this informa-tion in processing word meanings [Stager amp Werker (1997)Nature 388 381ndash382] The present study used event-relatedpotentials (ERPs) to examine developmental changes from14 to 20 months in brain activity important in processingphonetic detail in the context of meaningful words ERPs werecompared to three types of words words whose meaningswere known by the child (eg lsquolsquobearrsquorsquo) nonsense words thatdiffered by an initial phoneme (eg lsquolsquogarersquorsquo) and nonsensewords that differed from the known words by more than onephoneme (eg lsquolsquokobersquorsquo) These results supported the behavio-ral findings suggesting that inexperienced word learners donot use information about phonetic detail when processing
word meanings For the 14-month-olds ERPs to known words(eg lsquolsquobearrsquorsquo) differed from ERPs to phonetically dissimilarnonsense words (eg lsquolsquokobersquorsquo) but did not differ from ERPs tophonetically similar nonsense words (eg lsquolsquogarersquorsquo) suggestingthat known words and similar mispronunciations wereprocessed as the same word In contrast for experiencedword learners (ie 20-month-olds) ERPs to known words(eg lsquolsquobearrsquorsquo) differed from those to both types of nonsensewords (lsquolsquogarersquorsquo and lsquolsquokobersquorsquo) Changes in the lateral distributionof ERP differences to known and unknown (nonce) wordsbetween 14 and 20 months replicated previous findings Thefindings suggested that vocabulary development is an impor-tant factor in the organization of neural systems linkedto processing phonetic detail within the context of wordcomprehension amp
INTRODUCTION
By their first birthday children are typically able torecognize and respond appropriately to as many as100 words (Fenson et al 1994) Controversy remainsas to just what children actually know about words atthis time To fully understand a word and to be aproductive member of the language community thechild needs to have a working representation of boththe meaning and the phonology (sounds) of the wordthat matches that of adult native speakers There is along and rich history of studies investigating the stepschildren go through in building semantic representa-tions (for a review see Naigles 2002) There have beenmany fewer empirical studies exploring developmentalchanges in phonological representations as childrenbuild a lexicon Prior to mapping words on to meaningattention to phonetic detail is evident From the firstdays of life children can discriminate well-formed sylla-bles differing in only a single phonetic feature (eg ta
vs da Eimas Siqueland Jusczyk amp Vigorito 1971) orin the sequence of segments (eg tap vs pat Ber-toncini amp Mehler 1981) By the end of the first year oflife childrenrsquos phonetic perception has become finelytuned to the properties of their native language asevident in significantly better discrimination of nativeover nonnative speech sound differences (eg Werkeramp Tees 1984) It would be reasonable to expect thenthat the child would use these well-honed speechperception sensitivities when first learning meaningfulwords However recent evidence suggests this may notbe the case The young child may confuse rather thandistinguish similar sounding syllables when first map-ping words to meanings
One of the first empirical studies investigating thisquestion involved a word recognition study by Halle andde Boysson-Bardies (1996) Jusczyk and Aslin (1995) hadshown that children of 8 months show a preference forlistening to word forms they had been familiarized to inthe lab (such as lsquolsquocuprsquorsquo) over phonetically similar non-sense words (such as lsquolsquotuprsquorsquo) in a head-turn preferenceprocedure Halle and de Boysson-Bardies (1994) extend-ed this to show that by 11 months children prefer wordsthat are highly frequent in the input over uncommon
1Emory University 2University of California at Davis 3StanfordUniversity 4University of British Columbia 5University ofOregon
D 2004 Massachusetts Institute of Technology Journal of Cognitive Neuroscience 168 pp 1452ndash1464
words even without pre-exposure in the laboratoryHowever children at this age appear to confuse highlyfrequent known words with phonetically similar noncewords as they also showed a preference for listening tothese phonetically similar nonce words over infrequentwords (Halle amp de Boysson-Bardies 1996) Halle and deBoysson-Bardies hypothesized that this tendency revealsthat in the early stages of word understanding childrenonly represent words globally and will confuse minimal-ly different words with the standard form Although ofgreat interest Halle and de Boysson-Bardies did not firstassess whether or not the children actually knew themeanings of the highly frequent words RecentlyWerker Cohen Lloyd Casasola and Stager (1998) de-veloped the switch task to directly test whether childrenuse their speech perception sensitivities differently insituations that requires a link to meaning than when insituations that require listening to words as meaninglessforms In this task children are habituated to two wordndashobject pairings (eg AA and BB) and tested on theirknowledge of this pairing by comparing looking time toa lsquolsquoswitchrsquorsquo (eg AB) and a lsquolsquosamersquorsquo (eg AA) trialWerker and colleagues found that children of 14 monthsof age can learn to associate two dissimilar soundingwords such as lsquolsquolifrsquorsquo and lsquolsquoneemrsquorsquo to two differentobjects but fail on this task when the words arephonetically similar words such as lsquolsquobihrsquorsquo versus lsquolsquodihrsquorsquo(Stager amp Werker 1997) A series of control studiesconfirmed that children of 14 months are capable ofdiscriminating these two nonce words in a discrimina-tion task that does not entail linking the words with anameable object This set of experiments is consistentwith the suggestion that when children of 14 monthslisten to words as acoustic forms discrimination ofphonetically similar words is readily apparent but ifchildren of this same age are required to map the wordson to meaning they no longer attend to the finephonetic detail
The inattention to fine phonetic detail in newlylearned words is short-lived By 17 and more consis-tently by 20 months of age children are able to learn tomap similar sounding words such as lsquolsquobihrsquorsquo and lsquolsquodihrsquorsquo onto two different objects (Werker Fennell Corcoran ampStager 2002 see also Bailey amp Plunkett 2002) Even14-month-old children with exceptionally large vocabu-laries perform successfully in this task On the basis ofthese results Stager and Werker (1997 see also Werkeramp Fennell 2004 Fennell amp Werker 2003) concludedthat because the typical child at 14 months is still anovice word learner the task of linking words to mean-ings is still very computationally intensive This leavesinadequate attentional resources for using the phoneticdetail in the word
These results are not without challenge In a recentseries of studies Swingley and Aslin (2000 2002) using adifferent procedure provided evidence that children ofboth 18 and 14 months of age can distinguish correct
from incorrect pronunciations of well-known wordsSwingley and Aslin presented children pairs of well-known objects (eg lsquolsquobabyrsquorsquo and lsquolsquodogrsquorsquo) on a computerscreen While viewing both objects the child heardeither a correct (eg lsquolsquobabyrsquorsquo) or incorrect pronunciation(eg lsquolsquovabyrsquorsquo) of one of the object labels The childrenrsquoslooking times to the visual lsquolsquomatchrsquorsquo which was the babyin both conditions were significantly delayed in themispronunciation condition as compared to the correctpronunciation condition thus indicating access to thefine phonetic detail in the word forms In some con-ditions the children also looked longer to the correctpicture after hearing the correct pronunciation1 thanafter hearing the mispronunciation This same overallpattern was reported both for children of 18ndash23 monthsof age (Swingley amp Aslin 2000) and for children aged14 months (Swingley amp Aslin 2002) in apparent contra-diction to the results of Werker et al (2002) and Stagerand Werker (1997) On the basis of these results Swin-gley and Aslin conclude that there is a strong continuitybetween speech perception and word learning and thateven in the initial stages of word learning children havenot only complete representations but also completeaccess to the phonetic detail evidenced in speech per-ception tasks (see also Swingley 2003)
This interpretation is open to question First eventhough children in the Swingley and Aslin task lookedlonger at the correct object when the word beingspoken correctly matched the object than when themispronunciation was heard their looking time to thelsquolsquomatchrsquorsquo was still greater than chance in the mispronun-ciation condition One interpretation of these results isthat the children treated both the correct and themispronounced versions of the word as acceptablelabels for the object but with perhaps a higher level ofactivation in the recognition of the correct than theincorrect pronunciation This interpretation would allowfor the possibility that children do notice the phoneticdetail about the shape of the word at some point in theprocessing stream but do not treat it as significant intheir final lexical representation of the word In otherwords although their speech perception capabilities areintact when a decision about the label for the object isrequired this phonetic detail is no longer included
Studies such as these using looking behavior haveprovided insight into child lexical knowledge They arenonetheless open to criticism This is especially true inthe current context In the many studies using thelsquolsquoswitchrsquorsquo task conclusions about lexical representationare drawn on the basis of a lack of a significant differencein looking time to the switch over the same trials in thetest phase It is always problematic to draw a positiveinference on the basis of a negative result In theSwingley and Aslin studies the looking time measurethat revealed the most consistent results differed acrossage In the children aged 18ndash23 months the mostconsistent measure was latency to look away from the
Mills et al 1453
mismatch whereas in the children aged 14 months totallooking time to the lsquolsquocorrectrsquorsquo object yielded the mostconsistent results It is also problematic to draw defin-itive conclusions when different dependent variables areused with different age groups Finally whenever look-ing behaviors are used as dependent variables it isdifficult to ascertain whether looking time differencesreflect differences in detection encoding or the finalrepresentation
Electrophysiological measures provide a useful com-plement to looking time measures Previous child ERPstudies (St George amp Mills 2001 Mills Coffey-Corina ampNeville 1993 1994 1997 Molfese 1989 1990 MolfeseWetzel amp Gill 1993) have shown different patterns ofneural activity to known versus unknown words inchildren as young as 12 months of age Mills andcolleagues compared event-related potentials (ERPs) towords whose meanings the child did and did notcomprehend Results revealed larger amplitude ERPsto the known versus the unknown words at 200ndash400 msec following word onset At 13ndash17 months thisamplitude difference was evident over both hemi-spheres over frontal temporal parietal and occipitalsites By 20 months the ERP difference was limited totemporal and parietal regions of the left hemisphereThe results suggested there were changes in the orga-nization of brain activity linked to word processingwithin that age range that may be linked to vocabularysize The children aged 20 months who were post(gt 150 words) vocabulary spurt showed more focallydistributed ERPs to known words whereas children withsmaller vocabularies (lt50 words) showed a more dis-tributed response indicating that the increasingly localnature of the differential ERP signature seen in olderchildren reflects greater sophistication in word knowl-edge rather than age per se Although the 200ndash400 mseclatency may seem early for indexing word meanings inchildren looking time paradigms with children havealso shown evidence for word recognition by the middleof the word (eg Fernald Swingley amp Pinto 2001)Moreover this time window is consistent with ERPstudies of lexical access and word comprehension inadults (eg Sereno Brewer amp OrsquoDonnell 2003 Osterh-out amp Holcomb 1995 Holcomb amp Neville 1991 Fisch-ler 1990 Kutas amp Hillyard 1980) In adults ERPdifferences to contextually appropriate versus inappro-priate words typically emerge around 200 msec (BrownHagoort amp Kutas 2000 Kutas amp Hillyard 1980) andhave been observed as early as 50 msec (Holcomb ampNeville 1991)
In this article we used an ERP paradigm similar toMills et al (1997) to examine the nature of the phoneticdetail in lexical representations in children of 14 and20 months of age with one exception Instead of pre-senting the children with randomly varying presenta-tions of 10 known and 10 unknown words we added 10unknown words that were phonetically similar to the
known words For example we presented children withknown words such as lsquolsquodog cat shoe milkrsquorsquo and soforth and phonetically dissimilar nonsense words suchas lsquolsquoneem blick zav kobersquorsquo As an extension we added aset of nonsense words that were phonetically similar tothe known words These lsquolsquophonemic contrastrsquorsquo noncewords included items such as lsquolsquobog gat zue and nilkrsquorsquoWe predicted that children of 14 and 20 months wouldshow larger amplitude ERPs to the known versus thephonetically dissimilar unknown words and that thiswould be seen across most electrode sites at 14 monthsbut would be more localized at 20 months Of particularinterest were ERPs to the phonemic contrast words Wereasoned that if there is sufficient detail in the lexicalrepresentation the ERP signature to these phonemiccontrast foils should be like that of phonetically dissim-ilar unknown words However if the lexical representa-tion is not detailed the phonemic contrast items shouldbe confused with known words
RESULTS
ERPs for all three word-types over anterior and posteriorregions of both the left and right hemispheres areshown for the 14- and 20-month-olds in Figures 1 and
Figure 1 ERPs to all three word-types for the sixteen 14-month-oldsover the anterior and posterior regions of the left and right
hemispheres ERPs to known words are shown in the solid lines to
phonemic contrasts in the dotted lines and to phonetically dissimilar
nonsense words in the dashed lines
1454 Journal of Cognitive Neuroscience Volume 16 Number 8
2 respectively The results are presented for two ERPcomponents the peak amplitude and latency of the firstpositive component P100 and the mean area of thenegativity between 200 and 400 msec poststimulusonset These components were chosen because ourprevious studies found them to be sensitive to vocabu-lary size andor word meanings Also in previous studiesthe N600ndashN900 differed for known and unknown wordsat 13ndash17 but not 20 months (Mills et al 1997) Visualinspection of the present data for the 14-month-olds(Figure 1) suggested differences in this time windowbetween the mean amplitude for the phonetically similarnonsense word and the other two word-types None ofthe analyses conducted on the later time windowreached significant levels There were no interactionswith age or vocabulary size Therefore to conservespace these analyses were not included
Data Analysis
A priori hypotheses predicted that the 14-month-oldsand 20-month-olds would show different patterns ofERPs to the phonemic contrast words relative to the
known and nonsense words Therefore the ERP datawere analyzed separately for the two age groups inrepeated-measures ANOVAs with Word-type (knownphonemic contrast nonsense words) Hemisphere (leftand right) and Electrode site (frontal anterior temporaltemporal parietal and occipital) as the within-subjectsfactors HuynhndashFeldt corrections were used for all therepeated-measures analyses We were particularly inter-ested in changes in the lateral distribution of the N200ndashN400 for the 14- and 20-month-olds to known wordscompared with the two types of nonsense words Theseplanned simple effects were analyzed for the 14- and 20-month-olds separately in repeated-measures ANOVAscomparing two word-types (known words vs phonemiccontrasts known vs nonsense words and phonemiccontrasts vs nonsense words) by hemisphere (left andright) and by electrode site (frontal anterior temporaltemporal parietal occipital) Age group differenceswere analyzed in a two-way ANOVA with Age group asthe between-subjects variable and Word-type (knownwords phonemic contrasts nonsense words) Hemi-sphere (left and right) and Electrode site (frontalanterior temporal temporal parietal and occipital) asthe within-subjects factors
Children Aged 14 Months
P100 The first positive component peaked at 125 msec(P100) The P100 peaked later and was larger overfrontal than posterior regions [electrode site latencyF(450) = 934 p lt 001 amplitude F(460) = 753 p lt001] There were no main effects or interactions forpeak latencies or amplitude among word-types or be-tween the left and right hemispheres
N200ndashN400 The mean amplitudes between 200 and400 msec (N200ndashN400) were larger over posterior thananterior regions [electrode site F(460) = 514 p lt01] and differed with word-type [F(230) = 376 p lt05] There were no main effects or interactions withhemisphere
Planned comparisons showed that the N200ndashN400 waslarger for known words than for nonsense words (ielsquolsquobearkobersquorsquo Figure 3 top) [F(115) = 494 p lt 05]Importantly the N200ndashN400 amplitude was also greaterfor the phonemic contrast words than for the nonsensewords (lsquolsquogarekobersquorsquo Figure 3 bottom) [F(115) = 697p lt 05] However the N200ndashN400 was not significant-ly different for known words versus phonemic contrastwords (ie lsquolsquobearrsquorsquolsquolsquogarersquorsquo Figure 3 middle) [F(115) =007 p = 80] Thus both known and phonemic con-trast words showed the same activity as lsquolsquoknownrsquorsquo andwere different from unknown lsquolsquononsensersquorsquo words
We also predicted age-related differences in the lateraldistribution of the N200ndashN400 effects As predicted forthe 14-month-olds the N200ndashN400 mean amplitudedifferences between the word-types were broadly dis-tributed over anterior and posterior regions of both the
Figure 2 ERPs to all three word-types for the seventeen
20-month-olds over the anterior and posterior regions of the left
and right hemispheres ERPs to known words are shown in the solid
lines to phonemic contrasts in the dotted lines and to phoneticallydissimilar nonsense words in the dashed lines
Mills et al 1455
left and right hemispheres [known vs nonsense lefthemisphere F(115) = 230 p lt 15 right hemisphereF(115) = 745 p lt 05 phonemic contrasts vs non-sense words left hemisphere F(115) = 458 p lt 05right hemisphere F(115) = 797 p lt 01]
Children Aged 20 Months
P100 The P100 peaked later and was larger over frontalthan posterior regions [latency F(116) = 700 p lt001 amplitude F(116) = 1407 p lt 001] There wasalso a main effect of word-type for the P100 amplitude[F(232) = 355 p lt 05] Examination of the amplitudedifferences across word-types showed that the P100 waslarger to known words than to nonsense words [knownvs nonsense F(116) = 707 p lt 05] and phonemiccontrasts [known vs phonemic contrasts F(116) =605 p lt 05] The P100 amplitude was larger overthe left than the right hemisphere but only overtemporal and parietal sites [Hemisphere Electrodesite F(464) = 300 p lt 05] There were no othermain effects or interactions
N200ndashN400 The N200ndashN400 amplitude differences tothe different word-types approached significance [word-type F(116) = 303 p = 06] The N200ndashN400 waslarger over the left than the right hemisphere at tempo-ral and parietal sites but larger over the right than theleft over the occipital regions [Hemisphere Electrodesite F(464) = 283 p lt 05]
Planned comparisons showed that the N200ndashN400was larger to known than nonsense words [F(116) =576 p lt 05] but only over left temporal [F(116) =826 p lt 01] left parietal [F(116) = 1062 p lt 01]and right frontal [F(116) = 602 p lt 05] sites (Figure 4top) The N200ndashN400 was also larger to known wordsversus phonemic contrast words over left temporal[F(116) = 724 p lt 05] and left parietal [F(116) =709 p lt 05] regions (Figure 4 middle) However themain effect of word-type for this comparison only ap-proached significance [F(116) = 270 p = 10] Therewere no significant differences in N200ndashN400 amplitudesto the phonemic contrasts and nonsense words(Figure 4 bottom) [F(116) = 042 ns] At 20 monthsthen the phonemic contrast (nonsense) words were no
Figure 3 For the 14-month-olds ERP differences are directly
compared to known words and nonsense words (top) known words
and phonemic contrasts (middle) and phonemic contrasts andphonetically dissimilar nonsense words (bottom) Significant
differences in N200ndashN400 mean amplitudes are shaded and enclosed in
the rectangle Left and temporal and parietal regions are shown on theleft and right sides of the figure respectively
Figure 4 For the 20-month-olds ERP differences are directly
compared to known words and nonsense words (top) known words
and phonemic contrasts (middle) and phonemic contrasts andphonetically dissimilar nonsense words (bottom) Significant
differences in N200ndashN400 mean amplitudes are shaded and enclosed in
the rectangle Left and temporal and parietal regions are shown on theleft and right sides of the figure respectively
1456 Journal of Cognitive Neuroscience Volume 16 Number 8
longer treated as known words but were instead treatedlike other nonsense words
Age Group Comparisons
There were no main effects or interactions with agewhen all three word-types and all electrode sites wereincluded in the analyses Because the 20-month-olds inthis study and in our previous studies only showed ERPdifferences in this time window to known versus un-known words at temporal and parietal sites a separateANOVA was conducted including only temporal andparietal sites that is with two levels of Age group (14and 20 months) as the between-subjects factor andWord-type (known phonemic contrasts and nonsensewords) Hemisphere (left and right) and Electrodesite (temporal and parietal) as the within-subjects fac-tors That analysis showed main effects for Word-type[F(131) = 659 p lt 001] and Hemisphere [F(131) =395 = 420 p lt 05] These effects were qualifiedby two interactions with group including a Group Word-type Hemisphere interaction [F(662) = 304p lt 05] and a Group Electrode site interaction[F(131) = 396 p lt 05] These interactions were fur-ther validated by the different patterns displayed by the14- and 20-month-olds separately
To further support the age-related effects describedabove separate ANOVAs were conducted with Age (14and 20 months) as the between-subjects factor andWord-type (known phonemic contrasts and nonsensewords) Hemisphere (left and right) and Electrode site(frontal anterior temporal temporal parietal and oc-cipital) as the within-subjects factors To conserve spaceonly main effects of and interactions with age arereported below
P100 There were no main effects or interactions withage for the P100 latency The age-related difference inthe lateral distribution of the P100 amplitude was sup-
ported by an Age Hemisphere Electrode siteinteraction [F(4124) = 275 p lt 05]
N200ndashN400 There were no main effects or interac-tions with age when all three word-types and all elec-trode sites were included in the analyses Because the20-month-olds in previous studies and in this study asdescribed above only showed ERP differences in thistime window to known versus unknown words at tem-poral and parietal sites a separate ANOVA was con-ducted including only temporal and parietal sitesnamely with two levels of Age group (14 and 20 months)as the between-subjects factor and Word-type (knownphonemic contrasts and nonsense words) Hemisphere(left and right) and Electrode site (temporal andparietal) as the within-subjects factors That analysisshowed two interactions with group including a Groupby Word-type Hemisphere interaction [F(662) =304 p lt 05] and a Group Electrode site interaction[F(131) = 396 p lt 05] These interactions supportthe findings described above showing different patternsof responsiveness to the different word-types by the 14-and 20-month-olds separately
Summary
Fourteen- and 20-month-olds showed different patternsof brain activity to known words (eg lsquolsquobearrsquorsquo) phone-mic contrasts (eg lsquolsquogarersquorsquo) and (phonetically dissimilar)nonsense words (eg lsquolsquokobersquorsquo) (Figure 5) As predictedby Stager and Werker (1997) 14-month-olds showedlarger N200ndashN400 amplitudes to known words com-pared with nonsense words (eg lsquolsquokobersquorsquo) Howeverthe N200ndashN400 to known words and the phonemiccontrast stimuli (eg lsquolsquobearrsquorsquo and lsquolsquogarersquorsquo) did not differfrom each other
Unlike the pattern seen at 14 months and as pre-dicted by Werker et al (2002) the 20-month-oldsshowed larger amplitude N200ndashN400 responses to
Figure 5 N200ndashN400 mean
areas averaged across temporal
and parietal regions of the leftand right hemispheres to the
three word-types for the
14- and 20-month-olds
Mills et al 1457
known words (eg lsquolsquobearrsquorsquo) over both phonemic con-trast (eg lsquolsquogarersquorsquo) and other nonsense words (eglsquolsquokobersquorsquo) Moreover the N200ndashN400 amplitudes to thephonemic contrast and other nonsense words (lsquolsquogarersquorsquoand lsquolsquokobersquorsquo) did not differ from each other Thesefindings also replicated previously observed lateral dif-ferences in the N200ndashN400 amplitudes to known versusunknown words between 14 and 20 months (Mills et al1997)
DISCUSSION
The results from the present study were consistent withand expanded on findings observed in previous behav-ioral and ERP studies At both 14 and 20 months of agethe pattern of ERP responses seen replicated that pre-viously reported by Mills et al (1993 1997) for knownversus unknown words At both ages there was a largeramplitude negative response from 200 to 400 msec toknown versus nonsense words And as shown before at14 months of age this response was broadly distributedacross the scalp whereas at 20 months it was observedprimarily over the left temporal and parietal electrodesites
The replication of the previous work by Mills andcolleagues puts us in a particularly strong position tointerpret the results from the phonemic contrast wordsAt 14 months of age the ERP signature to phonemiccontrast nonsense words (eg lsquolsquogarersquorsquo or lsquolsquobogrsquorsquo) isindistinguishable from that to known words (eg lsquolsquobearrsquorsquoand lsquolsquodogrsquorsquo) and significantly greater in amplitude thanthat to dissimilar nonsense words (eg lsquolsquolifrsquorsquo) Thus at14 months of age the neural response appears toindicate a mistaken recognition of a minimal pair mis-pronunciation of a known word as the word itself Thisresult is consistent with that seen in the switch task at14 months (Werker et al 2002 Stager amp Werker 1997)The results with the ERP task are particularly convincingbecause here we see a significant increase in the ampli-tude of the response rather than a failure to increaselooking time At 20 months of age the ERP response isalso consistent with results seen in the switch task at thesame age (Werker et al 2002) At 20 months the ERPsignature to phonemic contrast nonsense words (eglsquolsquogarersquorsquo and lsquolsquobogrsquorsquo) is significantly different from that totheir known counterparts (eg lsquolsquobearrsquorsquo and lsquolsquodogrsquorsquo) andindistinguishable from that to phonetically dissimilarnonsense words (eg lsquolsquoneemrsquorsquo or lsquolsquolifrsquorsquo) Thus at20 months the evoked response indicates that childrenare showing the lsquolsquoword recognitionrsquorsquo ERP response onlyto known words that are phonologically correct in alldetails By 20 months then the confusion with similar-sounding known words is no longer present indicatingthat words in the lexicon are not only fully specified interms of phonological detail but that such detail isavailable even when listening to words in tasks that donot provide contextual or pictorial support for the word
meaning and may not even involve full attention to theword
One difference between the present study and thebehavioral studies by Werker and colleagues and Swin-gley and Aslin is that the behavioral studies used bothpictures and words whereas the present study pre-sented a series of words without a picture context Anegative going ERP wave within the 200ndash500 msec timewindow has also been shown to index word meaningin cross-modal matchmismatch paradigms using pic-tures and words in 14- and 20-month-old children (MillsConboy amp Paton in press) and real objects and noveltrained words in 14-month-old children (Molfese Morseamp Peters 1990) We chose to use the Mills et al (1997)paradigm rather than a pictureword matchmismatchparadigm for several reasons The pictureword matchmismatch paradigm elicits a large amplitude bilateralnegative component starting at 200 msec and peakingaround 500 msec when the subsequent word does notmatch the picture at both 14 and 20 months (MillsConboy et al in press) This ERP effect is most likelywithin the N400 family observed in children and adults toviolations of semantic expectancy (Holcomb Coffey ampNeville 1992 Kutas amp Hillyard 1980) In adults a similarparadigm elicited a phonological mismatch (PMN) towords and nonwords from 250 to 347 msec over rightfrontal regions and from 347 to 638 msec over symmet-rical centro-parietal regions (Connolly Service DrsquoArcyKujala amp Alho 2001) One concern was that the N400 andPMN effects elicited by a pictureword matchmismatchparadigm might swamp more subtle differences in thelatency amplitudes and distributions of ERPs related tothe different word-types and group differences The Millset al paradigm was chosen because it showed ERP dif-ferences to known and unknown words that varied withword-type age and vocabulary size The methodologicaldifferences should be taken into consideration whencomparing the effects However the ERP findings areconsistent with and provide additional support for theage-related differences reported by Stager et al (1997)
The robust differences in the ERP pattern at 14and 20 months of age could be explained at manylevels Several studies have shown that a negativepeak at around 200 msec is sensitive to phonetic varia-tion in voice onset time (Simos Molfese amp Brenden1997 Molfese amp Molfese 1988) place of articulation(Dehaene-Lambertz amp Dehaene 1994 Molfese Burger-Judisch amp Hans 1991) and acoustic cues in nonspeechstimuli (Dehaene-Lambertz 2000 Simos amp Molfese1997) in very young children and even newborns Oneinterpretation is that ERP differences between word-types are related to the acoustic or phonological featuresof the different word-types and that age-related changesare due to differences in the childrenrsquos ability to mem-orize the different word lists Although phonological dis-crimination can modulate the amplitude of the N200 incertain paradigms it is not necessarily the only factor
1458 Journal of Cognitive Neuroscience Volume 16 Number 8
that may contribute to the amplitude of this componentHere we argue that age-related changes in a top-downprocess linked to word meaning is the dominant factormodulating the amplitude of the N200ndashN400 rather thanphonological discrimination First if ERP differences be-tween conditions were due solely to phonological fac-tors then both age groups would have displayed N200amplitude differences between the known words andphonetic contrasts Additionally the known and phone-mic contrasts were balanced for the types of initialconsonants in the number of stop consonants nasalsand fricatives and did not differ in their phonologicalneighbors for other words frequently understood bychildren in this age range (see Methods) Therefore itis difficult to explain why the phonemic contrasts pat-terned with the known words at 14 months and thenonsense words at 20 months based solely on acousticaland phonemic differences A second line of evidencesuggesting that the ERP differences observed here can-not be explained by phonological differences betweenword-types comes from a study of bilingual toddlersusing the same paradigm In that study the N200ndashN400did not differ between Spanish and English for either theknown or unknown words (Conboy amp Mills 2000 Con-boy 2003) ERP differences were linked to word meaning(known vs unknown words) language dominance andtotal vocabulary size but not phonological differencesbetween Spanish and English If the N200ndashN400 indexedthe phonological aspects of the words amplitude differ-ences would be expected between Spanish and EnglishA third and perhaps the strongest line of evidencecomes from a training study with 20-month-old childrenin which nonsense words were either repeated or pairedwith an object (Mills Plunkett Prat amp Schafer in press)After the training phase when objects were no longerpresent the amplitude of the N200ndashN400 to the newlylearned words increased relative to before training butbecame more positive for nonsense words that wererepeated the same number of times Because the stimuliwere counterbalanced across participants the increasedamplitude of the N200ndashN400 to the newly learned wordswas modulated by its association with a meaningfulstimulus and cannot be explained by phonological dif-ferences between word-types
Another interpretation of the findings is that at 14months of age the phonetic detail distinguishing oneword from another is simply not available in the lexicalrepresentation Such an explanation would be compat-ible with theories of phonological development thatposit an initial under-specification of the informationin the lexicon that is only gradually filled in as thevocabulary expands (eg Brown amp Matthews 1997)Although possible we think this explanation is unlikelygiven the work by Swingley and Aslin (2002) showingthat children of 14 months do seem to show someevidence of use of phonetic detail in word recognitiontasks via longer looking time to the matching object
when a correct versus a mispronounced version of aword is given The Swingley and Aslin work involvesshowing the children two objects (eg a car and a dog)and asking them to find either the correctly labeledobject (eg lsquolsquodogrsquorsquo) or a mispronunciation of that sameobject (eg lsquolsquobogrsquorsquo) More recently Fennell and Werker(2003) showed that when tested on two phonemiccontrast words that they already know well (eg lsquolsquoballrsquorsquoand lsquolsquodollrsquorsquo) the child of 14 months can succeed even inthe switch task These two studies show that at somelevel in the system full phonetic specification of well-known words is represented even at 14 months
The successes in the two-choice (Swingley and Aslin)behavioral lsquolsquomispronunciationrsquorsquo task and in the switchtask using two well-known words in comparison to boththe switch word-learning task and the ERP word recog-nition task are compatible with the attentional resourcelimitation hypothesis offered by Werker and colleagues(Werker et al 2002 Werker amp Fennell 2004 Fennell ampWerker 2003 Stager amp Werker 1997) According to thisexplanation the task of linking a word to an object ischallenging for a novice word learner All the detail ispicked up but the computational difficulty of actuallymapping the word on to the object makes it difficultto hold all the information in mind As such one sees aU-shaped function in the pattern of data The child of7ndash8 months who is not yet actively engaged in mappingwords to referents is better able to attend to fine pho-netic detail than is the child of 14 months who is at-tempting to make and remember the link (see Stager ampWerker 1997 see also Halle amp de Boysson-Bardies1996 Jusczyk amp Aslin 1995) Becoming a word learnerchanges the task for the older child This increases theprocessing demands and interferes with the childrsquosability to access detail The information is picked upin perception it is just not available for access in lexicalrecall tasks After the child becomes a more accom-plished word learner and has a working phonemicinventory to guide information access the relevantphonetic differences lsquolsquostand outrsquorsquo as salient and impor-tant to the lexical entry Or as suggested by Nazzi andBertoncini (2003) once the child passes a criticaljuncture in word learning sophistication moving froma simple associationist to a referential word learneraccess to phonological detail becomes possible Priorto that point however any detection of that detail isat best fleeting As such it may be seen in on-lineprocessing tasks or in tasks that provide full support torecall memory (Fennell amp Werker 2003 Swingley ampAslin 2002) but is not evident in tasks that involvelearning new words (Werker et al 2002 Stager ampWerker 1997) nor is it evident in word recognitiontasks such as the ERP task used here where there is noobject to facilitate activation of the full memory trace
Support for this explanation can be seen in thecurrent ERP study In this study children of 20 months(but not 14 months) showed a significant P100 response
Mills et al 1459
to the known words over both the nonsense words andthe phonemic contrasts words The P100 is thought toindex sensory and attentional processes This is the firsttime Mills and colleagues (eg Mills et al 1993 1997)have observed a significantly greater P100 response toknown than to unknown words in the ERP word recog-nition studies The amplitude of the P100 is known to bemodulated by effects of attention in both adults (Luck ampHillyard 2000) and children (Richards amp Hunter 2001)One interpretation of this finding is that the inclusion ofphonemic contrast words in the dataset increased atten-tion to early phonological differences To distinguish theknown words from the phonemic contrast foils theyhad to increase their vigilance to the sensory andperceptual detail thus yielding a significant P100 re-sponse in this ERP word recognition task
Directions for Further Research
A focus for future work will be to determine preciselywhat changes between 14 and 20 months allow the moreaccomplished word learner to access and use full pho-netic detail across a wide range of word recognition andword learning situations In a recent training study MillsPlunkett et al (in press) addressed this question byexamining the effects of experience on the lateral distri-bution of the N200ndashN400 differences between knownand unknown words Mills et al asked whether observedchanges in the lateral distribution in ERP differencesto known versus unknown words between 13 and20 months (Mills et al 1997 and replicated in the pre-sent study) reflect the availability of specialized brainsystems for word recognition (as might be available in amore accomplished word learner) or increasing knowl-edge of particular words by testing 20-month-old chil-dren who varied in vocabulary size on newly learnedwords ERPs to novel words that had been paired withan object during a pretest training phase were comparedto ERPs to novel words that had been repeated the samenumber of times but without the objectword pairingThe results supported a mixed model ERP differences tonewly learned words compared to repeated-but-not-trained words showed a bilateral distribution acrossthe whole sample This finding was consistent with thehypothesis that the asymmetrical distribution of ERP dif-ferences to known and unknown words is linked to theamount of experience with individual words Howeverchildren with the largest vocabularies showed a leftgreater than right distribution of ERP differences Thelatter finding suggested that expertise with word learn-ing might also affect the specialization of language-relevant brain activity even in the service of learningnew words
If a more expert word learning system is becomingavailable (Nazzi amp Bertoncini 2003) in the older andormore experienced word learner this might also permit
greater direction of attention to the relevant phonolog-ical information in the word learning and access situa-tion Indeed as we argued at the beginning of thisarticle perhaps part of becoming a more expert wordlearner is knowing just which properties of a word aredefinitional in a particular linguistic community It maybe the emergence of this greater understanding of justwhat distinguishes words in our language that allows theolder more accomplished word learner to not onlyperceive but also lsquolsquomarkrsquorsquo and lsquolsquousersquorsquo the phonologicaldetail in word learning and word recognition tasks
In previous studies Mills and colleagues have re-ported a link between vocabulary size and the distribu-tion of the N200ndashN400 amplitude difference to knownand unknown words between 14 and 20 months of ageSimilarly Werker et al (2002) reported a significantcorrelation between performance on the CDI at both14 and 17 months and performance in the minimal pairswitch task There was no significant correlation with theCDI in the present study The lack of a significantcorrelation here could reflect lack of variability in thedata perhaps because the age groups selected are notyet in transition We predict that testing of a group ofchildren intermediate in age and vocabulary sizes to thegroups tested here would be more likely to reveal such apattern of findings
In summary using an ERP design we extended andhelped explain the pattern of findings seen in earlybehavioral tasks assessing the phonological detail repre-sented and used in early word learners The results ofthe current ERP study indicate that at the earliest stagesof word learning children treat minimal pair mispronun-ciations of known words as acceptable instances of thatword This provides strong evidence that in the decisionstage of word recognition novice word learners accept abroader range of pronunciations of the word as accept-able than do older children With increasing age andincreasing language sophistication the amount of pho-nological detail easily accessed in the representation ofwords increases helping to avoid mapping mistakes andto facilitate more rapid acquisition of a vocabulary
METHODS
Participants and Settings
Participants were tested in three locations at the Uni-versity of California San Diego at the University ofOregon Eugene Oregon and at the University of BritishColumbia Vancouver Canada Approximately one-thirdof the children in each age group were tested at eachsite Testing at the Vancouver site was conducted in amobile ERP lab belonging to the University of OregonChildren from the San Diego and Eugene areas wererecruited through advertisements in a local magazineposters displayed in the area requests at play groupsand referrals from parents whose children had partici-
1460 Journal of Cognitive Neuroscience Volume 16 Number 8
pated in our studies Children from the Vancouver areawere recruited through the University of British Colum-bia child Studies Center database Parents of all childrensigned consent forms consistent with the human sub-jects internal review boards at the university at whichthey were tested Parents were given $500 to defraytransportation costs and children were given a small toyin appreciation of their participation Only full-term(gt36 weeks of gestation) healthy children with mono-lingual experience with English participated in the studychildren with a family history of language impairmentwere excluded from the study
Data from 16 children aged 14 months (9 girls meanage = 14 months 15 days SD = 97 days range = 14months 1 day to 15 months 1 day) and 17 children aged20 to 21 months (9 girls mean age = 20 months 11 daysSD = 131 days age range = 19 months 14 days to 20months 27 days) were retained for analysis in the studyAn additional 18 children (12 boys nine 14-month-olds)were tested but were excluded from the analyses due totoo few artifact-free trials (n = 10) excessive crying (n =3) reversed polarity of the ERP response (n = 3 seenote)2 or refusal to wear the electrocap (n = 2)
Stimuli
The stimuli were naturally spoken in a female voice anddigitized at 16 bits 44 kHz sampling rate The stimuliconsisted of three types of words (a) 10 words whosemeanings were understood by the child (known wordseg lsquolsquomilkrsquorsquo) (b) 10 nonsense words that differed fromthe known words in the initial phonemic contrast (pho-nemic contrast words eg lsquolsquonilkrsquorsquo) and (c) 10 wordsthat differed phonetically from known words in allvowels and consonants (nonsense words eg lsquolsquoneemrsquorsquo)All stimuli were matched on word duration and numberof syllables The mean durations for the known wordsphonetic contrasts and nonsense words were 783 msecSD = 144 865 msec SD = 169 861 msec SD = 96[F(227) = 109 p = 34] Ideally all word-types wouldbe matched on manner of articulation (eg all stopconsonants all nasals all laterals all fricatives or allbalanced) However children at this age have very smallvocabularies and we were limited to common words thatalmost all children at this age would know Amongchildren first words are a disproportionate number ofwords that begin with stop consonants For many ofthose words minimal pair differences are still words(eg lsquolsquoballrsquorsquondashlsquolsquodollrsquorsquo) Thus we were constrained in ourselection of stimulus items For the known words therewere seven stop consonants two nasals and one frica-tive For the phonemic contrasts there are also sevenstop consonants two nasals and one fricative For thenonsense words there are three stop consonants oneaffricative two nasals two fricatives and two laterals Acomplete list of words is shown in Table 1
To evaluate at what point the known words couldphysically be distinguishable from the other word-typeswe examined two aspects of uniqueness (a) at whatpoint the phonetically dissimilar nonsense words be-come unique from their real word counterparts (egwhen does lsquolsquobearrsquorsquo become unique from lsquolsquogarersquorsquo) and (b)when the words might become unique from otherwords that would be in a babyrsquos vocabulary at this ageThe uniqueness point for initial consonants (the pointthat separates that consonant from other consonants)was within 20 msec of the beginning of the word forfricatives (s z f) 40 msec for stop consonants (b d gp t k) 60 msec for affricatives (j) and 20 msec for nasals(m n) and laterals (l r) To further examine theuniqueness points we compared each stimulus usedin the study relative to other words that might be in thechildrsquos vocabulary with the words in the MacArthur CDIthat would be considered close phonological neighborsTo this end we compared our stimuli to the number ofwords in the CDI with (a) the same consonant includingconsonant clusters and (b) the same consonant exclud-ing consonant clusters and (c) the same consonant plusthe vowel For comparisons a and b there are the mostneighbors for the known words (a = 474 b = 375) anintermediate amount for the phonemic contrast words(a = 371 b = 298) and the least for the nonsense words(a = 275 b = 233) The best comparison is to count thenumber of items that begin with the same consonantplus vowel (CV) There were no significant differences inthe number of CV neighbors for the three word-types(known = 16 phonemic contrasts = 12 nonsense =18) Additionally because of co-articulatory effects theacoustic energy for a given consonant varies as a func-tion of the following vowel (Jusczyk 1997) Thus evenwords beginning with the same consonant (eg lsquolsquobookrsquorsquoand lsquolsquobottlersquorsquo) differ in shape and spectral frequenciesand can be discriminated from each other as well as
Table 1 Stimuli
Known Words Phonemic Contrasts Nonsense
Bear Gare Kobe
Ball Pall Lif
Book Dook Neem
Bottle Pottle Fipe
Cup Tup Mon
Cat Gat Tek
Dog Bog Riss
Milk Nilk Keed
Nose Mose Jud
Shoe Zhu Zav
Mills et al 1461
their close phonological neighbors from the beginningof the word
Procedure
Language Assessment
Within one week prior to testing parents were asked tocomplete the MacArthur CDI (Fenson et al 1994)which provided an estimate of the childrsquos vocabularysize and percentile ranking relative to other children ofthe same age To ensure that the words to be used in thestudy as lsquolsquoknownrsquorsquo were comprehended by the childparents also completed a vocabulary checklist ratingscale indicating how sure they were that their childunderstood andor produced each word on a scale of1 (very sure they did not know that word) to 4 (very suretheir child understoodproduced a given word in avariety of different contexts and with different exem-plars) Additionally the children were asked to identify apicture of each word to be used as a known word from atwo-choice picture book All lsquolsquoknownrsquorsquo words used hadreceived a rating of 4 and were correctly identified in thepicture-pointing task
Electrophysiological Recording
The EEG was recorded using tin electrodes affixed to anelastic cap (Electro-Cap International) from sites overfrontal (F7 and F8) anterior temporal (50 of thedistance from F78 and T34) temporal (33 of thedistance from T34 to C34) parietal (50 of the dis-tance between T34 and P34) and occipital (O1 andO2) regions of the left and right hemispheres Addition-ally the electrooculogram was recorded from electrodesplaced over and under the eye to reject trials on whichblinks and vertical eye movement occurred and fromleft and right frontal electrodes to reject trials on whichhorizontal eye movement occurred Impedances werekept below 5 k and were balanced (within 1 k) acrossthe left and right hemispheres at any given position TheEEG was amplified by SA Instruments amplifiers with abandpass of 01 to 100 Hz and sampled continuouslyevery 4 msec All electrodes were referenced to linkedmastoids3 Averages of the EEG were conducted using2-sec epochs (ie 100 msec prestimulus and 1900 msecpoststimulus) The averaged ERPs were also digitallyfiltered off-line with a 60-Hz low-pass filter
Testing
Ten known 10 phonetically similar nonsense words(referred to as phonemic contrasts) and 10 phoneticallydissimilar nonsense (referred to as nonsense words)words were each presented six times in random orderfor a total of 60 trials per condition During testingchildren sat on their parentrsquos lap and listened to words
presented from a speaker located behind a movingpuppet in a puppet theater Words were presented ata variable rate between 1800 and 3000 msec SOA
Artifact Rejection
Artifact rejection was conducted off-line using a com-puter program to reject blinks and horizontal eye move-ment and amplifier blocking Individual thresholds wereset for each child based on visual inspection of the EEGepochs time-locked to each stimulus A mean of 50 ofthe trials were rejected due to eye and movementartifact The number of artifact-free trials per word-typeretained for analysis ranged from 12 to 53 (SD = 113)out of a possible 60 trials per condition There were nosignificant differences in the percentage of trials rejectedfor the different experimental conditions age groupsor sex
Measurement of ERP Components
Peak latencies and amplitudes were quantified by com-puter with reference to the 100-msec prestimulus base-line for the maximum negative or positive point in aspecified time window The time windows for eachcomponent were set according to the criteria used inour previous studies the first positive componentcalled the P100 was defined as the most positivedeflection between 50 and 175 msec The P100 indexesauditory sensory processing Moreover the lateral dis-tribution of the P100 has been shown to vary withpercentile ranking on the MacArthur CDI score Chil-dren who score at the 50th percentile or higher for theirage show a P100 left greater than right asymmetry Themean amplitude within the time window 200ndash400 afterword onset was quantified by computer with referenceto the 100-msec prestimulus baseline This time windowwas chosen because it had been shown to differ forknown and unknown words in our previous studies
Acknowledgments
This research was conducted while the first author was at theProject in Cognitive and Neural Development at the Universityof California at San Diego The research was supported by NIHgrant DC0048111 to H Neville from NIDCD NSERC (NaturalSciences and Engineering Research Council of Canada) grantto J Werker and HFSP (Human Frontierrsquos Science Program)grant to J Werker and H Neville The third author RenateZangl was funded in part by two postdoctoral grants (J-1646J-1817) from the Austrian Science Foundation
We thank Amy Adamson Greg Appelbaum Teresa Mitchelland Lisa Sanders (in alphabetical order not necessarily in orderof magnitude of contribution) for their help in acquiring andanalyzing the data and Chris Fennell and Liz Lewis for theircomments on an earlier version of the manuscript We areespecially indebted to the children and their parents whoparticipated in these studies and their parents without whomthe research would not be possible
1462 Journal of Cognitive Neuroscience Volume 16 Number 8
Reprint requests should be sent to Debra L Mills who is nowat the Department of Psychology 532 Kilgo Circle EmoryUniversity Atlanta GA 30322 or via e-mail dmills2emoryedu
Notes
1 Unlike the switch task the visual fixation task is an on-linetask with two simultaneously presented choices The twoindicators of success are shorter latency to look away from theincorrect object and longer looking times overall to the correctmatch The habituation phase in the lsquolsquoswitchrsquorsquo task leads to theprediction of a novelty preference in the test phase (ie longerlooking to the incorrect pairing)2 In previous studies Mills and colleagues typically findapproximately 10ndash15 of children show a reversed ERPresponse with a larger amplitude ERP response to unknownthan to known words In previous work Mills and colleagueshave treated those children as noise in the data and have keptthem in for the data analyses Ultimately we want tounderstand the nature of lexical representations in this groupof children showing an aberrant ERP response For the currentstudy however in order to compare the ERP response tophonemic contrasts to known words we felt it necessary toinclude in our sample only those children who showed themuch more typical pattern of response resulting in the ex-clusion of three children All effects reported here show thesame pattern with these children included but the variabilityfrom their inclusion did reduce the differences in somecomparisons3 We are aware of the controversies surrounding the use oflinked mastoids Linked mastoids were used here to increasethe number of active cites given the number of amplifiersavailable and to provide consistency with previous studies Apilot study using similar auditory stimuli was conducted toexamine possible distortions in the distribution of scalp activityresulting from forced linkage This was determined by re-cording from one mastoid using the other as a reference andlinking the mastoids off-line These pilot data were comparedwith the data recorded using linked mastoids and did not yieldsignificant differences
REFERENCES
Bailey T M amp Plunkett K (2002) Phonological specificity inearly words Cognitive Development 17 1265ndash1282
Bertoncini J amp Mehler J (1981) Syllables as units in infantspeech perception Infant Behavior and Development 4247ndash260
Brown C amp Matthews J (1997) The role of feature geometryin the development of phonemic contrasts In S J Hannahsamp M Young-Scholten (Eds) Focus on phonologicalacquisition (pp 67ndash112) Amsterdam Benjamins
Brown C M Hagoort P amp Kutas M (2000) Postlexicalintegration processes in language comprehension Evidencefrom brain-imaging research In M Gazzaniga (Ed) The newcognitive neurosciences (2nd ed pp 881ndash895)Cambridge MIT Press
Conboy B (2003) Patterns of language processing and growthin early EnglishndashSpanish bilingualism (Doctoral dissertationUniversity of California San Diego and San Diego StateUniversity 2000) Dissertation Abstracts InternationalB The Sciences and Engineering 63 (UMI No 5193)
Conboy B amp Mills D L (2000) Cerebral specialization forlanguage comprehension in bilingual toddlers CognitiveNeuroscience Society 2000 Annual Meeting AbstractsSupplement of the Journal of Cognitive Neuroscience 17
Connolly J F Service E DrsquoArcy R C N Kujala A Alho K(2001) Phonological aspects of word recognition as revealedby high-resolution spatio-temporal brain mappingNeuroReport 12 237ndash243
Dehaene-Lambertz G (2000) Cerebral specialization forspeech and non-speech stimuli in infants Journal ofCognitive Neuroscience 12 449ndash460
Dehaene-Lambertz G amp Dehaene S (1994) Speed andcerebral correlates of syllable discrimination in infantsNature 370 292ndash294
Eimas P D Siqueland E R Jusczyk P amp Vigorito J (1971)Speech perception in infants Science 171 303ndash306
Fennell C E amp Werker J F (2003) Early word learnersrsquoability to access phonetic detail in well-known wordsLanguage and Speech 46 245ndash264
Fenson L Dale P S Reznick J S Bates E Thal D Jamp Pethick S J (1994) Variability in early communicativedevelopment Monographs of the Society for Research inChild Development 59 1ndash173
Fernald A Swingley D amp Pinto J P (2001) When half aword is enough Infants can recognize spoken words usingpartial phonetic information Child Development 721003ndash1015
Fischler I S (1990) Comprehending language withevent-related potentials In J W Rohrbaugh R Parasuramanamp R Johnson (Eds) Event-related brain potentials(pp 165ndash170) New York Oxford University Press
Halle P amp de Boysson-Bardies B (1994) Emergence of anearly receptive lexicon Infantsrsquo recognition of wordsInfant Behavior and Development 17 119ndash129
Halle P amp de Boysson-Bardies B (1996) The format ofrepresentation of recognized words in infantsrsquo earlyreceptive lexicon Infant Behavior and Development 19463ndash481
Holcomb P J amp Neville H J (1991) The electrophysiologyof spoken sentence processing Psychobiology 19286ndash300
Holcomb P J Coffey S A amp Neville H J (1992) Visual andauditory sentence processing A developmental analysisusing event-related brain potentials DevelopmentalNeuropsychology 8 203ndash241
Jusczyk P W (1997) The discovery of spoken languageCambridge MIT Press
Jusczyk P W amp Aslin R N (1995) Infantsrsquo detection ofsound patterns of words in fluent speech CognitivePsychology 29 1ndash23
Kutas M amp Hillyard S (1980) Reading senseless sentencesBrain potentials ref lect semantic incongruity Science 207203ndash205
Luck S amp Hillyard S (2000) The operation of selectiveattention at multiple stages of processing Evidence fromhuman and monkey electrophysiology In M Gazzaniga(Ed) The new cognitive neurosciences (2nd edpp 687ndash700) Cambridge MIT Press
Mills D L Coffey-Corina S A amp Neville H J (1997)Language comprehension and cerebral specialization from13ndash20 months [(Special issue on origins of languagedisorders)] In D Thal amp J Reilly (Eds) DevelopmentalNeuropsychology 13 397ndash446
Mills D M Coffey-Corina S A amp Neville H J (1994)Variability in cerebral organization during primary languageacquisition In G Dawson amp K Fischer (Eds) Humanbehavior and the developing brain (pp 427ndash455) NewYork Guilford Publications
Mills D M Coffey-Corina S A amp Neville H J (1993)Language acquisition and cerebral specialization in20-month-old infants Journal of Cognitive Neuroscience 5326ndash342
Mills et al 1463
Mills D L Conboy B amp Paton C (in press) Do changes inbrain organization reflect shifts in symbolic functioning InL Namy (Ed) Symbol use and symbolic representationMahwah NJ Erlbaum
Mills D L Plunkett K Prat C amp Schafer G (in press)Watching the infant brain learn words Effects ofvocabulary size and experience Cognitive Development
Molfese D L (1989) Electrophysiological correlates of wordmeanings in 14-month-old human infants DevelopmentalNeuropsychology 5 79ndash103
Molfese D L (1990) Auditory evoked responses recordedfrom 16-month-old human infants to words they did anddid not know Brain and Language 38 345ndash363
Molfese D L Burger-Judisch L M amp Hans L L (1991)Consonant discrimination by newborn infantsElectrophysiological differences DevelopmentalNeuropsychology 7 177ndash195
Molfese D L amp Molfese V J (1988) Right hemisphereresponses from preschool children to temporal cuescontained in speech and nonspeech materialsElectrophysiological correlates Brain and Language33 245ndash259
Molfese D L Morse P A amp Peters C J (1990) Auditoryevoked responses to names for different objectsCross-modal processing as a basis for infant languageacquisition Developmental Psychology 26 780ndash795
Molfese D L Wetzel W F amp Gill L A (1993) Known versusunknown word discriminations in 12-month-old humaninfants Electrophysiological correlates DevelopmentalNeuropsychology 9 241ndash258
Naigles L R (2002) Form is easy meaning is hard Resolvinga paradox in early child language Cognition 8 157ndash199
Nazzi T amp Bertoncini J (2003) Before and after thevocabulary spurt Two modes of word acquisitionDevelopmental Science 6 136ndash142
Osterhout L amp Holcomb P J (1995) Event-related potentialsand language comprehension In M D Rugg amp M G HColes (Eds) Electrophyisology of mind (pp 171ndash215)Oxford Oxford University Press
Richards J E amp Hunter S K (2001) Testing neural models ofthe development of infant attention DevelopmentalPsychobiology 40 226ndash236
Sereno S C Brewer C C amp OrsquoDonnell P J (2003)Context effects in word recognition Evidence for earlyinteractive processing Psychological Science 14328ndash333
Simos P G amp Molfese D L (1997) Electrophysiologicalresponses from a temporal order continuum in the newborninfant Neuropsychologia 35 89ndash98
Simos P G Molfese D L amp Brenden R A (1997) Behavioraland electrophysiological indices of voicing-cuediscrimination Laterality patterns and developmentBrain and Language 57 122ndash150
St George M amp Mills D L (2001) Electrophysiologicalstudies of language development In J Weissenborn ampB Hoehle (Eds) Language acquisition and languagedisorders (pp 247ndash259) Amsterdam John BenjaminsPublishing
Stager C L amp Werker J F (1997) Infants listen for morephonetic detail in speech perception than in word-learningtasks Nature 388 381ndash382
Swingley D (2003) Phonetic detail in the developing lexiconLanguage and Speech 46 265ndash294
Swingley D amp Aslin R N (2000) Spoken word recognitionand lexical representation in very young children Cognition76 147ndash166
Swingley D amp Aslin R N (2002) Lexical neighborhoodsand the wordndashform representations of 14-month-oldsPsychological Science 13 480ndash484
Werker J F Cohen L B Lloyd V L Casasola M amp StagerC L (1998) Acquisition of wordndashobject associations by14-month-old infants Developmental Psychology 341289ndash1309
Werker J F amp Fennell C E (2004) From listening to soundsto listening to words Early steps in word learning In G Hallamp S Waxman (Eds) Weaving a lexicon (pp 79ndash109)Cambridge MA MIT Press
Werker J F Fennell C E T Corcoran K M amp Stager C L(2002) Infantsrsquo ability to learn phonetically similar wordsEffects of age and vocabulary size Infancy 3 1ndash30
Werker J F amp Tees R C (1984) Cross-language speechperception Evidence for perceptual reorganization duringthe first year of life Infant Behavior and Development 749ndash63
1464 Journal of Cognitive Neuroscience Volume 16 Number 8
words even without pre-exposure in the laboratoryHowever children at this age appear to confuse highlyfrequent known words with phonetically similar noncewords as they also showed a preference for listening tothese phonetically similar nonce words over infrequentwords (Halle amp de Boysson-Bardies 1996) Halle and deBoysson-Bardies hypothesized that this tendency revealsthat in the early stages of word understanding childrenonly represent words globally and will confuse minimal-ly different words with the standard form Although ofgreat interest Halle and de Boysson-Bardies did not firstassess whether or not the children actually knew themeanings of the highly frequent words RecentlyWerker Cohen Lloyd Casasola and Stager (1998) de-veloped the switch task to directly test whether childrenuse their speech perception sensitivities differently insituations that requires a link to meaning than when insituations that require listening to words as meaninglessforms In this task children are habituated to two wordndashobject pairings (eg AA and BB) and tested on theirknowledge of this pairing by comparing looking time toa lsquolsquoswitchrsquorsquo (eg AB) and a lsquolsquosamersquorsquo (eg AA) trialWerker and colleagues found that children of 14 monthsof age can learn to associate two dissimilar soundingwords such as lsquolsquolifrsquorsquo and lsquolsquoneemrsquorsquo to two differentobjects but fail on this task when the words arephonetically similar words such as lsquolsquobihrsquorsquo versus lsquolsquodihrsquorsquo(Stager amp Werker 1997) A series of control studiesconfirmed that children of 14 months are capable ofdiscriminating these two nonce words in a discrimina-tion task that does not entail linking the words with anameable object This set of experiments is consistentwith the suggestion that when children of 14 monthslisten to words as acoustic forms discrimination ofphonetically similar words is readily apparent but ifchildren of this same age are required to map the wordson to meaning they no longer attend to the finephonetic detail
The inattention to fine phonetic detail in newlylearned words is short-lived By 17 and more consis-tently by 20 months of age children are able to learn tomap similar sounding words such as lsquolsquobihrsquorsquo and lsquolsquodihrsquorsquo onto two different objects (Werker Fennell Corcoran ampStager 2002 see also Bailey amp Plunkett 2002) Even14-month-old children with exceptionally large vocabu-laries perform successfully in this task On the basis ofthese results Stager and Werker (1997 see also Werkeramp Fennell 2004 Fennell amp Werker 2003) concludedthat because the typical child at 14 months is still anovice word learner the task of linking words to mean-ings is still very computationally intensive This leavesinadequate attentional resources for using the phoneticdetail in the word
These results are not without challenge In a recentseries of studies Swingley and Aslin (2000 2002) using adifferent procedure provided evidence that children ofboth 18 and 14 months of age can distinguish correct
from incorrect pronunciations of well-known wordsSwingley and Aslin presented children pairs of well-known objects (eg lsquolsquobabyrsquorsquo and lsquolsquodogrsquorsquo) on a computerscreen While viewing both objects the child heardeither a correct (eg lsquolsquobabyrsquorsquo) or incorrect pronunciation(eg lsquolsquovabyrsquorsquo) of one of the object labels The childrenrsquoslooking times to the visual lsquolsquomatchrsquorsquo which was the babyin both conditions were significantly delayed in themispronunciation condition as compared to the correctpronunciation condition thus indicating access to thefine phonetic detail in the word forms In some con-ditions the children also looked longer to the correctpicture after hearing the correct pronunciation1 thanafter hearing the mispronunciation This same overallpattern was reported both for children of 18ndash23 monthsof age (Swingley amp Aslin 2000) and for children aged14 months (Swingley amp Aslin 2002) in apparent contra-diction to the results of Werker et al (2002) and Stagerand Werker (1997) On the basis of these results Swin-gley and Aslin conclude that there is a strong continuitybetween speech perception and word learning and thateven in the initial stages of word learning children havenot only complete representations but also completeaccess to the phonetic detail evidenced in speech per-ception tasks (see also Swingley 2003)
This interpretation is open to question First eventhough children in the Swingley and Aslin task lookedlonger at the correct object when the word beingspoken correctly matched the object than when themispronunciation was heard their looking time to thelsquolsquomatchrsquorsquo was still greater than chance in the mispronun-ciation condition One interpretation of these results isthat the children treated both the correct and themispronounced versions of the word as acceptablelabels for the object but with perhaps a higher level ofactivation in the recognition of the correct than theincorrect pronunciation This interpretation would allowfor the possibility that children do notice the phoneticdetail about the shape of the word at some point in theprocessing stream but do not treat it as significant intheir final lexical representation of the word In otherwords although their speech perception capabilities areintact when a decision about the label for the object isrequired this phonetic detail is no longer included
Studies such as these using looking behavior haveprovided insight into child lexical knowledge They arenonetheless open to criticism This is especially true inthe current context In the many studies using thelsquolsquoswitchrsquorsquo task conclusions about lexical representationare drawn on the basis of a lack of a significant differencein looking time to the switch over the same trials in thetest phase It is always problematic to draw a positiveinference on the basis of a negative result In theSwingley and Aslin studies the looking time measurethat revealed the most consistent results differed acrossage In the children aged 18ndash23 months the mostconsistent measure was latency to look away from the
Mills et al 1453
mismatch whereas in the children aged 14 months totallooking time to the lsquolsquocorrectrsquorsquo object yielded the mostconsistent results It is also problematic to draw defin-itive conclusions when different dependent variables areused with different age groups Finally whenever look-ing behaviors are used as dependent variables it isdifficult to ascertain whether looking time differencesreflect differences in detection encoding or the finalrepresentation
Electrophysiological measures provide a useful com-plement to looking time measures Previous child ERPstudies (St George amp Mills 2001 Mills Coffey-Corina ampNeville 1993 1994 1997 Molfese 1989 1990 MolfeseWetzel amp Gill 1993) have shown different patterns ofneural activity to known versus unknown words inchildren as young as 12 months of age Mills andcolleagues compared event-related potentials (ERPs) towords whose meanings the child did and did notcomprehend Results revealed larger amplitude ERPsto the known versus the unknown words at 200ndash400 msec following word onset At 13ndash17 months thisamplitude difference was evident over both hemi-spheres over frontal temporal parietal and occipitalsites By 20 months the ERP difference was limited totemporal and parietal regions of the left hemisphereThe results suggested there were changes in the orga-nization of brain activity linked to word processingwithin that age range that may be linked to vocabularysize The children aged 20 months who were post(gt 150 words) vocabulary spurt showed more focallydistributed ERPs to known words whereas children withsmaller vocabularies (lt50 words) showed a more dis-tributed response indicating that the increasingly localnature of the differential ERP signature seen in olderchildren reflects greater sophistication in word knowl-edge rather than age per se Although the 200ndash400 mseclatency may seem early for indexing word meanings inchildren looking time paradigms with children havealso shown evidence for word recognition by the middleof the word (eg Fernald Swingley amp Pinto 2001)Moreover this time window is consistent with ERPstudies of lexical access and word comprehension inadults (eg Sereno Brewer amp OrsquoDonnell 2003 Osterh-out amp Holcomb 1995 Holcomb amp Neville 1991 Fisch-ler 1990 Kutas amp Hillyard 1980) In adults ERPdifferences to contextually appropriate versus inappro-priate words typically emerge around 200 msec (BrownHagoort amp Kutas 2000 Kutas amp Hillyard 1980) andhave been observed as early as 50 msec (Holcomb ampNeville 1991)
In this article we used an ERP paradigm similar toMills et al (1997) to examine the nature of the phoneticdetail in lexical representations in children of 14 and20 months of age with one exception Instead of pre-senting the children with randomly varying presenta-tions of 10 known and 10 unknown words we added 10unknown words that were phonetically similar to the
known words For example we presented children withknown words such as lsquolsquodog cat shoe milkrsquorsquo and soforth and phonetically dissimilar nonsense words suchas lsquolsquoneem blick zav kobersquorsquo As an extension we added aset of nonsense words that were phonetically similar tothe known words These lsquolsquophonemic contrastrsquorsquo noncewords included items such as lsquolsquobog gat zue and nilkrsquorsquoWe predicted that children of 14 and 20 months wouldshow larger amplitude ERPs to the known versus thephonetically dissimilar unknown words and that thiswould be seen across most electrode sites at 14 monthsbut would be more localized at 20 months Of particularinterest were ERPs to the phonemic contrast words Wereasoned that if there is sufficient detail in the lexicalrepresentation the ERP signature to these phonemiccontrast foils should be like that of phonetically dissim-ilar unknown words However if the lexical representa-tion is not detailed the phonemic contrast items shouldbe confused with known words
RESULTS
ERPs for all three word-types over anterior and posteriorregions of both the left and right hemispheres areshown for the 14- and 20-month-olds in Figures 1 and
Figure 1 ERPs to all three word-types for the sixteen 14-month-oldsover the anterior and posterior regions of the left and right
hemispheres ERPs to known words are shown in the solid lines to
phonemic contrasts in the dotted lines and to phonetically dissimilar
nonsense words in the dashed lines
1454 Journal of Cognitive Neuroscience Volume 16 Number 8
2 respectively The results are presented for two ERPcomponents the peak amplitude and latency of the firstpositive component P100 and the mean area of thenegativity between 200 and 400 msec poststimulusonset These components were chosen because ourprevious studies found them to be sensitive to vocabu-lary size andor word meanings Also in previous studiesthe N600ndashN900 differed for known and unknown wordsat 13ndash17 but not 20 months (Mills et al 1997) Visualinspection of the present data for the 14-month-olds(Figure 1) suggested differences in this time windowbetween the mean amplitude for the phonetically similarnonsense word and the other two word-types None ofthe analyses conducted on the later time windowreached significant levels There were no interactionswith age or vocabulary size Therefore to conservespace these analyses were not included
Data Analysis
A priori hypotheses predicted that the 14-month-oldsand 20-month-olds would show different patterns ofERPs to the phonemic contrast words relative to the
known and nonsense words Therefore the ERP datawere analyzed separately for the two age groups inrepeated-measures ANOVAs with Word-type (knownphonemic contrast nonsense words) Hemisphere (leftand right) and Electrode site (frontal anterior temporaltemporal parietal and occipital) as the within-subjectsfactors HuynhndashFeldt corrections were used for all therepeated-measures analyses We were particularly inter-ested in changes in the lateral distribution of the N200ndashN400 for the 14- and 20-month-olds to known wordscompared with the two types of nonsense words Theseplanned simple effects were analyzed for the 14- and 20-month-olds separately in repeated-measures ANOVAscomparing two word-types (known words vs phonemiccontrasts known vs nonsense words and phonemiccontrasts vs nonsense words) by hemisphere (left andright) and by electrode site (frontal anterior temporaltemporal parietal occipital) Age group differenceswere analyzed in a two-way ANOVA with Age group asthe between-subjects variable and Word-type (knownwords phonemic contrasts nonsense words) Hemi-sphere (left and right) and Electrode site (frontalanterior temporal temporal parietal and occipital) asthe within-subjects factors
Children Aged 14 Months
P100 The first positive component peaked at 125 msec(P100) The P100 peaked later and was larger overfrontal than posterior regions [electrode site latencyF(450) = 934 p lt 001 amplitude F(460) = 753 p lt001] There were no main effects or interactions forpeak latencies or amplitude among word-types or be-tween the left and right hemispheres
N200ndashN400 The mean amplitudes between 200 and400 msec (N200ndashN400) were larger over posterior thananterior regions [electrode site F(460) = 514 p lt01] and differed with word-type [F(230) = 376 p lt05] There were no main effects or interactions withhemisphere
Planned comparisons showed that the N200ndashN400 waslarger for known words than for nonsense words (ielsquolsquobearkobersquorsquo Figure 3 top) [F(115) = 494 p lt 05]Importantly the N200ndashN400 amplitude was also greaterfor the phonemic contrast words than for the nonsensewords (lsquolsquogarekobersquorsquo Figure 3 bottom) [F(115) = 697p lt 05] However the N200ndashN400 was not significant-ly different for known words versus phonemic contrastwords (ie lsquolsquobearrsquorsquolsquolsquogarersquorsquo Figure 3 middle) [F(115) =007 p = 80] Thus both known and phonemic con-trast words showed the same activity as lsquolsquoknownrsquorsquo andwere different from unknown lsquolsquononsensersquorsquo words
We also predicted age-related differences in the lateraldistribution of the N200ndashN400 effects As predicted forthe 14-month-olds the N200ndashN400 mean amplitudedifferences between the word-types were broadly dis-tributed over anterior and posterior regions of both the
Figure 2 ERPs to all three word-types for the seventeen
20-month-olds over the anterior and posterior regions of the left
and right hemispheres ERPs to known words are shown in the solid
lines to phonemic contrasts in the dotted lines and to phoneticallydissimilar nonsense words in the dashed lines
Mills et al 1455
left and right hemispheres [known vs nonsense lefthemisphere F(115) = 230 p lt 15 right hemisphereF(115) = 745 p lt 05 phonemic contrasts vs non-sense words left hemisphere F(115) = 458 p lt 05right hemisphere F(115) = 797 p lt 01]
Children Aged 20 Months
P100 The P100 peaked later and was larger over frontalthan posterior regions [latency F(116) = 700 p lt001 amplitude F(116) = 1407 p lt 001] There wasalso a main effect of word-type for the P100 amplitude[F(232) = 355 p lt 05] Examination of the amplitudedifferences across word-types showed that the P100 waslarger to known words than to nonsense words [knownvs nonsense F(116) = 707 p lt 05] and phonemiccontrasts [known vs phonemic contrasts F(116) =605 p lt 05] The P100 amplitude was larger overthe left than the right hemisphere but only overtemporal and parietal sites [Hemisphere Electrodesite F(464) = 300 p lt 05] There were no othermain effects or interactions
N200ndashN400 The N200ndashN400 amplitude differences tothe different word-types approached significance [word-type F(116) = 303 p = 06] The N200ndashN400 waslarger over the left than the right hemisphere at tempo-ral and parietal sites but larger over the right than theleft over the occipital regions [Hemisphere Electrodesite F(464) = 283 p lt 05]
Planned comparisons showed that the N200ndashN400was larger to known than nonsense words [F(116) =576 p lt 05] but only over left temporal [F(116) =826 p lt 01] left parietal [F(116) = 1062 p lt 01]and right frontal [F(116) = 602 p lt 05] sites (Figure 4top) The N200ndashN400 was also larger to known wordsversus phonemic contrast words over left temporal[F(116) = 724 p lt 05] and left parietal [F(116) =709 p lt 05] regions (Figure 4 middle) However themain effect of word-type for this comparison only ap-proached significance [F(116) = 270 p = 10] Therewere no significant differences in N200ndashN400 amplitudesto the phonemic contrasts and nonsense words(Figure 4 bottom) [F(116) = 042 ns] At 20 monthsthen the phonemic contrast (nonsense) words were no
Figure 3 For the 14-month-olds ERP differences are directly
compared to known words and nonsense words (top) known words
and phonemic contrasts (middle) and phonemic contrasts andphonetically dissimilar nonsense words (bottom) Significant
differences in N200ndashN400 mean amplitudes are shaded and enclosed in
the rectangle Left and temporal and parietal regions are shown on theleft and right sides of the figure respectively
Figure 4 For the 20-month-olds ERP differences are directly
compared to known words and nonsense words (top) known words
and phonemic contrasts (middle) and phonemic contrasts andphonetically dissimilar nonsense words (bottom) Significant
differences in N200ndashN400 mean amplitudes are shaded and enclosed in
the rectangle Left and temporal and parietal regions are shown on theleft and right sides of the figure respectively
1456 Journal of Cognitive Neuroscience Volume 16 Number 8
longer treated as known words but were instead treatedlike other nonsense words
Age Group Comparisons
There were no main effects or interactions with agewhen all three word-types and all electrode sites wereincluded in the analyses Because the 20-month-olds inthis study and in our previous studies only showed ERPdifferences in this time window to known versus un-known words at temporal and parietal sites a separateANOVA was conducted including only temporal andparietal sites that is with two levels of Age group (14and 20 months) as the between-subjects factor andWord-type (known phonemic contrasts and nonsensewords) Hemisphere (left and right) and Electrodesite (temporal and parietal) as the within-subjects fac-tors That analysis showed main effects for Word-type[F(131) = 659 p lt 001] and Hemisphere [F(131) =395 = 420 p lt 05] These effects were qualifiedby two interactions with group including a Group Word-type Hemisphere interaction [F(662) = 304p lt 05] and a Group Electrode site interaction[F(131) = 396 p lt 05] These interactions were fur-ther validated by the different patterns displayed by the14- and 20-month-olds separately
To further support the age-related effects describedabove separate ANOVAs were conducted with Age (14and 20 months) as the between-subjects factor andWord-type (known phonemic contrasts and nonsensewords) Hemisphere (left and right) and Electrode site(frontal anterior temporal temporal parietal and oc-cipital) as the within-subjects factors To conserve spaceonly main effects of and interactions with age arereported below
P100 There were no main effects or interactions withage for the P100 latency The age-related difference inthe lateral distribution of the P100 amplitude was sup-
ported by an Age Hemisphere Electrode siteinteraction [F(4124) = 275 p lt 05]
N200ndashN400 There were no main effects or interac-tions with age when all three word-types and all elec-trode sites were included in the analyses Because the20-month-olds in previous studies and in this study asdescribed above only showed ERP differences in thistime window to known versus unknown words at tem-poral and parietal sites a separate ANOVA was con-ducted including only temporal and parietal sitesnamely with two levels of Age group (14 and 20 months)as the between-subjects factor and Word-type (knownphonemic contrasts and nonsense words) Hemisphere(left and right) and Electrode site (temporal andparietal) as the within-subjects factors That analysisshowed two interactions with group including a Groupby Word-type Hemisphere interaction [F(662) =304 p lt 05] and a Group Electrode site interaction[F(131) = 396 p lt 05] These interactions supportthe findings described above showing different patternsof responsiveness to the different word-types by the 14-and 20-month-olds separately
Summary
Fourteen- and 20-month-olds showed different patternsof brain activity to known words (eg lsquolsquobearrsquorsquo) phone-mic contrasts (eg lsquolsquogarersquorsquo) and (phonetically dissimilar)nonsense words (eg lsquolsquokobersquorsquo) (Figure 5) As predictedby Stager and Werker (1997) 14-month-olds showedlarger N200ndashN400 amplitudes to known words com-pared with nonsense words (eg lsquolsquokobersquorsquo) Howeverthe N200ndashN400 to known words and the phonemiccontrast stimuli (eg lsquolsquobearrsquorsquo and lsquolsquogarersquorsquo) did not differfrom each other
Unlike the pattern seen at 14 months and as pre-dicted by Werker et al (2002) the 20-month-oldsshowed larger amplitude N200ndashN400 responses to
Figure 5 N200ndashN400 mean
areas averaged across temporal
and parietal regions of the leftand right hemispheres to the
three word-types for the
14- and 20-month-olds
Mills et al 1457
known words (eg lsquolsquobearrsquorsquo) over both phonemic con-trast (eg lsquolsquogarersquorsquo) and other nonsense words (eglsquolsquokobersquorsquo) Moreover the N200ndashN400 amplitudes to thephonemic contrast and other nonsense words (lsquolsquogarersquorsquoand lsquolsquokobersquorsquo) did not differ from each other Thesefindings also replicated previously observed lateral dif-ferences in the N200ndashN400 amplitudes to known versusunknown words between 14 and 20 months (Mills et al1997)
DISCUSSION
The results from the present study were consistent withand expanded on findings observed in previous behav-ioral and ERP studies At both 14 and 20 months of agethe pattern of ERP responses seen replicated that pre-viously reported by Mills et al (1993 1997) for knownversus unknown words At both ages there was a largeramplitude negative response from 200 to 400 msec toknown versus nonsense words And as shown before at14 months of age this response was broadly distributedacross the scalp whereas at 20 months it was observedprimarily over the left temporal and parietal electrodesites
The replication of the previous work by Mills andcolleagues puts us in a particularly strong position tointerpret the results from the phonemic contrast wordsAt 14 months of age the ERP signature to phonemiccontrast nonsense words (eg lsquolsquogarersquorsquo or lsquolsquobogrsquorsquo) isindistinguishable from that to known words (eg lsquolsquobearrsquorsquoand lsquolsquodogrsquorsquo) and significantly greater in amplitude thanthat to dissimilar nonsense words (eg lsquolsquolifrsquorsquo) Thus at14 months of age the neural response appears toindicate a mistaken recognition of a minimal pair mis-pronunciation of a known word as the word itself Thisresult is consistent with that seen in the switch task at14 months (Werker et al 2002 Stager amp Werker 1997)The results with the ERP task are particularly convincingbecause here we see a significant increase in the ampli-tude of the response rather than a failure to increaselooking time At 20 months of age the ERP response isalso consistent with results seen in the switch task at thesame age (Werker et al 2002) At 20 months the ERPsignature to phonemic contrast nonsense words (eglsquolsquogarersquorsquo and lsquolsquobogrsquorsquo) is significantly different from that totheir known counterparts (eg lsquolsquobearrsquorsquo and lsquolsquodogrsquorsquo) andindistinguishable from that to phonetically dissimilarnonsense words (eg lsquolsquoneemrsquorsquo or lsquolsquolifrsquorsquo) Thus at20 months the evoked response indicates that childrenare showing the lsquolsquoword recognitionrsquorsquo ERP response onlyto known words that are phonologically correct in alldetails By 20 months then the confusion with similar-sounding known words is no longer present indicatingthat words in the lexicon are not only fully specified interms of phonological detail but that such detail isavailable even when listening to words in tasks that donot provide contextual or pictorial support for the word
meaning and may not even involve full attention to theword
One difference between the present study and thebehavioral studies by Werker and colleagues and Swin-gley and Aslin is that the behavioral studies used bothpictures and words whereas the present study pre-sented a series of words without a picture context Anegative going ERP wave within the 200ndash500 msec timewindow has also been shown to index word meaningin cross-modal matchmismatch paradigms using pic-tures and words in 14- and 20-month-old children (MillsConboy amp Paton in press) and real objects and noveltrained words in 14-month-old children (Molfese Morseamp Peters 1990) We chose to use the Mills et al (1997)paradigm rather than a pictureword matchmismatchparadigm for several reasons The pictureword matchmismatch paradigm elicits a large amplitude bilateralnegative component starting at 200 msec and peakingaround 500 msec when the subsequent word does notmatch the picture at both 14 and 20 months (MillsConboy et al in press) This ERP effect is most likelywithin the N400 family observed in children and adults toviolations of semantic expectancy (Holcomb Coffey ampNeville 1992 Kutas amp Hillyard 1980) In adults a similarparadigm elicited a phonological mismatch (PMN) towords and nonwords from 250 to 347 msec over rightfrontal regions and from 347 to 638 msec over symmet-rical centro-parietal regions (Connolly Service DrsquoArcyKujala amp Alho 2001) One concern was that the N400 andPMN effects elicited by a pictureword matchmismatchparadigm might swamp more subtle differences in thelatency amplitudes and distributions of ERPs related tothe different word-types and group differences The Millset al paradigm was chosen because it showed ERP dif-ferences to known and unknown words that varied withword-type age and vocabulary size The methodologicaldifferences should be taken into consideration whencomparing the effects However the ERP findings areconsistent with and provide additional support for theage-related differences reported by Stager et al (1997)
The robust differences in the ERP pattern at 14and 20 months of age could be explained at manylevels Several studies have shown that a negativepeak at around 200 msec is sensitive to phonetic varia-tion in voice onset time (Simos Molfese amp Brenden1997 Molfese amp Molfese 1988) place of articulation(Dehaene-Lambertz amp Dehaene 1994 Molfese Burger-Judisch amp Hans 1991) and acoustic cues in nonspeechstimuli (Dehaene-Lambertz 2000 Simos amp Molfese1997) in very young children and even newborns Oneinterpretation is that ERP differences between word-types are related to the acoustic or phonological featuresof the different word-types and that age-related changesare due to differences in the childrenrsquos ability to mem-orize the different word lists Although phonological dis-crimination can modulate the amplitude of the N200 incertain paradigms it is not necessarily the only factor
1458 Journal of Cognitive Neuroscience Volume 16 Number 8
that may contribute to the amplitude of this componentHere we argue that age-related changes in a top-downprocess linked to word meaning is the dominant factormodulating the amplitude of the N200ndashN400 rather thanphonological discrimination First if ERP differences be-tween conditions were due solely to phonological fac-tors then both age groups would have displayed N200amplitude differences between the known words andphonetic contrasts Additionally the known and phone-mic contrasts were balanced for the types of initialconsonants in the number of stop consonants nasalsand fricatives and did not differ in their phonologicalneighbors for other words frequently understood bychildren in this age range (see Methods) Therefore itis difficult to explain why the phonemic contrasts pat-terned with the known words at 14 months and thenonsense words at 20 months based solely on acousticaland phonemic differences A second line of evidencesuggesting that the ERP differences observed here can-not be explained by phonological differences betweenword-types comes from a study of bilingual toddlersusing the same paradigm In that study the N200ndashN400did not differ between Spanish and English for either theknown or unknown words (Conboy amp Mills 2000 Con-boy 2003) ERP differences were linked to word meaning(known vs unknown words) language dominance andtotal vocabulary size but not phonological differencesbetween Spanish and English If the N200ndashN400 indexedthe phonological aspects of the words amplitude differ-ences would be expected between Spanish and EnglishA third and perhaps the strongest line of evidencecomes from a training study with 20-month-old childrenin which nonsense words were either repeated or pairedwith an object (Mills Plunkett Prat amp Schafer in press)After the training phase when objects were no longerpresent the amplitude of the N200ndashN400 to the newlylearned words increased relative to before training butbecame more positive for nonsense words that wererepeated the same number of times Because the stimuliwere counterbalanced across participants the increasedamplitude of the N200ndashN400 to the newly learned wordswas modulated by its association with a meaningfulstimulus and cannot be explained by phonological dif-ferences between word-types
Another interpretation of the findings is that at 14months of age the phonetic detail distinguishing oneword from another is simply not available in the lexicalrepresentation Such an explanation would be compat-ible with theories of phonological development thatposit an initial under-specification of the informationin the lexicon that is only gradually filled in as thevocabulary expands (eg Brown amp Matthews 1997)Although possible we think this explanation is unlikelygiven the work by Swingley and Aslin (2002) showingthat children of 14 months do seem to show someevidence of use of phonetic detail in word recognitiontasks via longer looking time to the matching object
when a correct versus a mispronounced version of aword is given The Swingley and Aslin work involvesshowing the children two objects (eg a car and a dog)and asking them to find either the correctly labeledobject (eg lsquolsquodogrsquorsquo) or a mispronunciation of that sameobject (eg lsquolsquobogrsquorsquo) More recently Fennell and Werker(2003) showed that when tested on two phonemiccontrast words that they already know well (eg lsquolsquoballrsquorsquoand lsquolsquodollrsquorsquo) the child of 14 months can succeed even inthe switch task These two studies show that at somelevel in the system full phonetic specification of well-known words is represented even at 14 months
The successes in the two-choice (Swingley and Aslin)behavioral lsquolsquomispronunciationrsquorsquo task and in the switchtask using two well-known words in comparison to boththe switch word-learning task and the ERP word recog-nition task are compatible with the attentional resourcelimitation hypothesis offered by Werker and colleagues(Werker et al 2002 Werker amp Fennell 2004 Fennell ampWerker 2003 Stager amp Werker 1997) According to thisexplanation the task of linking a word to an object ischallenging for a novice word learner All the detail ispicked up but the computational difficulty of actuallymapping the word on to the object makes it difficultto hold all the information in mind As such one sees aU-shaped function in the pattern of data The child of7ndash8 months who is not yet actively engaged in mappingwords to referents is better able to attend to fine pho-netic detail than is the child of 14 months who is at-tempting to make and remember the link (see Stager ampWerker 1997 see also Halle amp de Boysson-Bardies1996 Jusczyk amp Aslin 1995) Becoming a word learnerchanges the task for the older child This increases theprocessing demands and interferes with the childrsquosability to access detail The information is picked upin perception it is just not available for access in lexicalrecall tasks After the child becomes a more accom-plished word learner and has a working phonemicinventory to guide information access the relevantphonetic differences lsquolsquostand outrsquorsquo as salient and impor-tant to the lexical entry Or as suggested by Nazzi andBertoncini (2003) once the child passes a criticaljuncture in word learning sophistication moving froma simple associationist to a referential word learneraccess to phonological detail becomes possible Priorto that point however any detection of that detail isat best fleeting As such it may be seen in on-lineprocessing tasks or in tasks that provide full support torecall memory (Fennell amp Werker 2003 Swingley ampAslin 2002) but is not evident in tasks that involvelearning new words (Werker et al 2002 Stager ampWerker 1997) nor is it evident in word recognitiontasks such as the ERP task used here where there is noobject to facilitate activation of the full memory trace
Support for this explanation can be seen in thecurrent ERP study In this study children of 20 months(but not 14 months) showed a significant P100 response
Mills et al 1459
to the known words over both the nonsense words andthe phonemic contrasts words The P100 is thought toindex sensory and attentional processes This is the firsttime Mills and colleagues (eg Mills et al 1993 1997)have observed a significantly greater P100 response toknown than to unknown words in the ERP word recog-nition studies The amplitude of the P100 is known to bemodulated by effects of attention in both adults (Luck ampHillyard 2000) and children (Richards amp Hunter 2001)One interpretation of this finding is that the inclusion ofphonemic contrast words in the dataset increased atten-tion to early phonological differences To distinguish theknown words from the phonemic contrast foils theyhad to increase their vigilance to the sensory andperceptual detail thus yielding a significant P100 re-sponse in this ERP word recognition task
Directions for Further Research
A focus for future work will be to determine preciselywhat changes between 14 and 20 months allow the moreaccomplished word learner to access and use full pho-netic detail across a wide range of word recognition andword learning situations In a recent training study MillsPlunkett et al (in press) addressed this question byexamining the effects of experience on the lateral distri-bution of the N200ndashN400 differences between knownand unknown words Mills et al asked whether observedchanges in the lateral distribution in ERP differencesto known versus unknown words between 13 and20 months (Mills et al 1997 and replicated in the pre-sent study) reflect the availability of specialized brainsystems for word recognition (as might be available in amore accomplished word learner) or increasing knowl-edge of particular words by testing 20-month-old chil-dren who varied in vocabulary size on newly learnedwords ERPs to novel words that had been paired withan object during a pretest training phase were comparedto ERPs to novel words that had been repeated the samenumber of times but without the objectword pairingThe results supported a mixed model ERP differences tonewly learned words compared to repeated-but-not-trained words showed a bilateral distribution acrossthe whole sample This finding was consistent with thehypothesis that the asymmetrical distribution of ERP dif-ferences to known and unknown words is linked to theamount of experience with individual words Howeverchildren with the largest vocabularies showed a leftgreater than right distribution of ERP differences Thelatter finding suggested that expertise with word learn-ing might also affect the specialization of language-relevant brain activity even in the service of learningnew words
If a more expert word learning system is becomingavailable (Nazzi amp Bertoncini 2003) in the older andormore experienced word learner this might also permit
greater direction of attention to the relevant phonolog-ical information in the word learning and access situa-tion Indeed as we argued at the beginning of thisarticle perhaps part of becoming a more expert wordlearner is knowing just which properties of a word aredefinitional in a particular linguistic community It maybe the emergence of this greater understanding of justwhat distinguishes words in our language that allows theolder more accomplished word learner to not onlyperceive but also lsquolsquomarkrsquorsquo and lsquolsquousersquorsquo the phonologicaldetail in word learning and word recognition tasks
In previous studies Mills and colleagues have re-ported a link between vocabulary size and the distribu-tion of the N200ndashN400 amplitude difference to knownand unknown words between 14 and 20 months of ageSimilarly Werker et al (2002) reported a significantcorrelation between performance on the CDI at both14 and 17 months and performance in the minimal pairswitch task There was no significant correlation with theCDI in the present study The lack of a significantcorrelation here could reflect lack of variability in thedata perhaps because the age groups selected are notyet in transition We predict that testing of a group ofchildren intermediate in age and vocabulary sizes to thegroups tested here would be more likely to reveal such apattern of findings
In summary using an ERP design we extended andhelped explain the pattern of findings seen in earlybehavioral tasks assessing the phonological detail repre-sented and used in early word learners The results ofthe current ERP study indicate that at the earliest stagesof word learning children treat minimal pair mispronun-ciations of known words as acceptable instances of thatword This provides strong evidence that in the decisionstage of word recognition novice word learners accept abroader range of pronunciations of the word as accept-able than do older children With increasing age andincreasing language sophistication the amount of pho-nological detail easily accessed in the representation ofwords increases helping to avoid mapping mistakes andto facilitate more rapid acquisition of a vocabulary
METHODS
Participants and Settings
Participants were tested in three locations at the Uni-versity of California San Diego at the University ofOregon Eugene Oregon and at the University of BritishColumbia Vancouver Canada Approximately one-thirdof the children in each age group were tested at eachsite Testing at the Vancouver site was conducted in amobile ERP lab belonging to the University of OregonChildren from the San Diego and Eugene areas wererecruited through advertisements in a local magazineposters displayed in the area requests at play groupsand referrals from parents whose children had partici-
1460 Journal of Cognitive Neuroscience Volume 16 Number 8
pated in our studies Children from the Vancouver areawere recruited through the University of British Colum-bia child Studies Center database Parents of all childrensigned consent forms consistent with the human sub-jects internal review boards at the university at whichthey were tested Parents were given $500 to defraytransportation costs and children were given a small toyin appreciation of their participation Only full-term(gt36 weeks of gestation) healthy children with mono-lingual experience with English participated in the studychildren with a family history of language impairmentwere excluded from the study
Data from 16 children aged 14 months (9 girls meanage = 14 months 15 days SD = 97 days range = 14months 1 day to 15 months 1 day) and 17 children aged20 to 21 months (9 girls mean age = 20 months 11 daysSD = 131 days age range = 19 months 14 days to 20months 27 days) were retained for analysis in the studyAn additional 18 children (12 boys nine 14-month-olds)were tested but were excluded from the analyses due totoo few artifact-free trials (n = 10) excessive crying (n =3) reversed polarity of the ERP response (n = 3 seenote)2 or refusal to wear the electrocap (n = 2)
Stimuli
The stimuli were naturally spoken in a female voice anddigitized at 16 bits 44 kHz sampling rate The stimuliconsisted of three types of words (a) 10 words whosemeanings were understood by the child (known wordseg lsquolsquomilkrsquorsquo) (b) 10 nonsense words that differed fromthe known words in the initial phonemic contrast (pho-nemic contrast words eg lsquolsquonilkrsquorsquo) and (c) 10 wordsthat differed phonetically from known words in allvowels and consonants (nonsense words eg lsquolsquoneemrsquorsquo)All stimuli were matched on word duration and numberof syllables The mean durations for the known wordsphonetic contrasts and nonsense words were 783 msecSD = 144 865 msec SD = 169 861 msec SD = 96[F(227) = 109 p = 34] Ideally all word-types wouldbe matched on manner of articulation (eg all stopconsonants all nasals all laterals all fricatives or allbalanced) However children at this age have very smallvocabularies and we were limited to common words thatalmost all children at this age would know Amongchildren first words are a disproportionate number ofwords that begin with stop consonants For many ofthose words minimal pair differences are still words(eg lsquolsquoballrsquorsquondashlsquolsquodollrsquorsquo) Thus we were constrained in ourselection of stimulus items For the known words therewere seven stop consonants two nasals and one frica-tive For the phonemic contrasts there are also sevenstop consonants two nasals and one fricative For thenonsense words there are three stop consonants oneaffricative two nasals two fricatives and two laterals Acomplete list of words is shown in Table 1
To evaluate at what point the known words couldphysically be distinguishable from the other word-typeswe examined two aspects of uniqueness (a) at whatpoint the phonetically dissimilar nonsense words be-come unique from their real word counterparts (egwhen does lsquolsquobearrsquorsquo become unique from lsquolsquogarersquorsquo) and (b)when the words might become unique from otherwords that would be in a babyrsquos vocabulary at this ageThe uniqueness point for initial consonants (the pointthat separates that consonant from other consonants)was within 20 msec of the beginning of the word forfricatives (s z f) 40 msec for stop consonants (b d gp t k) 60 msec for affricatives (j) and 20 msec for nasals(m n) and laterals (l r) To further examine theuniqueness points we compared each stimulus usedin the study relative to other words that might be in thechildrsquos vocabulary with the words in the MacArthur CDIthat would be considered close phonological neighborsTo this end we compared our stimuli to the number ofwords in the CDI with (a) the same consonant includingconsonant clusters and (b) the same consonant exclud-ing consonant clusters and (c) the same consonant plusthe vowel For comparisons a and b there are the mostneighbors for the known words (a = 474 b = 375) anintermediate amount for the phonemic contrast words(a = 371 b = 298) and the least for the nonsense words(a = 275 b = 233) The best comparison is to count thenumber of items that begin with the same consonantplus vowel (CV) There were no significant differences inthe number of CV neighbors for the three word-types(known = 16 phonemic contrasts = 12 nonsense =18) Additionally because of co-articulatory effects theacoustic energy for a given consonant varies as a func-tion of the following vowel (Jusczyk 1997) Thus evenwords beginning with the same consonant (eg lsquolsquobookrsquorsquoand lsquolsquobottlersquorsquo) differ in shape and spectral frequenciesand can be discriminated from each other as well as
Table 1 Stimuli
Known Words Phonemic Contrasts Nonsense
Bear Gare Kobe
Ball Pall Lif
Book Dook Neem
Bottle Pottle Fipe
Cup Tup Mon
Cat Gat Tek
Dog Bog Riss
Milk Nilk Keed
Nose Mose Jud
Shoe Zhu Zav
Mills et al 1461
their close phonological neighbors from the beginningof the word
Procedure
Language Assessment
Within one week prior to testing parents were asked tocomplete the MacArthur CDI (Fenson et al 1994)which provided an estimate of the childrsquos vocabularysize and percentile ranking relative to other children ofthe same age To ensure that the words to be used in thestudy as lsquolsquoknownrsquorsquo were comprehended by the childparents also completed a vocabulary checklist ratingscale indicating how sure they were that their childunderstood andor produced each word on a scale of1 (very sure they did not know that word) to 4 (very suretheir child understoodproduced a given word in avariety of different contexts and with different exem-plars) Additionally the children were asked to identify apicture of each word to be used as a known word from atwo-choice picture book All lsquolsquoknownrsquorsquo words used hadreceived a rating of 4 and were correctly identified in thepicture-pointing task
Electrophysiological Recording
The EEG was recorded using tin electrodes affixed to anelastic cap (Electro-Cap International) from sites overfrontal (F7 and F8) anterior temporal (50 of thedistance from F78 and T34) temporal (33 of thedistance from T34 to C34) parietal (50 of the dis-tance between T34 and P34) and occipital (O1 andO2) regions of the left and right hemispheres Addition-ally the electrooculogram was recorded from electrodesplaced over and under the eye to reject trials on whichblinks and vertical eye movement occurred and fromleft and right frontal electrodes to reject trials on whichhorizontal eye movement occurred Impedances werekept below 5 k and were balanced (within 1 k) acrossthe left and right hemispheres at any given position TheEEG was amplified by SA Instruments amplifiers with abandpass of 01 to 100 Hz and sampled continuouslyevery 4 msec All electrodes were referenced to linkedmastoids3 Averages of the EEG were conducted using2-sec epochs (ie 100 msec prestimulus and 1900 msecpoststimulus) The averaged ERPs were also digitallyfiltered off-line with a 60-Hz low-pass filter
Testing
Ten known 10 phonetically similar nonsense words(referred to as phonemic contrasts) and 10 phoneticallydissimilar nonsense (referred to as nonsense words)words were each presented six times in random orderfor a total of 60 trials per condition During testingchildren sat on their parentrsquos lap and listened to words
presented from a speaker located behind a movingpuppet in a puppet theater Words were presented ata variable rate between 1800 and 3000 msec SOA
Artifact Rejection
Artifact rejection was conducted off-line using a com-puter program to reject blinks and horizontal eye move-ment and amplifier blocking Individual thresholds wereset for each child based on visual inspection of the EEGepochs time-locked to each stimulus A mean of 50 ofthe trials were rejected due to eye and movementartifact The number of artifact-free trials per word-typeretained for analysis ranged from 12 to 53 (SD = 113)out of a possible 60 trials per condition There were nosignificant differences in the percentage of trials rejectedfor the different experimental conditions age groupsor sex
Measurement of ERP Components
Peak latencies and amplitudes were quantified by com-puter with reference to the 100-msec prestimulus base-line for the maximum negative or positive point in aspecified time window The time windows for eachcomponent were set according to the criteria used inour previous studies the first positive componentcalled the P100 was defined as the most positivedeflection between 50 and 175 msec The P100 indexesauditory sensory processing Moreover the lateral dis-tribution of the P100 has been shown to vary withpercentile ranking on the MacArthur CDI score Chil-dren who score at the 50th percentile or higher for theirage show a P100 left greater than right asymmetry Themean amplitude within the time window 200ndash400 afterword onset was quantified by computer with referenceto the 100-msec prestimulus baseline This time windowwas chosen because it had been shown to differ forknown and unknown words in our previous studies
Acknowledgments
This research was conducted while the first author was at theProject in Cognitive and Neural Development at the Universityof California at San Diego The research was supported by NIHgrant DC0048111 to H Neville from NIDCD NSERC (NaturalSciences and Engineering Research Council of Canada) grantto J Werker and HFSP (Human Frontierrsquos Science Program)grant to J Werker and H Neville The third author RenateZangl was funded in part by two postdoctoral grants (J-1646J-1817) from the Austrian Science Foundation
We thank Amy Adamson Greg Appelbaum Teresa Mitchelland Lisa Sanders (in alphabetical order not necessarily in orderof magnitude of contribution) for their help in acquiring andanalyzing the data and Chris Fennell and Liz Lewis for theircomments on an earlier version of the manuscript We areespecially indebted to the children and their parents whoparticipated in these studies and their parents without whomthe research would not be possible
1462 Journal of Cognitive Neuroscience Volume 16 Number 8
Reprint requests should be sent to Debra L Mills who is nowat the Department of Psychology 532 Kilgo Circle EmoryUniversity Atlanta GA 30322 or via e-mail dmills2emoryedu
Notes
1 Unlike the switch task the visual fixation task is an on-linetask with two simultaneously presented choices The twoindicators of success are shorter latency to look away from theincorrect object and longer looking times overall to the correctmatch The habituation phase in the lsquolsquoswitchrsquorsquo task leads to theprediction of a novelty preference in the test phase (ie longerlooking to the incorrect pairing)2 In previous studies Mills and colleagues typically findapproximately 10ndash15 of children show a reversed ERPresponse with a larger amplitude ERP response to unknownthan to known words In previous work Mills and colleagueshave treated those children as noise in the data and have keptthem in for the data analyses Ultimately we want tounderstand the nature of lexical representations in this groupof children showing an aberrant ERP response For the currentstudy however in order to compare the ERP response tophonemic contrasts to known words we felt it necessary toinclude in our sample only those children who showed themuch more typical pattern of response resulting in the ex-clusion of three children All effects reported here show thesame pattern with these children included but the variabilityfrom their inclusion did reduce the differences in somecomparisons3 We are aware of the controversies surrounding the use oflinked mastoids Linked mastoids were used here to increasethe number of active cites given the number of amplifiersavailable and to provide consistency with previous studies Apilot study using similar auditory stimuli was conducted toexamine possible distortions in the distribution of scalp activityresulting from forced linkage This was determined by re-cording from one mastoid using the other as a reference andlinking the mastoids off-line These pilot data were comparedwith the data recorded using linked mastoids and did not yieldsignificant differences
REFERENCES
Bailey T M amp Plunkett K (2002) Phonological specificity inearly words Cognitive Development 17 1265ndash1282
Bertoncini J amp Mehler J (1981) Syllables as units in infantspeech perception Infant Behavior and Development 4247ndash260
Brown C amp Matthews J (1997) The role of feature geometryin the development of phonemic contrasts In S J Hannahsamp M Young-Scholten (Eds) Focus on phonologicalacquisition (pp 67ndash112) Amsterdam Benjamins
Brown C M Hagoort P amp Kutas M (2000) Postlexicalintegration processes in language comprehension Evidencefrom brain-imaging research In M Gazzaniga (Ed) The newcognitive neurosciences (2nd ed pp 881ndash895)Cambridge MIT Press
Conboy B (2003) Patterns of language processing and growthin early EnglishndashSpanish bilingualism (Doctoral dissertationUniversity of California San Diego and San Diego StateUniversity 2000) Dissertation Abstracts InternationalB The Sciences and Engineering 63 (UMI No 5193)
Conboy B amp Mills D L (2000) Cerebral specialization forlanguage comprehension in bilingual toddlers CognitiveNeuroscience Society 2000 Annual Meeting AbstractsSupplement of the Journal of Cognitive Neuroscience 17
Connolly J F Service E DrsquoArcy R C N Kujala A Alho K(2001) Phonological aspects of word recognition as revealedby high-resolution spatio-temporal brain mappingNeuroReport 12 237ndash243
Dehaene-Lambertz G (2000) Cerebral specialization forspeech and non-speech stimuli in infants Journal ofCognitive Neuroscience 12 449ndash460
Dehaene-Lambertz G amp Dehaene S (1994) Speed andcerebral correlates of syllable discrimination in infantsNature 370 292ndash294
Eimas P D Siqueland E R Jusczyk P amp Vigorito J (1971)Speech perception in infants Science 171 303ndash306
Fennell C E amp Werker J F (2003) Early word learnersrsquoability to access phonetic detail in well-known wordsLanguage and Speech 46 245ndash264
Fenson L Dale P S Reznick J S Bates E Thal D Jamp Pethick S J (1994) Variability in early communicativedevelopment Monographs of the Society for Research inChild Development 59 1ndash173
Fernald A Swingley D amp Pinto J P (2001) When half aword is enough Infants can recognize spoken words usingpartial phonetic information Child Development 721003ndash1015
Fischler I S (1990) Comprehending language withevent-related potentials In J W Rohrbaugh R Parasuramanamp R Johnson (Eds) Event-related brain potentials(pp 165ndash170) New York Oxford University Press
Halle P amp de Boysson-Bardies B (1994) Emergence of anearly receptive lexicon Infantsrsquo recognition of wordsInfant Behavior and Development 17 119ndash129
Halle P amp de Boysson-Bardies B (1996) The format ofrepresentation of recognized words in infantsrsquo earlyreceptive lexicon Infant Behavior and Development 19463ndash481
Holcomb P J amp Neville H J (1991) The electrophysiologyof spoken sentence processing Psychobiology 19286ndash300
Holcomb P J Coffey S A amp Neville H J (1992) Visual andauditory sentence processing A developmental analysisusing event-related brain potentials DevelopmentalNeuropsychology 8 203ndash241
Jusczyk P W (1997) The discovery of spoken languageCambridge MIT Press
Jusczyk P W amp Aslin R N (1995) Infantsrsquo detection ofsound patterns of words in fluent speech CognitivePsychology 29 1ndash23
Kutas M amp Hillyard S (1980) Reading senseless sentencesBrain potentials ref lect semantic incongruity Science 207203ndash205
Luck S amp Hillyard S (2000) The operation of selectiveattention at multiple stages of processing Evidence fromhuman and monkey electrophysiology In M Gazzaniga(Ed) The new cognitive neurosciences (2nd edpp 687ndash700) Cambridge MIT Press
Mills D L Coffey-Corina S A amp Neville H J (1997)Language comprehension and cerebral specialization from13ndash20 months [(Special issue on origins of languagedisorders)] In D Thal amp J Reilly (Eds) DevelopmentalNeuropsychology 13 397ndash446
Mills D M Coffey-Corina S A amp Neville H J (1994)Variability in cerebral organization during primary languageacquisition In G Dawson amp K Fischer (Eds) Humanbehavior and the developing brain (pp 427ndash455) NewYork Guilford Publications
Mills D M Coffey-Corina S A amp Neville H J (1993)Language acquisition and cerebral specialization in20-month-old infants Journal of Cognitive Neuroscience 5326ndash342
Mills et al 1463
Mills D L Conboy B amp Paton C (in press) Do changes inbrain organization reflect shifts in symbolic functioning InL Namy (Ed) Symbol use and symbolic representationMahwah NJ Erlbaum
Mills D L Plunkett K Prat C amp Schafer G (in press)Watching the infant brain learn words Effects ofvocabulary size and experience Cognitive Development
Molfese D L (1989) Electrophysiological correlates of wordmeanings in 14-month-old human infants DevelopmentalNeuropsychology 5 79ndash103
Molfese D L (1990) Auditory evoked responses recordedfrom 16-month-old human infants to words they did anddid not know Brain and Language 38 345ndash363
Molfese D L Burger-Judisch L M amp Hans L L (1991)Consonant discrimination by newborn infantsElectrophysiological differences DevelopmentalNeuropsychology 7 177ndash195
Molfese D L amp Molfese V J (1988) Right hemisphereresponses from preschool children to temporal cuescontained in speech and nonspeech materialsElectrophysiological correlates Brain and Language33 245ndash259
Molfese D L Morse P A amp Peters C J (1990) Auditoryevoked responses to names for different objectsCross-modal processing as a basis for infant languageacquisition Developmental Psychology 26 780ndash795
Molfese D L Wetzel W F amp Gill L A (1993) Known versusunknown word discriminations in 12-month-old humaninfants Electrophysiological correlates DevelopmentalNeuropsychology 9 241ndash258
Naigles L R (2002) Form is easy meaning is hard Resolvinga paradox in early child language Cognition 8 157ndash199
Nazzi T amp Bertoncini J (2003) Before and after thevocabulary spurt Two modes of word acquisitionDevelopmental Science 6 136ndash142
Osterhout L amp Holcomb P J (1995) Event-related potentialsand language comprehension In M D Rugg amp M G HColes (Eds) Electrophyisology of mind (pp 171ndash215)Oxford Oxford University Press
Richards J E amp Hunter S K (2001) Testing neural models ofthe development of infant attention DevelopmentalPsychobiology 40 226ndash236
Sereno S C Brewer C C amp OrsquoDonnell P J (2003)Context effects in word recognition Evidence for earlyinteractive processing Psychological Science 14328ndash333
Simos P G amp Molfese D L (1997) Electrophysiologicalresponses from a temporal order continuum in the newborninfant Neuropsychologia 35 89ndash98
Simos P G Molfese D L amp Brenden R A (1997) Behavioraland electrophysiological indices of voicing-cuediscrimination Laterality patterns and developmentBrain and Language 57 122ndash150
St George M amp Mills D L (2001) Electrophysiologicalstudies of language development In J Weissenborn ampB Hoehle (Eds) Language acquisition and languagedisorders (pp 247ndash259) Amsterdam John BenjaminsPublishing
Stager C L amp Werker J F (1997) Infants listen for morephonetic detail in speech perception than in word-learningtasks Nature 388 381ndash382
Swingley D (2003) Phonetic detail in the developing lexiconLanguage and Speech 46 265ndash294
Swingley D amp Aslin R N (2000) Spoken word recognitionand lexical representation in very young children Cognition76 147ndash166
Swingley D amp Aslin R N (2002) Lexical neighborhoodsand the wordndashform representations of 14-month-oldsPsychological Science 13 480ndash484
Werker J F Cohen L B Lloyd V L Casasola M amp StagerC L (1998) Acquisition of wordndashobject associations by14-month-old infants Developmental Psychology 341289ndash1309
Werker J F amp Fennell C E (2004) From listening to soundsto listening to words Early steps in word learning In G Hallamp S Waxman (Eds) Weaving a lexicon (pp 79ndash109)Cambridge MA MIT Press
Werker J F Fennell C E T Corcoran K M amp Stager C L(2002) Infantsrsquo ability to learn phonetically similar wordsEffects of age and vocabulary size Infancy 3 1ndash30
Werker J F amp Tees R C (1984) Cross-language speechperception Evidence for perceptual reorganization duringthe first year of life Infant Behavior and Development 749ndash63
1464 Journal of Cognitive Neuroscience Volume 16 Number 8
mismatch whereas in the children aged 14 months totallooking time to the lsquolsquocorrectrsquorsquo object yielded the mostconsistent results It is also problematic to draw defin-itive conclusions when different dependent variables areused with different age groups Finally whenever look-ing behaviors are used as dependent variables it isdifficult to ascertain whether looking time differencesreflect differences in detection encoding or the finalrepresentation
Electrophysiological measures provide a useful com-plement to looking time measures Previous child ERPstudies (St George amp Mills 2001 Mills Coffey-Corina ampNeville 1993 1994 1997 Molfese 1989 1990 MolfeseWetzel amp Gill 1993) have shown different patterns ofneural activity to known versus unknown words inchildren as young as 12 months of age Mills andcolleagues compared event-related potentials (ERPs) towords whose meanings the child did and did notcomprehend Results revealed larger amplitude ERPsto the known versus the unknown words at 200ndash400 msec following word onset At 13ndash17 months thisamplitude difference was evident over both hemi-spheres over frontal temporal parietal and occipitalsites By 20 months the ERP difference was limited totemporal and parietal regions of the left hemisphereThe results suggested there were changes in the orga-nization of brain activity linked to word processingwithin that age range that may be linked to vocabularysize The children aged 20 months who were post(gt 150 words) vocabulary spurt showed more focallydistributed ERPs to known words whereas children withsmaller vocabularies (lt50 words) showed a more dis-tributed response indicating that the increasingly localnature of the differential ERP signature seen in olderchildren reflects greater sophistication in word knowl-edge rather than age per se Although the 200ndash400 mseclatency may seem early for indexing word meanings inchildren looking time paradigms with children havealso shown evidence for word recognition by the middleof the word (eg Fernald Swingley amp Pinto 2001)Moreover this time window is consistent with ERPstudies of lexical access and word comprehension inadults (eg Sereno Brewer amp OrsquoDonnell 2003 Osterh-out amp Holcomb 1995 Holcomb amp Neville 1991 Fisch-ler 1990 Kutas amp Hillyard 1980) In adults ERPdifferences to contextually appropriate versus inappro-priate words typically emerge around 200 msec (BrownHagoort amp Kutas 2000 Kutas amp Hillyard 1980) andhave been observed as early as 50 msec (Holcomb ampNeville 1991)
In this article we used an ERP paradigm similar toMills et al (1997) to examine the nature of the phoneticdetail in lexical representations in children of 14 and20 months of age with one exception Instead of pre-senting the children with randomly varying presenta-tions of 10 known and 10 unknown words we added 10unknown words that were phonetically similar to the
known words For example we presented children withknown words such as lsquolsquodog cat shoe milkrsquorsquo and soforth and phonetically dissimilar nonsense words suchas lsquolsquoneem blick zav kobersquorsquo As an extension we added aset of nonsense words that were phonetically similar tothe known words These lsquolsquophonemic contrastrsquorsquo noncewords included items such as lsquolsquobog gat zue and nilkrsquorsquoWe predicted that children of 14 and 20 months wouldshow larger amplitude ERPs to the known versus thephonetically dissimilar unknown words and that thiswould be seen across most electrode sites at 14 monthsbut would be more localized at 20 months Of particularinterest were ERPs to the phonemic contrast words Wereasoned that if there is sufficient detail in the lexicalrepresentation the ERP signature to these phonemiccontrast foils should be like that of phonetically dissim-ilar unknown words However if the lexical representa-tion is not detailed the phonemic contrast items shouldbe confused with known words
RESULTS
ERPs for all three word-types over anterior and posteriorregions of both the left and right hemispheres areshown for the 14- and 20-month-olds in Figures 1 and
Figure 1 ERPs to all three word-types for the sixteen 14-month-oldsover the anterior and posterior regions of the left and right
hemispheres ERPs to known words are shown in the solid lines to
phonemic contrasts in the dotted lines and to phonetically dissimilar
nonsense words in the dashed lines
1454 Journal of Cognitive Neuroscience Volume 16 Number 8
2 respectively The results are presented for two ERPcomponents the peak amplitude and latency of the firstpositive component P100 and the mean area of thenegativity between 200 and 400 msec poststimulusonset These components were chosen because ourprevious studies found them to be sensitive to vocabu-lary size andor word meanings Also in previous studiesthe N600ndashN900 differed for known and unknown wordsat 13ndash17 but not 20 months (Mills et al 1997) Visualinspection of the present data for the 14-month-olds(Figure 1) suggested differences in this time windowbetween the mean amplitude for the phonetically similarnonsense word and the other two word-types None ofthe analyses conducted on the later time windowreached significant levels There were no interactionswith age or vocabulary size Therefore to conservespace these analyses were not included
Data Analysis
A priori hypotheses predicted that the 14-month-oldsand 20-month-olds would show different patterns ofERPs to the phonemic contrast words relative to the
known and nonsense words Therefore the ERP datawere analyzed separately for the two age groups inrepeated-measures ANOVAs with Word-type (knownphonemic contrast nonsense words) Hemisphere (leftand right) and Electrode site (frontal anterior temporaltemporal parietal and occipital) as the within-subjectsfactors HuynhndashFeldt corrections were used for all therepeated-measures analyses We were particularly inter-ested in changes in the lateral distribution of the N200ndashN400 for the 14- and 20-month-olds to known wordscompared with the two types of nonsense words Theseplanned simple effects were analyzed for the 14- and 20-month-olds separately in repeated-measures ANOVAscomparing two word-types (known words vs phonemiccontrasts known vs nonsense words and phonemiccontrasts vs nonsense words) by hemisphere (left andright) and by electrode site (frontal anterior temporaltemporal parietal occipital) Age group differenceswere analyzed in a two-way ANOVA with Age group asthe between-subjects variable and Word-type (knownwords phonemic contrasts nonsense words) Hemi-sphere (left and right) and Electrode site (frontalanterior temporal temporal parietal and occipital) asthe within-subjects factors
Children Aged 14 Months
P100 The first positive component peaked at 125 msec(P100) The P100 peaked later and was larger overfrontal than posterior regions [electrode site latencyF(450) = 934 p lt 001 amplitude F(460) = 753 p lt001] There were no main effects or interactions forpeak latencies or amplitude among word-types or be-tween the left and right hemispheres
N200ndashN400 The mean amplitudes between 200 and400 msec (N200ndashN400) were larger over posterior thananterior regions [electrode site F(460) = 514 p lt01] and differed with word-type [F(230) = 376 p lt05] There were no main effects or interactions withhemisphere
Planned comparisons showed that the N200ndashN400 waslarger for known words than for nonsense words (ielsquolsquobearkobersquorsquo Figure 3 top) [F(115) = 494 p lt 05]Importantly the N200ndashN400 amplitude was also greaterfor the phonemic contrast words than for the nonsensewords (lsquolsquogarekobersquorsquo Figure 3 bottom) [F(115) = 697p lt 05] However the N200ndashN400 was not significant-ly different for known words versus phonemic contrastwords (ie lsquolsquobearrsquorsquolsquolsquogarersquorsquo Figure 3 middle) [F(115) =007 p = 80] Thus both known and phonemic con-trast words showed the same activity as lsquolsquoknownrsquorsquo andwere different from unknown lsquolsquononsensersquorsquo words
We also predicted age-related differences in the lateraldistribution of the N200ndashN400 effects As predicted forthe 14-month-olds the N200ndashN400 mean amplitudedifferences between the word-types were broadly dis-tributed over anterior and posterior regions of both the
Figure 2 ERPs to all three word-types for the seventeen
20-month-olds over the anterior and posterior regions of the left
and right hemispheres ERPs to known words are shown in the solid
lines to phonemic contrasts in the dotted lines and to phoneticallydissimilar nonsense words in the dashed lines
Mills et al 1455
left and right hemispheres [known vs nonsense lefthemisphere F(115) = 230 p lt 15 right hemisphereF(115) = 745 p lt 05 phonemic contrasts vs non-sense words left hemisphere F(115) = 458 p lt 05right hemisphere F(115) = 797 p lt 01]
Children Aged 20 Months
P100 The P100 peaked later and was larger over frontalthan posterior regions [latency F(116) = 700 p lt001 amplitude F(116) = 1407 p lt 001] There wasalso a main effect of word-type for the P100 amplitude[F(232) = 355 p lt 05] Examination of the amplitudedifferences across word-types showed that the P100 waslarger to known words than to nonsense words [knownvs nonsense F(116) = 707 p lt 05] and phonemiccontrasts [known vs phonemic contrasts F(116) =605 p lt 05] The P100 amplitude was larger overthe left than the right hemisphere but only overtemporal and parietal sites [Hemisphere Electrodesite F(464) = 300 p lt 05] There were no othermain effects or interactions
N200ndashN400 The N200ndashN400 amplitude differences tothe different word-types approached significance [word-type F(116) = 303 p = 06] The N200ndashN400 waslarger over the left than the right hemisphere at tempo-ral and parietal sites but larger over the right than theleft over the occipital regions [Hemisphere Electrodesite F(464) = 283 p lt 05]
Planned comparisons showed that the N200ndashN400was larger to known than nonsense words [F(116) =576 p lt 05] but only over left temporal [F(116) =826 p lt 01] left parietal [F(116) = 1062 p lt 01]and right frontal [F(116) = 602 p lt 05] sites (Figure 4top) The N200ndashN400 was also larger to known wordsversus phonemic contrast words over left temporal[F(116) = 724 p lt 05] and left parietal [F(116) =709 p lt 05] regions (Figure 4 middle) However themain effect of word-type for this comparison only ap-proached significance [F(116) = 270 p = 10] Therewere no significant differences in N200ndashN400 amplitudesto the phonemic contrasts and nonsense words(Figure 4 bottom) [F(116) = 042 ns] At 20 monthsthen the phonemic contrast (nonsense) words were no
Figure 3 For the 14-month-olds ERP differences are directly
compared to known words and nonsense words (top) known words
and phonemic contrasts (middle) and phonemic contrasts andphonetically dissimilar nonsense words (bottom) Significant
differences in N200ndashN400 mean amplitudes are shaded and enclosed in
the rectangle Left and temporal and parietal regions are shown on theleft and right sides of the figure respectively
Figure 4 For the 20-month-olds ERP differences are directly
compared to known words and nonsense words (top) known words
and phonemic contrasts (middle) and phonemic contrasts andphonetically dissimilar nonsense words (bottom) Significant
differences in N200ndashN400 mean amplitudes are shaded and enclosed in
the rectangle Left and temporal and parietal regions are shown on theleft and right sides of the figure respectively
1456 Journal of Cognitive Neuroscience Volume 16 Number 8
longer treated as known words but were instead treatedlike other nonsense words
Age Group Comparisons
There were no main effects or interactions with agewhen all three word-types and all electrode sites wereincluded in the analyses Because the 20-month-olds inthis study and in our previous studies only showed ERPdifferences in this time window to known versus un-known words at temporal and parietal sites a separateANOVA was conducted including only temporal andparietal sites that is with two levels of Age group (14and 20 months) as the between-subjects factor andWord-type (known phonemic contrasts and nonsensewords) Hemisphere (left and right) and Electrodesite (temporal and parietal) as the within-subjects fac-tors That analysis showed main effects for Word-type[F(131) = 659 p lt 001] and Hemisphere [F(131) =395 = 420 p lt 05] These effects were qualifiedby two interactions with group including a Group Word-type Hemisphere interaction [F(662) = 304p lt 05] and a Group Electrode site interaction[F(131) = 396 p lt 05] These interactions were fur-ther validated by the different patterns displayed by the14- and 20-month-olds separately
To further support the age-related effects describedabove separate ANOVAs were conducted with Age (14and 20 months) as the between-subjects factor andWord-type (known phonemic contrasts and nonsensewords) Hemisphere (left and right) and Electrode site(frontal anterior temporal temporal parietal and oc-cipital) as the within-subjects factors To conserve spaceonly main effects of and interactions with age arereported below
P100 There were no main effects or interactions withage for the P100 latency The age-related difference inthe lateral distribution of the P100 amplitude was sup-
ported by an Age Hemisphere Electrode siteinteraction [F(4124) = 275 p lt 05]
N200ndashN400 There were no main effects or interac-tions with age when all three word-types and all elec-trode sites were included in the analyses Because the20-month-olds in previous studies and in this study asdescribed above only showed ERP differences in thistime window to known versus unknown words at tem-poral and parietal sites a separate ANOVA was con-ducted including only temporal and parietal sitesnamely with two levels of Age group (14 and 20 months)as the between-subjects factor and Word-type (knownphonemic contrasts and nonsense words) Hemisphere(left and right) and Electrode site (temporal andparietal) as the within-subjects factors That analysisshowed two interactions with group including a Groupby Word-type Hemisphere interaction [F(662) =304 p lt 05] and a Group Electrode site interaction[F(131) = 396 p lt 05] These interactions supportthe findings described above showing different patternsof responsiveness to the different word-types by the 14-and 20-month-olds separately
Summary
Fourteen- and 20-month-olds showed different patternsof brain activity to known words (eg lsquolsquobearrsquorsquo) phone-mic contrasts (eg lsquolsquogarersquorsquo) and (phonetically dissimilar)nonsense words (eg lsquolsquokobersquorsquo) (Figure 5) As predictedby Stager and Werker (1997) 14-month-olds showedlarger N200ndashN400 amplitudes to known words com-pared with nonsense words (eg lsquolsquokobersquorsquo) Howeverthe N200ndashN400 to known words and the phonemiccontrast stimuli (eg lsquolsquobearrsquorsquo and lsquolsquogarersquorsquo) did not differfrom each other
Unlike the pattern seen at 14 months and as pre-dicted by Werker et al (2002) the 20-month-oldsshowed larger amplitude N200ndashN400 responses to
Figure 5 N200ndashN400 mean
areas averaged across temporal
and parietal regions of the leftand right hemispheres to the
three word-types for the
14- and 20-month-olds
Mills et al 1457
known words (eg lsquolsquobearrsquorsquo) over both phonemic con-trast (eg lsquolsquogarersquorsquo) and other nonsense words (eglsquolsquokobersquorsquo) Moreover the N200ndashN400 amplitudes to thephonemic contrast and other nonsense words (lsquolsquogarersquorsquoand lsquolsquokobersquorsquo) did not differ from each other Thesefindings also replicated previously observed lateral dif-ferences in the N200ndashN400 amplitudes to known versusunknown words between 14 and 20 months (Mills et al1997)
DISCUSSION
The results from the present study were consistent withand expanded on findings observed in previous behav-ioral and ERP studies At both 14 and 20 months of agethe pattern of ERP responses seen replicated that pre-viously reported by Mills et al (1993 1997) for knownversus unknown words At both ages there was a largeramplitude negative response from 200 to 400 msec toknown versus nonsense words And as shown before at14 months of age this response was broadly distributedacross the scalp whereas at 20 months it was observedprimarily over the left temporal and parietal electrodesites
The replication of the previous work by Mills andcolleagues puts us in a particularly strong position tointerpret the results from the phonemic contrast wordsAt 14 months of age the ERP signature to phonemiccontrast nonsense words (eg lsquolsquogarersquorsquo or lsquolsquobogrsquorsquo) isindistinguishable from that to known words (eg lsquolsquobearrsquorsquoand lsquolsquodogrsquorsquo) and significantly greater in amplitude thanthat to dissimilar nonsense words (eg lsquolsquolifrsquorsquo) Thus at14 months of age the neural response appears toindicate a mistaken recognition of a minimal pair mis-pronunciation of a known word as the word itself Thisresult is consistent with that seen in the switch task at14 months (Werker et al 2002 Stager amp Werker 1997)The results with the ERP task are particularly convincingbecause here we see a significant increase in the ampli-tude of the response rather than a failure to increaselooking time At 20 months of age the ERP response isalso consistent with results seen in the switch task at thesame age (Werker et al 2002) At 20 months the ERPsignature to phonemic contrast nonsense words (eglsquolsquogarersquorsquo and lsquolsquobogrsquorsquo) is significantly different from that totheir known counterparts (eg lsquolsquobearrsquorsquo and lsquolsquodogrsquorsquo) andindistinguishable from that to phonetically dissimilarnonsense words (eg lsquolsquoneemrsquorsquo or lsquolsquolifrsquorsquo) Thus at20 months the evoked response indicates that childrenare showing the lsquolsquoword recognitionrsquorsquo ERP response onlyto known words that are phonologically correct in alldetails By 20 months then the confusion with similar-sounding known words is no longer present indicatingthat words in the lexicon are not only fully specified interms of phonological detail but that such detail isavailable even when listening to words in tasks that donot provide contextual or pictorial support for the word
meaning and may not even involve full attention to theword
One difference between the present study and thebehavioral studies by Werker and colleagues and Swin-gley and Aslin is that the behavioral studies used bothpictures and words whereas the present study pre-sented a series of words without a picture context Anegative going ERP wave within the 200ndash500 msec timewindow has also been shown to index word meaningin cross-modal matchmismatch paradigms using pic-tures and words in 14- and 20-month-old children (MillsConboy amp Paton in press) and real objects and noveltrained words in 14-month-old children (Molfese Morseamp Peters 1990) We chose to use the Mills et al (1997)paradigm rather than a pictureword matchmismatchparadigm for several reasons The pictureword matchmismatch paradigm elicits a large amplitude bilateralnegative component starting at 200 msec and peakingaround 500 msec when the subsequent word does notmatch the picture at both 14 and 20 months (MillsConboy et al in press) This ERP effect is most likelywithin the N400 family observed in children and adults toviolations of semantic expectancy (Holcomb Coffey ampNeville 1992 Kutas amp Hillyard 1980) In adults a similarparadigm elicited a phonological mismatch (PMN) towords and nonwords from 250 to 347 msec over rightfrontal regions and from 347 to 638 msec over symmet-rical centro-parietal regions (Connolly Service DrsquoArcyKujala amp Alho 2001) One concern was that the N400 andPMN effects elicited by a pictureword matchmismatchparadigm might swamp more subtle differences in thelatency amplitudes and distributions of ERPs related tothe different word-types and group differences The Millset al paradigm was chosen because it showed ERP dif-ferences to known and unknown words that varied withword-type age and vocabulary size The methodologicaldifferences should be taken into consideration whencomparing the effects However the ERP findings areconsistent with and provide additional support for theage-related differences reported by Stager et al (1997)
The robust differences in the ERP pattern at 14and 20 months of age could be explained at manylevels Several studies have shown that a negativepeak at around 200 msec is sensitive to phonetic varia-tion in voice onset time (Simos Molfese amp Brenden1997 Molfese amp Molfese 1988) place of articulation(Dehaene-Lambertz amp Dehaene 1994 Molfese Burger-Judisch amp Hans 1991) and acoustic cues in nonspeechstimuli (Dehaene-Lambertz 2000 Simos amp Molfese1997) in very young children and even newborns Oneinterpretation is that ERP differences between word-types are related to the acoustic or phonological featuresof the different word-types and that age-related changesare due to differences in the childrenrsquos ability to mem-orize the different word lists Although phonological dis-crimination can modulate the amplitude of the N200 incertain paradigms it is not necessarily the only factor
1458 Journal of Cognitive Neuroscience Volume 16 Number 8
that may contribute to the amplitude of this componentHere we argue that age-related changes in a top-downprocess linked to word meaning is the dominant factormodulating the amplitude of the N200ndashN400 rather thanphonological discrimination First if ERP differences be-tween conditions were due solely to phonological fac-tors then both age groups would have displayed N200amplitude differences between the known words andphonetic contrasts Additionally the known and phone-mic contrasts were balanced for the types of initialconsonants in the number of stop consonants nasalsand fricatives and did not differ in their phonologicalneighbors for other words frequently understood bychildren in this age range (see Methods) Therefore itis difficult to explain why the phonemic contrasts pat-terned with the known words at 14 months and thenonsense words at 20 months based solely on acousticaland phonemic differences A second line of evidencesuggesting that the ERP differences observed here can-not be explained by phonological differences betweenword-types comes from a study of bilingual toddlersusing the same paradigm In that study the N200ndashN400did not differ between Spanish and English for either theknown or unknown words (Conboy amp Mills 2000 Con-boy 2003) ERP differences were linked to word meaning(known vs unknown words) language dominance andtotal vocabulary size but not phonological differencesbetween Spanish and English If the N200ndashN400 indexedthe phonological aspects of the words amplitude differ-ences would be expected between Spanish and EnglishA third and perhaps the strongest line of evidencecomes from a training study with 20-month-old childrenin which nonsense words were either repeated or pairedwith an object (Mills Plunkett Prat amp Schafer in press)After the training phase when objects were no longerpresent the amplitude of the N200ndashN400 to the newlylearned words increased relative to before training butbecame more positive for nonsense words that wererepeated the same number of times Because the stimuliwere counterbalanced across participants the increasedamplitude of the N200ndashN400 to the newly learned wordswas modulated by its association with a meaningfulstimulus and cannot be explained by phonological dif-ferences between word-types
Another interpretation of the findings is that at 14months of age the phonetic detail distinguishing oneword from another is simply not available in the lexicalrepresentation Such an explanation would be compat-ible with theories of phonological development thatposit an initial under-specification of the informationin the lexicon that is only gradually filled in as thevocabulary expands (eg Brown amp Matthews 1997)Although possible we think this explanation is unlikelygiven the work by Swingley and Aslin (2002) showingthat children of 14 months do seem to show someevidence of use of phonetic detail in word recognitiontasks via longer looking time to the matching object
when a correct versus a mispronounced version of aword is given The Swingley and Aslin work involvesshowing the children two objects (eg a car and a dog)and asking them to find either the correctly labeledobject (eg lsquolsquodogrsquorsquo) or a mispronunciation of that sameobject (eg lsquolsquobogrsquorsquo) More recently Fennell and Werker(2003) showed that when tested on two phonemiccontrast words that they already know well (eg lsquolsquoballrsquorsquoand lsquolsquodollrsquorsquo) the child of 14 months can succeed even inthe switch task These two studies show that at somelevel in the system full phonetic specification of well-known words is represented even at 14 months
The successes in the two-choice (Swingley and Aslin)behavioral lsquolsquomispronunciationrsquorsquo task and in the switchtask using two well-known words in comparison to boththe switch word-learning task and the ERP word recog-nition task are compatible with the attentional resourcelimitation hypothesis offered by Werker and colleagues(Werker et al 2002 Werker amp Fennell 2004 Fennell ampWerker 2003 Stager amp Werker 1997) According to thisexplanation the task of linking a word to an object ischallenging for a novice word learner All the detail ispicked up but the computational difficulty of actuallymapping the word on to the object makes it difficultto hold all the information in mind As such one sees aU-shaped function in the pattern of data The child of7ndash8 months who is not yet actively engaged in mappingwords to referents is better able to attend to fine pho-netic detail than is the child of 14 months who is at-tempting to make and remember the link (see Stager ampWerker 1997 see also Halle amp de Boysson-Bardies1996 Jusczyk amp Aslin 1995) Becoming a word learnerchanges the task for the older child This increases theprocessing demands and interferes with the childrsquosability to access detail The information is picked upin perception it is just not available for access in lexicalrecall tasks After the child becomes a more accom-plished word learner and has a working phonemicinventory to guide information access the relevantphonetic differences lsquolsquostand outrsquorsquo as salient and impor-tant to the lexical entry Or as suggested by Nazzi andBertoncini (2003) once the child passes a criticaljuncture in word learning sophistication moving froma simple associationist to a referential word learneraccess to phonological detail becomes possible Priorto that point however any detection of that detail isat best fleeting As such it may be seen in on-lineprocessing tasks or in tasks that provide full support torecall memory (Fennell amp Werker 2003 Swingley ampAslin 2002) but is not evident in tasks that involvelearning new words (Werker et al 2002 Stager ampWerker 1997) nor is it evident in word recognitiontasks such as the ERP task used here where there is noobject to facilitate activation of the full memory trace
Support for this explanation can be seen in thecurrent ERP study In this study children of 20 months(but not 14 months) showed a significant P100 response
Mills et al 1459
to the known words over both the nonsense words andthe phonemic contrasts words The P100 is thought toindex sensory and attentional processes This is the firsttime Mills and colleagues (eg Mills et al 1993 1997)have observed a significantly greater P100 response toknown than to unknown words in the ERP word recog-nition studies The amplitude of the P100 is known to bemodulated by effects of attention in both adults (Luck ampHillyard 2000) and children (Richards amp Hunter 2001)One interpretation of this finding is that the inclusion ofphonemic contrast words in the dataset increased atten-tion to early phonological differences To distinguish theknown words from the phonemic contrast foils theyhad to increase their vigilance to the sensory andperceptual detail thus yielding a significant P100 re-sponse in this ERP word recognition task
Directions for Further Research
A focus for future work will be to determine preciselywhat changes between 14 and 20 months allow the moreaccomplished word learner to access and use full pho-netic detail across a wide range of word recognition andword learning situations In a recent training study MillsPlunkett et al (in press) addressed this question byexamining the effects of experience on the lateral distri-bution of the N200ndashN400 differences between knownand unknown words Mills et al asked whether observedchanges in the lateral distribution in ERP differencesto known versus unknown words between 13 and20 months (Mills et al 1997 and replicated in the pre-sent study) reflect the availability of specialized brainsystems for word recognition (as might be available in amore accomplished word learner) or increasing knowl-edge of particular words by testing 20-month-old chil-dren who varied in vocabulary size on newly learnedwords ERPs to novel words that had been paired withan object during a pretest training phase were comparedto ERPs to novel words that had been repeated the samenumber of times but without the objectword pairingThe results supported a mixed model ERP differences tonewly learned words compared to repeated-but-not-trained words showed a bilateral distribution acrossthe whole sample This finding was consistent with thehypothesis that the asymmetrical distribution of ERP dif-ferences to known and unknown words is linked to theamount of experience with individual words Howeverchildren with the largest vocabularies showed a leftgreater than right distribution of ERP differences Thelatter finding suggested that expertise with word learn-ing might also affect the specialization of language-relevant brain activity even in the service of learningnew words
If a more expert word learning system is becomingavailable (Nazzi amp Bertoncini 2003) in the older andormore experienced word learner this might also permit
greater direction of attention to the relevant phonolog-ical information in the word learning and access situa-tion Indeed as we argued at the beginning of thisarticle perhaps part of becoming a more expert wordlearner is knowing just which properties of a word aredefinitional in a particular linguistic community It maybe the emergence of this greater understanding of justwhat distinguishes words in our language that allows theolder more accomplished word learner to not onlyperceive but also lsquolsquomarkrsquorsquo and lsquolsquousersquorsquo the phonologicaldetail in word learning and word recognition tasks
In previous studies Mills and colleagues have re-ported a link between vocabulary size and the distribu-tion of the N200ndashN400 amplitude difference to knownand unknown words between 14 and 20 months of ageSimilarly Werker et al (2002) reported a significantcorrelation between performance on the CDI at both14 and 17 months and performance in the minimal pairswitch task There was no significant correlation with theCDI in the present study The lack of a significantcorrelation here could reflect lack of variability in thedata perhaps because the age groups selected are notyet in transition We predict that testing of a group ofchildren intermediate in age and vocabulary sizes to thegroups tested here would be more likely to reveal such apattern of findings
In summary using an ERP design we extended andhelped explain the pattern of findings seen in earlybehavioral tasks assessing the phonological detail repre-sented and used in early word learners The results ofthe current ERP study indicate that at the earliest stagesof word learning children treat minimal pair mispronun-ciations of known words as acceptable instances of thatword This provides strong evidence that in the decisionstage of word recognition novice word learners accept abroader range of pronunciations of the word as accept-able than do older children With increasing age andincreasing language sophistication the amount of pho-nological detail easily accessed in the representation ofwords increases helping to avoid mapping mistakes andto facilitate more rapid acquisition of a vocabulary
METHODS
Participants and Settings
Participants were tested in three locations at the Uni-versity of California San Diego at the University ofOregon Eugene Oregon and at the University of BritishColumbia Vancouver Canada Approximately one-thirdof the children in each age group were tested at eachsite Testing at the Vancouver site was conducted in amobile ERP lab belonging to the University of OregonChildren from the San Diego and Eugene areas wererecruited through advertisements in a local magazineposters displayed in the area requests at play groupsand referrals from parents whose children had partici-
1460 Journal of Cognitive Neuroscience Volume 16 Number 8
pated in our studies Children from the Vancouver areawere recruited through the University of British Colum-bia child Studies Center database Parents of all childrensigned consent forms consistent with the human sub-jects internal review boards at the university at whichthey were tested Parents were given $500 to defraytransportation costs and children were given a small toyin appreciation of their participation Only full-term(gt36 weeks of gestation) healthy children with mono-lingual experience with English participated in the studychildren with a family history of language impairmentwere excluded from the study
Data from 16 children aged 14 months (9 girls meanage = 14 months 15 days SD = 97 days range = 14months 1 day to 15 months 1 day) and 17 children aged20 to 21 months (9 girls mean age = 20 months 11 daysSD = 131 days age range = 19 months 14 days to 20months 27 days) were retained for analysis in the studyAn additional 18 children (12 boys nine 14-month-olds)were tested but were excluded from the analyses due totoo few artifact-free trials (n = 10) excessive crying (n =3) reversed polarity of the ERP response (n = 3 seenote)2 or refusal to wear the electrocap (n = 2)
Stimuli
The stimuli were naturally spoken in a female voice anddigitized at 16 bits 44 kHz sampling rate The stimuliconsisted of three types of words (a) 10 words whosemeanings were understood by the child (known wordseg lsquolsquomilkrsquorsquo) (b) 10 nonsense words that differed fromthe known words in the initial phonemic contrast (pho-nemic contrast words eg lsquolsquonilkrsquorsquo) and (c) 10 wordsthat differed phonetically from known words in allvowels and consonants (nonsense words eg lsquolsquoneemrsquorsquo)All stimuli were matched on word duration and numberof syllables The mean durations for the known wordsphonetic contrasts and nonsense words were 783 msecSD = 144 865 msec SD = 169 861 msec SD = 96[F(227) = 109 p = 34] Ideally all word-types wouldbe matched on manner of articulation (eg all stopconsonants all nasals all laterals all fricatives or allbalanced) However children at this age have very smallvocabularies and we were limited to common words thatalmost all children at this age would know Amongchildren first words are a disproportionate number ofwords that begin with stop consonants For many ofthose words minimal pair differences are still words(eg lsquolsquoballrsquorsquondashlsquolsquodollrsquorsquo) Thus we were constrained in ourselection of stimulus items For the known words therewere seven stop consonants two nasals and one frica-tive For the phonemic contrasts there are also sevenstop consonants two nasals and one fricative For thenonsense words there are three stop consonants oneaffricative two nasals two fricatives and two laterals Acomplete list of words is shown in Table 1
To evaluate at what point the known words couldphysically be distinguishable from the other word-typeswe examined two aspects of uniqueness (a) at whatpoint the phonetically dissimilar nonsense words be-come unique from their real word counterparts (egwhen does lsquolsquobearrsquorsquo become unique from lsquolsquogarersquorsquo) and (b)when the words might become unique from otherwords that would be in a babyrsquos vocabulary at this ageThe uniqueness point for initial consonants (the pointthat separates that consonant from other consonants)was within 20 msec of the beginning of the word forfricatives (s z f) 40 msec for stop consonants (b d gp t k) 60 msec for affricatives (j) and 20 msec for nasals(m n) and laterals (l r) To further examine theuniqueness points we compared each stimulus usedin the study relative to other words that might be in thechildrsquos vocabulary with the words in the MacArthur CDIthat would be considered close phonological neighborsTo this end we compared our stimuli to the number ofwords in the CDI with (a) the same consonant includingconsonant clusters and (b) the same consonant exclud-ing consonant clusters and (c) the same consonant plusthe vowel For comparisons a and b there are the mostneighbors for the known words (a = 474 b = 375) anintermediate amount for the phonemic contrast words(a = 371 b = 298) and the least for the nonsense words(a = 275 b = 233) The best comparison is to count thenumber of items that begin with the same consonantplus vowel (CV) There were no significant differences inthe number of CV neighbors for the three word-types(known = 16 phonemic contrasts = 12 nonsense =18) Additionally because of co-articulatory effects theacoustic energy for a given consonant varies as a func-tion of the following vowel (Jusczyk 1997) Thus evenwords beginning with the same consonant (eg lsquolsquobookrsquorsquoand lsquolsquobottlersquorsquo) differ in shape and spectral frequenciesand can be discriminated from each other as well as
Table 1 Stimuli
Known Words Phonemic Contrasts Nonsense
Bear Gare Kobe
Ball Pall Lif
Book Dook Neem
Bottle Pottle Fipe
Cup Tup Mon
Cat Gat Tek
Dog Bog Riss
Milk Nilk Keed
Nose Mose Jud
Shoe Zhu Zav
Mills et al 1461
their close phonological neighbors from the beginningof the word
Procedure
Language Assessment
Within one week prior to testing parents were asked tocomplete the MacArthur CDI (Fenson et al 1994)which provided an estimate of the childrsquos vocabularysize and percentile ranking relative to other children ofthe same age To ensure that the words to be used in thestudy as lsquolsquoknownrsquorsquo were comprehended by the childparents also completed a vocabulary checklist ratingscale indicating how sure they were that their childunderstood andor produced each word on a scale of1 (very sure they did not know that word) to 4 (very suretheir child understoodproduced a given word in avariety of different contexts and with different exem-plars) Additionally the children were asked to identify apicture of each word to be used as a known word from atwo-choice picture book All lsquolsquoknownrsquorsquo words used hadreceived a rating of 4 and were correctly identified in thepicture-pointing task
Electrophysiological Recording
The EEG was recorded using tin electrodes affixed to anelastic cap (Electro-Cap International) from sites overfrontal (F7 and F8) anterior temporal (50 of thedistance from F78 and T34) temporal (33 of thedistance from T34 to C34) parietal (50 of the dis-tance between T34 and P34) and occipital (O1 andO2) regions of the left and right hemispheres Addition-ally the electrooculogram was recorded from electrodesplaced over and under the eye to reject trials on whichblinks and vertical eye movement occurred and fromleft and right frontal electrodes to reject trials on whichhorizontal eye movement occurred Impedances werekept below 5 k and were balanced (within 1 k) acrossthe left and right hemispheres at any given position TheEEG was amplified by SA Instruments amplifiers with abandpass of 01 to 100 Hz and sampled continuouslyevery 4 msec All electrodes were referenced to linkedmastoids3 Averages of the EEG were conducted using2-sec epochs (ie 100 msec prestimulus and 1900 msecpoststimulus) The averaged ERPs were also digitallyfiltered off-line with a 60-Hz low-pass filter
Testing
Ten known 10 phonetically similar nonsense words(referred to as phonemic contrasts) and 10 phoneticallydissimilar nonsense (referred to as nonsense words)words were each presented six times in random orderfor a total of 60 trials per condition During testingchildren sat on their parentrsquos lap and listened to words
presented from a speaker located behind a movingpuppet in a puppet theater Words were presented ata variable rate between 1800 and 3000 msec SOA
Artifact Rejection
Artifact rejection was conducted off-line using a com-puter program to reject blinks and horizontal eye move-ment and amplifier blocking Individual thresholds wereset for each child based on visual inspection of the EEGepochs time-locked to each stimulus A mean of 50 ofthe trials were rejected due to eye and movementartifact The number of artifact-free trials per word-typeretained for analysis ranged from 12 to 53 (SD = 113)out of a possible 60 trials per condition There were nosignificant differences in the percentage of trials rejectedfor the different experimental conditions age groupsor sex
Measurement of ERP Components
Peak latencies and amplitudes were quantified by com-puter with reference to the 100-msec prestimulus base-line for the maximum negative or positive point in aspecified time window The time windows for eachcomponent were set according to the criteria used inour previous studies the first positive componentcalled the P100 was defined as the most positivedeflection between 50 and 175 msec The P100 indexesauditory sensory processing Moreover the lateral dis-tribution of the P100 has been shown to vary withpercentile ranking on the MacArthur CDI score Chil-dren who score at the 50th percentile or higher for theirage show a P100 left greater than right asymmetry Themean amplitude within the time window 200ndash400 afterword onset was quantified by computer with referenceto the 100-msec prestimulus baseline This time windowwas chosen because it had been shown to differ forknown and unknown words in our previous studies
Acknowledgments
This research was conducted while the first author was at theProject in Cognitive and Neural Development at the Universityof California at San Diego The research was supported by NIHgrant DC0048111 to H Neville from NIDCD NSERC (NaturalSciences and Engineering Research Council of Canada) grantto J Werker and HFSP (Human Frontierrsquos Science Program)grant to J Werker and H Neville The third author RenateZangl was funded in part by two postdoctoral grants (J-1646J-1817) from the Austrian Science Foundation
We thank Amy Adamson Greg Appelbaum Teresa Mitchelland Lisa Sanders (in alphabetical order not necessarily in orderof magnitude of contribution) for their help in acquiring andanalyzing the data and Chris Fennell and Liz Lewis for theircomments on an earlier version of the manuscript We areespecially indebted to the children and their parents whoparticipated in these studies and their parents without whomthe research would not be possible
1462 Journal of Cognitive Neuroscience Volume 16 Number 8
Reprint requests should be sent to Debra L Mills who is nowat the Department of Psychology 532 Kilgo Circle EmoryUniversity Atlanta GA 30322 or via e-mail dmills2emoryedu
Notes
1 Unlike the switch task the visual fixation task is an on-linetask with two simultaneously presented choices The twoindicators of success are shorter latency to look away from theincorrect object and longer looking times overall to the correctmatch The habituation phase in the lsquolsquoswitchrsquorsquo task leads to theprediction of a novelty preference in the test phase (ie longerlooking to the incorrect pairing)2 In previous studies Mills and colleagues typically findapproximately 10ndash15 of children show a reversed ERPresponse with a larger amplitude ERP response to unknownthan to known words In previous work Mills and colleagueshave treated those children as noise in the data and have keptthem in for the data analyses Ultimately we want tounderstand the nature of lexical representations in this groupof children showing an aberrant ERP response For the currentstudy however in order to compare the ERP response tophonemic contrasts to known words we felt it necessary toinclude in our sample only those children who showed themuch more typical pattern of response resulting in the ex-clusion of three children All effects reported here show thesame pattern with these children included but the variabilityfrom their inclusion did reduce the differences in somecomparisons3 We are aware of the controversies surrounding the use oflinked mastoids Linked mastoids were used here to increasethe number of active cites given the number of amplifiersavailable and to provide consistency with previous studies Apilot study using similar auditory stimuli was conducted toexamine possible distortions in the distribution of scalp activityresulting from forced linkage This was determined by re-cording from one mastoid using the other as a reference andlinking the mastoids off-line These pilot data were comparedwith the data recorded using linked mastoids and did not yieldsignificant differences
REFERENCES
Bailey T M amp Plunkett K (2002) Phonological specificity inearly words Cognitive Development 17 1265ndash1282
Bertoncini J amp Mehler J (1981) Syllables as units in infantspeech perception Infant Behavior and Development 4247ndash260
Brown C amp Matthews J (1997) The role of feature geometryin the development of phonemic contrasts In S J Hannahsamp M Young-Scholten (Eds) Focus on phonologicalacquisition (pp 67ndash112) Amsterdam Benjamins
Brown C M Hagoort P amp Kutas M (2000) Postlexicalintegration processes in language comprehension Evidencefrom brain-imaging research In M Gazzaniga (Ed) The newcognitive neurosciences (2nd ed pp 881ndash895)Cambridge MIT Press
Conboy B (2003) Patterns of language processing and growthin early EnglishndashSpanish bilingualism (Doctoral dissertationUniversity of California San Diego and San Diego StateUniversity 2000) Dissertation Abstracts InternationalB The Sciences and Engineering 63 (UMI No 5193)
Conboy B amp Mills D L (2000) Cerebral specialization forlanguage comprehension in bilingual toddlers CognitiveNeuroscience Society 2000 Annual Meeting AbstractsSupplement of the Journal of Cognitive Neuroscience 17
Connolly J F Service E DrsquoArcy R C N Kujala A Alho K(2001) Phonological aspects of word recognition as revealedby high-resolution spatio-temporal brain mappingNeuroReport 12 237ndash243
Dehaene-Lambertz G (2000) Cerebral specialization forspeech and non-speech stimuli in infants Journal ofCognitive Neuroscience 12 449ndash460
Dehaene-Lambertz G amp Dehaene S (1994) Speed andcerebral correlates of syllable discrimination in infantsNature 370 292ndash294
Eimas P D Siqueland E R Jusczyk P amp Vigorito J (1971)Speech perception in infants Science 171 303ndash306
Fennell C E amp Werker J F (2003) Early word learnersrsquoability to access phonetic detail in well-known wordsLanguage and Speech 46 245ndash264
Fenson L Dale P S Reznick J S Bates E Thal D Jamp Pethick S J (1994) Variability in early communicativedevelopment Monographs of the Society for Research inChild Development 59 1ndash173
Fernald A Swingley D amp Pinto J P (2001) When half aword is enough Infants can recognize spoken words usingpartial phonetic information Child Development 721003ndash1015
Fischler I S (1990) Comprehending language withevent-related potentials In J W Rohrbaugh R Parasuramanamp R Johnson (Eds) Event-related brain potentials(pp 165ndash170) New York Oxford University Press
Halle P amp de Boysson-Bardies B (1994) Emergence of anearly receptive lexicon Infantsrsquo recognition of wordsInfant Behavior and Development 17 119ndash129
Halle P amp de Boysson-Bardies B (1996) The format ofrepresentation of recognized words in infantsrsquo earlyreceptive lexicon Infant Behavior and Development 19463ndash481
Holcomb P J amp Neville H J (1991) The electrophysiologyof spoken sentence processing Psychobiology 19286ndash300
Holcomb P J Coffey S A amp Neville H J (1992) Visual andauditory sentence processing A developmental analysisusing event-related brain potentials DevelopmentalNeuropsychology 8 203ndash241
Jusczyk P W (1997) The discovery of spoken languageCambridge MIT Press
Jusczyk P W amp Aslin R N (1995) Infantsrsquo detection ofsound patterns of words in fluent speech CognitivePsychology 29 1ndash23
Kutas M amp Hillyard S (1980) Reading senseless sentencesBrain potentials ref lect semantic incongruity Science 207203ndash205
Luck S amp Hillyard S (2000) The operation of selectiveattention at multiple stages of processing Evidence fromhuman and monkey electrophysiology In M Gazzaniga(Ed) The new cognitive neurosciences (2nd edpp 687ndash700) Cambridge MIT Press
Mills D L Coffey-Corina S A amp Neville H J (1997)Language comprehension and cerebral specialization from13ndash20 months [(Special issue on origins of languagedisorders)] In D Thal amp J Reilly (Eds) DevelopmentalNeuropsychology 13 397ndash446
Mills D M Coffey-Corina S A amp Neville H J (1994)Variability in cerebral organization during primary languageacquisition In G Dawson amp K Fischer (Eds) Humanbehavior and the developing brain (pp 427ndash455) NewYork Guilford Publications
Mills D M Coffey-Corina S A amp Neville H J (1993)Language acquisition and cerebral specialization in20-month-old infants Journal of Cognitive Neuroscience 5326ndash342
Mills et al 1463
Mills D L Conboy B amp Paton C (in press) Do changes inbrain organization reflect shifts in symbolic functioning InL Namy (Ed) Symbol use and symbolic representationMahwah NJ Erlbaum
Mills D L Plunkett K Prat C amp Schafer G (in press)Watching the infant brain learn words Effects ofvocabulary size and experience Cognitive Development
Molfese D L (1989) Electrophysiological correlates of wordmeanings in 14-month-old human infants DevelopmentalNeuropsychology 5 79ndash103
Molfese D L (1990) Auditory evoked responses recordedfrom 16-month-old human infants to words they did anddid not know Brain and Language 38 345ndash363
Molfese D L Burger-Judisch L M amp Hans L L (1991)Consonant discrimination by newborn infantsElectrophysiological differences DevelopmentalNeuropsychology 7 177ndash195
Molfese D L amp Molfese V J (1988) Right hemisphereresponses from preschool children to temporal cuescontained in speech and nonspeech materialsElectrophysiological correlates Brain and Language33 245ndash259
Molfese D L Morse P A amp Peters C J (1990) Auditoryevoked responses to names for different objectsCross-modal processing as a basis for infant languageacquisition Developmental Psychology 26 780ndash795
Molfese D L Wetzel W F amp Gill L A (1993) Known versusunknown word discriminations in 12-month-old humaninfants Electrophysiological correlates DevelopmentalNeuropsychology 9 241ndash258
Naigles L R (2002) Form is easy meaning is hard Resolvinga paradox in early child language Cognition 8 157ndash199
Nazzi T amp Bertoncini J (2003) Before and after thevocabulary spurt Two modes of word acquisitionDevelopmental Science 6 136ndash142
Osterhout L amp Holcomb P J (1995) Event-related potentialsand language comprehension In M D Rugg amp M G HColes (Eds) Electrophyisology of mind (pp 171ndash215)Oxford Oxford University Press
Richards J E amp Hunter S K (2001) Testing neural models ofthe development of infant attention DevelopmentalPsychobiology 40 226ndash236
Sereno S C Brewer C C amp OrsquoDonnell P J (2003)Context effects in word recognition Evidence for earlyinteractive processing Psychological Science 14328ndash333
Simos P G amp Molfese D L (1997) Electrophysiologicalresponses from a temporal order continuum in the newborninfant Neuropsychologia 35 89ndash98
Simos P G Molfese D L amp Brenden R A (1997) Behavioraland electrophysiological indices of voicing-cuediscrimination Laterality patterns and developmentBrain and Language 57 122ndash150
St George M amp Mills D L (2001) Electrophysiologicalstudies of language development In J Weissenborn ampB Hoehle (Eds) Language acquisition and languagedisorders (pp 247ndash259) Amsterdam John BenjaminsPublishing
Stager C L amp Werker J F (1997) Infants listen for morephonetic detail in speech perception than in word-learningtasks Nature 388 381ndash382
Swingley D (2003) Phonetic detail in the developing lexiconLanguage and Speech 46 265ndash294
Swingley D amp Aslin R N (2000) Spoken word recognitionand lexical representation in very young children Cognition76 147ndash166
Swingley D amp Aslin R N (2002) Lexical neighborhoodsand the wordndashform representations of 14-month-oldsPsychological Science 13 480ndash484
Werker J F Cohen L B Lloyd V L Casasola M amp StagerC L (1998) Acquisition of wordndashobject associations by14-month-old infants Developmental Psychology 341289ndash1309
Werker J F amp Fennell C E (2004) From listening to soundsto listening to words Early steps in word learning In G Hallamp S Waxman (Eds) Weaving a lexicon (pp 79ndash109)Cambridge MA MIT Press
Werker J F Fennell C E T Corcoran K M amp Stager C L(2002) Infantsrsquo ability to learn phonetically similar wordsEffects of age and vocabulary size Infancy 3 1ndash30
Werker J F amp Tees R C (1984) Cross-language speechperception Evidence for perceptual reorganization duringthe first year of life Infant Behavior and Development 749ndash63
1464 Journal of Cognitive Neuroscience Volume 16 Number 8
2 respectively The results are presented for two ERPcomponents the peak amplitude and latency of the firstpositive component P100 and the mean area of thenegativity between 200 and 400 msec poststimulusonset These components were chosen because ourprevious studies found them to be sensitive to vocabu-lary size andor word meanings Also in previous studiesthe N600ndashN900 differed for known and unknown wordsat 13ndash17 but not 20 months (Mills et al 1997) Visualinspection of the present data for the 14-month-olds(Figure 1) suggested differences in this time windowbetween the mean amplitude for the phonetically similarnonsense word and the other two word-types None ofthe analyses conducted on the later time windowreached significant levels There were no interactionswith age or vocabulary size Therefore to conservespace these analyses were not included
Data Analysis
A priori hypotheses predicted that the 14-month-oldsand 20-month-olds would show different patterns ofERPs to the phonemic contrast words relative to the
known and nonsense words Therefore the ERP datawere analyzed separately for the two age groups inrepeated-measures ANOVAs with Word-type (knownphonemic contrast nonsense words) Hemisphere (leftand right) and Electrode site (frontal anterior temporaltemporal parietal and occipital) as the within-subjectsfactors HuynhndashFeldt corrections were used for all therepeated-measures analyses We were particularly inter-ested in changes in the lateral distribution of the N200ndashN400 for the 14- and 20-month-olds to known wordscompared with the two types of nonsense words Theseplanned simple effects were analyzed for the 14- and 20-month-olds separately in repeated-measures ANOVAscomparing two word-types (known words vs phonemiccontrasts known vs nonsense words and phonemiccontrasts vs nonsense words) by hemisphere (left andright) and by electrode site (frontal anterior temporaltemporal parietal occipital) Age group differenceswere analyzed in a two-way ANOVA with Age group asthe between-subjects variable and Word-type (knownwords phonemic contrasts nonsense words) Hemi-sphere (left and right) and Electrode site (frontalanterior temporal temporal parietal and occipital) asthe within-subjects factors
Children Aged 14 Months
P100 The first positive component peaked at 125 msec(P100) The P100 peaked later and was larger overfrontal than posterior regions [electrode site latencyF(450) = 934 p lt 001 amplitude F(460) = 753 p lt001] There were no main effects or interactions forpeak latencies or amplitude among word-types or be-tween the left and right hemispheres
N200ndashN400 The mean amplitudes between 200 and400 msec (N200ndashN400) were larger over posterior thananterior regions [electrode site F(460) = 514 p lt01] and differed with word-type [F(230) = 376 p lt05] There were no main effects or interactions withhemisphere
Planned comparisons showed that the N200ndashN400 waslarger for known words than for nonsense words (ielsquolsquobearkobersquorsquo Figure 3 top) [F(115) = 494 p lt 05]Importantly the N200ndashN400 amplitude was also greaterfor the phonemic contrast words than for the nonsensewords (lsquolsquogarekobersquorsquo Figure 3 bottom) [F(115) = 697p lt 05] However the N200ndashN400 was not significant-ly different for known words versus phonemic contrastwords (ie lsquolsquobearrsquorsquolsquolsquogarersquorsquo Figure 3 middle) [F(115) =007 p = 80] Thus both known and phonemic con-trast words showed the same activity as lsquolsquoknownrsquorsquo andwere different from unknown lsquolsquononsensersquorsquo words
We also predicted age-related differences in the lateraldistribution of the N200ndashN400 effects As predicted forthe 14-month-olds the N200ndashN400 mean amplitudedifferences between the word-types were broadly dis-tributed over anterior and posterior regions of both the
Figure 2 ERPs to all three word-types for the seventeen
20-month-olds over the anterior and posterior regions of the left
and right hemispheres ERPs to known words are shown in the solid
lines to phonemic contrasts in the dotted lines and to phoneticallydissimilar nonsense words in the dashed lines
Mills et al 1455
left and right hemispheres [known vs nonsense lefthemisphere F(115) = 230 p lt 15 right hemisphereF(115) = 745 p lt 05 phonemic contrasts vs non-sense words left hemisphere F(115) = 458 p lt 05right hemisphere F(115) = 797 p lt 01]
Children Aged 20 Months
P100 The P100 peaked later and was larger over frontalthan posterior regions [latency F(116) = 700 p lt001 amplitude F(116) = 1407 p lt 001] There wasalso a main effect of word-type for the P100 amplitude[F(232) = 355 p lt 05] Examination of the amplitudedifferences across word-types showed that the P100 waslarger to known words than to nonsense words [knownvs nonsense F(116) = 707 p lt 05] and phonemiccontrasts [known vs phonemic contrasts F(116) =605 p lt 05] The P100 amplitude was larger overthe left than the right hemisphere but only overtemporal and parietal sites [Hemisphere Electrodesite F(464) = 300 p lt 05] There were no othermain effects or interactions
N200ndashN400 The N200ndashN400 amplitude differences tothe different word-types approached significance [word-type F(116) = 303 p = 06] The N200ndashN400 waslarger over the left than the right hemisphere at tempo-ral and parietal sites but larger over the right than theleft over the occipital regions [Hemisphere Electrodesite F(464) = 283 p lt 05]
Planned comparisons showed that the N200ndashN400was larger to known than nonsense words [F(116) =576 p lt 05] but only over left temporal [F(116) =826 p lt 01] left parietal [F(116) = 1062 p lt 01]and right frontal [F(116) = 602 p lt 05] sites (Figure 4top) The N200ndashN400 was also larger to known wordsversus phonemic contrast words over left temporal[F(116) = 724 p lt 05] and left parietal [F(116) =709 p lt 05] regions (Figure 4 middle) However themain effect of word-type for this comparison only ap-proached significance [F(116) = 270 p = 10] Therewere no significant differences in N200ndashN400 amplitudesto the phonemic contrasts and nonsense words(Figure 4 bottom) [F(116) = 042 ns] At 20 monthsthen the phonemic contrast (nonsense) words were no
Figure 3 For the 14-month-olds ERP differences are directly
compared to known words and nonsense words (top) known words
and phonemic contrasts (middle) and phonemic contrasts andphonetically dissimilar nonsense words (bottom) Significant
differences in N200ndashN400 mean amplitudes are shaded and enclosed in
the rectangle Left and temporal and parietal regions are shown on theleft and right sides of the figure respectively
Figure 4 For the 20-month-olds ERP differences are directly
compared to known words and nonsense words (top) known words
and phonemic contrasts (middle) and phonemic contrasts andphonetically dissimilar nonsense words (bottom) Significant
differences in N200ndashN400 mean amplitudes are shaded and enclosed in
the rectangle Left and temporal and parietal regions are shown on theleft and right sides of the figure respectively
1456 Journal of Cognitive Neuroscience Volume 16 Number 8
longer treated as known words but were instead treatedlike other nonsense words
Age Group Comparisons
There were no main effects or interactions with agewhen all three word-types and all electrode sites wereincluded in the analyses Because the 20-month-olds inthis study and in our previous studies only showed ERPdifferences in this time window to known versus un-known words at temporal and parietal sites a separateANOVA was conducted including only temporal andparietal sites that is with two levels of Age group (14and 20 months) as the between-subjects factor andWord-type (known phonemic contrasts and nonsensewords) Hemisphere (left and right) and Electrodesite (temporal and parietal) as the within-subjects fac-tors That analysis showed main effects for Word-type[F(131) = 659 p lt 001] and Hemisphere [F(131) =395 = 420 p lt 05] These effects were qualifiedby two interactions with group including a Group Word-type Hemisphere interaction [F(662) = 304p lt 05] and a Group Electrode site interaction[F(131) = 396 p lt 05] These interactions were fur-ther validated by the different patterns displayed by the14- and 20-month-olds separately
To further support the age-related effects describedabove separate ANOVAs were conducted with Age (14and 20 months) as the between-subjects factor andWord-type (known phonemic contrasts and nonsensewords) Hemisphere (left and right) and Electrode site(frontal anterior temporal temporal parietal and oc-cipital) as the within-subjects factors To conserve spaceonly main effects of and interactions with age arereported below
P100 There were no main effects or interactions withage for the P100 latency The age-related difference inthe lateral distribution of the P100 amplitude was sup-
ported by an Age Hemisphere Electrode siteinteraction [F(4124) = 275 p lt 05]
N200ndashN400 There were no main effects or interac-tions with age when all three word-types and all elec-trode sites were included in the analyses Because the20-month-olds in previous studies and in this study asdescribed above only showed ERP differences in thistime window to known versus unknown words at tem-poral and parietal sites a separate ANOVA was con-ducted including only temporal and parietal sitesnamely with two levels of Age group (14 and 20 months)as the between-subjects factor and Word-type (knownphonemic contrasts and nonsense words) Hemisphere(left and right) and Electrode site (temporal andparietal) as the within-subjects factors That analysisshowed two interactions with group including a Groupby Word-type Hemisphere interaction [F(662) =304 p lt 05] and a Group Electrode site interaction[F(131) = 396 p lt 05] These interactions supportthe findings described above showing different patternsof responsiveness to the different word-types by the 14-and 20-month-olds separately
Summary
Fourteen- and 20-month-olds showed different patternsof brain activity to known words (eg lsquolsquobearrsquorsquo) phone-mic contrasts (eg lsquolsquogarersquorsquo) and (phonetically dissimilar)nonsense words (eg lsquolsquokobersquorsquo) (Figure 5) As predictedby Stager and Werker (1997) 14-month-olds showedlarger N200ndashN400 amplitudes to known words com-pared with nonsense words (eg lsquolsquokobersquorsquo) Howeverthe N200ndashN400 to known words and the phonemiccontrast stimuli (eg lsquolsquobearrsquorsquo and lsquolsquogarersquorsquo) did not differfrom each other
Unlike the pattern seen at 14 months and as pre-dicted by Werker et al (2002) the 20-month-oldsshowed larger amplitude N200ndashN400 responses to
Figure 5 N200ndashN400 mean
areas averaged across temporal
and parietal regions of the leftand right hemispheres to the
three word-types for the
14- and 20-month-olds
Mills et al 1457
known words (eg lsquolsquobearrsquorsquo) over both phonemic con-trast (eg lsquolsquogarersquorsquo) and other nonsense words (eglsquolsquokobersquorsquo) Moreover the N200ndashN400 amplitudes to thephonemic contrast and other nonsense words (lsquolsquogarersquorsquoand lsquolsquokobersquorsquo) did not differ from each other Thesefindings also replicated previously observed lateral dif-ferences in the N200ndashN400 amplitudes to known versusunknown words between 14 and 20 months (Mills et al1997)
DISCUSSION
The results from the present study were consistent withand expanded on findings observed in previous behav-ioral and ERP studies At both 14 and 20 months of agethe pattern of ERP responses seen replicated that pre-viously reported by Mills et al (1993 1997) for knownversus unknown words At both ages there was a largeramplitude negative response from 200 to 400 msec toknown versus nonsense words And as shown before at14 months of age this response was broadly distributedacross the scalp whereas at 20 months it was observedprimarily over the left temporal and parietal electrodesites
The replication of the previous work by Mills andcolleagues puts us in a particularly strong position tointerpret the results from the phonemic contrast wordsAt 14 months of age the ERP signature to phonemiccontrast nonsense words (eg lsquolsquogarersquorsquo or lsquolsquobogrsquorsquo) isindistinguishable from that to known words (eg lsquolsquobearrsquorsquoand lsquolsquodogrsquorsquo) and significantly greater in amplitude thanthat to dissimilar nonsense words (eg lsquolsquolifrsquorsquo) Thus at14 months of age the neural response appears toindicate a mistaken recognition of a minimal pair mis-pronunciation of a known word as the word itself Thisresult is consistent with that seen in the switch task at14 months (Werker et al 2002 Stager amp Werker 1997)The results with the ERP task are particularly convincingbecause here we see a significant increase in the ampli-tude of the response rather than a failure to increaselooking time At 20 months of age the ERP response isalso consistent with results seen in the switch task at thesame age (Werker et al 2002) At 20 months the ERPsignature to phonemic contrast nonsense words (eglsquolsquogarersquorsquo and lsquolsquobogrsquorsquo) is significantly different from that totheir known counterparts (eg lsquolsquobearrsquorsquo and lsquolsquodogrsquorsquo) andindistinguishable from that to phonetically dissimilarnonsense words (eg lsquolsquoneemrsquorsquo or lsquolsquolifrsquorsquo) Thus at20 months the evoked response indicates that childrenare showing the lsquolsquoword recognitionrsquorsquo ERP response onlyto known words that are phonologically correct in alldetails By 20 months then the confusion with similar-sounding known words is no longer present indicatingthat words in the lexicon are not only fully specified interms of phonological detail but that such detail isavailable even when listening to words in tasks that donot provide contextual or pictorial support for the word
meaning and may not even involve full attention to theword
One difference between the present study and thebehavioral studies by Werker and colleagues and Swin-gley and Aslin is that the behavioral studies used bothpictures and words whereas the present study pre-sented a series of words without a picture context Anegative going ERP wave within the 200ndash500 msec timewindow has also been shown to index word meaningin cross-modal matchmismatch paradigms using pic-tures and words in 14- and 20-month-old children (MillsConboy amp Paton in press) and real objects and noveltrained words in 14-month-old children (Molfese Morseamp Peters 1990) We chose to use the Mills et al (1997)paradigm rather than a pictureword matchmismatchparadigm for several reasons The pictureword matchmismatch paradigm elicits a large amplitude bilateralnegative component starting at 200 msec and peakingaround 500 msec when the subsequent word does notmatch the picture at both 14 and 20 months (MillsConboy et al in press) This ERP effect is most likelywithin the N400 family observed in children and adults toviolations of semantic expectancy (Holcomb Coffey ampNeville 1992 Kutas amp Hillyard 1980) In adults a similarparadigm elicited a phonological mismatch (PMN) towords and nonwords from 250 to 347 msec over rightfrontal regions and from 347 to 638 msec over symmet-rical centro-parietal regions (Connolly Service DrsquoArcyKujala amp Alho 2001) One concern was that the N400 andPMN effects elicited by a pictureword matchmismatchparadigm might swamp more subtle differences in thelatency amplitudes and distributions of ERPs related tothe different word-types and group differences The Millset al paradigm was chosen because it showed ERP dif-ferences to known and unknown words that varied withword-type age and vocabulary size The methodologicaldifferences should be taken into consideration whencomparing the effects However the ERP findings areconsistent with and provide additional support for theage-related differences reported by Stager et al (1997)
The robust differences in the ERP pattern at 14and 20 months of age could be explained at manylevels Several studies have shown that a negativepeak at around 200 msec is sensitive to phonetic varia-tion in voice onset time (Simos Molfese amp Brenden1997 Molfese amp Molfese 1988) place of articulation(Dehaene-Lambertz amp Dehaene 1994 Molfese Burger-Judisch amp Hans 1991) and acoustic cues in nonspeechstimuli (Dehaene-Lambertz 2000 Simos amp Molfese1997) in very young children and even newborns Oneinterpretation is that ERP differences between word-types are related to the acoustic or phonological featuresof the different word-types and that age-related changesare due to differences in the childrenrsquos ability to mem-orize the different word lists Although phonological dis-crimination can modulate the amplitude of the N200 incertain paradigms it is not necessarily the only factor
1458 Journal of Cognitive Neuroscience Volume 16 Number 8
that may contribute to the amplitude of this componentHere we argue that age-related changes in a top-downprocess linked to word meaning is the dominant factormodulating the amplitude of the N200ndashN400 rather thanphonological discrimination First if ERP differences be-tween conditions were due solely to phonological fac-tors then both age groups would have displayed N200amplitude differences between the known words andphonetic contrasts Additionally the known and phone-mic contrasts were balanced for the types of initialconsonants in the number of stop consonants nasalsand fricatives and did not differ in their phonologicalneighbors for other words frequently understood bychildren in this age range (see Methods) Therefore itis difficult to explain why the phonemic contrasts pat-terned with the known words at 14 months and thenonsense words at 20 months based solely on acousticaland phonemic differences A second line of evidencesuggesting that the ERP differences observed here can-not be explained by phonological differences betweenword-types comes from a study of bilingual toddlersusing the same paradigm In that study the N200ndashN400did not differ between Spanish and English for either theknown or unknown words (Conboy amp Mills 2000 Con-boy 2003) ERP differences were linked to word meaning(known vs unknown words) language dominance andtotal vocabulary size but not phonological differencesbetween Spanish and English If the N200ndashN400 indexedthe phonological aspects of the words amplitude differ-ences would be expected between Spanish and EnglishA third and perhaps the strongest line of evidencecomes from a training study with 20-month-old childrenin which nonsense words were either repeated or pairedwith an object (Mills Plunkett Prat amp Schafer in press)After the training phase when objects were no longerpresent the amplitude of the N200ndashN400 to the newlylearned words increased relative to before training butbecame more positive for nonsense words that wererepeated the same number of times Because the stimuliwere counterbalanced across participants the increasedamplitude of the N200ndashN400 to the newly learned wordswas modulated by its association with a meaningfulstimulus and cannot be explained by phonological dif-ferences between word-types
Another interpretation of the findings is that at 14months of age the phonetic detail distinguishing oneword from another is simply not available in the lexicalrepresentation Such an explanation would be compat-ible with theories of phonological development thatposit an initial under-specification of the informationin the lexicon that is only gradually filled in as thevocabulary expands (eg Brown amp Matthews 1997)Although possible we think this explanation is unlikelygiven the work by Swingley and Aslin (2002) showingthat children of 14 months do seem to show someevidence of use of phonetic detail in word recognitiontasks via longer looking time to the matching object
when a correct versus a mispronounced version of aword is given The Swingley and Aslin work involvesshowing the children two objects (eg a car and a dog)and asking them to find either the correctly labeledobject (eg lsquolsquodogrsquorsquo) or a mispronunciation of that sameobject (eg lsquolsquobogrsquorsquo) More recently Fennell and Werker(2003) showed that when tested on two phonemiccontrast words that they already know well (eg lsquolsquoballrsquorsquoand lsquolsquodollrsquorsquo) the child of 14 months can succeed even inthe switch task These two studies show that at somelevel in the system full phonetic specification of well-known words is represented even at 14 months
The successes in the two-choice (Swingley and Aslin)behavioral lsquolsquomispronunciationrsquorsquo task and in the switchtask using two well-known words in comparison to boththe switch word-learning task and the ERP word recog-nition task are compatible with the attentional resourcelimitation hypothesis offered by Werker and colleagues(Werker et al 2002 Werker amp Fennell 2004 Fennell ampWerker 2003 Stager amp Werker 1997) According to thisexplanation the task of linking a word to an object ischallenging for a novice word learner All the detail ispicked up but the computational difficulty of actuallymapping the word on to the object makes it difficultto hold all the information in mind As such one sees aU-shaped function in the pattern of data The child of7ndash8 months who is not yet actively engaged in mappingwords to referents is better able to attend to fine pho-netic detail than is the child of 14 months who is at-tempting to make and remember the link (see Stager ampWerker 1997 see also Halle amp de Boysson-Bardies1996 Jusczyk amp Aslin 1995) Becoming a word learnerchanges the task for the older child This increases theprocessing demands and interferes with the childrsquosability to access detail The information is picked upin perception it is just not available for access in lexicalrecall tasks After the child becomes a more accom-plished word learner and has a working phonemicinventory to guide information access the relevantphonetic differences lsquolsquostand outrsquorsquo as salient and impor-tant to the lexical entry Or as suggested by Nazzi andBertoncini (2003) once the child passes a criticaljuncture in word learning sophistication moving froma simple associationist to a referential word learneraccess to phonological detail becomes possible Priorto that point however any detection of that detail isat best fleeting As such it may be seen in on-lineprocessing tasks or in tasks that provide full support torecall memory (Fennell amp Werker 2003 Swingley ampAslin 2002) but is not evident in tasks that involvelearning new words (Werker et al 2002 Stager ampWerker 1997) nor is it evident in word recognitiontasks such as the ERP task used here where there is noobject to facilitate activation of the full memory trace
Support for this explanation can be seen in thecurrent ERP study In this study children of 20 months(but not 14 months) showed a significant P100 response
Mills et al 1459
to the known words over both the nonsense words andthe phonemic contrasts words The P100 is thought toindex sensory and attentional processes This is the firsttime Mills and colleagues (eg Mills et al 1993 1997)have observed a significantly greater P100 response toknown than to unknown words in the ERP word recog-nition studies The amplitude of the P100 is known to bemodulated by effects of attention in both adults (Luck ampHillyard 2000) and children (Richards amp Hunter 2001)One interpretation of this finding is that the inclusion ofphonemic contrast words in the dataset increased atten-tion to early phonological differences To distinguish theknown words from the phonemic contrast foils theyhad to increase their vigilance to the sensory andperceptual detail thus yielding a significant P100 re-sponse in this ERP word recognition task
Directions for Further Research
A focus for future work will be to determine preciselywhat changes between 14 and 20 months allow the moreaccomplished word learner to access and use full pho-netic detail across a wide range of word recognition andword learning situations In a recent training study MillsPlunkett et al (in press) addressed this question byexamining the effects of experience on the lateral distri-bution of the N200ndashN400 differences between knownand unknown words Mills et al asked whether observedchanges in the lateral distribution in ERP differencesto known versus unknown words between 13 and20 months (Mills et al 1997 and replicated in the pre-sent study) reflect the availability of specialized brainsystems for word recognition (as might be available in amore accomplished word learner) or increasing knowl-edge of particular words by testing 20-month-old chil-dren who varied in vocabulary size on newly learnedwords ERPs to novel words that had been paired withan object during a pretest training phase were comparedto ERPs to novel words that had been repeated the samenumber of times but without the objectword pairingThe results supported a mixed model ERP differences tonewly learned words compared to repeated-but-not-trained words showed a bilateral distribution acrossthe whole sample This finding was consistent with thehypothesis that the asymmetrical distribution of ERP dif-ferences to known and unknown words is linked to theamount of experience with individual words Howeverchildren with the largest vocabularies showed a leftgreater than right distribution of ERP differences Thelatter finding suggested that expertise with word learn-ing might also affect the specialization of language-relevant brain activity even in the service of learningnew words
If a more expert word learning system is becomingavailable (Nazzi amp Bertoncini 2003) in the older andormore experienced word learner this might also permit
greater direction of attention to the relevant phonolog-ical information in the word learning and access situa-tion Indeed as we argued at the beginning of thisarticle perhaps part of becoming a more expert wordlearner is knowing just which properties of a word aredefinitional in a particular linguistic community It maybe the emergence of this greater understanding of justwhat distinguishes words in our language that allows theolder more accomplished word learner to not onlyperceive but also lsquolsquomarkrsquorsquo and lsquolsquousersquorsquo the phonologicaldetail in word learning and word recognition tasks
In previous studies Mills and colleagues have re-ported a link between vocabulary size and the distribu-tion of the N200ndashN400 amplitude difference to knownand unknown words between 14 and 20 months of ageSimilarly Werker et al (2002) reported a significantcorrelation between performance on the CDI at both14 and 17 months and performance in the minimal pairswitch task There was no significant correlation with theCDI in the present study The lack of a significantcorrelation here could reflect lack of variability in thedata perhaps because the age groups selected are notyet in transition We predict that testing of a group ofchildren intermediate in age and vocabulary sizes to thegroups tested here would be more likely to reveal such apattern of findings
In summary using an ERP design we extended andhelped explain the pattern of findings seen in earlybehavioral tasks assessing the phonological detail repre-sented and used in early word learners The results ofthe current ERP study indicate that at the earliest stagesof word learning children treat minimal pair mispronun-ciations of known words as acceptable instances of thatword This provides strong evidence that in the decisionstage of word recognition novice word learners accept abroader range of pronunciations of the word as accept-able than do older children With increasing age andincreasing language sophistication the amount of pho-nological detail easily accessed in the representation ofwords increases helping to avoid mapping mistakes andto facilitate more rapid acquisition of a vocabulary
METHODS
Participants and Settings
Participants were tested in three locations at the Uni-versity of California San Diego at the University ofOregon Eugene Oregon and at the University of BritishColumbia Vancouver Canada Approximately one-thirdof the children in each age group were tested at eachsite Testing at the Vancouver site was conducted in amobile ERP lab belonging to the University of OregonChildren from the San Diego and Eugene areas wererecruited through advertisements in a local magazineposters displayed in the area requests at play groupsand referrals from parents whose children had partici-
1460 Journal of Cognitive Neuroscience Volume 16 Number 8
pated in our studies Children from the Vancouver areawere recruited through the University of British Colum-bia child Studies Center database Parents of all childrensigned consent forms consistent with the human sub-jects internal review boards at the university at whichthey were tested Parents were given $500 to defraytransportation costs and children were given a small toyin appreciation of their participation Only full-term(gt36 weeks of gestation) healthy children with mono-lingual experience with English participated in the studychildren with a family history of language impairmentwere excluded from the study
Data from 16 children aged 14 months (9 girls meanage = 14 months 15 days SD = 97 days range = 14months 1 day to 15 months 1 day) and 17 children aged20 to 21 months (9 girls mean age = 20 months 11 daysSD = 131 days age range = 19 months 14 days to 20months 27 days) were retained for analysis in the studyAn additional 18 children (12 boys nine 14-month-olds)were tested but were excluded from the analyses due totoo few artifact-free trials (n = 10) excessive crying (n =3) reversed polarity of the ERP response (n = 3 seenote)2 or refusal to wear the electrocap (n = 2)
Stimuli
The stimuli were naturally spoken in a female voice anddigitized at 16 bits 44 kHz sampling rate The stimuliconsisted of three types of words (a) 10 words whosemeanings were understood by the child (known wordseg lsquolsquomilkrsquorsquo) (b) 10 nonsense words that differed fromthe known words in the initial phonemic contrast (pho-nemic contrast words eg lsquolsquonilkrsquorsquo) and (c) 10 wordsthat differed phonetically from known words in allvowels and consonants (nonsense words eg lsquolsquoneemrsquorsquo)All stimuli were matched on word duration and numberof syllables The mean durations for the known wordsphonetic contrasts and nonsense words were 783 msecSD = 144 865 msec SD = 169 861 msec SD = 96[F(227) = 109 p = 34] Ideally all word-types wouldbe matched on manner of articulation (eg all stopconsonants all nasals all laterals all fricatives or allbalanced) However children at this age have very smallvocabularies and we were limited to common words thatalmost all children at this age would know Amongchildren first words are a disproportionate number ofwords that begin with stop consonants For many ofthose words minimal pair differences are still words(eg lsquolsquoballrsquorsquondashlsquolsquodollrsquorsquo) Thus we were constrained in ourselection of stimulus items For the known words therewere seven stop consonants two nasals and one frica-tive For the phonemic contrasts there are also sevenstop consonants two nasals and one fricative For thenonsense words there are three stop consonants oneaffricative two nasals two fricatives and two laterals Acomplete list of words is shown in Table 1
To evaluate at what point the known words couldphysically be distinguishable from the other word-typeswe examined two aspects of uniqueness (a) at whatpoint the phonetically dissimilar nonsense words be-come unique from their real word counterparts (egwhen does lsquolsquobearrsquorsquo become unique from lsquolsquogarersquorsquo) and (b)when the words might become unique from otherwords that would be in a babyrsquos vocabulary at this ageThe uniqueness point for initial consonants (the pointthat separates that consonant from other consonants)was within 20 msec of the beginning of the word forfricatives (s z f) 40 msec for stop consonants (b d gp t k) 60 msec for affricatives (j) and 20 msec for nasals(m n) and laterals (l r) To further examine theuniqueness points we compared each stimulus usedin the study relative to other words that might be in thechildrsquos vocabulary with the words in the MacArthur CDIthat would be considered close phonological neighborsTo this end we compared our stimuli to the number ofwords in the CDI with (a) the same consonant includingconsonant clusters and (b) the same consonant exclud-ing consonant clusters and (c) the same consonant plusthe vowel For comparisons a and b there are the mostneighbors for the known words (a = 474 b = 375) anintermediate amount for the phonemic contrast words(a = 371 b = 298) and the least for the nonsense words(a = 275 b = 233) The best comparison is to count thenumber of items that begin with the same consonantplus vowel (CV) There were no significant differences inthe number of CV neighbors for the three word-types(known = 16 phonemic contrasts = 12 nonsense =18) Additionally because of co-articulatory effects theacoustic energy for a given consonant varies as a func-tion of the following vowel (Jusczyk 1997) Thus evenwords beginning with the same consonant (eg lsquolsquobookrsquorsquoand lsquolsquobottlersquorsquo) differ in shape and spectral frequenciesand can be discriminated from each other as well as
Table 1 Stimuli
Known Words Phonemic Contrasts Nonsense
Bear Gare Kobe
Ball Pall Lif
Book Dook Neem
Bottle Pottle Fipe
Cup Tup Mon
Cat Gat Tek
Dog Bog Riss
Milk Nilk Keed
Nose Mose Jud
Shoe Zhu Zav
Mills et al 1461
their close phonological neighbors from the beginningof the word
Procedure
Language Assessment
Within one week prior to testing parents were asked tocomplete the MacArthur CDI (Fenson et al 1994)which provided an estimate of the childrsquos vocabularysize and percentile ranking relative to other children ofthe same age To ensure that the words to be used in thestudy as lsquolsquoknownrsquorsquo were comprehended by the childparents also completed a vocabulary checklist ratingscale indicating how sure they were that their childunderstood andor produced each word on a scale of1 (very sure they did not know that word) to 4 (very suretheir child understoodproduced a given word in avariety of different contexts and with different exem-plars) Additionally the children were asked to identify apicture of each word to be used as a known word from atwo-choice picture book All lsquolsquoknownrsquorsquo words used hadreceived a rating of 4 and were correctly identified in thepicture-pointing task
Electrophysiological Recording
The EEG was recorded using tin electrodes affixed to anelastic cap (Electro-Cap International) from sites overfrontal (F7 and F8) anterior temporal (50 of thedistance from F78 and T34) temporal (33 of thedistance from T34 to C34) parietal (50 of the dis-tance between T34 and P34) and occipital (O1 andO2) regions of the left and right hemispheres Addition-ally the electrooculogram was recorded from electrodesplaced over and under the eye to reject trials on whichblinks and vertical eye movement occurred and fromleft and right frontal electrodes to reject trials on whichhorizontal eye movement occurred Impedances werekept below 5 k and were balanced (within 1 k) acrossthe left and right hemispheres at any given position TheEEG was amplified by SA Instruments amplifiers with abandpass of 01 to 100 Hz and sampled continuouslyevery 4 msec All electrodes were referenced to linkedmastoids3 Averages of the EEG were conducted using2-sec epochs (ie 100 msec prestimulus and 1900 msecpoststimulus) The averaged ERPs were also digitallyfiltered off-line with a 60-Hz low-pass filter
Testing
Ten known 10 phonetically similar nonsense words(referred to as phonemic contrasts) and 10 phoneticallydissimilar nonsense (referred to as nonsense words)words were each presented six times in random orderfor a total of 60 trials per condition During testingchildren sat on their parentrsquos lap and listened to words
presented from a speaker located behind a movingpuppet in a puppet theater Words were presented ata variable rate between 1800 and 3000 msec SOA
Artifact Rejection
Artifact rejection was conducted off-line using a com-puter program to reject blinks and horizontal eye move-ment and amplifier blocking Individual thresholds wereset for each child based on visual inspection of the EEGepochs time-locked to each stimulus A mean of 50 ofthe trials were rejected due to eye and movementartifact The number of artifact-free trials per word-typeretained for analysis ranged from 12 to 53 (SD = 113)out of a possible 60 trials per condition There were nosignificant differences in the percentage of trials rejectedfor the different experimental conditions age groupsor sex
Measurement of ERP Components
Peak latencies and amplitudes were quantified by com-puter with reference to the 100-msec prestimulus base-line for the maximum negative or positive point in aspecified time window The time windows for eachcomponent were set according to the criteria used inour previous studies the first positive componentcalled the P100 was defined as the most positivedeflection between 50 and 175 msec The P100 indexesauditory sensory processing Moreover the lateral dis-tribution of the P100 has been shown to vary withpercentile ranking on the MacArthur CDI score Chil-dren who score at the 50th percentile or higher for theirage show a P100 left greater than right asymmetry Themean amplitude within the time window 200ndash400 afterword onset was quantified by computer with referenceto the 100-msec prestimulus baseline This time windowwas chosen because it had been shown to differ forknown and unknown words in our previous studies
Acknowledgments
This research was conducted while the first author was at theProject in Cognitive and Neural Development at the Universityof California at San Diego The research was supported by NIHgrant DC0048111 to H Neville from NIDCD NSERC (NaturalSciences and Engineering Research Council of Canada) grantto J Werker and HFSP (Human Frontierrsquos Science Program)grant to J Werker and H Neville The third author RenateZangl was funded in part by two postdoctoral grants (J-1646J-1817) from the Austrian Science Foundation
We thank Amy Adamson Greg Appelbaum Teresa Mitchelland Lisa Sanders (in alphabetical order not necessarily in orderof magnitude of contribution) for their help in acquiring andanalyzing the data and Chris Fennell and Liz Lewis for theircomments on an earlier version of the manuscript We areespecially indebted to the children and their parents whoparticipated in these studies and their parents without whomthe research would not be possible
1462 Journal of Cognitive Neuroscience Volume 16 Number 8
Reprint requests should be sent to Debra L Mills who is nowat the Department of Psychology 532 Kilgo Circle EmoryUniversity Atlanta GA 30322 or via e-mail dmills2emoryedu
Notes
1 Unlike the switch task the visual fixation task is an on-linetask with two simultaneously presented choices The twoindicators of success are shorter latency to look away from theincorrect object and longer looking times overall to the correctmatch The habituation phase in the lsquolsquoswitchrsquorsquo task leads to theprediction of a novelty preference in the test phase (ie longerlooking to the incorrect pairing)2 In previous studies Mills and colleagues typically findapproximately 10ndash15 of children show a reversed ERPresponse with a larger amplitude ERP response to unknownthan to known words In previous work Mills and colleagueshave treated those children as noise in the data and have keptthem in for the data analyses Ultimately we want tounderstand the nature of lexical representations in this groupof children showing an aberrant ERP response For the currentstudy however in order to compare the ERP response tophonemic contrasts to known words we felt it necessary toinclude in our sample only those children who showed themuch more typical pattern of response resulting in the ex-clusion of three children All effects reported here show thesame pattern with these children included but the variabilityfrom their inclusion did reduce the differences in somecomparisons3 We are aware of the controversies surrounding the use oflinked mastoids Linked mastoids were used here to increasethe number of active cites given the number of amplifiersavailable and to provide consistency with previous studies Apilot study using similar auditory stimuli was conducted toexamine possible distortions in the distribution of scalp activityresulting from forced linkage This was determined by re-cording from one mastoid using the other as a reference andlinking the mastoids off-line These pilot data were comparedwith the data recorded using linked mastoids and did not yieldsignificant differences
REFERENCES
Bailey T M amp Plunkett K (2002) Phonological specificity inearly words Cognitive Development 17 1265ndash1282
Bertoncini J amp Mehler J (1981) Syllables as units in infantspeech perception Infant Behavior and Development 4247ndash260
Brown C amp Matthews J (1997) The role of feature geometryin the development of phonemic contrasts In S J Hannahsamp M Young-Scholten (Eds) Focus on phonologicalacquisition (pp 67ndash112) Amsterdam Benjamins
Brown C M Hagoort P amp Kutas M (2000) Postlexicalintegration processes in language comprehension Evidencefrom brain-imaging research In M Gazzaniga (Ed) The newcognitive neurosciences (2nd ed pp 881ndash895)Cambridge MIT Press
Conboy B (2003) Patterns of language processing and growthin early EnglishndashSpanish bilingualism (Doctoral dissertationUniversity of California San Diego and San Diego StateUniversity 2000) Dissertation Abstracts InternationalB The Sciences and Engineering 63 (UMI No 5193)
Conboy B amp Mills D L (2000) Cerebral specialization forlanguage comprehension in bilingual toddlers CognitiveNeuroscience Society 2000 Annual Meeting AbstractsSupplement of the Journal of Cognitive Neuroscience 17
Connolly J F Service E DrsquoArcy R C N Kujala A Alho K(2001) Phonological aspects of word recognition as revealedby high-resolution spatio-temporal brain mappingNeuroReport 12 237ndash243
Dehaene-Lambertz G (2000) Cerebral specialization forspeech and non-speech stimuli in infants Journal ofCognitive Neuroscience 12 449ndash460
Dehaene-Lambertz G amp Dehaene S (1994) Speed andcerebral correlates of syllable discrimination in infantsNature 370 292ndash294
Eimas P D Siqueland E R Jusczyk P amp Vigorito J (1971)Speech perception in infants Science 171 303ndash306
Fennell C E amp Werker J F (2003) Early word learnersrsquoability to access phonetic detail in well-known wordsLanguage and Speech 46 245ndash264
Fenson L Dale P S Reznick J S Bates E Thal D Jamp Pethick S J (1994) Variability in early communicativedevelopment Monographs of the Society for Research inChild Development 59 1ndash173
Fernald A Swingley D amp Pinto J P (2001) When half aword is enough Infants can recognize spoken words usingpartial phonetic information Child Development 721003ndash1015
Fischler I S (1990) Comprehending language withevent-related potentials In J W Rohrbaugh R Parasuramanamp R Johnson (Eds) Event-related brain potentials(pp 165ndash170) New York Oxford University Press
Halle P amp de Boysson-Bardies B (1994) Emergence of anearly receptive lexicon Infantsrsquo recognition of wordsInfant Behavior and Development 17 119ndash129
Halle P amp de Boysson-Bardies B (1996) The format ofrepresentation of recognized words in infantsrsquo earlyreceptive lexicon Infant Behavior and Development 19463ndash481
Holcomb P J amp Neville H J (1991) The electrophysiologyof spoken sentence processing Psychobiology 19286ndash300
Holcomb P J Coffey S A amp Neville H J (1992) Visual andauditory sentence processing A developmental analysisusing event-related brain potentials DevelopmentalNeuropsychology 8 203ndash241
Jusczyk P W (1997) The discovery of spoken languageCambridge MIT Press
Jusczyk P W amp Aslin R N (1995) Infantsrsquo detection ofsound patterns of words in fluent speech CognitivePsychology 29 1ndash23
Kutas M amp Hillyard S (1980) Reading senseless sentencesBrain potentials ref lect semantic incongruity Science 207203ndash205
Luck S amp Hillyard S (2000) The operation of selectiveattention at multiple stages of processing Evidence fromhuman and monkey electrophysiology In M Gazzaniga(Ed) The new cognitive neurosciences (2nd edpp 687ndash700) Cambridge MIT Press
Mills D L Coffey-Corina S A amp Neville H J (1997)Language comprehension and cerebral specialization from13ndash20 months [(Special issue on origins of languagedisorders)] In D Thal amp J Reilly (Eds) DevelopmentalNeuropsychology 13 397ndash446
Mills D M Coffey-Corina S A amp Neville H J (1994)Variability in cerebral organization during primary languageacquisition In G Dawson amp K Fischer (Eds) Humanbehavior and the developing brain (pp 427ndash455) NewYork Guilford Publications
Mills D M Coffey-Corina S A amp Neville H J (1993)Language acquisition and cerebral specialization in20-month-old infants Journal of Cognitive Neuroscience 5326ndash342
Mills et al 1463
Mills D L Conboy B amp Paton C (in press) Do changes inbrain organization reflect shifts in symbolic functioning InL Namy (Ed) Symbol use and symbolic representationMahwah NJ Erlbaum
Mills D L Plunkett K Prat C amp Schafer G (in press)Watching the infant brain learn words Effects ofvocabulary size and experience Cognitive Development
Molfese D L (1989) Electrophysiological correlates of wordmeanings in 14-month-old human infants DevelopmentalNeuropsychology 5 79ndash103
Molfese D L (1990) Auditory evoked responses recordedfrom 16-month-old human infants to words they did anddid not know Brain and Language 38 345ndash363
Molfese D L Burger-Judisch L M amp Hans L L (1991)Consonant discrimination by newborn infantsElectrophysiological differences DevelopmentalNeuropsychology 7 177ndash195
Molfese D L amp Molfese V J (1988) Right hemisphereresponses from preschool children to temporal cuescontained in speech and nonspeech materialsElectrophysiological correlates Brain and Language33 245ndash259
Molfese D L Morse P A amp Peters C J (1990) Auditoryevoked responses to names for different objectsCross-modal processing as a basis for infant languageacquisition Developmental Psychology 26 780ndash795
Molfese D L Wetzel W F amp Gill L A (1993) Known versusunknown word discriminations in 12-month-old humaninfants Electrophysiological correlates DevelopmentalNeuropsychology 9 241ndash258
Naigles L R (2002) Form is easy meaning is hard Resolvinga paradox in early child language Cognition 8 157ndash199
Nazzi T amp Bertoncini J (2003) Before and after thevocabulary spurt Two modes of word acquisitionDevelopmental Science 6 136ndash142
Osterhout L amp Holcomb P J (1995) Event-related potentialsand language comprehension In M D Rugg amp M G HColes (Eds) Electrophyisology of mind (pp 171ndash215)Oxford Oxford University Press
Richards J E amp Hunter S K (2001) Testing neural models ofthe development of infant attention DevelopmentalPsychobiology 40 226ndash236
Sereno S C Brewer C C amp OrsquoDonnell P J (2003)Context effects in word recognition Evidence for earlyinteractive processing Psychological Science 14328ndash333
Simos P G amp Molfese D L (1997) Electrophysiologicalresponses from a temporal order continuum in the newborninfant Neuropsychologia 35 89ndash98
Simos P G Molfese D L amp Brenden R A (1997) Behavioraland electrophysiological indices of voicing-cuediscrimination Laterality patterns and developmentBrain and Language 57 122ndash150
St George M amp Mills D L (2001) Electrophysiologicalstudies of language development In J Weissenborn ampB Hoehle (Eds) Language acquisition and languagedisorders (pp 247ndash259) Amsterdam John BenjaminsPublishing
Stager C L amp Werker J F (1997) Infants listen for morephonetic detail in speech perception than in word-learningtasks Nature 388 381ndash382
Swingley D (2003) Phonetic detail in the developing lexiconLanguage and Speech 46 265ndash294
Swingley D amp Aslin R N (2000) Spoken word recognitionand lexical representation in very young children Cognition76 147ndash166
Swingley D amp Aslin R N (2002) Lexical neighborhoodsand the wordndashform representations of 14-month-oldsPsychological Science 13 480ndash484
Werker J F Cohen L B Lloyd V L Casasola M amp StagerC L (1998) Acquisition of wordndashobject associations by14-month-old infants Developmental Psychology 341289ndash1309
Werker J F amp Fennell C E (2004) From listening to soundsto listening to words Early steps in word learning In G Hallamp S Waxman (Eds) Weaving a lexicon (pp 79ndash109)Cambridge MA MIT Press
Werker J F Fennell C E T Corcoran K M amp Stager C L(2002) Infantsrsquo ability to learn phonetically similar wordsEffects of age and vocabulary size Infancy 3 1ndash30
Werker J F amp Tees R C (1984) Cross-language speechperception Evidence for perceptual reorganization duringthe first year of life Infant Behavior and Development 749ndash63
1464 Journal of Cognitive Neuroscience Volume 16 Number 8
left and right hemispheres [known vs nonsense lefthemisphere F(115) = 230 p lt 15 right hemisphereF(115) = 745 p lt 05 phonemic contrasts vs non-sense words left hemisphere F(115) = 458 p lt 05right hemisphere F(115) = 797 p lt 01]
Children Aged 20 Months
P100 The P100 peaked later and was larger over frontalthan posterior regions [latency F(116) = 700 p lt001 amplitude F(116) = 1407 p lt 001] There wasalso a main effect of word-type for the P100 amplitude[F(232) = 355 p lt 05] Examination of the amplitudedifferences across word-types showed that the P100 waslarger to known words than to nonsense words [knownvs nonsense F(116) = 707 p lt 05] and phonemiccontrasts [known vs phonemic contrasts F(116) =605 p lt 05] The P100 amplitude was larger overthe left than the right hemisphere but only overtemporal and parietal sites [Hemisphere Electrodesite F(464) = 300 p lt 05] There were no othermain effects or interactions
N200ndashN400 The N200ndashN400 amplitude differences tothe different word-types approached significance [word-type F(116) = 303 p = 06] The N200ndashN400 waslarger over the left than the right hemisphere at tempo-ral and parietal sites but larger over the right than theleft over the occipital regions [Hemisphere Electrodesite F(464) = 283 p lt 05]
Planned comparisons showed that the N200ndashN400was larger to known than nonsense words [F(116) =576 p lt 05] but only over left temporal [F(116) =826 p lt 01] left parietal [F(116) = 1062 p lt 01]and right frontal [F(116) = 602 p lt 05] sites (Figure 4top) The N200ndashN400 was also larger to known wordsversus phonemic contrast words over left temporal[F(116) = 724 p lt 05] and left parietal [F(116) =709 p lt 05] regions (Figure 4 middle) However themain effect of word-type for this comparison only ap-proached significance [F(116) = 270 p = 10] Therewere no significant differences in N200ndashN400 amplitudesto the phonemic contrasts and nonsense words(Figure 4 bottom) [F(116) = 042 ns] At 20 monthsthen the phonemic contrast (nonsense) words were no
Figure 3 For the 14-month-olds ERP differences are directly
compared to known words and nonsense words (top) known words
and phonemic contrasts (middle) and phonemic contrasts andphonetically dissimilar nonsense words (bottom) Significant
differences in N200ndashN400 mean amplitudes are shaded and enclosed in
the rectangle Left and temporal and parietal regions are shown on theleft and right sides of the figure respectively
Figure 4 For the 20-month-olds ERP differences are directly
compared to known words and nonsense words (top) known words
and phonemic contrasts (middle) and phonemic contrasts andphonetically dissimilar nonsense words (bottom) Significant
differences in N200ndashN400 mean amplitudes are shaded and enclosed in
the rectangle Left and temporal and parietal regions are shown on theleft and right sides of the figure respectively
1456 Journal of Cognitive Neuroscience Volume 16 Number 8
longer treated as known words but were instead treatedlike other nonsense words
Age Group Comparisons
There were no main effects or interactions with agewhen all three word-types and all electrode sites wereincluded in the analyses Because the 20-month-olds inthis study and in our previous studies only showed ERPdifferences in this time window to known versus un-known words at temporal and parietal sites a separateANOVA was conducted including only temporal andparietal sites that is with two levels of Age group (14and 20 months) as the between-subjects factor andWord-type (known phonemic contrasts and nonsensewords) Hemisphere (left and right) and Electrodesite (temporal and parietal) as the within-subjects fac-tors That analysis showed main effects for Word-type[F(131) = 659 p lt 001] and Hemisphere [F(131) =395 = 420 p lt 05] These effects were qualifiedby two interactions with group including a Group Word-type Hemisphere interaction [F(662) = 304p lt 05] and a Group Electrode site interaction[F(131) = 396 p lt 05] These interactions were fur-ther validated by the different patterns displayed by the14- and 20-month-olds separately
To further support the age-related effects describedabove separate ANOVAs were conducted with Age (14and 20 months) as the between-subjects factor andWord-type (known phonemic contrasts and nonsensewords) Hemisphere (left and right) and Electrode site(frontal anterior temporal temporal parietal and oc-cipital) as the within-subjects factors To conserve spaceonly main effects of and interactions with age arereported below
P100 There were no main effects or interactions withage for the P100 latency The age-related difference inthe lateral distribution of the P100 amplitude was sup-
ported by an Age Hemisphere Electrode siteinteraction [F(4124) = 275 p lt 05]
N200ndashN400 There were no main effects or interac-tions with age when all three word-types and all elec-trode sites were included in the analyses Because the20-month-olds in previous studies and in this study asdescribed above only showed ERP differences in thistime window to known versus unknown words at tem-poral and parietal sites a separate ANOVA was con-ducted including only temporal and parietal sitesnamely with two levels of Age group (14 and 20 months)as the between-subjects factor and Word-type (knownphonemic contrasts and nonsense words) Hemisphere(left and right) and Electrode site (temporal andparietal) as the within-subjects factors That analysisshowed two interactions with group including a Groupby Word-type Hemisphere interaction [F(662) =304 p lt 05] and a Group Electrode site interaction[F(131) = 396 p lt 05] These interactions supportthe findings described above showing different patternsof responsiveness to the different word-types by the 14-and 20-month-olds separately
Summary
Fourteen- and 20-month-olds showed different patternsof brain activity to known words (eg lsquolsquobearrsquorsquo) phone-mic contrasts (eg lsquolsquogarersquorsquo) and (phonetically dissimilar)nonsense words (eg lsquolsquokobersquorsquo) (Figure 5) As predictedby Stager and Werker (1997) 14-month-olds showedlarger N200ndashN400 amplitudes to known words com-pared with nonsense words (eg lsquolsquokobersquorsquo) Howeverthe N200ndashN400 to known words and the phonemiccontrast stimuli (eg lsquolsquobearrsquorsquo and lsquolsquogarersquorsquo) did not differfrom each other
Unlike the pattern seen at 14 months and as pre-dicted by Werker et al (2002) the 20-month-oldsshowed larger amplitude N200ndashN400 responses to
Figure 5 N200ndashN400 mean
areas averaged across temporal
and parietal regions of the leftand right hemispheres to the
three word-types for the
14- and 20-month-olds
Mills et al 1457
known words (eg lsquolsquobearrsquorsquo) over both phonemic con-trast (eg lsquolsquogarersquorsquo) and other nonsense words (eglsquolsquokobersquorsquo) Moreover the N200ndashN400 amplitudes to thephonemic contrast and other nonsense words (lsquolsquogarersquorsquoand lsquolsquokobersquorsquo) did not differ from each other Thesefindings also replicated previously observed lateral dif-ferences in the N200ndashN400 amplitudes to known versusunknown words between 14 and 20 months (Mills et al1997)
DISCUSSION
The results from the present study were consistent withand expanded on findings observed in previous behav-ioral and ERP studies At both 14 and 20 months of agethe pattern of ERP responses seen replicated that pre-viously reported by Mills et al (1993 1997) for knownversus unknown words At both ages there was a largeramplitude negative response from 200 to 400 msec toknown versus nonsense words And as shown before at14 months of age this response was broadly distributedacross the scalp whereas at 20 months it was observedprimarily over the left temporal and parietal electrodesites
The replication of the previous work by Mills andcolleagues puts us in a particularly strong position tointerpret the results from the phonemic contrast wordsAt 14 months of age the ERP signature to phonemiccontrast nonsense words (eg lsquolsquogarersquorsquo or lsquolsquobogrsquorsquo) isindistinguishable from that to known words (eg lsquolsquobearrsquorsquoand lsquolsquodogrsquorsquo) and significantly greater in amplitude thanthat to dissimilar nonsense words (eg lsquolsquolifrsquorsquo) Thus at14 months of age the neural response appears toindicate a mistaken recognition of a minimal pair mis-pronunciation of a known word as the word itself Thisresult is consistent with that seen in the switch task at14 months (Werker et al 2002 Stager amp Werker 1997)The results with the ERP task are particularly convincingbecause here we see a significant increase in the ampli-tude of the response rather than a failure to increaselooking time At 20 months of age the ERP response isalso consistent with results seen in the switch task at thesame age (Werker et al 2002) At 20 months the ERPsignature to phonemic contrast nonsense words (eglsquolsquogarersquorsquo and lsquolsquobogrsquorsquo) is significantly different from that totheir known counterparts (eg lsquolsquobearrsquorsquo and lsquolsquodogrsquorsquo) andindistinguishable from that to phonetically dissimilarnonsense words (eg lsquolsquoneemrsquorsquo or lsquolsquolifrsquorsquo) Thus at20 months the evoked response indicates that childrenare showing the lsquolsquoword recognitionrsquorsquo ERP response onlyto known words that are phonologically correct in alldetails By 20 months then the confusion with similar-sounding known words is no longer present indicatingthat words in the lexicon are not only fully specified interms of phonological detail but that such detail isavailable even when listening to words in tasks that donot provide contextual or pictorial support for the word
meaning and may not even involve full attention to theword
One difference between the present study and thebehavioral studies by Werker and colleagues and Swin-gley and Aslin is that the behavioral studies used bothpictures and words whereas the present study pre-sented a series of words without a picture context Anegative going ERP wave within the 200ndash500 msec timewindow has also been shown to index word meaningin cross-modal matchmismatch paradigms using pic-tures and words in 14- and 20-month-old children (MillsConboy amp Paton in press) and real objects and noveltrained words in 14-month-old children (Molfese Morseamp Peters 1990) We chose to use the Mills et al (1997)paradigm rather than a pictureword matchmismatchparadigm for several reasons The pictureword matchmismatch paradigm elicits a large amplitude bilateralnegative component starting at 200 msec and peakingaround 500 msec when the subsequent word does notmatch the picture at both 14 and 20 months (MillsConboy et al in press) This ERP effect is most likelywithin the N400 family observed in children and adults toviolations of semantic expectancy (Holcomb Coffey ampNeville 1992 Kutas amp Hillyard 1980) In adults a similarparadigm elicited a phonological mismatch (PMN) towords and nonwords from 250 to 347 msec over rightfrontal regions and from 347 to 638 msec over symmet-rical centro-parietal regions (Connolly Service DrsquoArcyKujala amp Alho 2001) One concern was that the N400 andPMN effects elicited by a pictureword matchmismatchparadigm might swamp more subtle differences in thelatency amplitudes and distributions of ERPs related tothe different word-types and group differences The Millset al paradigm was chosen because it showed ERP dif-ferences to known and unknown words that varied withword-type age and vocabulary size The methodologicaldifferences should be taken into consideration whencomparing the effects However the ERP findings areconsistent with and provide additional support for theage-related differences reported by Stager et al (1997)
The robust differences in the ERP pattern at 14and 20 months of age could be explained at manylevels Several studies have shown that a negativepeak at around 200 msec is sensitive to phonetic varia-tion in voice onset time (Simos Molfese amp Brenden1997 Molfese amp Molfese 1988) place of articulation(Dehaene-Lambertz amp Dehaene 1994 Molfese Burger-Judisch amp Hans 1991) and acoustic cues in nonspeechstimuli (Dehaene-Lambertz 2000 Simos amp Molfese1997) in very young children and even newborns Oneinterpretation is that ERP differences between word-types are related to the acoustic or phonological featuresof the different word-types and that age-related changesare due to differences in the childrenrsquos ability to mem-orize the different word lists Although phonological dis-crimination can modulate the amplitude of the N200 incertain paradigms it is not necessarily the only factor
1458 Journal of Cognitive Neuroscience Volume 16 Number 8
that may contribute to the amplitude of this componentHere we argue that age-related changes in a top-downprocess linked to word meaning is the dominant factormodulating the amplitude of the N200ndashN400 rather thanphonological discrimination First if ERP differences be-tween conditions were due solely to phonological fac-tors then both age groups would have displayed N200amplitude differences between the known words andphonetic contrasts Additionally the known and phone-mic contrasts were balanced for the types of initialconsonants in the number of stop consonants nasalsand fricatives and did not differ in their phonologicalneighbors for other words frequently understood bychildren in this age range (see Methods) Therefore itis difficult to explain why the phonemic contrasts pat-terned with the known words at 14 months and thenonsense words at 20 months based solely on acousticaland phonemic differences A second line of evidencesuggesting that the ERP differences observed here can-not be explained by phonological differences betweenword-types comes from a study of bilingual toddlersusing the same paradigm In that study the N200ndashN400did not differ between Spanish and English for either theknown or unknown words (Conboy amp Mills 2000 Con-boy 2003) ERP differences were linked to word meaning(known vs unknown words) language dominance andtotal vocabulary size but not phonological differencesbetween Spanish and English If the N200ndashN400 indexedthe phonological aspects of the words amplitude differ-ences would be expected between Spanish and EnglishA third and perhaps the strongest line of evidencecomes from a training study with 20-month-old childrenin which nonsense words were either repeated or pairedwith an object (Mills Plunkett Prat amp Schafer in press)After the training phase when objects were no longerpresent the amplitude of the N200ndashN400 to the newlylearned words increased relative to before training butbecame more positive for nonsense words that wererepeated the same number of times Because the stimuliwere counterbalanced across participants the increasedamplitude of the N200ndashN400 to the newly learned wordswas modulated by its association with a meaningfulstimulus and cannot be explained by phonological dif-ferences between word-types
Another interpretation of the findings is that at 14months of age the phonetic detail distinguishing oneword from another is simply not available in the lexicalrepresentation Such an explanation would be compat-ible with theories of phonological development thatposit an initial under-specification of the informationin the lexicon that is only gradually filled in as thevocabulary expands (eg Brown amp Matthews 1997)Although possible we think this explanation is unlikelygiven the work by Swingley and Aslin (2002) showingthat children of 14 months do seem to show someevidence of use of phonetic detail in word recognitiontasks via longer looking time to the matching object
when a correct versus a mispronounced version of aword is given The Swingley and Aslin work involvesshowing the children two objects (eg a car and a dog)and asking them to find either the correctly labeledobject (eg lsquolsquodogrsquorsquo) or a mispronunciation of that sameobject (eg lsquolsquobogrsquorsquo) More recently Fennell and Werker(2003) showed that when tested on two phonemiccontrast words that they already know well (eg lsquolsquoballrsquorsquoand lsquolsquodollrsquorsquo) the child of 14 months can succeed even inthe switch task These two studies show that at somelevel in the system full phonetic specification of well-known words is represented even at 14 months
The successes in the two-choice (Swingley and Aslin)behavioral lsquolsquomispronunciationrsquorsquo task and in the switchtask using two well-known words in comparison to boththe switch word-learning task and the ERP word recog-nition task are compatible with the attentional resourcelimitation hypothesis offered by Werker and colleagues(Werker et al 2002 Werker amp Fennell 2004 Fennell ampWerker 2003 Stager amp Werker 1997) According to thisexplanation the task of linking a word to an object ischallenging for a novice word learner All the detail ispicked up but the computational difficulty of actuallymapping the word on to the object makes it difficultto hold all the information in mind As such one sees aU-shaped function in the pattern of data The child of7ndash8 months who is not yet actively engaged in mappingwords to referents is better able to attend to fine pho-netic detail than is the child of 14 months who is at-tempting to make and remember the link (see Stager ampWerker 1997 see also Halle amp de Boysson-Bardies1996 Jusczyk amp Aslin 1995) Becoming a word learnerchanges the task for the older child This increases theprocessing demands and interferes with the childrsquosability to access detail The information is picked upin perception it is just not available for access in lexicalrecall tasks After the child becomes a more accom-plished word learner and has a working phonemicinventory to guide information access the relevantphonetic differences lsquolsquostand outrsquorsquo as salient and impor-tant to the lexical entry Or as suggested by Nazzi andBertoncini (2003) once the child passes a criticaljuncture in word learning sophistication moving froma simple associationist to a referential word learneraccess to phonological detail becomes possible Priorto that point however any detection of that detail isat best fleeting As such it may be seen in on-lineprocessing tasks or in tasks that provide full support torecall memory (Fennell amp Werker 2003 Swingley ampAslin 2002) but is not evident in tasks that involvelearning new words (Werker et al 2002 Stager ampWerker 1997) nor is it evident in word recognitiontasks such as the ERP task used here where there is noobject to facilitate activation of the full memory trace
Support for this explanation can be seen in thecurrent ERP study In this study children of 20 months(but not 14 months) showed a significant P100 response
Mills et al 1459
to the known words over both the nonsense words andthe phonemic contrasts words The P100 is thought toindex sensory and attentional processes This is the firsttime Mills and colleagues (eg Mills et al 1993 1997)have observed a significantly greater P100 response toknown than to unknown words in the ERP word recog-nition studies The amplitude of the P100 is known to bemodulated by effects of attention in both adults (Luck ampHillyard 2000) and children (Richards amp Hunter 2001)One interpretation of this finding is that the inclusion ofphonemic contrast words in the dataset increased atten-tion to early phonological differences To distinguish theknown words from the phonemic contrast foils theyhad to increase their vigilance to the sensory andperceptual detail thus yielding a significant P100 re-sponse in this ERP word recognition task
Directions for Further Research
A focus for future work will be to determine preciselywhat changes between 14 and 20 months allow the moreaccomplished word learner to access and use full pho-netic detail across a wide range of word recognition andword learning situations In a recent training study MillsPlunkett et al (in press) addressed this question byexamining the effects of experience on the lateral distri-bution of the N200ndashN400 differences between knownand unknown words Mills et al asked whether observedchanges in the lateral distribution in ERP differencesto known versus unknown words between 13 and20 months (Mills et al 1997 and replicated in the pre-sent study) reflect the availability of specialized brainsystems for word recognition (as might be available in amore accomplished word learner) or increasing knowl-edge of particular words by testing 20-month-old chil-dren who varied in vocabulary size on newly learnedwords ERPs to novel words that had been paired withan object during a pretest training phase were comparedto ERPs to novel words that had been repeated the samenumber of times but without the objectword pairingThe results supported a mixed model ERP differences tonewly learned words compared to repeated-but-not-trained words showed a bilateral distribution acrossthe whole sample This finding was consistent with thehypothesis that the asymmetrical distribution of ERP dif-ferences to known and unknown words is linked to theamount of experience with individual words Howeverchildren with the largest vocabularies showed a leftgreater than right distribution of ERP differences Thelatter finding suggested that expertise with word learn-ing might also affect the specialization of language-relevant brain activity even in the service of learningnew words
If a more expert word learning system is becomingavailable (Nazzi amp Bertoncini 2003) in the older andormore experienced word learner this might also permit
greater direction of attention to the relevant phonolog-ical information in the word learning and access situa-tion Indeed as we argued at the beginning of thisarticle perhaps part of becoming a more expert wordlearner is knowing just which properties of a word aredefinitional in a particular linguistic community It maybe the emergence of this greater understanding of justwhat distinguishes words in our language that allows theolder more accomplished word learner to not onlyperceive but also lsquolsquomarkrsquorsquo and lsquolsquousersquorsquo the phonologicaldetail in word learning and word recognition tasks
In previous studies Mills and colleagues have re-ported a link between vocabulary size and the distribu-tion of the N200ndashN400 amplitude difference to knownand unknown words between 14 and 20 months of ageSimilarly Werker et al (2002) reported a significantcorrelation between performance on the CDI at both14 and 17 months and performance in the minimal pairswitch task There was no significant correlation with theCDI in the present study The lack of a significantcorrelation here could reflect lack of variability in thedata perhaps because the age groups selected are notyet in transition We predict that testing of a group ofchildren intermediate in age and vocabulary sizes to thegroups tested here would be more likely to reveal such apattern of findings
In summary using an ERP design we extended andhelped explain the pattern of findings seen in earlybehavioral tasks assessing the phonological detail repre-sented and used in early word learners The results ofthe current ERP study indicate that at the earliest stagesof word learning children treat minimal pair mispronun-ciations of known words as acceptable instances of thatword This provides strong evidence that in the decisionstage of word recognition novice word learners accept abroader range of pronunciations of the word as accept-able than do older children With increasing age andincreasing language sophistication the amount of pho-nological detail easily accessed in the representation ofwords increases helping to avoid mapping mistakes andto facilitate more rapid acquisition of a vocabulary
METHODS
Participants and Settings
Participants were tested in three locations at the Uni-versity of California San Diego at the University ofOregon Eugene Oregon and at the University of BritishColumbia Vancouver Canada Approximately one-thirdof the children in each age group were tested at eachsite Testing at the Vancouver site was conducted in amobile ERP lab belonging to the University of OregonChildren from the San Diego and Eugene areas wererecruited through advertisements in a local magazineposters displayed in the area requests at play groupsand referrals from parents whose children had partici-
1460 Journal of Cognitive Neuroscience Volume 16 Number 8
pated in our studies Children from the Vancouver areawere recruited through the University of British Colum-bia child Studies Center database Parents of all childrensigned consent forms consistent with the human sub-jects internal review boards at the university at whichthey were tested Parents were given $500 to defraytransportation costs and children were given a small toyin appreciation of their participation Only full-term(gt36 weeks of gestation) healthy children with mono-lingual experience with English participated in the studychildren with a family history of language impairmentwere excluded from the study
Data from 16 children aged 14 months (9 girls meanage = 14 months 15 days SD = 97 days range = 14months 1 day to 15 months 1 day) and 17 children aged20 to 21 months (9 girls mean age = 20 months 11 daysSD = 131 days age range = 19 months 14 days to 20months 27 days) were retained for analysis in the studyAn additional 18 children (12 boys nine 14-month-olds)were tested but were excluded from the analyses due totoo few artifact-free trials (n = 10) excessive crying (n =3) reversed polarity of the ERP response (n = 3 seenote)2 or refusal to wear the electrocap (n = 2)
Stimuli
The stimuli were naturally spoken in a female voice anddigitized at 16 bits 44 kHz sampling rate The stimuliconsisted of three types of words (a) 10 words whosemeanings were understood by the child (known wordseg lsquolsquomilkrsquorsquo) (b) 10 nonsense words that differed fromthe known words in the initial phonemic contrast (pho-nemic contrast words eg lsquolsquonilkrsquorsquo) and (c) 10 wordsthat differed phonetically from known words in allvowels and consonants (nonsense words eg lsquolsquoneemrsquorsquo)All stimuli were matched on word duration and numberof syllables The mean durations for the known wordsphonetic contrasts and nonsense words were 783 msecSD = 144 865 msec SD = 169 861 msec SD = 96[F(227) = 109 p = 34] Ideally all word-types wouldbe matched on manner of articulation (eg all stopconsonants all nasals all laterals all fricatives or allbalanced) However children at this age have very smallvocabularies and we were limited to common words thatalmost all children at this age would know Amongchildren first words are a disproportionate number ofwords that begin with stop consonants For many ofthose words minimal pair differences are still words(eg lsquolsquoballrsquorsquondashlsquolsquodollrsquorsquo) Thus we were constrained in ourselection of stimulus items For the known words therewere seven stop consonants two nasals and one frica-tive For the phonemic contrasts there are also sevenstop consonants two nasals and one fricative For thenonsense words there are three stop consonants oneaffricative two nasals two fricatives and two laterals Acomplete list of words is shown in Table 1
To evaluate at what point the known words couldphysically be distinguishable from the other word-typeswe examined two aspects of uniqueness (a) at whatpoint the phonetically dissimilar nonsense words be-come unique from their real word counterparts (egwhen does lsquolsquobearrsquorsquo become unique from lsquolsquogarersquorsquo) and (b)when the words might become unique from otherwords that would be in a babyrsquos vocabulary at this ageThe uniqueness point for initial consonants (the pointthat separates that consonant from other consonants)was within 20 msec of the beginning of the word forfricatives (s z f) 40 msec for stop consonants (b d gp t k) 60 msec for affricatives (j) and 20 msec for nasals(m n) and laterals (l r) To further examine theuniqueness points we compared each stimulus usedin the study relative to other words that might be in thechildrsquos vocabulary with the words in the MacArthur CDIthat would be considered close phonological neighborsTo this end we compared our stimuli to the number ofwords in the CDI with (a) the same consonant includingconsonant clusters and (b) the same consonant exclud-ing consonant clusters and (c) the same consonant plusthe vowel For comparisons a and b there are the mostneighbors for the known words (a = 474 b = 375) anintermediate amount for the phonemic contrast words(a = 371 b = 298) and the least for the nonsense words(a = 275 b = 233) The best comparison is to count thenumber of items that begin with the same consonantplus vowel (CV) There were no significant differences inthe number of CV neighbors for the three word-types(known = 16 phonemic contrasts = 12 nonsense =18) Additionally because of co-articulatory effects theacoustic energy for a given consonant varies as a func-tion of the following vowel (Jusczyk 1997) Thus evenwords beginning with the same consonant (eg lsquolsquobookrsquorsquoand lsquolsquobottlersquorsquo) differ in shape and spectral frequenciesand can be discriminated from each other as well as
Table 1 Stimuli
Known Words Phonemic Contrasts Nonsense
Bear Gare Kobe
Ball Pall Lif
Book Dook Neem
Bottle Pottle Fipe
Cup Tup Mon
Cat Gat Tek
Dog Bog Riss
Milk Nilk Keed
Nose Mose Jud
Shoe Zhu Zav
Mills et al 1461
their close phonological neighbors from the beginningof the word
Procedure
Language Assessment
Within one week prior to testing parents were asked tocomplete the MacArthur CDI (Fenson et al 1994)which provided an estimate of the childrsquos vocabularysize and percentile ranking relative to other children ofthe same age To ensure that the words to be used in thestudy as lsquolsquoknownrsquorsquo were comprehended by the childparents also completed a vocabulary checklist ratingscale indicating how sure they were that their childunderstood andor produced each word on a scale of1 (very sure they did not know that word) to 4 (very suretheir child understoodproduced a given word in avariety of different contexts and with different exem-plars) Additionally the children were asked to identify apicture of each word to be used as a known word from atwo-choice picture book All lsquolsquoknownrsquorsquo words used hadreceived a rating of 4 and were correctly identified in thepicture-pointing task
Electrophysiological Recording
The EEG was recorded using tin electrodes affixed to anelastic cap (Electro-Cap International) from sites overfrontal (F7 and F8) anterior temporal (50 of thedistance from F78 and T34) temporal (33 of thedistance from T34 to C34) parietal (50 of the dis-tance between T34 and P34) and occipital (O1 andO2) regions of the left and right hemispheres Addition-ally the electrooculogram was recorded from electrodesplaced over and under the eye to reject trials on whichblinks and vertical eye movement occurred and fromleft and right frontal electrodes to reject trials on whichhorizontal eye movement occurred Impedances werekept below 5 k and were balanced (within 1 k) acrossthe left and right hemispheres at any given position TheEEG was amplified by SA Instruments amplifiers with abandpass of 01 to 100 Hz and sampled continuouslyevery 4 msec All electrodes were referenced to linkedmastoids3 Averages of the EEG were conducted using2-sec epochs (ie 100 msec prestimulus and 1900 msecpoststimulus) The averaged ERPs were also digitallyfiltered off-line with a 60-Hz low-pass filter
Testing
Ten known 10 phonetically similar nonsense words(referred to as phonemic contrasts) and 10 phoneticallydissimilar nonsense (referred to as nonsense words)words were each presented six times in random orderfor a total of 60 trials per condition During testingchildren sat on their parentrsquos lap and listened to words
presented from a speaker located behind a movingpuppet in a puppet theater Words were presented ata variable rate between 1800 and 3000 msec SOA
Artifact Rejection
Artifact rejection was conducted off-line using a com-puter program to reject blinks and horizontal eye move-ment and amplifier blocking Individual thresholds wereset for each child based on visual inspection of the EEGepochs time-locked to each stimulus A mean of 50 ofthe trials were rejected due to eye and movementartifact The number of artifact-free trials per word-typeretained for analysis ranged from 12 to 53 (SD = 113)out of a possible 60 trials per condition There were nosignificant differences in the percentage of trials rejectedfor the different experimental conditions age groupsor sex
Measurement of ERP Components
Peak latencies and amplitudes were quantified by com-puter with reference to the 100-msec prestimulus base-line for the maximum negative or positive point in aspecified time window The time windows for eachcomponent were set according to the criteria used inour previous studies the first positive componentcalled the P100 was defined as the most positivedeflection between 50 and 175 msec The P100 indexesauditory sensory processing Moreover the lateral dis-tribution of the P100 has been shown to vary withpercentile ranking on the MacArthur CDI score Chil-dren who score at the 50th percentile or higher for theirage show a P100 left greater than right asymmetry Themean amplitude within the time window 200ndash400 afterword onset was quantified by computer with referenceto the 100-msec prestimulus baseline This time windowwas chosen because it had been shown to differ forknown and unknown words in our previous studies
Acknowledgments
This research was conducted while the first author was at theProject in Cognitive and Neural Development at the Universityof California at San Diego The research was supported by NIHgrant DC0048111 to H Neville from NIDCD NSERC (NaturalSciences and Engineering Research Council of Canada) grantto J Werker and HFSP (Human Frontierrsquos Science Program)grant to J Werker and H Neville The third author RenateZangl was funded in part by two postdoctoral grants (J-1646J-1817) from the Austrian Science Foundation
We thank Amy Adamson Greg Appelbaum Teresa Mitchelland Lisa Sanders (in alphabetical order not necessarily in orderof magnitude of contribution) for their help in acquiring andanalyzing the data and Chris Fennell and Liz Lewis for theircomments on an earlier version of the manuscript We areespecially indebted to the children and their parents whoparticipated in these studies and their parents without whomthe research would not be possible
1462 Journal of Cognitive Neuroscience Volume 16 Number 8
Reprint requests should be sent to Debra L Mills who is nowat the Department of Psychology 532 Kilgo Circle EmoryUniversity Atlanta GA 30322 or via e-mail dmills2emoryedu
Notes
1 Unlike the switch task the visual fixation task is an on-linetask with two simultaneously presented choices The twoindicators of success are shorter latency to look away from theincorrect object and longer looking times overall to the correctmatch The habituation phase in the lsquolsquoswitchrsquorsquo task leads to theprediction of a novelty preference in the test phase (ie longerlooking to the incorrect pairing)2 In previous studies Mills and colleagues typically findapproximately 10ndash15 of children show a reversed ERPresponse with a larger amplitude ERP response to unknownthan to known words In previous work Mills and colleagueshave treated those children as noise in the data and have keptthem in for the data analyses Ultimately we want tounderstand the nature of lexical representations in this groupof children showing an aberrant ERP response For the currentstudy however in order to compare the ERP response tophonemic contrasts to known words we felt it necessary toinclude in our sample only those children who showed themuch more typical pattern of response resulting in the ex-clusion of three children All effects reported here show thesame pattern with these children included but the variabilityfrom their inclusion did reduce the differences in somecomparisons3 We are aware of the controversies surrounding the use oflinked mastoids Linked mastoids were used here to increasethe number of active cites given the number of amplifiersavailable and to provide consistency with previous studies Apilot study using similar auditory stimuli was conducted toexamine possible distortions in the distribution of scalp activityresulting from forced linkage This was determined by re-cording from one mastoid using the other as a reference andlinking the mastoids off-line These pilot data were comparedwith the data recorded using linked mastoids and did not yieldsignificant differences
REFERENCES
Bailey T M amp Plunkett K (2002) Phonological specificity inearly words Cognitive Development 17 1265ndash1282
Bertoncini J amp Mehler J (1981) Syllables as units in infantspeech perception Infant Behavior and Development 4247ndash260
Brown C amp Matthews J (1997) The role of feature geometryin the development of phonemic contrasts In S J Hannahsamp M Young-Scholten (Eds) Focus on phonologicalacquisition (pp 67ndash112) Amsterdam Benjamins
Brown C M Hagoort P amp Kutas M (2000) Postlexicalintegration processes in language comprehension Evidencefrom brain-imaging research In M Gazzaniga (Ed) The newcognitive neurosciences (2nd ed pp 881ndash895)Cambridge MIT Press
Conboy B (2003) Patterns of language processing and growthin early EnglishndashSpanish bilingualism (Doctoral dissertationUniversity of California San Diego and San Diego StateUniversity 2000) Dissertation Abstracts InternationalB The Sciences and Engineering 63 (UMI No 5193)
Conboy B amp Mills D L (2000) Cerebral specialization forlanguage comprehension in bilingual toddlers CognitiveNeuroscience Society 2000 Annual Meeting AbstractsSupplement of the Journal of Cognitive Neuroscience 17
Connolly J F Service E DrsquoArcy R C N Kujala A Alho K(2001) Phonological aspects of word recognition as revealedby high-resolution spatio-temporal brain mappingNeuroReport 12 237ndash243
Dehaene-Lambertz G (2000) Cerebral specialization forspeech and non-speech stimuli in infants Journal ofCognitive Neuroscience 12 449ndash460
Dehaene-Lambertz G amp Dehaene S (1994) Speed andcerebral correlates of syllable discrimination in infantsNature 370 292ndash294
Eimas P D Siqueland E R Jusczyk P amp Vigorito J (1971)Speech perception in infants Science 171 303ndash306
Fennell C E amp Werker J F (2003) Early word learnersrsquoability to access phonetic detail in well-known wordsLanguage and Speech 46 245ndash264
Fenson L Dale P S Reznick J S Bates E Thal D Jamp Pethick S J (1994) Variability in early communicativedevelopment Monographs of the Society for Research inChild Development 59 1ndash173
Fernald A Swingley D amp Pinto J P (2001) When half aword is enough Infants can recognize spoken words usingpartial phonetic information Child Development 721003ndash1015
Fischler I S (1990) Comprehending language withevent-related potentials In J W Rohrbaugh R Parasuramanamp R Johnson (Eds) Event-related brain potentials(pp 165ndash170) New York Oxford University Press
Halle P amp de Boysson-Bardies B (1994) Emergence of anearly receptive lexicon Infantsrsquo recognition of wordsInfant Behavior and Development 17 119ndash129
Halle P amp de Boysson-Bardies B (1996) The format ofrepresentation of recognized words in infantsrsquo earlyreceptive lexicon Infant Behavior and Development 19463ndash481
Holcomb P J amp Neville H J (1991) The electrophysiologyof spoken sentence processing Psychobiology 19286ndash300
Holcomb P J Coffey S A amp Neville H J (1992) Visual andauditory sentence processing A developmental analysisusing event-related brain potentials DevelopmentalNeuropsychology 8 203ndash241
Jusczyk P W (1997) The discovery of spoken languageCambridge MIT Press
Jusczyk P W amp Aslin R N (1995) Infantsrsquo detection ofsound patterns of words in fluent speech CognitivePsychology 29 1ndash23
Kutas M amp Hillyard S (1980) Reading senseless sentencesBrain potentials ref lect semantic incongruity Science 207203ndash205
Luck S amp Hillyard S (2000) The operation of selectiveattention at multiple stages of processing Evidence fromhuman and monkey electrophysiology In M Gazzaniga(Ed) The new cognitive neurosciences (2nd edpp 687ndash700) Cambridge MIT Press
Mills D L Coffey-Corina S A amp Neville H J (1997)Language comprehension and cerebral specialization from13ndash20 months [(Special issue on origins of languagedisorders)] In D Thal amp J Reilly (Eds) DevelopmentalNeuropsychology 13 397ndash446
Mills D M Coffey-Corina S A amp Neville H J (1994)Variability in cerebral organization during primary languageacquisition In G Dawson amp K Fischer (Eds) Humanbehavior and the developing brain (pp 427ndash455) NewYork Guilford Publications
Mills D M Coffey-Corina S A amp Neville H J (1993)Language acquisition and cerebral specialization in20-month-old infants Journal of Cognitive Neuroscience 5326ndash342
Mills et al 1463
Mills D L Conboy B amp Paton C (in press) Do changes inbrain organization reflect shifts in symbolic functioning InL Namy (Ed) Symbol use and symbolic representationMahwah NJ Erlbaum
Mills D L Plunkett K Prat C amp Schafer G (in press)Watching the infant brain learn words Effects ofvocabulary size and experience Cognitive Development
Molfese D L (1989) Electrophysiological correlates of wordmeanings in 14-month-old human infants DevelopmentalNeuropsychology 5 79ndash103
Molfese D L (1990) Auditory evoked responses recordedfrom 16-month-old human infants to words they did anddid not know Brain and Language 38 345ndash363
Molfese D L Burger-Judisch L M amp Hans L L (1991)Consonant discrimination by newborn infantsElectrophysiological differences DevelopmentalNeuropsychology 7 177ndash195
Molfese D L amp Molfese V J (1988) Right hemisphereresponses from preschool children to temporal cuescontained in speech and nonspeech materialsElectrophysiological correlates Brain and Language33 245ndash259
Molfese D L Morse P A amp Peters C J (1990) Auditoryevoked responses to names for different objectsCross-modal processing as a basis for infant languageacquisition Developmental Psychology 26 780ndash795
Molfese D L Wetzel W F amp Gill L A (1993) Known versusunknown word discriminations in 12-month-old humaninfants Electrophysiological correlates DevelopmentalNeuropsychology 9 241ndash258
Naigles L R (2002) Form is easy meaning is hard Resolvinga paradox in early child language Cognition 8 157ndash199
Nazzi T amp Bertoncini J (2003) Before and after thevocabulary spurt Two modes of word acquisitionDevelopmental Science 6 136ndash142
Osterhout L amp Holcomb P J (1995) Event-related potentialsand language comprehension In M D Rugg amp M G HColes (Eds) Electrophyisology of mind (pp 171ndash215)Oxford Oxford University Press
Richards J E amp Hunter S K (2001) Testing neural models ofthe development of infant attention DevelopmentalPsychobiology 40 226ndash236
Sereno S C Brewer C C amp OrsquoDonnell P J (2003)Context effects in word recognition Evidence for earlyinteractive processing Psychological Science 14328ndash333
Simos P G amp Molfese D L (1997) Electrophysiologicalresponses from a temporal order continuum in the newborninfant Neuropsychologia 35 89ndash98
Simos P G Molfese D L amp Brenden R A (1997) Behavioraland electrophysiological indices of voicing-cuediscrimination Laterality patterns and developmentBrain and Language 57 122ndash150
St George M amp Mills D L (2001) Electrophysiologicalstudies of language development In J Weissenborn ampB Hoehle (Eds) Language acquisition and languagedisorders (pp 247ndash259) Amsterdam John BenjaminsPublishing
Stager C L amp Werker J F (1997) Infants listen for morephonetic detail in speech perception than in word-learningtasks Nature 388 381ndash382
Swingley D (2003) Phonetic detail in the developing lexiconLanguage and Speech 46 265ndash294
Swingley D amp Aslin R N (2000) Spoken word recognitionand lexical representation in very young children Cognition76 147ndash166
Swingley D amp Aslin R N (2002) Lexical neighborhoodsand the wordndashform representations of 14-month-oldsPsychological Science 13 480ndash484
Werker J F Cohen L B Lloyd V L Casasola M amp StagerC L (1998) Acquisition of wordndashobject associations by14-month-old infants Developmental Psychology 341289ndash1309
Werker J F amp Fennell C E (2004) From listening to soundsto listening to words Early steps in word learning In G Hallamp S Waxman (Eds) Weaving a lexicon (pp 79ndash109)Cambridge MA MIT Press
Werker J F Fennell C E T Corcoran K M amp Stager C L(2002) Infantsrsquo ability to learn phonetically similar wordsEffects of age and vocabulary size Infancy 3 1ndash30
Werker J F amp Tees R C (1984) Cross-language speechperception Evidence for perceptual reorganization duringthe first year of life Infant Behavior and Development 749ndash63
1464 Journal of Cognitive Neuroscience Volume 16 Number 8
longer treated as known words but were instead treatedlike other nonsense words
Age Group Comparisons
There were no main effects or interactions with agewhen all three word-types and all electrode sites wereincluded in the analyses Because the 20-month-olds inthis study and in our previous studies only showed ERPdifferences in this time window to known versus un-known words at temporal and parietal sites a separateANOVA was conducted including only temporal andparietal sites that is with two levels of Age group (14and 20 months) as the between-subjects factor andWord-type (known phonemic contrasts and nonsensewords) Hemisphere (left and right) and Electrodesite (temporal and parietal) as the within-subjects fac-tors That analysis showed main effects for Word-type[F(131) = 659 p lt 001] and Hemisphere [F(131) =395 = 420 p lt 05] These effects were qualifiedby two interactions with group including a Group Word-type Hemisphere interaction [F(662) = 304p lt 05] and a Group Electrode site interaction[F(131) = 396 p lt 05] These interactions were fur-ther validated by the different patterns displayed by the14- and 20-month-olds separately
To further support the age-related effects describedabove separate ANOVAs were conducted with Age (14and 20 months) as the between-subjects factor andWord-type (known phonemic contrasts and nonsensewords) Hemisphere (left and right) and Electrode site(frontal anterior temporal temporal parietal and oc-cipital) as the within-subjects factors To conserve spaceonly main effects of and interactions with age arereported below
P100 There were no main effects or interactions withage for the P100 latency The age-related difference inthe lateral distribution of the P100 amplitude was sup-
ported by an Age Hemisphere Electrode siteinteraction [F(4124) = 275 p lt 05]
N200ndashN400 There were no main effects or interac-tions with age when all three word-types and all elec-trode sites were included in the analyses Because the20-month-olds in previous studies and in this study asdescribed above only showed ERP differences in thistime window to known versus unknown words at tem-poral and parietal sites a separate ANOVA was con-ducted including only temporal and parietal sitesnamely with two levels of Age group (14 and 20 months)as the between-subjects factor and Word-type (knownphonemic contrasts and nonsense words) Hemisphere(left and right) and Electrode site (temporal andparietal) as the within-subjects factors That analysisshowed two interactions with group including a Groupby Word-type Hemisphere interaction [F(662) =304 p lt 05] and a Group Electrode site interaction[F(131) = 396 p lt 05] These interactions supportthe findings described above showing different patternsof responsiveness to the different word-types by the 14-and 20-month-olds separately
Summary
Fourteen- and 20-month-olds showed different patternsof brain activity to known words (eg lsquolsquobearrsquorsquo) phone-mic contrasts (eg lsquolsquogarersquorsquo) and (phonetically dissimilar)nonsense words (eg lsquolsquokobersquorsquo) (Figure 5) As predictedby Stager and Werker (1997) 14-month-olds showedlarger N200ndashN400 amplitudes to known words com-pared with nonsense words (eg lsquolsquokobersquorsquo) Howeverthe N200ndashN400 to known words and the phonemiccontrast stimuli (eg lsquolsquobearrsquorsquo and lsquolsquogarersquorsquo) did not differfrom each other
Unlike the pattern seen at 14 months and as pre-dicted by Werker et al (2002) the 20-month-oldsshowed larger amplitude N200ndashN400 responses to
Figure 5 N200ndashN400 mean
areas averaged across temporal
and parietal regions of the leftand right hemispheres to the
three word-types for the
14- and 20-month-olds
Mills et al 1457
known words (eg lsquolsquobearrsquorsquo) over both phonemic con-trast (eg lsquolsquogarersquorsquo) and other nonsense words (eglsquolsquokobersquorsquo) Moreover the N200ndashN400 amplitudes to thephonemic contrast and other nonsense words (lsquolsquogarersquorsquoand lsquolsquokobersquorsquo) did not differ from each other Thesefindings also replicated previously observed lateral dif-ferences in the N200ndashN400 amplitudes to known versusunknown words between 14 and 20 months (Mills et al1997)
DISCUSSION
The results from the present study were consistent withand expanded on findings observed in previous behav-ioral and ERP studies At both 14 and 20 months of agethe pattern of ERP responses seen replicated that pre-viously reported by Mills et al (1993 1997) for knownversus unknown words At both ages there was a largeramplitude negative response from 200 to 400 msec toknown versus nonsense words And as shown before at14 months of age this response was broadly distributedacross the scalp whereas at 20 months it was observedprimarily over the left temporal and parietal electrodesites
The replication of the previous work by Mills andcolleagues puts us in a particularly strong position tointerpret the results from the phonemic contrast wordsAt 14 months of age the ERP signature to phonemiccontrast nonsense words (eg lsquolsquogarersquorsquo or lsquolsquobogrsquorsquo) isindistinguishable from that to known words (eg lsquolsquobearrsquorsquoand lsquolsquodogrsquorsquo) and significantly greater in amplitude thanthat to dissimilar nonsense words (eg lsquolsquolifrsquorsquo) Thus at14 months of age the neural response appears toindicate a mistaken recognition of a minimal pair mis-pronunciation of a known word as the word itself Thisresult is consistent with that seen in the switch task at14 months (Werker et al 2002 Stager amp Werker 1997)The results with the ERP task are particularly convincingbecause here we see a significant increase in the ampli-tude of the response rather than a failure to increaselooking time At 20 months of age the ERP response isalso consistent with results seen in the switch task at thesame age (Werker et al 2002) At 20 months the ERPsignature to phonemic contrast nonsense words (eglsquolsquogarersquorsquo and lsquolsquobogrsquorsquo) is significantly different from that totheir known counterparts (eg lsquolsquobearrsquorsquo and lsquolsquodogrsquorsquo) andindistinguishable from that to phonetically dissimilarnonsense words (eg lsquolsquoneemrsquorsquo or lsquolsquolifrsquorsquo) Thus at20 months the evoked response indicates that childrenare showing the lsquolsquoword recognitionrsquorsquo ERP response onlyto known words that are phonologically correct in alldetails By 20 months then the confusion with similar-sounding known words is no longer present indicatingthat words in the lexicon are not only fully specified interms of phonological detail but that such detail isavailable even when listening to words in tasks that donot provide contextual or pictorial support for the word
meaning and may not even involve full attention to theword
One difference between the present study and thebehavioral studies by Werker and colleagues and Swin-gley and Aslin is that the behavioral studies used bothpictures and words whereas the present study pre-sented a series of words without a picture context Anegative going ERP wave within the 200ndash500 msec timewindow has also been shown to index word meaningin cross-modal matchmismatch paradigms using pic-tures and words in 14- and 20-month-old children (MillsConboy amp Paton in press) and real objects and noveltrained words in 14-month-old children (Molfese Morseamp Peters 1990) We chose to use the Mills et al (1997)paradigm rather than a pictureword matchmismatchparadigm for several reasons The pictureword matchmismatch paradigm elicits a large amplitude bilateralnegative component starting at 200 msec and peakingaround 500 msec when the subsequent word does notmatch the picture at both 14 and 20 months (MillsConboy et al in press) This ERP effect is most likelywithin the N400 family observed in children and adults toviolations of semantic expectancy (Holcomb Coffey ampNeville 1992 Kutas amp Hillyard 1980) In adults a similarparadigm elicited a phonological mismatch (PMN) towords and nonwords from 250 to 347 msec over rightfrontal regions and from 347 to 638 msec over symmet-rical centro-parietal regions (Connolly Service DrsquoArcyKujala amp Alho 2001) One concern was that the N400 andPMN effects elicited by a pictureword matchmismatchparadigm might swamp more subtle differences in thelatency amplitudes and distributions of ERPs related tothe different word-types and group differences The Millset al paradigm was chosen because it showed ERP dif-ferences to known and unknown words that varied withword-type age and vocabulary size The methodologicaldifferences should be taken into consideration whencomparing the effects However the ERP findings areconsistent with and provide additional support for theage-related differences reported by Stager et al (1997)
The robust differences in the ERP pattern at 14and 20 months of age could be explained at manylevels Several studies have shown that a negativepeak at around 200 msec is sensitive to phonetic varia-tion in voice onset time (Simos Molfese amp Brenden1997 Molfese amp Molfese 1988) place of articulation(Dehaene-Lambertz amp Dehaene 1994 Molfese Burger-Judisch amp Hans 1991) and acoustic cues in nonspeechstimuli (Dehaene-Lambertz 2000 Simos amp Molfese1997) in very young children and even newborns Oneinterpretation is that ERP differences between word-types are related to the acoustic or phonological featuresof the different word-types and that age-related changesare due to differences in the childrenrsquos ability to mem-orize the different word lists Although phonological dis-crimination can modulate the amplitude of the N200 incertain paradigms it is not necessarily the only factor
1458 Journal of Cognitive Neuroscience Volume 16 Number 8
that may contribute to the amplitude of this componentHere we argue that age-related changes in a top-downprocess linked to word meaning is the dominant factormodulating the amplitude of the N200ndashN400 rather thanphonological discrimination First if ERP differences be-tween conditions were due solely to phonological fac-tors then both age groups would have displayed N200amplitude differences between the known words andphonetic contrasts Additionally the known and phone-mic contrasts were balanced for the types of initialconsonants in the number of stop consonants nasalsand fricatives and did not differ in their phonologicalneighbors for other words frequently understood bychildren in this age range (see Methods) Therefore itis difficult to explain why the phonemic contrasts pat-terned with the known words at 14 months and thenonsense words at 20 months based solely on acousticaland phonemic differences A second line of evidencesuggesting that the ERP differences observed here can-not be explained by phonological differences betweenword-types comes from a study of bilingual toddlersusing the same paradigm In that study the N200ndashN400did not differ between Spanish and English for either theknown or unknown words (Conboy amp Mills 2000 Con-boy 2003) ERP differences were linked to word meaning(known vs unknown words) language dominance andtotal vocabulary size but not phonological differencesbetween Spanish and English If the N200ndashN400 indexedthe phonological aspects of the words amplitude differ-ences would be expected between Spanish and EnglishA third and perhaps the strongest line of evidencecomes from a training study with 20-month-old childrenin which nonsense words were either repeated or pairedwith an object (Mills Plunkett Prat amp Schafer in press)After the training phase when objects were no longerpresent the amplitude of the N200ndashN400 to the newlylearned words increased relative to before training butbecame more positive for nonsense words that wererepeated the same number of times Because the stimuliwere counterbalanced across participants the increasedamplitude of the N200ndashN400 to the newly learned wordswas modulated by its association with a meaningfulstimulus and cannot be explained by phonological dif-ferences between word-types
Another interpretation of the findings is that at 14months of age the phonetic detail distinguishing oneword from another is simply not available in the lexicalrepresentation Such an explanation would be compat-ible with theories of phonological development thatposit an initial under-specification of the informationin the lexicon that is only gradually filled in as thevocabulary expands (eg Brown amp Matthews 1997)Although possible we think this explanation is unlikelygiven the work by Swingley and Aslin (2002) showingthat children of 14 months do seem to show someevidence of use of phonetic detail in word recognitiontasks via longer looking time to the matching object
when a correct versus a mispronounced version of aword is given The Swingley and Aslin work involvesshowing the children two objects (eg a car and a dog)and asking them to find either the correctly labeledobject (eg lsquolsquodogrsquorsquo) or a mispronunciation of that sameobject (eg lsquolsquobogrsquorsquo) More recently Fennell and Werker(2003) showed that when tested on two phonemiccontrast words that they already know well (eg lsquolsquoballrsquorsquoand lsquolsquodollrsquorsquo) the child of 14 months can succeed even inthe switch task These two studies show that at somelevel in the system full phonetic specification of well-known words is represented even at 14 months
The successes in the two-choice (Swingley and Aslin)behavioral lsquolsquomispronunciationrsquorsquo task and in the switchtask using two well-known words in comparison to boththe switch word-learning task and the ERP word recog-nition task are compatible with the attentional resourcelimitation hypothesis offered by Werker and colleagues(Werker et al 2002 Werker amp Fennell 2004 Fennell ampWerker 2003 Stager amp Werker 1997) According to thisexplanation the task of linking a word to an object ischallenging for a novice word learner All the detail ispicked up but the computational difficulty of actuallymapping the word on to the object makes it difficultto hold all the information in mind As such one sees aU-shaped function in the pattern of data The child of7ndash8 months who is not yet actively engaged in mappingwords to referents is better able to attend to fine pho-netic detail than is the child of 14 months who is at-tempting to make and remember the link (see Stager ampWerker 1997 see also Halle amp de Boysson-Bardies1996 Jusczyk amp Aslin 1995) Becoming a word learnerchanges the task for the older child This increases theprocessing demands and interferes with the childrsquosability to access detail The information is picked upin perception it is just not available for access in lexicalrecall tasks After the child becomes a more accom-plished word learner and has a working phonemicinventory to guide information access the relevantphonetic differences lsquolsquostand outrsquorsquo as salient and impor-tant to the lexical entry Or as suggested by Nazzi andBertoncini (2003) once the child passes a criticaljuncture in word learning sophistication moving froma simple associationist to a referential word learneraccess to phonological detail becomes possible Priorto that point however any detection of that detail isat best fleeting As such it may be seen in on-lineprocessing tasks or in tasks that provide full support torecall memory (Fennell amp Werker 2003 Swingley ampAslin 2002) but is not evident in tasks that involvelearning new words (Werker et al 2002 Stager ampWerker 1997) nor is it evident in word recognitiontasks such as the ERP task used here where there is noobject to facilitate activation of the full memory trace
Support for this explanation can be seen in thecurrent ERP study In this study children of 20 months(but not 14 months) showed a significant P100 response
Mills et al 1459
to the known words over both the nonsense words andthe phonemic contrasts words The P100 is thought toindex sensory and attentional processes This is the firsttime Mills and colleagues (eg Mills et al 1993 1997)have observed a significantly greater P100 response toknown than to unknown words in the ERP word recog-nition studies The amplitude of the P100 is known to bemodulated by effects of attention in both adults (Luck ampHillyard 2000) and children (Richards amp Hunter 2001)One interpretation of this finding is that the inclusion ofphonemic contrast words in the dataset increased atten-tion to early phonological differences To distinguish theknown words from the phonemic contrast foils theyhad to increase their vigilance to the sensory andperceptual detail thus yielding a significant P100 re-sponse in this ERP word recognition task
Directions for Further Research
A focus for future work will be to determine preciselywhat changes between 14 and 20 months allow the moreaccomplished word learner to access and use full pho-netic detail across a wide range of word recognition andword learning situations In a recent training study MillsPlunkett et al (in press) addressed this question byexamining the effects of experience on the lateral distri-bution of the N200ndashN400 differences between knownand unknown words Mills et al asked whether observedchanges in the lateral distribution in ERP differencesto known versus unknown words between 13 and20 months (Mills et al 1997 and replicated in the pre-sent study) reflect the availability of specialized brainsystems for word recognition (as might be available in amore accomplished word learner) or increasing knowl-edge of particular words by testing 20-month-old chil-dren who varied in vocabulary size on newly learnedwords ERPs to novel words that had been paired withan object during a pretest training phase were comparedto ERPs to novel words that had been repeated the samenumber of times but without the objectword pairingThe results supported a mixed model ERP differences tonewly learned words compared to repeated-but-not-trained words showed a bilateral distribution acrossthe whole sample This finding was consistent with thehypothesis that the asymmetrical distribution of ERP dif-ferences to known and unknown words is linked to theamount of experience with individual words Howeverchildren with the largest vocabularies showed a leftgreater than right distribution of ERP differences Thelatter finding suggested that expertise with word learn-ing might also affect the specialization of language-relevant brain activity even in the service of learningnew words
If a more expert word learning system is becomingavailable (Nazzi amp Bertoncini 2003) in the older andormore experienced word learner this might also permit
greater direction of attention to the relevant phonolog-ical information in the word learning and access situa-tion Indeed as we argued at the beginning of thisarticle perhaps part of becoming a more expert wordlearner is knowing just which properties of a word aredefinitional in a particular linguistic community It maybe the emergence of this greater understanding of justwhat distinguishes words in our language that allows theolder more accomplished word learner to not onlyperceive but also lsquolsquomarkrsquorsquo and lsquolsquousersquorsquo the phonologicaldetail in word learning and word recognition tasks
In previous studies Mills and colleagues have re-ported a link between vocabulary size and the distribu-tion of the N200ndashN400 amplitude difference to knownand unknown words between 14 and 20 months of ageSimilarly Werker et al (2002) reported a significantcorrelation between performance on the CDI at both14 and 17 months and performance in the minimal pairswitch task There was no significant correlation with theCDI in the present study The lack of a significantcorrelation here could reflect lack of variability in thedata perhaps because the age groups selected are notyet in transition We predict that testing of a group ofchildren intermediate in age and vocabulary sizes to thegroups tested here would be more likely to reveal such apattern of findings
In summary using an ERP design we extended andhelped explain the pattern of findings seen in earlybehavioral tasks assessing the phonological detail repre-sented and used in early word learners The results ofthe current ERP study indicate that at the earliest stagesof word learning children treat minimal pair mispronun-ciations of known words as acceptable instances of thatword This provides strong evidence that in the decisionstage of word recognition novice word learners accept abroader range of pronunciations of the word as accept-able than do older children With increasing age andincreasing language sophistication the amount of pho-nological detail easily accessed in the representation ofwords increases helping to avoid mapping mistakes andto facilitate more rapid acquisition of a vocabulary
METHODS
Participants and Settings
Participants were tested in three locations at the Uni-versity of California San Diego at the University ofOregon Eugene Oregon and at the University of BritishColumbia Vancouver Canada Approximately one-thirdof the children in each age group were tested at eachsite Testing at the Vancouver site was conducted in amobile ERP lab belonging to the University of OregonChildren from the San Diego and Eugene areas wererecruited through advertisements in a local magazineposters displayed in the area requests at play groupsand referrals from parents whose children had partici-
1460 Journal of Cognitive Neuroscience Volume 16 Number 8
pated in our studies Children from the Vancouver areawere recruited through the University of British Colum-bia child Studies Center database Parents of all childrensigned consent forms consistent with the human sub-jects internal review boards at the university at whichthey were tested Parents were given $500 to defraytransportation costs and children were given a small toyin appreciation of their participation Only full-term(gt36 weeks of gestation) healthy children with mono-lingual experience with English participated in the studychildren with a family history of language impairmentwere excluded from the study
Data from 16 children aged 14 months (9 girls meanage = 14 months 15 days SD = 97 days range = 14months 1 day to 15 months 1 day) and 17 children aged20 to 21 months (9 girls mean age = 20 months 11 daysSD = 131 days age range = 19 months 14 days to 20months 27 days) were retained for analysis in the studyAn additional 18 children (12 boys nine 14-month-olds)were tested but were excluded from the analyses due totoo few artifact-free trials (n = 10) excessive crying (n =3) reversed polarity of the ERP response (n = 3 seenote)2 or refusal to wear the electrocap (n = 2)
Stimuli
The stimuli were naturally spoken in a female voice anddigitized at 16 bits 44 kHz sampling rate The stimuliconsisted of three types of words (a) 10 words whosemeanings were understood by the child (known wordseg lsquolsquomilkrsquorsquo) (b) 10 nonsense words that differed fromthe known words in the initial phonemic contrast (pho-nemic contrast words eg lsquolsquonilkrsquorsquo) and (c) 10 wordsthat differed phonetically from known words in allvowels and consonants (nonsense words eg lsquolsquoneemrsquorsquo)All stimuli were matched on word duration and numberof syllables The mean durations for the known wordsphonetic contrasts and nonsense words were 783 msecSD = 144 865 msec SD = 169 861 msec SD = 96[F(227) = 109 p = 34] Ideally all word-types wouldbe matched on manner of articulation (eg all stopconsonants all nasals all laterals all fricatives or allbalanced) However children at this age have very smallvocabularies and we were limited to common words thatalmost all children at this age would know Amongchildren first words are a disproportionate number ofwords that begin with stop consonants For many ofthose words minimal pair differences are still words(eg lsquolsquoballrsquorsquondashlsquolsquodollrsquorsquo) Thus we were constrained in ourselection of stimulus items For the known words therewere seven stop consonants two nasals and one frica-tive For the phonemic contrasts there are also sevenstop consonants two nasals and one fricative For thenonsense words there are three stop consonants oneaffricative two nasals two fricatives and two laterals Acomplete list of words is shown in Table 1
To evaluate at what point the known words couldphysically be distinguishable from the other word-typeswe examined two aspects of uniqueness (a) at whatpoint the phonetically dissimilar nonsense words be-come unique from their real word counterparts (egwhen does lsquolsquobearrsquorsquo become unique from lsquolsquogarersquorsquo) and (b)when the words might become unique from otherwords that would be in a babyrsquos vocabulary at this ageThe uniqueness point for initial consonants (the pointthat separates that consonant from other consonants)was within 20 msec of the beginning of the word forfricatives (s z f) 40 msec for stop consonants (b d gp t k) 60 msec for affricatives (j) and 20 msec for nasals(m n) and laterals (l r) To further examine theuniqueness points we compared each stimulus usedin the study relative to other words that might be in thechildrsquos vocabulary with the words in the MacArthur CDIthat would be considered close phonological neighborsTo this end we compared our stimuli to the number ofwords in the CDI with (a) the same consonant includingconsonant clusters and (b) the same consonant exclud-ing consonant clusters and (c) the same consonant plusthe vowel For comparisons a and b there are the mostneighbors for the known words (a = 474 b = 375) anintermediate amount for the phonemic contrast words(a = 371 b = 298) and the least for the nonsense words(a = 275 b = 233) The best comparison is to count thenumber of items that begin with the same consonantplus vowel (CV) There were no significant differences inthe number of CV neighbors for the three word-types(known = 16 phonemic contrasts = 12 nonsense =18) Additionally because of co-articulatory effects theacoustic energy for a given consonant varies as a func-tion of the following vowel (Jusczyk 1997) Thus evenwords beginning with the same consonant (eg lsquolsquobookrsquorsquoand lsquolsquobottlersquorsquo) differ in shape and spectral frequenciesand can be discriminated from each other as well as
Table 1 Stimuli
Known Words Phonemic Contrasts Nonsense
Bear Gare Kobe
Ball Pall Lif
Book Dook Neem
Bottle Pottle Fipe
Cup Tup Mon
Cat Gat Tek
Dog Bog Riss
Milk Nilk Keed
Nose Mose Jud
Shoe Zhu Zav
Mills et al 1461
their close phonological neighbors from the beginningof the word
Procedure
Language Assessment
Within one week prior to testing parents were asked tocomplete the MacArthur CDI (Fenson et al 1994)which provided an estimate of the childrsquos vocabularysize and percentile ranking relative to other children ofthe same age To ensure that the words to be used in thestudy as lsquolsquoknownrsquorsquo were comprehended by the childparents also completed a vocabulary checklist ratingscale indicating how sure they were that their childunderstood andor produced each word on a scale of1 (very sure they did not know that word) to 4 (very suretheir child understoodproduced a given word in avariety of different contexts and with different exem-plars) Additionally the children were asked to identify apicture of each word to be used as a known word from atwo-choice picture book All lsquolsquoknownrsquorsquo words used hadreceived a rating of 4 and were correctly identified in thepicture-pointing task
Electrophysiological Recording
The EEG was recorded using tin electrodes affixed to anelastic cap (Electro-Cap International) from sites overfrontal (F7 and F8) anterior temporal (50 of thedistance from F78 and T34) temporal (33 of thedistance from T34 to C34) parietal (50 of the dis-tance between T34 and P34) and occipital (O1 andO2) regions of the left and right hemispheres Addition-ally the electrooculogram was recorded from electrodesplaced over and under the eye to reject trials on whichblinks and vertical eye movement occurred and fromleft and right frontal electrodes to reject trials on whichhorizontal eye movement occurred Impedances werekept below 5 k and were balanced (within 1 k) acrossthe left and right hemispheres at any given position TheEEG was amplified by SA Instruments amplifiers with abandpass of 01 to 100 Hz and sampled continuouslyevery 4 msec All electrodes were referenced to linkedmastoids3 Averages of the EEG were conducted using2-sec epochs (ie 100 msec prestimulus and 1900 msecpoststimulus) The averaged ERPs were also digitallyfiltered off-line with a 60-Hz low-pass filter
Testing
Ten known 10 phonetically similar nonsense words(referred to as phonemic contrasts) and 10 phoneticallydissimilar nonsense (referred to as nonsense words)words were each presented six times in random orderfor a total of 60 trials per condition During testingchildren sat on their parentrsquos lap and listened to words
presented from a speaker located behind a movingpuppet in a puppet theater Words were presented ata variable rate between 1800 and 3000 msec SOA
Artifact Rejection
Artifact rejection was conducted off-line using a com-puter program to reject blinks and horizontal eye move-ment and amplifier blocking Individual thresholds wereset for each child based on visual inspection of the EEGepochs time-locked to each stimulus A mean of 50 ofthe trials were rejected due to eye and movementartifact The number of artifact-free trials per word-typeretained for analysis ranged from 12 to 53 (SD = 113)out of a possible 60 trials per condition There were nosignificant differences in the percentage of trials rejectedfor the different experimental conditions age groupsor sex
Measurement of ERP Components
Peak latencies and amplitudes were quantified by com-puter with reference to the 100-msec prestimulus base-line for the maximum negative or positive point in aspecified time window The time windows for eachcomponent were set according to the criteria used inour previous studies the first positive componentcalled the P100 was defined as the most positivedeflection between 50 and 175 msec The P100 indexesauditory sensory processing Moreover the lateral dis-tribution of the P100 has been shown to vary withpercentile ranking on the MacArthur CDI score Chil-dren who score at the 50th percentile or higher for theirage show a P100 left greater than right asymmetry Themean amplitude within the time window 200ndash400 afterword onset was quantified by computer with referenceto the 100-msec prestimulus baseline This time windowwas chosen because it had been shown to differ forknown and unknown words in our previous studies
Acknowledgments
This research was conducted while the first author was at theProject in Cognitive and Neural Development at the Universityof California at San Diego The research was supported by NIHgrant DC0048111 to H Neville from NIDCD NSERC (NaturalSciences and Engineering Research Council of Canada) grantto J Werker and HFSP (Human Frontierrsquos Science Program)grant to J Werker and H Neville The third author RenateZangl was funded in part by two postdoctoral grants (J-1646J-1817) from the Austrian Science Foundation
We thank Amy Adamson Greg Appelbaum Teresa Mitchelland Lisa Sanders (in alphabetical order not necessarily in orderof magnitude of contribution) for their help in acquiring andanalyzing the data and Chris Fennell and Liz Lewis for theircomments on an earlier version of the manuscript We areespecially indebted to the children and their parents whoparticipated in these studies and their parents without whomthe research would not be possible
1462 Journal of Cognitive Neuroscience Volume 16 Number 8
Reprint requests should be sent to Debra L Mills who is nowat the Department of Psychology 532 Kilgo Circle EmoryUniversity Atlanta GA 30322 or via e-mail dmills2emoryedu
Notes
1 Unlike the switch task the visual fixation task is an on-linetask with two simultaneously presented choices The twoindicators of success are shorter latency to look away from theincorrect object and longer looking times overall to the correctmatch The habituation phase in the lsquolsquoswitchrsquorsquo task leads to theprediction of a novelty preference in the test phase (ie longerlooking to the incorrect pairing)2 In previous studies Mills and colleagues typically findapproximately 10ndash15 of children show a reversed ERPresponse with a larger amplitude ERP response to unknownthan to known words In previous work Mills and colleagueshave treated those children as noise in the data and have keptthem in for the data analyses Ultimately we want tounderstand the nature of lexical representations in this groupof children showing an aberrant ERP response For the currentstudy however in order to compare the ERP response tophonemic contrasts to known words we felt it necessary toinclude in our sample only those children who showed themuch more typical pattern of response resulting in the ex-clusion of three children All effects reported here show thesame pattern with these children included but the variabilityfrom their inclusion did reduce the differences in somecomparisons3 We are aware of the controversies surrounding the use oflinked mastoids Linked mastoids were used here to increasethe number of active cites given the number of amplifiersavailable and to provide consistency with previous studies Apilot study using similar auditory stimuli was conducted toexamine possible distortions in the distribution of scalp activityresulting from forced linkage This was determined by re-cording from one mastoid using the other as a reference andlinking the mastoids off-line These pilot data were comparedwith the data recorded using linked mastoids and did not yieldsignificant differences
REFERENCES
Bailey T M amp Plunkett K (2002) Phonological specificity inearly words Cognitive Development 17 1265ndash1282
Bertoncini J amp Mehler J (1981) Syllables as units in infantspeech perception Infant Behavior and Development 4247ndash260
Brown C amp Matthews J (1997) The role of feature geometryin the development of phonemic contrasts In S J Hannahsamp M Young-Scholten (Eds) Focus on phonologicalacquisition (pp 67ndash112) Amsterdam Benjamins
Brown C M Hagoort P amp Kutas M (2000) Postlexicalintegration processes in language comprehension Evidencefrom brain-imaging research In M Gazzaniga (Ed) The newcognitive neurosciences (2nd ed pp 881ndash895)Cambridge MIT Press
Conboy B (2003) Patterns of language processing and growthin early EnglishndashSpanish bilingualism (Doctoral dissertationUniversity of California San Diego and San Diego StateUniversity 2000) Dissertation Abstracts InternationalB The Sciences and Engineering 63 (UMI No 5193)
Conboy B amp Mills D L (2000) Cerebral specialization forlanguage comprehension in bilingual toddlers CognitiveNeuroscience Society 2000 Annual Meeting AbstractsSupplement of the Journal of Cognitive Neuroscience 17
Connolly J F Service E DrsquoArcy R C N Kujala A Alho K(2001) Phonological aspects of word recognition as revealedby high-resolution spatio-temporal brain mappingNeuroReport 12 237ndash243
Dehaene-Lambertz G (2000) Cerebral specialization forspeech and non-speech stimuli in infants Journal ofCognitive Neuroscience 12 449ndash460
Dehaene-Lambertz G amp Dehaene S (1994) Speed andcerebral correlates of syllable discrimination in infantsNature 370 292ndash294
Eimas P D Siqueland E R Jusczyk P amp Vigorito J (1971)Speech perception in infants Science 171 303ndash306
Fennell C E amp Werker J F (2003) Early word learnersrsquoability to access phonetic detail in well-known wordsLanguage and Speech 46 245ndash264
Fenson L Dale P S Reznick J S Bates E Thal D Jamp Pethick S J (1994) Variability in early communicativedevelopment Monographs of the Society for Research inChild Development 59 1ndash173
Fernald A Swingley D amp Pinto J P (2001) When half aword is enough Infants can recognize spoken words usingpartial phonetic information Child Development 721003ndash1015
Fischler I S (1990) Comprehending language withevent-related potentials In J W Rohrbaugh R Parasuramanamp R Johnson (Eds) Event-related brain potentials(pp 165ndash170) New York Oxford University Press
Halle P amp de Boysson-Bardies B (1994) Emergence of anearly receptive lexicon Infantsrsquo recognition of wordsInfant Behavior and Development 17 119ndash129
Halle P amp de Boysson-Bardies B (1996) The format ofrepresentation of recognized words in infantsrsquo earlyreceptive lexicon Infant Behavior and Development 19463ndash481
Holcomb P J amp Neville H J (1991) The electrophysiologyof spoken sentence processing Psychobiology 19286ndash300
Holcomb P J Coffey S A amp Neville H J (1992) Visual andauditory sentence processing A developmental analysisusing event-related brain potentials DevelopmentalNeuropsychology 8 203ndash241
Jusczyk P W (1997) The discovery of spoken languageCambridge MIT Press
Jusczyk P W amp Aslin R N (1995) Infantsrsquo detection ofsound patterns of words in fluent speech CognitivePsychology 29 1ndash23
Kutas M amp Hillyard S (1980) Reading senseless sentencesBrain potentials ref lect semantic incongruity Science 207203ndash205
Luck S amp Hillyard S (2000) The operation of selectiveattention at multiple stages of processing Evidence fromhuman and monkey electrophysiology In M Gazzaniga(Ed) The new cognitive neurosciences (2nd edpp 687ndash700) Cambridge MIT Press
Mills D L Coffey-Corina S A amp Neville H J (1997)Language comprehension and cerebral specialization from13ndash20 months [(Special issue on origins of languagedisorders)] In D Thal amp J Reilly (Eds) DevelopmentalNeuropsychology 13 397ndash446
Mills D M Coffey-Corina S A amp Neville H J (1994)Variability in cerebral organization during primary languageacquisition In G Dawson amp K Fischer (Eds) Humanbehavior and the developing brain (pp 427ndash455) NewYork Guilford Publications
Mills D M Coffey-Corina S A amp Neville H J (1993)Language acquisition and cerebral specialization in20-month-old infants Journal of Cognitive Neuroscience 5326ndash342
Mills et al 1463
Mills D L Conboy B amp Paton C (in press) Do changes inbrain organization reflect shifts in symbolic functioning InL Namy (Ed) Symbol use and symbolic representationMahwah NJ Erlbaum
Mills D L Plunkett K Prat C amp Schafer G (in press)Watching the infant brain learn words Effects ofvocabulary size and experience Cognitive Development
Molfese D L (1989) Electrophysiological correlates of wordmeanings in 14-month-old human infants DevelopmentalNeuropsychology 5 79ndash103
Molfese D L (1990) Auditory evoked responses recordedfrom 16-month-old human infants to words they did anddid not know Brain and Language 38 345ndash363
Molfese D L Burger-Judisch L M amp Hans L L (1991)Consonant discrimination by newborn infantsElectrophysiological differences DevelopmentalNeuropsychology 7 177ndash195
Molfese D L amp Molfese V J (1988) Right hemisphereresponses from preschool children to temporal cuescontained in speech and nonspeech materialsElectrophysiological correlates Brain and Language33 245ndash259
Molfese D L Morse P A amp Peters C J (1990) Auditoryevoked responses to names for different objectsCross-modal processing as a basis for infant languageacquisition Developmental Psychology 26 780ndash795
Molfese D L Wetzel W F amp Gill L A (1993) Known versusunknown word discriminations in 12-month-old humaninfants Electrophysiological correlates DevelopmentalNeuropsychology 9 241ndash258
Naigles L R (2002) Form is easy meaning is hard Resolvinga paradox in early child language Cognition 8 157ndash199
Nazzi T amp Bertoncini J (2003) Before and after thevocabulary spurt Two modes of word acquisitionDevelopmental Science 6 136ndash142
Osterhout L amp Holcomb P J (1995) Event-related potentialsand language comprehension In M D Rugg amp M G HColes (Eds) Electrophyisology of mind (pp 171ndash215)Oxford Oxford University Press
Richards J E amp Hunter S K (2001) Testing neural models ofthe development of infant attention DevelopmentalPsychobiology 40 226ndash236
Sereno S C Brewer C C amp OrsquoDonnell P J (2003)Context effects in word recognition Evidence for earlyinteractive processing Psychological Science 14328ndash333
Simos P G amp Molfese D L (1997) Electrophysiologicalresponses from a temporal order continuum in the newborninfant Neuropsychologia 35 89ndash98
Simos P G Molfese D L amp Brenden R A (1997) Behavioraland electrophysiological indices of voicing-cuediscrimination Laterality patterns and developmentBrain and Language 57 122ndash150
St George M amp Mills D L (2001) Electrophysiologicalstudies of language development In J Weissenborn ampB Hoehle (Eds) Language acquisition and languagedisorders (pp 247ndash259) Amsterdam John BenjaminsPublishing
Stager C L amp Werker J F (1997) Infants listen for morephonetic detail in speech perception than in word-learningtasks Nature 388 381ndash382
Swingley D (2003) Phonetic detail in the developing lexiconLanguage and Speech 46 265ndash294
Swingley D amp Aslin R N (2000) Spoken word recognitionand lexical representation in very young children Cognition76 147ndash166
Swingley D amp Aslin R N (2002) Lexical neighborhoodsand the wordndashform representations of 14-month-oldsPsychological Science 13 480ndash484
Werker J F Cohen L B Lloyd V L Casasola M amp StagerC L (1998) Acquisition of wordndashobject associations by14-month-old infants Developmental Psychology 341289ndash1309
Werker J F amp Fennell C E (2004) From listening to soundsto listening to words Early steps in word learning In G Hallamp S Waxman (Eds) Weaving a lexicon (pp 79ndash109)Cambridge MA MIT Press
Werker J F Fennell C E T Corcoran K M amp Stager C L(2002) Infantsrsquo ability to learn phonetically similar wordsEffects of age and vocabulary size Infancy 3 1ndash30
Werker J F amp Tees R C (1984) Cross-language speechperception Evidence for perceptual reorganization duringthe first year of life Infant Behavior and Development 749ndash63
1464 Journal of Cognitive Neuroscience Volume 16 Number 8
known words (eg lsquolsquobearrsquorsquo) over both phonemic con-trast (eg lsquolsquogarersquorsquo) and other nonsense words (eglsquolsquokobersquorsquo) Moreover the N200ndashN400 amplitudes to thephonemic contrast and other nonsense words (lsquolsquogarersquorsquoand lsquolsquokobersquorsquo) did not differ from each other Thesefindings also replicated previously observed lateral dif-ferences in the N200ndashN400 amplitudes to known versusunknown words between 14 and 20 months (Mills et al1997)
DISCUSSION
The results from the present study were consistent withand expanded on findings observed in previous behav-ioral and ERP studies At both 14 and 20 months of agethe pattern of ERP responses seen replicated that pre-viously reported by Mills et al (1993 1997) for knownversus unknown words At both ages there was a largeramplitude negative response from 200 to 400 msec toknown versus nonsense words And as shown before at14 months of age this response was broadly distributedacross the scalp whereas at 20 months it was observedprimarily over the left temporal and parietal electrodesites
The replication of the previous work by Mills andcolleagues puts us in a particularly strong position tointerpret the results from the phonemic contrast wordsAt 14 months of age the ERP signature to phonemiccontrast nonsense words (eg lsquolsquogarersquorsquo or lsquolsquobogrsquorsquo) isindistinguishable from that to known words (eg lsquolsquobearrsquorsquoand lsquolsquodogrsquorsquo) and significantly greater in amplitude thanthat to dissimilar nonsense words (eg lsquolsquolifrsquorsquo) Thus at14 months of age the neural response appears toindicate a mistaken recognition of a minimal pair mis-pronunciation of a known word as the word itself Thisresult is consistent with that seen in the switch task at14 months (Werker et al 2002 Stager amp Werker 1997)The results with the ERP task are particularly convincingbecause here we see a significant increase in the ampli-tude of the response rather than a failure to increaselooking time At 20 months of age the ERP response isalso consistent with results seen in the switch task at thesame age (Werker et al 2002) At 20 months the ERPsignature to phonemic contrast nonsense words (eglsquolsquogarersquorsquo and lsquolsquobogrsquorsquo) is significantly different from that totheir known counterparts (eg lsquolsquobearrsquorsquo and lsquolsquodogrsquorsquo) andindistinguishable from that to phonetically dissimilarnonsense words (eg lsquolsquoneemrsquorsquo or lsquolsquolifrsquorsquo) Thus at20 months the evoked response indicates that childrenare showing the lsquolsquoword recognitionrsquorsquo ERP response onlyto known words that are phonologically correct in alldetails By 20 months then the confusion with similar-sounding known words is no longer present indicatingthat words in the lexicon are not only fully specified interms of phonological detail but that such detail isavailable even when listening to words in tasks that donot provide contextual or pictorial support for the word
meaning and may not even involve full attention to theword
One difference between the present study and thebehavioral studies by Werker and colleagues and Swin-gley and Aslin is that the behavioral studies used bothpictures and words whereas the present study pre-sented a series of words without a picture context Anegative going ERP wave within the 200ndash500 msec timewindow has also been shown to index word meaningin cross-modal matchmismatch paradigms using pic-tures and words in 14- and 20-month-old children (MillsConboy amp Paton in press) and real objects and noveltrained words in 14-month-old children (Molfese Morseamp Peters 1990) We chose to use the Mills et al (1997)paradigm rather than a pictureword matchmismatchparadigm for several reasons The pictureword matchmismatch paradigm elicits a large amplitude bilateralnegative component starting at 200 msec and peakingaround 500 msec when the subsequent word does notmatch the picture at both 14 and 20 months (MillsConboy et al in press) This ERP effect is most likelywithin the N400 family observed in children and adults toviolations of semantic expectancy (Holcomb Coffey ampNeville 1992 Kutas amp Hillyard 1980) In adults a similarparadigm elicited a phonological mismatch (PMN) towords and nonwords from 250 to 347 msec over rightfrontal regions and from 347 to 638 msec over symmet-rical centro-parietal regions (Connolly Service DrsquoArcyKujala amp Alho 2001) One concern was that the N400 andPMN effects elicited by a pictureword matchmismatchparadigm might swamp more subtle differences in thelatency amplitudes and distributions of ERPs related tothe different word-types and group differences The Millset al paradigm was chosen because it showed ERP dif-ferences to known and unknown words that varied withword-type age and vocabulary size The methodologicaldifferences should be taken into consideration whencomparing the effects However the ERP findings areconsistent with and provide additional support for theage-related differences reported by Stager et al (1997)
The robust differences in the ERP pattern at 14and 20 months of age could be explained at manylevels Several studies have shown that a negativepeak at around 200 msec is sensitive to phonetic varia-tion in voice onset time (Simos Molfese amp Brenden1997 Molfese amp Molfese 1988) place of articulation(Dehaene-Lambertz amp Dehaene 1994 Molfese Burger-Judisch amp Hans 1991) and acoustic cues in nonspeechstimuli (Dehaene-Lambertz 2000 Simos amp Molfese1997) in very young children and even newborns Oneinterpretation is that ERP differences between word-types are related to the acoustic or phonological featuresof the different word-types and that age-related changesare due to differences in the childrenrsquos ability to mem-orize the different word lists Although phonological dis-crimination can modulate the amplitude of the N200 incertain paradigms it is not necessarily the only factor
1458 Journal of Cognitive Neuroscience Volume 16 Number 8
that may contribute to the amplitude of this componentHere we argue that age-related changes in a top-downprocess linked to word meaning is the dominant factormodulating the amplitude of the N200ndashN400 rather thanphonological discrimination First if ERP differences be-tween conditions were due solely to phonological fac-tors then both age groups would have displayed N200amplitude differences between the known words andphonetic contrasts Additionally the known and phone-mic contrasts were balanced for the types of initialconsonants in the number of stop consonants nasalsand fricatives and did not differ in their phonologicalneighbors for other words frequently understood bychildren in this age range (see Methods) Therefore itis difficult to explain why the phonemic contrasts pat-terned with the known words at 14 months and thenonsense words at 20 months based solely on acousticaland phonemic differences A second line of evidencesuggesting that the ERP differences observed here can-not be explained by phonological differences betweenword-types comes from a study of bilingual toddlersusing the same paradigm In that study the N200ndashN400did not differ between Spanish and English for either theknown or unknown words (Conboy amp Mills 2000 Con-boy 2003) ERP differences were linked to word meaning(known vs unknown words) language dominance andtotal vocabulary size but not phonological differencesbetween Spanish and English If the N200ndashN400 indexedthe phonological aspects of the words amplitude differ-ences would be expected between Spanish and EnglishA third and perhaps the strongest line of evidencecomes from a training study with 20-month-old childrenin which nonsense words were either repeated or pairedwith an object (Mills Plunkett Prat amp Schafer in press)After the training phase when objects were no longerpresent the amplitude of the N200ndashN400 to the newlylearned words increased relative to before training butbecame more positive for nonsense words that wererepeated the same number of times Because the stimuliwere counterbalanced across participants the increasedamplitude of the N200ndashN400 to the newly learned wordswas modulated by its association with a meaningfulstimulus and cannot be explained by phonological dif-ferences between word-types
Another interpretation of the findings is that at 14months of age the phonetic detail distinguishing oneword from another is simply not available in the lexicalrepresentation Such an explanation would be compat-ible with theories of phonological development thatposit an initial under-specification of the informationin the lexicon that is only gradually filled in as thevocabulary expands (eg Brown amp Matthews 1997)Although possible we think this explanation is unlikelygiven the work by Swingley and Aslin (2002) showingthat children of 14 months do seem to show someevidence of use of phonetic detail in word recognitiontasks via longer looking time to the matching object
when a correct versus a mispronounced version of aword is given The Swingley and Aslin work involvesshowing the children two objects (eg a car and a dog)and asking them to find either the correctly labeledobject (eg lsquolsquodogrsquorsquo) or a mispronunciation of that sameobject (eg lsquolsquobogrsquorsquo) More recently Fennell and Werker(2003) showed that when tested on two phonemiccontrast words that they already know well (eg lsquolsquoballrsquorsquoand lsquolsquodollrsquorsquo) the child of 14 months can succeed even inthe switch task These two studies show that at somelevel in the system full phonetic specification of well-known words is represented even at 14 months
The successes in the two-choice (Swingley and Aslin)behavioral lsquolsquomispronunciationrsquorsquo task and in the switchtask using two well-known words in comparison to boththe switch word-learning task and the ERP word recog-nition task are compatible with the attentional resourcelimitation hypothesis offered by Werker and colleagues(Werker et al 2002 Werker amp Fennell 2004 Fennell ampWerker 2003 Stager amp Werker 1997) According to thisexplanation the task of linking a word to an object ischallenging for a novice word learner All the detail ispicked up but the computational difficulty of actuallymapping the word on to the object makes it difficultto hold all the information in mind As such one sees aU-shaped function in the pattern of data The child of7ndash8 months who is not yet actively engaged in mappingwords to referents is better able to attend to fine pho-netic detail than is the child of 14 months who is at-tempting to make and remember the link (see Stager ampWerker 1997 see also Halle amp de Boysson-Bardies1996 Jusczyk amp Aslin 1995) Becoming a word learnerchanges the task for the older child This increases theprocessing demands and interferes with the childrsquosability to access detail The information is picked upin perception it is just not available for access in lexicalrecall tasks After the child becomes a more accom-plished word learner and has a working phonemicinventory to guide information access the relevantphonetic differences lsquolsquostand outrsquorsquo as salient and impor-tant to the lexical entry Or as suggested by Nazzi andBertoncini (2003) once the child passes a criticaljuncture in word learning sophistication moving froma simple associationist to a referential word learneraccess to phonological detail becomes possible Priorto that point however any detection of that detail isat best fleeting As such it may be seen in on-lineprocessing tasks or in tasks that provide full support torecall memory (Fennell amp Werker 2003 Swingley ampAslin 2002) but is not evident in tasks that involvelearning new words (Werker et al 2002 Stager ampWerker 1997) nor is it evident in word recognitiontasks such as the ERP task used here where there is noobject to facilitate activation of the full memory trace
Support for this explanation can be seen in thecurrent ERP study In this study children of 20 months(but not 14 months) showed a significant P100 response
Mills et al 1459
to the known words over both the nonsense words andthe phonemic contrasts words The P100 is thought toindex sensory and attentional processes This is the firsttime Mills and colleagues (eg Mills et al 1993 1997)have observed a significantly greater P100 response toknown than to unknown words in the ERP word recog-nition studies The amplitude of the P100 is known to bemodulated by effects of attention in both adults (Luck ampHillyard 2000) and children (Richards amp Hunter 2001)One interpretation of this finding is that the inclusion ofphonemic contrast words in the dataset increased atten-tion to early phonological differences To distinguish theknown words from the phonemic contrast foils theyhad to increase their vigilance to the sensory andperceptual detail thus yielding a significant P100 re-sponse in this ERP word recognition task
Directions for Further Research
A focus for future work will be to determine preciselywhat changes between 14 and 20 months allow the moreaccomplished word learner to access and use full pho-netic detail across a wide range of word recognition andword learning situations In a recent training study MillsPlunkett et al (in press) addressed this question byexamining the effects of experience on the lateral distri-bution of the N200ndashN400 differences between knownand unknown words Mills et al asked whether observedchanges in the lateral distribution in ERP differencesto known versus unknown words between 13 and20 months (Mills et al 1997 and replicated in the pre-sent study) reflect the availability of specialized brainsystems for word recognition (as might be available in amore accomplished word learner) or increasing knowl-edge of particular words by testing 20-month-old chil-dren who varied in vocabulary size on newly learnedwords ERPs to novel words that had been paired withan object during a pretest training phase were comparedto ERPs to novel words that had been repeated the samenumber of times but without the objectword pairingThe results supported a mixed model ERP differences tonewly learned words compared to repeated-but-not-trained words showed a bilateral distribution acrossthe whole sample This finding was consistent with thehypothesis that the asymmetrical distribution of ERP dif-ferences to known and unknown words is linked to theamount of experience with individual words Howeverchildren with the largest vocabularies showed a leftgreater than right distribution of ERP differences Thelatter finding suggested that expertise with word learn-ing might also affect the specialization of language-relevant brain activity even in the service of learningnew words
If a more expert word learning system is becomingavailable (Nazzi amp Bertoncini 2003) in the older andormore experienced word learner this might also permit
greater direction of attention to the relevant phonolog-ical information in the word learning and access situa-tion Indeed as we argued at the beginning of thisarticle perhaps part of becoming a more expert wordlearner is knowing just which properties of a word aredefinitional in a particular linguistic community It maybe the emergence of this greater understanding of justwhat distinguishes words in our language that allows theolder more accomplished word learner to not onlyperceive but also lsquolsquomarkrsquorsquo and lsquolsquousersquorsquo the phonologicaldetail in word learning and word recognition tasks
In previous studies Mills and colleagues have re-ported a link between vocabulary size and the distribu-tion of the N200ndashN400 amplitude difference to knownand unknown words between 14 and 20 months of ageSimilarly Werker et al (2002) reported a significantcorrelation between performance on the CDI at both14 and 17 months and performance in the minimal pairswitch task There was no significant correlation with theCDI in the present study The lack of a significantcorrelation here could reflect lack of variability in thedata perhaps because the age groups selected are notyet in transition We predict that testing of a group ofchildren intermediate in age and vocabulary sizes to thegroups tested here would be more likely to reveal such apattern of findings
In summary using an ERP design we extended andhelped explain the pattern of findings seen in earlybehavioral tasks assessing the phonological detail repre-sented and used in early word learners The results ofthe current ERP study indicate that at the earliest stagesof word learning children treat minimal pair mispronun-ciations of known words as acceptable instances of thatword This provides strong evidence that in the decisionstage of word recognition novice word learners accept abroader range of pronunciations of the word as accept-able than do older children With increasing age andincreasing language sophistication the amount of pho-nological detail easily accessed in the representation ofwords increases helping to avoid mapping mistakes andto facilitate more rapid acquisition of a vocabulary
METHODS
Participants and Settings
Participants were tested in three locations at the Uni-versity of California San Diego at the University ofOregon Eugene Oregon and at the University of BritishColumbia Vancouver Canada Approximately one-thirdof the children in each age group were tested at eachsite Testing at the Vancouver site was conducted in amobile ERP lab belonging to the University of OregonChildren from the San Diego and Eugene areas wererecruited through advertisements in a local magazineposters displayed in the area requests at play groupsand referrals from parents whose children had partici-
1460 Journal of Cognitive Neuroscience Volume 16 Number 8
pated in our studies Children from the Vancouver areawere recruited through the University of British Colum-bia child Studies Center database Parents of all childrensigned consent forms consistent with the human sub-jects internal review boards at the university at whichthey were tested Parents were given $500 to defraytransportation costs and children were given a small toyin appreciation of their participation Only full-term(gt36 weeks of gestation) healthy children with mono-lingual experience with English participated in the studychildren with a family history of language impairmentwere excluded from the study
Data from 16 children aged 14 months (9 girls meanage = 14 months 15 days SD = 97 days range = 14months 1 day to 15 months 1 day) and 17 children aged20 to 21 months (9 girls mean age = 20 months 11 daysSD = 131 days age range = 19 months 14 days to 20months 27 days) were retained for analysis in the studyAn additional 18 children (12 boys nine 14-month-olds)were tested but were excluded from the analyses due totoo few artifact-free trials (n = 10) excessive crying (n =3) reversed polarity of the ERP response (n = 3 seenote)2 or refusal to wear the electrocap (n = 2)
Stimuli
The stimuli were naturally spoken in a female voice anddigitized at 16 bits 44 kHz sampling rate The stimuliconsisted of three types of words (a) 10 words whosemeanings were understood by the child (known wordseg lsquolsquomilkrsquorsquo) (b) 10 nonsense words that differed fromthe known words in the initial phonemic contrast (pho-nemic contrast words eg lsquolsquonilkrsquorsquo) and (c) 10 wordsthat differed phonetically from known words in allvowels and consonants (nonsense words eg lsquolsquoneemrsquorsquo)All stimuli were matched on word duration and numberof syllables The mean durations for the known wordsphonetic contrasts and nonsense words were 783 msecSD = 144 865 msec SD = 169 861 msec SD = 96[F(227) = 109 p = 34] Ideally all word-types wouldbe matched on manner of articulation (eg all stopconsonants all nasals all laterals all fricatives or allbalanced) However children at this age have very smallvocabularies and we were limited to common words thatalmost all children at this age would know Amongchildren first words are a disproportionate number ofwords that begin with stop consonants For many ofthose words minimal pair differences are still words(eg lsquolsquoballrsquorsquondashlsquolsquodollrsquorsquo) Thus we were constrained in ourselection of stimulus items For the known words therewere seven stop consonants two nasals and one frica-tive For the phonemic contrasts there are also sevenstop consonants two nasals and one fricative For thenonsense words there are three stop consonants oneaffricative two nasals two fricatives and two laterals Acomplete list of words is shown in Table 1
To evaluate at what point the known words couldphysically be distinguishable from the other word-typeswe examined two aspects of uniqueness (a) at whatpoint the phonetically dissimilar nonsense words be-come unique from their real word counterparts (egwhen does lsquolsquobearrsquorsquo become unique from lsquolsquogarersquorsquo) and (b)when the words might become unique from otherwords that would be in a babyrsquos vocabulary at this ageThe uniqueness point for initial consonants (the pointthat separates that consonant from other consonants)was within 20 msec of the beginning of the word forfricatives (s z f) 40 msec for stop consonants (b d gp t k) 60 msec for affricatives (j) and 20 msec for nasals(m n) and laterals (l r) To further examine theuniqueness points we compared each stimulus usedin the study relative to other words that might be in thechildrsquos vocabulary with the words in the MacArthur CDIthat would be considered close phonological neighborsTo this end we compared our stimuli to the number ofwords in the CDI with (a) the same consonant includingconsonant clusters and (b) the same consonant exclud-ing consonant clusters and (c) the same consonant plusthe vowel For comparisons a and b there are the mostneighbors for the known words (a = 474 b = 375) anintermediate amount for the phonemic contrast words(a = 371 b = 298) and the least for the nonsense words(a = 275 b = 233) The best comparison is to count thenumber of items that begin with the same consonantplus vowel (CV) There were no significant differences inthe number of CV neighbors for the three word-types(known = 16 phonemic contrasts = 12 nonsense =18) Additionally because of co-articulatory effects theacoustic energy for a given consonant varies as a func-tion of the following vowel (Jusczyk 1997) Thus evenwords beginning with the same consonant (eg lsquolsquobookrsquorsquoand lsquolsquobottlersquorsquo) differ in shape and spectral frequenciesand can be discriminated from each other as well as
Table 1 Stimuli
Known Words Phonemic Contrasts Nonsense
Bear Gare Kobe
Ball Pall Lif
Book Dook Neem
Bottle Pottle Fipe
Cup Tup Mon
Cat Gat Tek
Dog Bog Riss
Milk Nilk Keed
Nose Mose Jud
Shoe Zhu Zav
Mills et al 1461
their close phonological neighbors from the beginningof the word
Procedure
Language Assessment
Within one week prior to testing parents were asked tocomplete the MacArthur CDI (Fenson et al 1994)which provided an estimate of the childrsquos vocabularysize and percentile ranking relative to other children ofthe same age To ensure that the words to be used in thestudy as lsquolsquoknownrsquorsquo were comprehended by the childparents also completed a vocabulary checklist ratingscale indicating how sure they were that their childunderstood andor produced each word on a scale of1 (very sure they did not know that word) to 4 (very suretheir child understoodproduced a given word in avariety of different contexts and with different exem-plars) Additionally the children were asked to identify apicture of each word to be used as a known word from atwo-choice picture book All lsquolsquoknownrsquorsquo words used hadreceived a rating of 4 and were correctly identified in thepicture-pointing task
Electrophysiological Recording
The EEG was recorded using tin electrodes affixed to anelastic cap (Electro-Cap International) from sites overfrontal (F7 and F8) anterior temporal (50 of thedistance from F78 and T34) temporal (33 of thedistance from T34 to C34) parietal (50 of the dis-tance between T34 and P34) and occipital (O1 andO2) regions of the left and right hemispheres Addition-ally the electrooculogram was recorded from electrodesplaced over and under the eye to reject trials on whichblinks and vertical eye movement occurred and fromleft and right frontal electrodes to reject trials on whichhorizontal eye movement occurred Impedances werekept below 5 k and were balanced (within 1 k) acrossthe left and right hemispheres at any given position TheEEG was amplified by SA Instruments amplifiers with abandpass of 01 to 100 Hz and sampled continuouslyevery 4 msec All electrodes were referenced to linkedmastoids3 Averages of the EEG were conducted using2-sec epochs (ie 100 msec prestimulus and 1900 msecpoststimulus) The averaged ERPs were also digitallyfiltered off-line with a 60-Hz low-pass filter
Testing
Ten known 10 phonetically similar nonsense words(referred to as phonemic contrasts) and 10 phoneticallydissimilar nonsense (referred to as nonsense words)words were each presented six times in random orderfor a total of 60 trials per condition During testingchildren sat on their parentrsquos lap and listened to words
presented from a speaker located behind a movingpuppet in a puppet theater Words were presented ata variable rate between 1800 and 3000 msec SOA
Artifact Rejection
Artifact rejection was conducted off-line using a com-puter program to reject blinks and horizontal eye move-ment and amplifier blocking Individual thresholds wereset for each child based on visual inspection of the EEGepochs time-locked to each stimulus A mean of 50 ofthe trials were rejected due to eye and movementartifact The number of artifact-free trials per word-typeretained for analysis ranged from 12 to 53 (SD = 113)out of a possible 60 trials per condition There were nosignificant differences in the percentage of trials rejectedfor the different experimental conditions age groupsor sex
Measurement of ERP Components
Peak latencies and amplitudes were quantified by com-puter with reference to the 100-msec prestimulus base-line for the maximum negative or positive point in aspecified time window The time windows for eachcomponent were set according to the criteria used inour previous studies the first positive componentcalled the P100 was defined as the most positivedeflection between 50 and 175 msec The P100 indexesauditory sensory processing Moreover the lateral dis-tribution of the P100 has been shown to vary withpercentile ranking on the MacArthur CDI score Chil-dren who score at the 50th percentile or higher for theirage show a P100 left greater than right asymmetry Themean amplitude within the time window 200ndash400 afterword onset was quantified by computer with referenceto the 100-msec prestimulus baseline This time windowwas chosen because it had been shown to differ forknown and unknown words in our previous studies
Acknowledgments
This research was conducted while the first author was at theProject in Cognitive and Neural Development at the Universityof California at San Diego The research was supported by NIHgrant DC0048111 to H Neville from NIDCD NSERC (NaturalSciences and Engineering Research Council of Canada) grantto J Werker and HFSP (Human Frontierrsquos Science Program)grant to J Werker and H Neville The third author RenateZangl was funded in part by two postdoctoral grants (J-1646J-1817) from the Austrian Science Foundation
We thank Amy Adamson Greg Appelbaum Teresa Mitchelland Lisa Sanders (in alphabetical order not necessarily in orderof magnitude of contribution) for their help in acquiring andanalyzing the data and Chris Fennell and Liz Lewis for theircomments on an earlier version of the manuscript We areespecially indebted to the children and their parents whoparticipated in these studies and their parents without whomthe research would not be possible
1462 Journal of Cognitive Neuroscience Volume 16 Number 8
Reprint requests should be sent to Debra L Mills who is nowat the Department of Psychology 532 Kilgo Circle EmoryUniversity Atlanta GA 30322 or via e-mail dmills2emoryedu
Notes
1 Unlike the switch task the visual fixation task is an on-linetask with two simultaneously presented choices The twoindicators of success are shorter latency to look away from theincorrect object and longer looking times overall to the correctmatch The habituation phase in the lsquolsquoswitchrsquorsquo task leads to theprediction of a novelty preference in the test phase (ie longerlooking to the incorrect pairing)2 In previous studies Mills and colleagues typically findapproximately 10ndash15 of children show a reversed ERPresponse with a larger amplitude ERP response to unknownthan to known words In previous work Mills and colleagueshave treated those children as noise in the data and have keptthem in for the data analyses Ultimately we want tounderstand the nature of lexical representations in this groupof children showing an aberrant ERP response For the currentstudy however in order to compare the ERP response tophonemic contrasts to known words we felt it necessary toinclude in our sample only those children who showed themuch more typical pattern of response resulting in the ex-clusion of three children All effects reported here show thesame pattern with these children included but the variabilityfrom their inclusion did reduce the differences in somecomparisons3 We are aware of the controversies surrounding the use oflinked mastoids Linked mastoids were used here to increasethe number of active cites given the number of amplifiersavailable and to provide consistency with previous studies Apilot study using similar auditory stimuli was conducted toexamine possible distortions in the distribution of scalp activityresulting from forced linkage This was determined by re-cording from one mastoid using the other as a reference andlinking the mastoids off-line These pilot data were comparedwith the data recorded using linked mastoids and did not yieldsignificant differences
REFERENCES
Bailey T M amp Plunkett K (2002) Phonological specificity inearly words Cognitive Development 17 1265ndash1282
Bertoncini J amp Mehler J (1981) Syllables as units in infantspeech perception Infant Behavior and Development 4247ndash260
Brown C amp Matthews J (1997) The role of feature geometryin the development of phonemic contrasts In S J Hannahsamp M Young-Scholten (Eds) Focus on phonologicalacquisition (pp 67ndash112) Amsterdam Benjamins
Brown C M Hagoort P amp Kutas M (2000) Postlexicalintegration processes in language comprehension Evidencefrom brain-imaging research In M Gazzaniga (Ed) The newcognitive neurosciences (2nd ed pp 881ndash895)Cambridge MIT Press
Conboy B (2003) Patterns of language processing and growthin early EnglishndashSpanish bilingualism (Doctoral dissertationUniversity of California San Diego and San Diego StateUniversity 2000) Dissertation Abstracts InternationalB The Sciences and Engineering 63 (UMI No 5193)
Conboy B amp Mills D L (2000) Cerebral specialization forlanguage comprehension in bilingual toddlers CognitiveNeuroscience Society 2000 Annual Meeting AbstractsSupplement of the Journal of Cognitive Neuroscience 17
Connolly J F Service E DrsquoArcy R C N Kujala A Alho K(2001) Phonological aspects of word recognition as revealedby high-resolution spatio-temporal brain mappingNeuroReport 12 237ndash243
Dehaene-Lambertz G (2000) Cerebral specialization forspeech and non-speech stimuli in infants Journal ofCognitive Neuroscience 12 449ndash460
Dehaene-Lambertz G amp Dehaene S (1994) Speed andcerebral correlates of syllable discrimination in infantsNature 370 292ndash294
Eimas P D Siqueland E R Jusczyk P amp Vigorito J (1971)Speech perception in infants Science 171 303ndash306
Fennell C E amp Werker J F (2003) Early word learnersrsquoability to access phonetic detail in well-known wordsLanguage and Speech 46 245ndash264
Fenson L Dale P S Reznick J S Bates E Thal D Jamp Pethick S J (1994) Variability in early communicativedevelopment Monographs of the Society for Research inChild Development 59 1ndash173
Fernald A Swingley D amp Pinto J P (2001) When half aword is enough Infants can recognize spoken words usingpartial phonetic information Child Development 721003ndash1015
Fischler I S (1990) Comprehending language withevent-related potentials In J W Rohrbaugh R Parasuramanamp R Johnson (Eds) Event-related brain potentials(pp 165ndash170) New York Oxford University Press
Halle P amp de Boysson-Bardies B (1994) Emergence of anearly receptive lexicon Infantsrsquo recognition of wordsInfant Behavior and Development 17 119ndash129
Halle P amp de Boysson-Bardies B (1996) The format ofrepresentation of recognized words in infantsrsquo earlyreceptive lexicon Infant Behavior and Development 19463ndash481
Holcomb P J amp Neville H J (1991) The electrophysiologyof spoken sentence processing Psychobiology 19286ndash300
Holcomb P J Coffey S A amp Neville H J (1992) Visual andauditory sentence processing A developmental analysisusing event-related brain potentials DevelopmentalNeuropsychology 8 203ndash241
Jusczyk P W (1997) The discovery of spoken languageCambridge MIT Press
Jusczyk P W amp Aslin R N (1995) Infantsrsquo detection ofsound patterns of words in fluent speech CognitivePsychology 29 1ndash23
Kutas M amp Hillyard S (1980) Reading senseless sentencesBrain potentials ref lect semantic incongruity Science 207203ndash205
Luck S amp Hillyard S (2000) The operation of selectiveattention at multiple stages of processing Evidence fromhuman and monkey electrophysiology In M Gazzaniga(Ed) The new cognitive neurosciences (2nd edpp 687ndash700) Cambridge MIT Press
Mills D L Coffey-Corina S A amp Neville H J (1997)Language comprehension and cerebral specialization from13ndash20 months [(Special issue on origins of languagedisorders)] In D Thal amp J Reilly (Eds) DevelopmentalNeuropsychology 13 397ndash446
Mills D M Coffey-Corina S A amp Neville H J (1994)Variability in cerebral organization during primary languageacquisition In G Dawson amp K Fischer (Eds) Humanbehavior and the developing brain (pp 427ndash455) NewYork Guilford Publications
Mills D M Coffey-Corina S A amp Neville H J (1993)Language acquisition and cerebral specialization in20-month-old infants Journal of Cognitive Neuroscience 5326ndash342
Mills et al 1463
Mills D L Conboy B amp Paton C (in press) Do changes inbrain organization reflect shifts in symbolic functioning InL Namy (Ed) Symbol use and symbolic representationMahwah NJ Erlbaum
Mills D L Plunkett K Prat C amp Schafer G (in press)Watching the infant brain learn words Effects ofvocabulary size and experience Cognitive Development
Molfese D L (1989) Electrophysiological correlates of wordmeanings in 14-month-old human infants DevelopmentalNeuropsychology 5 79ndash103
Molfese D L (1990) Auditory evoked responses recordedfrom 16-month-old human infants to words they did anddid not know Brain and Language 38 345ndash363
Molfese D L Burger-Judisch L M amp Hans L L (1991)Consonant discrimination by newborn infantsElectrophysiological differences DevelopmentalNeuropsychology 7 177ndash195
Molfese D L amp Molfese V J (1988) Right hemisphereresponses from preschool children to temporal cuescontained in speech and nonspeech materialsElectrophysiological correlates Brain and Language33 245ndash259
Molfese D L Morse P A amp Peters C J (1990) Auditoryevoked responses to names for different objectsCross-modal processing as a basis for infant languageacquisition Developmental Psychology 26 780ndash795
Molfese D L Wetzel W F amp Gill L A (1993) Known versusunknown word discriminations in 12-month-old humaninfants Electrophysiological correlates DevelopmentalNeuropsychology 9 241ndash258
Naigles L R (2002) Form is easy meaning is hard Resolvinga paradox in early child language Cognition 8 157ndash199
Nazzi T amp Bertoncini J (2003) Before and after thevocabulary spurt Two modes of word acquisitionDevelopmental Science 6 136ndash142
Osterhout L amp Holcomb P J (1995) Event-related potentialsand language comprehension In M D Rugg amp M G HColes (Eds) Electrophyisology of mind (pp 171ndash215)Oxford Oxford University Press
Richards J E amp Hunter S K (2001) Testing neural models ofthe development of infant attention DevelopmentalPsychobiology 40 226ndash236
Sereno S C Brewer C C amp OrsquoDonnell P J (2003)Context effects in word recognition Evidence for earlyinteractive processing Psychological Science 14328ndash333
Simos P G amp Molfese D L (1997) Electrophysiologicalresponses from a temporal order continuum in the newborninfant Neuropsychologia 35 89ndash98
Simos P G Molfese D L amp Brenden R A (1997) Behavioraland electrophysiological indices of voicing-cuediscrimination Laterality patterns and developmentBrain and Language 57 122ndash150
St George M amp Mills D L (2001) Electrophysiologicalstudies of language development In J Weissenborn ampB Hoehle (Eds) Language acquisition and languagedisorders (pp 247ndash259) Amsterdam John BenjaminsPublishing
Stager C L amp Werker J F (1997) Infants listen for morephonetic detail in speech perception than in word-learningtasks Nature 388 381ndash382
Swingley D (2003) Phonetic detail in the developing lexiconLanguage and Speech 46 265ndash294
Swingley D amp Aslin R N (2000) Spoken word recognitionand lexical representation in very young children Cognition76 147ndash166
Swingley D amp Aslin R N (2002) Lexical neighborhoodsand the wordndashform representations of 14-month-oldsPsychological Science 13 480ndash484
Werker J F Cohen L B Lloyd V L Casasola M amp StagerC L (1998) Acquisition of wordndashobject associations by14-month-old infants Developmental Psychology 341289ndash1309
Werker J F amp Fennell C E (2004) From listening to soundsto listening to words Early steps in word learning In G Hallamp S Waxman (Eds) Weaving a lexicon (pp 79ndash109)Cambridge MA MIT Press
Werker J F Fennell C E T Corcoran K M amp Stager C L(2002) Infantsrsquo ability to learn phonetically similar wordsEffects of age and vocabulary size Infancy 3 1ndash30
Werker J F amp Tees R C (1984) Cross-language speechperception Evidence for perceptual reorganization duringthe first year of life Infant Behavior and Development 749ndash63
1464 Journal of Cognitive Neuroscience Volume 16 Number 8
that may contribute to the amplitude of this componentHere we argue that age-related changes in a top-downprocess linked to word meaning is the dominant factormodulating the amplitude of the N200ndashN400 rather thanphonological discrimination First if ERP differences be-tween conditions were due solely to phonological fac-tors then both age groups would have displayed N200amplitude differences between the known words andphonetic contrasts Additionally the known and phone-mic contrasts were balanced for the types of initialconsonants in the number of stop consonants nasalsand fricatives and did not differ in their phonologicalneighbors for other words frequently understood bychildren in this age range (see Methods) Therefore itis difficult to explain why the phonemic contrasts pat-terned with the known words at 14 months and thenonsense words at 20 months based solely on acousticaland phonemic differences A second line of evidencesuggesting that the ERP differences observed here can-not be explained by phonological differences betweenword-types comes from a study of bilingual toddlersusing the same paradigm In that study the N200ndashN400did not differ between Spanish and English for either theknown or unknown words (Conboy amp Mills 2000 Con-boy 2003) ERP differences were linked to word meaning(known vs unknown words) language dominance andtotal vocabulary size but not phonological differencesbetween Spanish and English If the N200ndashN400 indexedthe phonological aspects of the words amplitude differ-ences would be expected between Spanish and EnglishA third and perhaps the strongest line of evidencecomes from a training study with 20-month-old childrenin which nonsense words were either repeated or pairedwith an object (Mills Plunkett Prat amp Schafer in press)After the training phase when objects were no longerpresent the amplitude of the N200ndashN400 to the newlylearned words increased relative to before training butbecame more positive for nonsense words that wererepeated the same number of times Because the stimuliwere counterbalanced across participants the increasedamplitude of the N200ndashN400 to the newly learned wordswas modulated by its association with a meaningfulstimulus and cannot be explained by phonological dif-ferences between word-types
Another interpretation of the findings is that at 14months of age the phonetic detail distinguishing oneword from another is simply not available in the lexicalrepresentation Such an explanation would be compat-ible with theories of phonological development thatposit an initial under-specification of the informationin the lexicon that is only gradually filled in as thevocabulary expands (eg Brown amp Matthews 1997)Although possible we think this explanation is unlikelygiven the work by Swingley and Aslin (2002) showingthat children of 14 months do seem to show someevidence of use of phonetic detail in word recognitiontasks via longer looking time to the matching object
when a correct versus a mispronounced version of aword is given The Swingley and Aslin work involvesshowing the children two objects (eg a car and a dog)and asking them to find either the correctly labeledobject (eg lsquolsquodogrsquorsquo) or a mispronunciation of that sameobject (eg lsquolsquobogrsquorsquo) More recently Fennell and Werker(2003) showed that when tested on two phonemiccontrast words that they already know well (eg lsquolsquoballrsquorsquoand lsquolsquodollrsquorsquo) the child of 14 months can succeed even inthe switch task These two studies show that at somelevel in the system full phonetic specification of well-known words is represented even at 14 months
The successes in the two-choice (Swingley and Aslin)behavioral lsquolsquomispronunciationrsquorsquo task and in the switchtask using two well-known words in comparison to boththe switch word-learning task and the ERP word recog-nition task are compatible with the attentional resourcelimitation hypothesis offered by Werker and colleagues(Werker et al 2002 Werker amp Fennell 2004 Fennell ampWerker 2003 Stager amp Werker 1997) According to thisexplanation the task of linking a word to an object ischallenging for a novice word learner All the detail ispicked up but the computational difficulty of actuallymapping the word on to the object makes it difficultto hold all the information in mind As such one sees aU-shaped function in the pattern of data The child of7ndash8 months who is not yet actively engaged in mappingwords to referents is better able to attend to fine pho-netic detail than is the child of 14 months who is at-tempting to make and remember the link (see Stager ampWerker 1997 see also Halle amp de Boysson-Bardies1996 Jusczyk amp Aslin 1995) Becoming a word learnerchanges the task for the older child This increases theprocessing demands and interferes with the childrsquosability to access detail The information is picked upin perception it is just not available for access in lexicalrecall tasks After the child becomes a more accom-plished word learner and has a working phonemicinventory to guide information access the relevantphonetic differences lsquolsquostand outrsquorsquo as salient and impor-tant to the lexical entry Or as suggested by Nazzi andBertoncini (2003) once the child passes a criticaljuncture in word learning sophistication moving froma simple associationist to a referential word learneraccess to phonological detail becomes possible Priorto that point however any detection of that detail isat best fleeting As such it may be seen in on-lineprocessing tasks or in tasks that provide full support torecall memory (Fennell amp Werker 2003 Swingley ampAslin 2002) but is not evident in tasks that involvelearning new words (Werker et al 2002 Stager ampWerker 1997) nor is it evident in word recognitiontasks such as the ERP task used here where there is noobject to facilitate activation of the full memory trace
Support for this explanation can be seen in thecurrent ERP study In this study children of 20 months(but not 14 months) showed a significant P100 response
Mills et al 1459
to the known words over both the nonsense words andthe phonemic contrasts words The P100 is thought toindex sensory and attentional processes This is the firsttime Mills and colleagues (eg Mills et al 1993 1997)have observed a significantly greater P100 response toknown than to unknown words in the ERP word recog-nition studies The amplitude of the P100 is known to bemodulated by effects of attention in both adults (Luck ampHillyard 2000) and children (Richards amp Hunter 2001)One interpretation of this finding is that the inclusion ofphonemic contrast words in the dataset increased atten-tion to early phonological differences To distinguish theknown words from the phonemic contrast foils theyhad to increase their vigilance to the sensory andperceptual detail thus yielding a significant P100 re-sponse in this ERP word recognition task
Directions for Further Research
A focus for future work will be to determine preciselywhat changes between 14 and 20 months allow the moreaccomplished word learner to access and use full pho-netic detail across a wide range of word recognition andword learning situations In a recent training study MillsPlunkett et al (in press) addressed this question byexamining the effects of experience on the lateral distri-bution of the N200ndashN400 differences between knownand unknown words Mills et al asked whether observedchanges in the lateral distribution in ERP differencesto known versus unknown words between 13 and20 months (Mills et al 1997 and replicated in the pre-sent study) reflect the availability of specialized brainsystems for word recognition (as might be available in amore accomplished word learner) or increasing knowl-edge of particular words by testing 20-month-old chil-dren who varied in vocabulary size on newly learnedwords ERPs to novel words that had been paired withan object during a pretest training phase were comparedto ERPs to novel words that had been repeated the samenumber of times but without the objectword pairingThe results supported a mixed model ERP differences tonewly learned words compared to repeated-but-not-trained words showed a bilateral distribution acrossthe whole sample This finding was consistent with thehypothesis that the asymmetrical distribution of ERP dif-ferences to known and unknown words is linked to theamount of experience with individual words Howeverchildren with the largest vocabularies showed a leftgreater than right distribution of ERP differences Thelatter finding suggested that expertise with word learn-ing might also affect the specialization of language-relevant brain activity even in the service of learningnew words
If a more expert word learning system is becomingavailable (Nazzi amp Bertoncini 2003) in the older andormore experienced word learner this might also permit
greater direction of attention to the relevant phonolog-ical information in the word learning and access situa-tion Indeed as we argued at the beginning of thisarticle perhaps part of becoming a more expert wordlearner is knowing just which properties of a word aredefinitional in a particular linguistic community It maybe the emergence of this greater understanding of justwhat distinguishes words in our language that allows theolder more accomplished word learner to not onlyperceive but also lsquolsquomarkrsquorsquo and lsquolsquousersquorsquo the phonologicaldetail in word learning and word recognition tasks
In previous studies Mills and colleagues have re-ported a link between vocabulary size and the distribu-tion of the N200ndashN400 amplitude difference to knownand unknown words between 14 and 20 months of ageSimilarly Werker et al (2002) reported a significantcorrelation between performance on the CDI at both14 and 17 months and performance in the minimal pairswitch task There was no significant correlation with theCDI in the present study The lack of a significantcorrelation here could reflect lack of variability in thedata perhaps because the age groups selected are notyet in transition We predict that testing of a group ofchildren intermediate in age and vocabulary sizes to thegroups tested here would be more likely to reveal such apattern of findings
In summary using an ERP design we extended andhelped explain the pattern of findings seen in earlybehavioral tasks assessing the phonological detail repre-sented and used in early word learners The results ofthe current ERP study indicate that at the earliest stagesof word learning children treat minimal pair mispronun-ciations of known words as acceptable instances of thatword This provides strong evidence that in the decisionstage of word recognition novice word learners accept abroader range of pronunciations of the word as accept-able than do older children With increasing age andincreasing language sophistication the amount of pho-nological detail easily accessed in the representation ofwords increases helping to avoid mapping mistakes andto facilitate more rapid acquisition of a vocabulary
METHODS
Participants and Settings
Participants were tested in three locations at the Uni-versity of California San Diego at the University ofOregon Eugene Oregon and at the University of BritishColumbia Vancouver Canada Approximately one-thirdof the children in each age group were tested at eachsite Testing at the Vancouver site was conducted in amobile ERP lab belonging to the University of OregonChildren from the San Diego and Eugene areas wererecruited through advertisements in a local magazineposters displayed in the area requests at play groupsand referrals from parents whose children had partici-
1460 Journal of Cognitive Neuroscience Volume 16 Number 8
pated in our studies Children from the Vancouver areawere recruited through the University of British Colum-bia child Studies Center database Parents of all childrensigned consent forms consistent with the human sub-jects internal review boards at the university at whichthey were tested Parents were given $500 to defraytransportation costs and children were given a small toyin appreciation of their participation Only full-term(gt36 weeks of gestation) healthy children with mono-lingual experience with English participated in the studychildren with a family history of language impairmentwere excluded from the study
Data from 16 children aged 14 months (9 girls meanage = 14 months 15 days SD = 97 days range = 14months 1 day to 15 months 1 day) and 17 children aged20 to 21 months (9 girls mean age = 20 months 11 daysSD = 131 days age range = 19 months 14 days to 20months 27 days) were retained for analysis in the studyAn additional 18 children (12 boys nine 14-month-olds)were tested but were excluded from the analyses due totoo few artifact-free trials (n = 10) excessive crying (n =3) reversed polarity of the ERP response (n = 3 seenote)2 or refusal to wear the electrocap (n = 2)
Stimuli
The stimuli were naturally spoken in a female voice anddigitized at 16 bits 44 kHz sampling rate The stimuliconsisted of three types of words (a) 10 words whosemeanings were understood by the child (known wordseg lsquolsquomilkrsquorsquo) (b) 10 nonsense words that differed fromthe known words in the initial phonemic contrast (pho-nemic contrast words eg lsquolsquonilkrsquorsquo) and (c) 10 wordsthat differed phonetically from known words in allvowels and consonants (nonsense words eg lsquolsquoneemrsquorsquo)All stimuli were matched on word duration and numberof syllables The mean durations for the known wordsphonetic contrasts and nonsense words were 783 msecSD = 144 865 msec SD = 169 861 msec SD = 96[F(227) = 109 p = 34] Ideally all word-types wouldbe matched on manner of articulation (eg all stopconsonants all nasals all laterals all fricatives or allbalanced) However children at this age have very smallvocabularies and we were limited to common words thatalmost all children at this age would know Amongchildren first words are a disproportionate number ofwords that begin with stop consonants For many ofthose words minimal pair differences are still words(eg lsquolsquoballrsquorsquondashlsquolsquodollrsquorsquo) Thus we were constrained in ourselection of stimulus items For the known words therewere seven stop consonants two nasals and one frica-tive For the phonemic contrasts there are also sevenstop consonants two nasals and one fricative For thenonsense words there are three stop consonants oneaffricative two nasals two fricatives and two laterals Acomplete list of words is shown in Table 1
To evaluate at what point the known words couldphysically be distinguishable from the other word-typeswe examined two aspects of uniqueness (a) at whatpoint the phonetically dissimilar nonsense words be-come unique from their real word counterparts (egwhen does lsquolsquobearrsquorsquo become unique from lsquolsquogarersquorsquo) and (b)when the words might become unique from otherwords that would be in a babyrsquos vocabulary at this ageThe uniqueness point for initial consonants (the pointthat separates that consonant from other consonants)was within 20 msec of the beginning of the word forfricatives (s z f) 40 msec for stop consonants (b d gp t k) 60 msec for affricatives (j) and 20 msec for nasals(m n) and laterals (l r) To further examine theuniqueness points we compared each stimulus usedin the study relative to other words that might be in thechildrsquos vocabulary with the words in the MacArthur CDIthat would be considered close phonological neighborsTo this end we compared our stimuli to the number ofwords in the CDI with (a) the same consonant includingconsonant clusters and (b) the same consonant exclud-ing consonant clusters and (c) the same consonant plusthe vowel For comparisons a and b there are the mostneighbors for the known words (a = 474 b = 375) anintermediate amount for the phonemic contrast words(a = 371 b = 298) and the least for the nonsense words(a = 275 b = 233) The best comparison is to count thenumber of items that begin with the same consonantplus vowel (CV) There were no significant differences inthe number of CV neighbors for the three word-types(known = 16 phonemic contrasts = 12 nonsense =18) Additionally because of co-articulatory effects theacoustic energy for a given consonant varies as a func-tion of the following vowel (Jusczyk 1997) Thus evenwords beginning with the same consonant (eg lsquolsquobookrsquorsquoand lsquolsquobottlersquorsquo) differ in shape and spectral frequenciesand can be discriminated from each other as well as
Table 1 Stimuli
Known Words Phonemic Contrasts Nonsense
Bear Gare Kobe
Ball Pall Lif
Book Dook Neem
Bottle Pottle Fipe
Cup Tup Mon
Cat Gat Tek
Dog Bog Riss
Milk Nilk Keed
Nose Mose Jud
Shoe Zhu Zav
Mills et al 1461
their close phonological neighbors from the beginningof the word
Procedure
Language Assessment
Within one week prior to testing parents were asked tocomplete the MacArthur CDI (Fenson et al 1994)which provided an estimate of the childrsquos vocabularysize and percentile ranking relative to other children ofthe same age To ensure that the words to be used in thestudy as lsquolsquoknownrsquorsquo were comprehended by the childparents also completed a vocabulary checklist ratingscale indicating how sure they were that their childunderstood andor produced each word on a scale of1 (very sure they did not know that word) to 4 (very suretheir child understoodproduced a given word in avariety of different contexts and with different exem-plars) Additionally the children were asked to identify apicture of each word to be used as a known word from atwo-choice picture book All lsquolsquoknownrsquorsquo words used hadreceived a rating of 4 and were correctly identified in thepicture-pointing task
Electrophysiological Recording
The EEG was recorded using tin electrodes affixed to anelastic cap (Electro-Cap International) from sites overfrontal (F7 and F8) anterior temporal (50 of thedistance from F78 and T34) temporal (33 of thedistance from T34 to C34) parietal (50 of the dis-tance between T34 and P34) and occipital (O1 andO2) regions of the left and right hemispheres Addition-ally the electrooculogram was recorded from electrodesplaced over and under the eye to reject trials on whichblinks and vertical eye movement occurred and fromleft and right frontal electrodes to reject trials on whichhorizontal eye movement occurred Impedances werekept below 5 k and were balanced (within 1 k) acrossthe left and right hemispheres at any given position TheEEG was amplified by SA Instruments amplifiers with abandpass of 01 to 100 Hz and sampled continuouslyevery 4 msec All electrodes were referenced to linkedmastoids3 Averages of the EEG were conducted using2-sec epochs (ie 100 msec prestimulus and 1900 msecpoststimulus) The averaged ERPs were also digitallyfiltered off-line with a 60-Hz low-pass filter
Testing
Ten known 10 phonetically similar nonsense words(referred to as phonemic contrasts) and 10 phoneticallydissimilar nonsense (referred to as nonsense words)words were each presented six times in random orderfor a total of 60 trials per condition During testingchildren sat on their parentrsquos lap and listened to words
presented from a speaker located behind a movingpuppet in a puppet theater Words were presented ata variable rate between 1800 and 3000 msec SOA
Artifact Rejection
Artifact rejection was conducted off-line using a com-puter program to reject blinks and horizontal eye move-ment and amplifier blocking Individual thresholds wereset for each child based on visual inspection of the EEGepochs time-locked to each stimulus A mean of 50 ofthe trials were rejected due to eye and movementartifact The number of artifact-free trials per word-typeretained for analysis ranged from 12 to 53 (SD = 113)out of a possible 60 trials per condition There were nosignificant differences in the percentage of trials rejectedfor the different experimental conditions age groupsor sex
Measurement of ERP Components
Peak latencies and amplitudes were quantified by com-puter with reference to the 100-msec prestimulus base-line for the maximum negative or positive point in aspecified time window The time windows for eachcomponent were set according to the criteria used inour previous studies the first positive componentcalled the P100 was defined as the most positivedeflection between 50 and 175 msec The P100 indexesauditory sensory processing Moreover the lateral dis-tribution of the P100 has been shown to vary withpercentile ranking on the MacArthur CDI score Chil-dren who score at the 50th percentile or higher for theirage show a P100 left greater than right asymmetry Themean amplitude within the time window 200ndash400 afterword onset was quantified by computer with referenceto the 100-msec prestimulus baseline This time windowwas chosen because it had been shown to differ forknown and unknown words in our previous studies
Acknowledgments
This research was conducted while the first author was at theProject in Cognitive and Neural Development at the Universityof California at San Diego The research was supported by NIHgrant DC0048111 to H Neville from NIDCD NSERC (NaturalSciences and Engineering Research Council of Canada) grantto J Werker and HFSP (Human Frontierrsquos Science Program)grant to J Werker and H Neville The third author RenateZangl was funded in part by two postdoctoral grants (J-1646J-1817) from the Austrian Science Foundation
We thank Amy Adamson Greg Appelbaum Teresa Mitchelland Lisa Sanders (in alphabetical order not necessarily in orderof magnitude of contribution) for their help in acquiring andanalyzing the data and Chris Fennell and Liz Lewis for theircomments on an earlier version of the manuscript We areespecially indebted to the children and their parents whoparticipated in these studies and their parents without whomthe research would not be possible
1462 Journal of Cognitive Neuroscience Volume 16 Number 8
Reprint requests should be sent to Debra L Mills who is nowat the Department of Psychology 532 Kilgo Circle EmoryUniversity Atlanta GA 30322 or via e-mail dmills2emoryedu
Notes
1 Unlike the switch task the visual fixation task is an on-linetask with two simultaneously presented choices The twoindicators of success are shorter latency to look away from theincorrect object and longer looking times overall to the correctmatch The habituation phase in the lsquolsquoswitchrsquorsquo task leads to theprediction of a novelty preference in the test phase (ie longerlooking to the incorrect pairing)2 In previous studies Mills and colleagues typically findapproximately 10ndash15 of children show a reversed ERPresponse with a larger amplitude ERP response to unknownthan to known words In previous work Mills and colleagueshave treated those children as noise in the data and have keptthem in for the data analyses Ultimately we want tounderstand the nature of lexical representations in this groupof children showing an aberrant ERP response For the currentstudy however in order to compare the ERP response tophonemic contrasts to known words we felt it necessary toinclude in our sample only those children who showed themuch more typical pattern of response resulting in the ex-clusion of three children All effects reported here show thesame pattern with these children included but the variabilityfrom their inclusion did reduce the differences in somecomparisons3 We are aware of the controversies surrounding the use oflinked mastoids Linked mastoids were used here to increasethe number of active cites given the number of amplifiersavailable and to provide consistency with previous studies Apilot study using similar auditory stimuli was conducted toexamine possible distortions in the distribution of scalp activityresulting from forced linkage This was determined by re-cording from one mastoid using the other as a reference andlinking the mastoids off-line These pilot data were comparedwith the data recorded using linked mastoids and did not yieldsignificant differences
REFERENCES
Bailey T M amp Plunkett K (2002) Phonological specificity inearly words Cognitive Development 17 1265ndash1282
Bertoncini J amp Mehler J (1981) Syllables as units in infantspeech perception Infant Behavior and Development 4247ndash260
Brown C amp Matthews J (1997) The role of feature geometryin the development of phonemic contrasts In S J Hannahsamp M Young-Scholten (Eds) Focus on phonologicalacquisition (pp 67ndash112) Amsterdam Benjamins
Brown C M Hagoort P amp Kutas M (2000) Postlexicalintegration processes in language comprehension Evidencefrom brain-imaging research In M Gazzaniga (Ed) The newcognitive neurosciences (2nd ed pp 881ndash895)Cambridge MIT Press
Conboy B (2003) Patterns of language processing and growthin early EnglishndashSpanish bilingualism (Doctoral dissertationUniversity of California San Diego and San Diego StateUniversity 2000) Dissertation Abstracts InternationalB The Sciences and Engineering 63 (UMI No 5193)
Conboy B amp Mills D L (2000) Cerebral specialization forlanguage comprehension in bilingual toddlers CognitiveNeuroscience Society 2000 Annual Meeting AbstractsSupplement of the Journal of Cognitive Neuroscience 17
Connolly J F Service E DrsquoArcy R C N Kujala A Alho K(2001) Phonological aspects of word recognition as revealedby high-resolution spatio-temporal brain mappingNeuroReport 12 237ndash243
Dehaene-Lambertz G (2000) Cerebral specialization forspeech and non-speech stimuli in infants Journal ofCognitive Neuroscience 12 449ndash460
Dehaene-Lambertz G amp Dehaene S (1994) Speed andcerebral correlates of syllable discrimination in infantsNature 370 292ndash294
Eimas P D Siqueland E R Jusczyk P amp Vigorito J (1971)Speech perception in infants Science 171 303ndash306
Fennell C E amp Werker J F (2003) Early word learnersrsquoability to access phonetic detail in well-known wordsLanguage and Speech 46 245ndash264
Fenson L Dale P S Reznick J S Bates E Thal D Jamp Pethick S J (1994) Variability in early communicativedevelopment Monographs of the Society for Research inChild Development 59 1ndash173
Fernald A Swingley D amp Pinto J P (2001) When half aword is enough Infants can recognize spoken words usingpartial phonetic information Child Development 721003ndash1015
Fischler I S (1990) Comprehending language withevent-related potentials In J W Rohrbaugh R Parasuramanamp R Johnson (Eds) Event-related brain potentials(pp 165ndash170) New York Oxford University Press
Halle P amp de Boysson-Bardies B (1994) Emergence of anearly receptive lexicon Infantsrsquo recognition of wordsInfant Behavior and Development 17 119ndash129
Halle P amp de Boysson-Bardies B (1996) The format ofrepresentation of recognized words in infantsrsquo earlyreceptive lexicon Infant Behavior and Development 19463ndash481
Holcomb P J amp Neville H J (1991) The electrophysiologyof spoken sentence processing Psychobiology 19286ndash300
Holcomb P J Coffey S A amp Neville H J (1992) Visual andauditory sentence processing A developmental analysisusing event-related brain potentials DevelopmentalNeuropsychology 8 203ndash241
Jusczyk P W (1997) The discovery of spoken languageCambridge MIT Press
Jusczyk P W amp Aslin R N (1995) Infantsrsquo detection ofsound patterns of words in fluent speech CognitivePsychology 29 1ndash23
Kutas M amp Hillyard S (1980) Reading senseless sentencesBrain potentials ref lect semantic incongruity Science 207203ndash205
Luck S amp Hillyard S (2000) The operation of selectiveattention at multiple stages of processing Evidence fromhuman and monkey electrophysiology In M Gazzaniga(Ed) The new cognitive neurosciences (2nd edpp 687ndash700) Cambridge MIT Press
Mills D L Coffey-Corina S A amp Neville H J (1997)Language comprehension and cerebral specialization from13ndash20 months [(Special issue on origins of languagedisorders)] In D Thal amp J Reilly (Eds) DevelopmentalNeuropsychology 13 397ndash446
Mills D M Coffey-Corina S A amp Neville H J (1994)Variability in cerebral organization during primary languageacquisition In G Dawson amp K Fischer (Eds) Humanbehavior and the developing brain (pp 427ndash455) NewYork Guilford Publications
Mills D M Coffey-Corina S A amp Neville H J (1993)Language acquisition and cerebral specialization in20-month-old infants Journal of Cognitive Neuroscience 5326ndash342
Mills et al 1463
Mills D L Conboy B amp Paton C (in press) Do changes inbrain organization reflect shifts in symbolic functioning InL Namy (Ed) Symbol use and symbolic representationMahwah NJ Erlbaum
Mills D L Plunkett K Prat C amp Schafer G (in press)Watching the infant brain learn words Effects ofvocabulary size and experience Cognitive Development
Molfese D L (1989) Electrophysiological correlates of wordmeanings in 14-month-old human infants DevelopmentalNeuropsychology 5 79ndash103
Molfese D L (1990) Auditory evoked responses recordedfrom 16-month-old human infants to words they did anddid not know Brain and Language 38 345ndash363
Molfese D L Burger-Judisch L M amp Hans L L (1991)Consonant discrimination by newborn infantsElectrophysiological differences DevelopmentalNeuropsychology 7 177ndash195
Molfese D L amp Molfese V J (1988) Right hemisphereresponses from preschool children to temporal cuescontained in speech and nonspeech materialsElectrophysiological correlates Brain and Language33 245ndash259
Molfese D L Morse P A amp Peters C J (1990) Auditoryevoked responses to names for different objectsCross-modal processing as a basis for infant languageacquisition Developmental Psychology 26 780ndash795
Molfese D L Wetzel W F amp Gill L A (1993) Known versusunknown word discriminations in 12-month-old humaninfants Electrophysiological correlates DevelopmentalNeuropsychology 9 241ndash258
Naigles L R (2002) Form is easy meaning is hard Resolvinga paradox in early child language Cognition 8 157ndash199
Nazzi T amp Bertoncini J (2003) Before and after thevocabulary spurt Two modes of word acquisitionDevelopmental Science 6 136ndash142
Osterhout L amp Holcomb P J (1995) Event-related potentialsand language comprehension In M D Rugg amp M G HColes (Eds) Electrophyisology of mind (pp 171ndash215)Oxford Oxford University Press
Richards J E amp Hunter S K (2001) Testing neural models ofthe development of infant attention DevelopmentalPsychobiology 40 226ndash236
Sereno S C Brewer C C amp OrsquoDonnell P J (2003)Context effects in word recognition Evidence for earlyinteractive processing Psychological Science 14328ndash333
Simos P G amp Molfese D L (1997) Electrophysiologicalresponses from a temporal order continuum in the newborninfant Neuropsychologia 35 89ndash98
Simos P G Molfese D L amp Brenden R A (1997) Behavioraland electrophysiological indices of voicing-cuediscrimination Laterality patterns and developmentBrain and Language 57 122ndash150
St George M amp Mills D L (2001) Electrophysiologicalstudies of language development In J Weissenborn ampB Hoehle (Eds) Language acquisition and languagedisorders (pp 247ndash259) Amsterdam John BenjaminsPublishing
Stager C L amp Werker J F (1997) Infants listen for morephonetic detail in speech perception than in word-learningtasks Nature 388 381ndash382
Swingley D (2003) Phonetic detail in the developing lexiconLanguage and Speech 46 265ndash294
Swingley D amp Aslin R N (2000) Spoken word recognitionand lexical representation in very young children Cognition76 147ndash166
Swingley D amp Aslin R N (2002) Lexical neighborhoodsand the wordndashform representations of 14-month-oldsPsychological Science 13 480ndash484
Werker J F Cohen L B Lloyd V L Casasola M amp StagerC L (1998) Acquisition of wordndashobject associations by14-month-old infants Developmental Psychology 341289ndash1309
Werker J F amp Fennell C E (2004) From listening to soundsto listening to words Early steps in word learning In G Hallamp S Waxman (Eds) Weaving a lexicon (pp 79ndash109)Cambridge MA MIT Press
Werker J F Fennell C E T Corcoran K M amp Stager C L(2002) Infantsrsquo ability to learn phonetically similar wordsEffects of age and vocabulary size Infancy 3 1ndash30
Werker J F amp Tees R C (1984) Cross-language speechperception Evidence for perceptual reorganization duringthe first year of life Infant Behavior and Development 749ndash63
1464 Journal of Cognitive Neuroscience Volume 16 Number 8
to the known words over both the nonsense words andthe phonemic contrasts words The P100 is thought toindex sensory and attentional processes This is the firsttime Mills and colleagues (eg Mills et al 1993 1997)have observed a significantly greater P100 response toknown than to unknown words in the ERP word recog-nition studies The amplitude of the P100 is known to bemodulated by effects of attention in both adults (Luck ampHillyard 2000) and children (Richards amp Hunter 2001)One interpretation of this finding is that the inclusion ofphonemic contrast words in the dataset increased atten-tion to early phonological differences To distinguish theknown words from the phonemic contrast foils theyhad to increase their vigilance to the sensory andperceptual detail thus yielding a significant P100 re-sponse in this ERP word recognition task
Directions for Further Research
A focus for future work will be to determine preciselywhat changes between 14 and 20 months allow the moreaccomplished word learner to access and use full pho-netic detail across a wide range of word recognition andword learning situations In a recent training study MillsPlunkett et al (in press) addressed this question byexamining the effects of experience on the lateral distri-bution of the N200ndashN400 differences between knownand unknown words Mills et al asked whether observedchanges in the lateral distribution in ERP differencesto known versus unknown words between 13 and20 months (Mills et al 1997 and replicated in the pre-sent study) reflect the availability of specialized brainsystems for word recognition (as might be available in amore accomplished word learner) or increasing knowl-edge of particular words by testing 20-month-old chil-dren who varied in vocabulary size on newly learnedwords ERPs to novel words that had been paired withan object during a pretest training phase were comparedto ERPs to novel words that had been repeated the samenumber of times but without the objectword pairingThe results supported a mixed model ERP differences tonewly learned words compared to repeated-but-not-trained words showed a bilateral distribution acrossthe whole sample This finding was consistent with thehypothesis that the asymmetrical distribution of ERP dif-ferences to known and unknown words is linked to theamount of experience with individual words Howeverchildren with the largest vocabularies showed a leftgreater than right distribution of ERP differences Thelatter finding suggested that expertise with word learn-ing might also affect the specialization of language-relevant brain activity even in the service of learningnew words
If a more expert word learning system is becomingavailable (Nazzi amp Bertoncini 2003) in the older andormore experienced word learner this might also permit
greater direction of attention to the relevant phonolog-ical information in the word learning and access situa-tion Indeed as we argued at the beginning of thisarticle perhaps part of becoming a more expert wordlearner is knowing just which properties of a word aredefinitional in a particular linguistic community It maybe the emergence of this greater understanding of justwhat distinguishes words in our language that allows theolder more accomplished word learner to not onlyperceive but also lsquolsquomarkrsquorsquo and lsquolsquousersquorsquo the phonologicaldetail in word learning and word recognition tasks
In previous studies Mills and colleagues have re-ported a link between vocabulary size and the distribu-tion of the N200ndashN400 amplitude difference to knownand unknown words between 14 and 20 months of ageSimilarly Werker et al (2002) reported a significantcorrelation between performance on the CDI at both14 and 17 months and performance in the minimal pairswitch task There was no significant correlation with theCDI in the present study The lack of a significantcorrelation here could reflect lack of variability in thedata perhaps because the age groups selected are notyet in transition We predict that testing of a group ofchildren intermediate in age and vocabulary sizes to thegroups tested here would be more likely to reveal such apattern of findings
In summary using an ERP design we extended andhelped explain the pattern of findings seen in earlybehavioral tasks assessing the phonological detail repre-sented and used in early word learners The results ofthe current ERP study indicate that at the earliest stagesof word learning children treat minimal pair mispronun-ciations of known words as acceptable instances of thatword This provides strong evidence that in the decisionstage of word recognition novice word learners accept abroader range of pronunciations of the word as accept-able than do older children With increasing age andincreasing language sophistication the amount of pho-nological detail easily accessed in the representation ofwords increases helping to avoid mapping mistakes andto facilitate more rapid acquisition of a vocabulary
METHODS
Participants and Settings
Participants were tested in three locations at the Uni-versity of California San Diego at the University ofOregon Eugene Oregon and at the University of BritishColumbia Vancouver Canada Approximately one-thirdof the children in each age group were tested at eachsite Testing at the Vancouver site was conducted in amobile ERP lab belonging to the University of OregonChildren from the San Diego and Eugene areas wererecruited through advertisements in a local magazineposters displayed in the area requests at play groupsand referrals from parents whose children had partici-
1460 Journal of Cognitive Neuroscience Volume 16 Number 8
pated in our studies Children from the Vancouver areawere recruited through the University of British Colum-bia child Studies Center database Parents of all childrensigned consent forms consistent with the human sub-jects internal review boards at the university at whichthey were tested Parents were given $500 to defraytransportation costs and children were given a small toyin appreciation of their participation Only full-term(gt36 weeks of gestation) healthy children with mono-lingual experience with English participated in the studychildren with a family history of language impairmentwere excluded from the study
Data from 16 children aged 14 months (9 girls meanage = 14 months 15 days SD = 97 days range = 14months 1 day to 15 months 1 day) and 17 children aged20 to 21 months (9 girls mean age = 20 months 11 daysSD = 131 days age range = 19 months 14 days to 20months 27 days) were retained for analysis in the studyAn additional 18 children (12 boys nine 14-month-olds)were tested but were excluded from the analyses due totoo few artifact-free trials (n = 10) excessive crying (n =3) reversed polarity of the ERP response (n = 3 seenote)2 or refusal to wear the electrocap (n = 2)
Stimuli
The stimuli were naturally spoken in a female voice anddigitized at 16 bits 44 kHz sampling rate The stimuliconsisted of three types of words (a) 10 words whosemeanings were understood by the child (known wordseg lsquolsquomilkrsquorsquo) (b) 10 nonsense words that differed fromthe known words in the initial phonemic contrast (pho-nemic contrast words eg lsquolsquonilkrsquorsquo) and (c) 10 wordsthat differed phonetically from known words in allvowels and consonants (nonsense words eg lsquolsquoneemrsquorsquo)All stimuli were matched on word duration and numberof syllables The mean durations for the known wordsphonetic contrasts and nonsense words were 783 msecSD = 144 865 msec SD = 169 861 msec SD = 96[F(227) = 109 p = 34] Ideally all word-types wouldbe matched on manner of articulation (eg all stopconsonants all nasals all laterals all fricatives or allbalanced) However children at this age have very smallvocabularies and we were limited to common words thatalmost all children at this age would know Amongchildren first words are a disproportionate number ofwords that begin with stop consonants For many ofthose words minimal pair differences are still words(eg lsquolsquoballrsquorsquondashlsquolsquodollrsquorsquo) Thus we were constrained in ourselection of stimulus items For the known words therewere seven stop consonants two nasals and one frica-tive For the phonemic contrasts there are also sevenstop consonants two nasals and one fricative For thenonsense words there are three stop consonants oneaffricative two nasals two fricatives and two laterals Acomplete list of words is shown in Table 1
To evaluate at what point the known words couldphysically be distinguishable from the other word-typeswe examined two aspects of uniqueness (a) at whatpoint the phonetically dissimilar nonsense words be-come unique from their real word counterparts (egwhen does lsquolsquobearrsquorsquo become unique from lsquolsquogarersquorsquo) and (b)when the words might become unique from otherwords that would be in a babyrsquos vocabulary at this ageThe uniqueness point for initial consonants (the pointthat separates that consonant from other consonants)was within 20 msec of the beginning of the word forfricatives (s z f) 40 msec for stop consonants (b d gp t k) 60 msec for affricatives (j) and 20 msec for nasals(m n) and laterals (l r) To further examine theuniqueness points we compared each stimulus usedin the study relative to other words that might be in thechildrsquos vocabulary with the words in the MacArthur CDIthat would be considered close phonological neighborsTo this end we compared our stimuli to the number ofwords in the CDI with (a) the same consonant includingconsonant clusters and (b) the same consonant exclud-ing consonant clusters and (c) the same consonant plusthe vowel For comparisons a and b there are the mostneighbors for the known words (a = 474 b = 375) anintermediate amount for the phonemic contrast words(a = 371 b = 298) and the least for the nonsense words(a = 275 b = 233) The best comparison is to count thenumber of items that begin with the same consonantplus vowel (CV) There were no significant differences inthe number of CV neighbors for the three word-types(known = 16 phonemic contrasts = 12 nonsense =18) Additionally because of co-articulatory effects theacoustic energy for a given consonant varies as a func-tion of the following vowel (Jusczyk 1997) Thus evenwords beginning with the same consonant (eg lsquolsquobookrsquorsquoand lsquolsquobottlersquorsquo) differ in shape and spectral frequenciesand can be discriminated from each other as well as
Table 1 Stimuli
Known Words Phonemic Contrasts Nonsense
Bear Gare Kobe
Ball Pall Lif
Book Dook Neem
Bottle Pottle Fipe
Cup Tup Mon
Cat Gat Tek
Dog Bog Riss
Milk Nilk Keed
Nose Mose Jud
Shoe Zhu Zav
Mills et al 1461
their close phonological neighbors from the beginningof the word
Procedure
Language Assessment
Within one week prior to testing parents were asked tocomplete the MacArthur CDI (Fenson et al 1994)which provided an estimate of the childrsquos vocabularysize and percentile ranking relative to other children ofthe same age To ensure that the words to be used in thestudy as lsquolsquoknownrsquorsquo were comprehended by the childparents also completed a vocabulary checklist ratingscale indicating how sure they were that their childunderstood andor produced each word on a scale of1 (very sure they did not know that word) to 4 (very suretheir child understoodproduced a given word in avariety of different contexts and with different exem-plars) Additionally the children were asked to identify apicture of each word to be used as a known word from atwo-choice picture book All lsquolsquoknownrsquorsquo words used hadreceived a rating of 4 and were correctly identified in thepicture-pointing task
Electrophysiological Recording
The EEG was recorded using tin electrodes affixed to anelastic cap (Electro-Cap International) from sites overfrontal (F7 and F8) anterior temporal (50 of thedistance from F78 and T34) temporal (33 of thedistance from T34 to C34) parietal (50 of the dis-tance between T34 and P34) and occipital (O1 andO2) regions of the left and right hemispheres Addition-ally the electrooculogram was recorded from electrodesplaced over and under the eye to reject trials on whichblinks and vertical eye movement occurred and fromleft and right frontal electrodes to reject trials on whichhorizontal eye movement occurred Impedances werekept below 5 k and were balanced (within 1 k) acrossthe left and right hemispheres at any given position TheEEG was amplified by SA Instruments amplifiers with abandpass of 01 to 100 Hz and sampled continuouslyevery 4 msec All electrodes were referenced to linkedmastoids3 Averages of the EEG were conducted using2-sec epochs (ie 100 msec prestimulus and 1900 msecpoststimulus) The averaged ERPs were also digitallyfiltered off-line with a 60-Hz low-pass filter
Testing
Ten known 10 phonetically similar nonsense words(referred to as phonemic contrasts) and 10 phoneticallydissimilar nonsense (referred to as nonsense words)words were each presented six times in random orderfor a total of 60 trials per condition During testingchildren sat on their parentrsquos lap and listened to words
presented from a speaker located behind a movingpuppet in a puppet theater Words were presented ata variable rate between 1800 and 3000 msec SOA
Artifact Rejection
Artifact rejection was conducted off-line using a com-puter program to reject blinks and horizontal eye move-ment and amplifier blocking Individual thresholds wereset for each child based on visual inspection of the EEGepochs time-locked to each stimulus A mean of 50 ofthe trials were rejected due to eye and movementartifact The number of artifact-free trials per word-typeretained for analysis ranged from 12 to 53 (SD = 113)out of a possible 60 trials per condition There were nosignificant differences in the percentage of trials rejectedfor the different experimental conditions age groupsor sex
Measurement of ERP Components
Peak latencies and amplitudes were quantified by com-puter with reference to the 100-msec prestimulus base-line for the maximum negative or positive point in aspecified time window The time windows for eachcomponent were set according to the criteria used inour previous studies the first positive componentcalled the P100 was defined as the most positivedeflection between 50 and 175 msec The P100 indexesauditory sensory processing Moreover the lateral dis-tribution of the P100 has been shown to vary withpercentile ranking on the MacArthur CDI score Chil-dren who score at the 50th percentile or higher for theirage show a P100 left greater than right asymmetry Themean amplitude within the time window 200ndash400 afterword onset was quantified by computer with referenceto the 100-msec prestimulus baseline This time windowwas chosen because it had been shown to differ forknown and unknown words in our previous studies
Acknowledgments
This research was conducted while the first author was at theProject in Cognitive and Neural Development at the Universityof California at San Diego The research was supported by NIHgrant DC0048111 to H Neville from NIDCD NSERC (NaturalSciences and Engineering Research Council of Canada) grantto J Werker and HFSP (Human Frontierrsquos Science Program)grant to J Werker and H Neville The third author RenateZangl was funded in part by two postdoctoral grants (J-1646J-1817) from the Austrian Science Foundation
We thank Amy Adamson Greg Appelbaum Teresa Mitchelland Lisa Sanders (in alphabetical order not necessarily in orderof magnitude of contribution) for their help in acquiring andanalyzing the data and Chris Fennell and Liz Lewis for theircomments on an earlier version of the manuscript We areespecially indebted to the children and their parents whoparticipated in these studies and their parents without whomthe research would not be possible
1462 Journal of Cognitive Neuroscience Volume 16 Number 8
Reprint requests should be sent to Debra L Mills who is nowat the Department of Psychology 532 Kilgo Circle EmoryUniversity Atlanta GA 30322 or via e-mail dmills2emoryedu
Notes
1 Unlike the switch task the visual fixation task is an on-linetask with two simultaneously presented choices The twoindicators of success are shorter latency to look away from theincorrect object and longer looking times overall to the correctmatch The habituation phase in the lsquolsquoswitchrsquorsquo task leads to theprediction of a novelty preference in the test phase (ie longerlooking to the incorrect pairing)2 In previous studies Mills and colleagues typically findapproximately 10ndash15 of children show a reversed ERPresponse with a larger amplitude ERP response to unknownthan to known words In previous work Mills and colleagueshave treated those children as noise in the data and have keptthem in for the data analyses Ultimately we want tounderstand the nature of lexical representations in this groupof children showing an aberrant ERP response For the currentstudy however in order to compare the ERP response tophonemic contrasts to known words we felt it necessary toinclude in our sample only those children who showed themuch more typical pattern of response resulting in the ex-clusion of three children All effects reported here show thesame pattern with these children included but the variabilityfrom their inclusion did reduce the differences in somecomparisons3 We are aware of the controversies surrounding the use oflinked mastoids Linked mastoids were used here to increasethe number of active cites given the number of amplifiersavailable and to provide consistency with previous studies Apilot study using similar auditory stimuli was conducted toexamine possible distortions in the distribution of scalp activityresulting from forced linkage This was determined by re-cording from one mastoid using the other as a reference andlinking the mastoids off-line These pilot data were comparedwith the data recorded using linked mastoids and did not yieldsignificant differences
REFERENCES
Bailey T M amp Plunkett K (2002) Phonological specificity inearly words Cognitive Development 17 1265ndash1282
Bertoncini J amp Mehler J (1981) Syllables as units in infantspeech perception Infant Behavior and Development 4247ndash260
Brown C amp Matthews J (1997) The role of feature geometryin the development of phonemic contrasts In S J Hannahsamp M Young-Scholten (Eds) Focus on phonologicalacquisition (pp 67ndash112) Amsterdam Benjamins
Brown C M Hagoort P amp Kutas M (2000) Postlexicalintegration processes in language comprehension Evidencefrom brain-imaging research In M Gazzaniga (Ed) The newcognitive neurosciences (2nd ed pp 881ndash895)Cambridge MIT Press
Conboy B (2003) Patterns of language processing and growthin early EnglishndashSpanish bilingualism (Doctoral dissertationUniversity of California San Diego and San Diego StateUniversity 2000) Dissertation Abstracts InternationalB The Sciences and Engineering 63 (UMI No 5193)
Conboy B amp Mills D L (2000) Cerebral specialization forlanguage comprehension in bilingual toddlers CognitiveNeuroscience Society 2000 Annual Meeting AbstractsSupplement of the Journal of Cognitive Neuroscience 17
Connolly J F Service E DrsquoArcy R C N Kujala A Alho K(2001) Phonological aspects of word recognition as revealedby high-resolution spatio-temporal brain mappingNeuroReport 12 237ndash243
Dehaene-Lambertz G (2000) Cerebral specialization forspeech and non-speech stimuli in infants Journal ofCognitive Neuroscience 12 449ndash460
Dehaene-Lambertz G amp Dehaene S (1994) Speed andcerebral correlates of syllable discrimination in infantsNature 370 292ndash294
Eimas P D Siqueland E R Jusczyk P amp Vigorito J (1971)Speech perception in infants Science 171 303ndash306
Fennell C E amp Werker J F (2003) Early word learnersrsquoability to access phonetic detail in well-known wordsLanguage and Speech 46 245ndash264
Fenson L Dale P S Reznick J S Bates E Thal D Jamp Pethick S J (1994) Variability in early communicativedevelopment Monographs of the Society for Research inChild Development 59 1ndash173
Fernald A Swingley D amp Pinto J P (2001) When half aword is enough Infants can recognize spoken words usingpartial phonetic information Child Development 721003ndash1015
Fischler I S (1990) Comprehending language withevent-related potentials In J W Rohrbaugh R Parasuramanamp R Johnson (Eds) Event-related brain potentials(pp 165ndash170) New York Oxford University Press
Halle P amp de Boysson-Bardies B (1994) Emergence of anearly receptive lexicon Infantsrsquo recognition of wordsInfant Behavior and Development 17 119ndash129
Halle P amp de Boysson-Bardies B (1996) The format ofrepresentation of recognized words in infantsrsquo earlyreceptive lexicon Infant Behavior and Development 19463ndash481
Holcomb P J amp Neville H J (1991) The electrophysiologyof spoken sentence processing Psychobiology 19286ndash300
Holcomb P J Coffey S A amp Neville H J (1992) Visual andauditory sentence processing A developmental analysisusing event-related brain potentials DevelopmentalNeuropsychology 8 203ndash241
Jusczyk P W (1997) The discovery of spoken languageCambridge MIT Press
Jusczyk P W amp Aslin R N (1995) Infantsrsquo detection ofsound patterns of words in fluent speech CognitivePsychology 29 1ndash23
Kutas M amp Hillyard S (1980) Reading senseless sentencesBrain potentials ref lect semantic incongruity Science 207203ndash205
Luck S amp Hillyard S (2000) The operation of selectiveattention at multiple stages of processing Evidence fromhuman and monkey electrophysiology In M Gazzaniga(Ed) The new cognitive neurosciences (2nd edpp 687ndash700) Cambridge MIT Press
Mills D L Coffey-Corina S A amp Neville H J (1997)Language comprehension and cerebral specialization from13ndash20 months [(Special issue on origins of languagedisorders)] In D Thal amp J Reilly (Eds) DevelopmentalNeuropsychology 13 397ndash446
Mills D M Coffey-Corina S A amp Neville H J (1994)Variability in cerebral organization during primary languageacquisition In G Dawson amp K Fischer (Eds) Humanbehavior and the developing brain (pp 427ndash455) NewYork Guilford Publications
Mills D M Coffey-Corina S A amp Neville H J (1993)Language acquisition and cerebral specialization in20-month-old infants Journal of Cognitive Neuroscience 5326ndash342
Mills et al 1463
Mills D L Conboy B amp Paton C (in press) Do changes inbrain organization reflect shifts in symbolic functioning InL Namy (Ed) Symbol use and symbolic representationMahwah NJ Erlbaum
Mills D L Plunkett K Prat C amp Schafer G (in press)Watching the infant brain learn words Effects ofvocabulary size and experience Cognitive Development
Molfese D L (1989) Electrophysiological correlates of wordmeanings in 14-month-old human infants DevelopmentalNeuropsychology 5 79ndash103
Molfese D L (1990) Auditory evoked responses recordedfrom 16-month-old human infants to words they did anddid not know Brain and Language 38 345ndash363
Molfese D L Burger-Judisch L M amp Hans L L (1991)Consonant discrimination by newborn infantsElectrophysiological differences DevelopmentalNeuropsychology 7 177ndash195
Molfese D L amp Molfese V J (1988) Right hemisphereresponses from preschool children to temporal cuescontained in speech and nonspeech materialsElectrophysiological correlates Brain and Language33 245ndash259
Molfese D L Morse P A amp Peters C J (1990) Auditoryevoked responses to names for different objectsCross-modal processing as a basis for infant languageacquisition Developmental Psychology 26 780ndash795
Molfese D L Wetzel W F amp Gill L A (1993) Known versusunknown word discriminations in 12-month-old humaninfants Electrophysiological correlates DevelopmentalNeuropsychology 9 241ndash258
Naigles L R (2002) Form is easy meaning is hard Resolvinga paradox in early child language Cognition 8 157ndash199
Nazzi T amp Bertoncini J (2003) Before and after thevocabulary spurt Two modes of word acquisitionDevelopmental Science 6 136ndash142
Osterhout L amp Holcomb P J (1995) Event-related potentialsand language comprehension In M D Rugg amp M G HColes (Eds) Electrophyisology of mind (pp 171ndash215)Oxford Oxford University Press
Richards J E amp Hunter S K (2001) Testing neural models ofthe development of infant attention DevelopmentalPsychobiology 40 226ndash236
Sereno S C Brewer C C amp OrsquoDonnell P J (2003)Context effects in word recognition Evidence for earlyinteractive processing Psychological Science 14328ndash333
Simos P G amp Molfese D L (1997) Electrophysiologicalresponses from a temporal order continuum in the newborninfant Neuropsychologia 35 89ndash98
Simos P G Molfese D L amp Brenden R A (1997) Behavioraland electrophysiological indices of voicing-cuediscrimination Laterality patterns and developmentBrain and Language 57 122ndash150
St George M amp Mills D L (2001) Electrophysiologicalstudies of language development In J Weissenborn ampB Hoehle (Eds) Language acquisition and languagedisorders (pp 247ndash259) Amsterdam John BenjaminsPublishing
Stager C L amp Werker J F (1997) Infants listen for morephonetic detail in speech perception than in word-learningtasks Nature 388 381ndash382
Swingley D (2003) Phonetic detail in the developing lexiconLanguage and Speech 46 265ndash294
Swingley D amp Aslin R N (2000) Spoken word recognitionand lexical representation in very young children Cognition76 147ndash166
Swingley D amp Aslin R N (2002) Lexical neighborhoodsand the wordndashform representations of 14-month-oldsPsychological Science 13 480ndash484
Werker J F Cohen L B Lloyd V L Casasola M amp StagerC L (1998) Acquisition of wordndashobject associations by14-month-old infants Developmental Psychology 341289ndash1309
Werker J F amp Fennell C E (2004) From listening to soundsto listening to words Early steps in word learning In G Hallamp S Waxman (Eds) Weaving a lexicon (pp 79ndash109)Cambridge MA MIT Press
Werker J F Fennell C E T Corcoran K M amp Stager C L(2002) Infantsrsquo ability to learn phonetically similar wordsEffects of age and vocabulary size Infancy 3 1ndash30
Werker J F amp Tees R C (1984) Cross-language speechperception Evidence for perceptual reorganization duringthe first year of life Infant Behavior and Development 749ndash63
1464 Journal of Cognitive Neuroscience Volume 16 Number 8
pated in our studies Children from the Vancouver areawere recruited through the University of British Colum-bia child Studies Center database Parents of all childrensigned consent forms consistent with the human sub-jects internal review boards at the university at whichthey were tested Parents were given $500 to defraytransportation costs and children were given a small toyin appreciation of their participation Only full-term(gt36 weeks of gestation) healthy children with mono-lingual experience with English participated in the studychildren with a family history of language impairmentwere excluded from the study
Data from 16 children aged 14 months (9 girls meanage = 14 months 15 days SD = 97 days range = 14months 1 day to 15 months 1 day) and 17 children aged20 to 21 months (9 girls mean age = 20 months 11 daysSD = 131 days age range = 19 months 14 days to 20months 27 days) were retained for analysis in the studyAn additional 18 children (12 boys nine 14-month-olds)were tested but were excluded from the analyses due totoo few artifact-free trials (n = 10) excessive crying (n =3) reversed polarity of the ERP response (n = 3 seenote)2 or refusal to wear the electrocap (n = 2)
Stimuli
The stimuli were naturally spoken in a female voice anddigitized at 16 bits 44 kHz sampling rate The stimuliconsisted of three types of words (a) 10 words whosemeanings were understood by the child (known wordseg lsquolsquomilkrsquorsquo) (b) 10 nonsense words that differed fromthe known words in the initial phonemic contrast (pho-nemic contrast words eg lsquolsquonilkrsquorsquo) and (c) 10 wordsthat differed phonetically from known words in allvowels and consonants (nonsense words eg lsquolsquoneemrsquorsquo)All stimuli were matched on word duration and numberof syllables The mean durations for the known wordsphonetic contrasts and nonsense words were 783 msecSD = 144 865 msec SD = 169 861 msec SD = 96[F(227) = 109 p = 34] Ideally all word-types wouldbe matched on manner of articulation (eg all stopconsonants all nasals all laterals all fricatives or allbalanced) However children at this age have very smallvocabularies and we were limited to common words thatalmost all children at this age would know Amongchildren first words are a disproportionate number ofwords that begin with stop consonants For many ofthose words minimal pair differences are still words(eg lsquolsquoballrsquorsquondashlsquolsquodollrsquorsquo) Thus we were constrained in ourselection of stimulus items For the known words therewere seven stop consonants two nasals and one frica-tive For the phonemic contrasts there are also sevenstop consonants two nasals and one fricative For thenonsense words there are three stop consonants oneaffricative two nasals two fricatives and two laterals Acomplete list of words is shown in Table 1
To evaluate at what point the known words couldphysically be distinguishable from the other word-typeswe examined two aspects of uniqueness (a) at whatpoint the phonetically dissimilar nonsense words be-come unique from their real word counterparts (egwhen does lsquolsquobearrsquorsquo become unique from lsquolsquogarersquorsquo) and (b)when the words might become unique from otherwords that would be in a babyrsquos vocabulary at this ageThe uniqueness point for initial consonants (the pointthat separates that consonant from other consonants)was within 20 msec of the beginning of the word forfricatives (s z f) 40 msec for stop consonants (b d gp t k) 60 msec for affricatives (j) and 20 msec for nasals(m n) and laterals (l r) To further examine theuniqueness points we compared each stimulus usedin the study relative to other words that might be in thechildrsquos vocabulary with the words in the MacArthur CDIthat would be considered close phonological neighborsTo this end we compared our stimuli to the number ofwords in the CDI with (a) the same consonant includingconsonant clusters and (b) the same consonant exclud-ing consonant clusters and (c) the same consonant plusthe vowel For comparisons a and b there are the mostneighbors for the known words (a = 474 b = 375) anintermediate amount for the phonemic contrast words(a = 371 b = 298) and the least for the nonsense words(a = 275 b = 233) The best comparison is to count thenumber of items that begin with the same consonantplus vowel (CV) There were no significant differences inthe number of CV neighbors for the three word-types(known = 16 phonemic contrasts = 12 nonsense =18) Additionally because of co-articulatory effects theacoustic energy for a given consonant varies as a func-tion of the following vowel (Jusczyk 1997) Thus evenwords beginning with the same consonant (eg lsquolsquobookrsquorsquoand lsquolsquobottlersquorsquo) differ in shape and spectral frequenciesand can be discriminated from each other as well as
Table 1 Stimuli
Known Words Phonemic Contrasts Nonsense
Bear Gare Kobe
Ball Pall Lif
Book Dook Neem
Bottle Pottle Fipe
Cup Tup Mon
Cat Gat Tek
Dog Bog Riss
Milk Nilk Keed
Nose Mose Jud
Shoe Zhu Zav
Mills et al 1461
their close phonological neighbors from the beginningof the word
Procedure
Language Assessment
Within one week prior to testing parents were asked tocomplete the MacArthur CDI (Fenson et al 1994)which provided an estimate of the childrsquos vocabularysize and percentile ranking relative to other children ofthe same age To ensure that the words to be used in thestudy as lsquolsquoknownrsquorsquo were comprehended by the childparents also completed a vocabulary checklist ratingscale indicating how sure they were that their childunderstood andor produced each word on a scale of1 (very sure they did not know that word) to 4 (very suretheir child understoodproduced a given word in avariety of different contexts and with different exem-plars) Additionally the children were asked to identify apicture of each word to be used as a known word from atwo-choice picture book All lsquolsquoknownrsquorsquo words used hadreceived a rating of 4 and were correctly identified in thepicture-pointing task
Electrophysiological Recording
The EEG was recorded using tin electrodes affixed to anelastic cap (Electro-Cap International) from sites overfrontal (F7 and F8) anterior temporal (50 of thedistance from F78 and T34) temporal (33 of thedistance from T34 to C34) parietal (50 of the dis-tance between T34 and P34) and occipital (O1 andO2) regions of the left and right hemispheres Addition-ally the electrooculogram was recorded from electrodesplaced over and under the eye to reject trials on whichblinks and vertical eye movement occurred and fromleft and right frontal electrodes to reject trials on whichhorizontal eye movement occurred Impedances werekept below 5 k and were balanced (within 1 k) acrossthe left and right hemispheres at any given position TheEEG was amplified by SA Instruments amplifiers with abandpass of 01 to 100 Hz and sampled continuouslyevery 4 msec All electrodes were referenced to linkedmastoids3 Averages of the EEG were conducted using2-sec epochs (ie 100 msec prestimulus and 1900 msecpoststimulus) The averaged ERPs were also digitallyfiltered off-line with a 60-Hz low-pass filter
Testing
Ten known 10 phonetically similar nonsense words(referred to as phonemic contrasts) and 10 phoneticallydissimilar nonsense (referred to as nonsense words)words were each presented six times in random orderfor a total of 60 trials per condition During testingchildren sat on their parentrsquos lap and listened to words
presented from a speaker located behind a movingpuppet in a puppet theater Words were presented ata variable rate between 1800 and 3000 msec SOA
Artifact Rejection
Artifact rejection was conducted off-line using a com-puter program to reject blinks and horizontal eye move-ment and amplifier blocking Individual thresholds wereset for each child based on visual inspection of the EEGepochs time-locked to each stimulus A mean of 50 ofthe trials were rejected due to eye and movementartifact The number of artifact-free trials per word-typeretained for analysis ranged from 12 to 53 (SD = 113)out of a possible 60 trials per condition There were nosignificant differences in the percentage of trials rejectedfor the different experimental conditions age groupsor sex
Measurement of ERP Components
Peak latencies and amplitudes were quantified by com-puter with reference to the 100-msec prestimulus base-line for the maximum negative or positive point in aspecified time window The time windows for eachcomponent were set according to the criteria used inour previous studies the first positive componentcalled the P100 was defined as the most positivedeflection between 50 and 175 msec The P100 indexesauditory sensory processing Moreover the lateral dis-tribution of the P100 has been shown to vary withpercentile ranking on the MacArthur CDI score Chil-dren who score at the 50th percentile or higher for theirage show a P100 left greater than right asymmetry Themean amplitude within the time window 200ndash400 afterword onset was quantified by computer with referenceto the 100-msec prestimulus baseline This time windowwas chosen because it had been shown to differ forknown and unknown words in our previous studies
Acknowledgments
This research was conducted while the first author was at theProject in Cognitive and Neural Development at the Universityof California at San Diego The research was supported by NIHgrant DC0048111 to H Neville from NIDCD NSERC (NaturalSciences and Engineering Research Council of Canada) grantto J Werker and HFSP (Human Frontierrsquos Science Program)grant to J Werker and H Neville The third author RenateZangl was funded in part by two postdoctoral grants (J-1646J-1817) from the Austrian Science Foundation
We thank Amy Adamson Greg Appelbaum Teresa Mitchelland Lisa Sanders (in alphabetical order not necessarily in orderof magnitude of contribution) for their help in acquiring andanalyzing the data and Chris Fennell and Liz Lewis for theircomments on an earlier version of the manuscript We areespecially indebted to the children and their parents whoparticipated in these studies and their parents without whomthe research would not be possible
1462 Journal of Cognitive Neuroscience Volume 16 Number 8
Reprint requests should be sent to Debra L Mills who is nowat the Department of Psychology 532 Kilgo Circle EmoryUniversity Atlanta GA 30322 or via e-mail dmills2emoryedu
Notes
1 Unlike the switch task the visual fixation task is an on-linetask with two simultaneously presented choices The twoindicators of success are shorter latency to look away from theincorrect object and longer looking times overall to the correctmatch The habituation phase in the lsquolsquoswitchrsquorsquo task leads to theprediction of a novelty preference in the test phase (ie longerlooking to the incorrect pairing)2 In previous studies Mills and colleagues typically findapproximately 10ndash15 of children show a reversed ERPresponse with a larger amplitude ERP response to unknownthan to known words In previous work Mills and colleagueshave treated those children as noise in the data and have keptthem in for the data analyses Ultimately we want tounderstand the nature of lexical representations in this groupof children showing an aberrant ERP response For the currentstudy however in order to compare the ERP response tophonemic contrasts to known words we felt it necessary toinclude in our sample only those children who showed themuch more typical pattern of response resulting in the ex-clusion of three children All effects reported here show thesame pattern with these children included but the variabilityfrom their inclusion did reduce the differences in somecomparisons3 We are aware of the controversies surrounding the use oflinked mastoids Linked mastoids were used here to increasethe number of active cites given the number of amplifiersavailable and to provide consistency with previous studies Apilot study using similar auditory stimuli was conducted toexamine possible distortions in the distribution of scalp activityresulting from forced linkage This was determined by re-cording from one mastoid using the other as a reference andlinking the mastoids off-line These pilot data were comparedwith the data recorded using linked mastoids and did not yieldsignificant differences
REFERENCES
Bailey T M amp Plunkett K (2002) Phonological specificity inearly words Cognitive Development 17 1265ndash1282
Bertoncini J amp Mehler J (1981) Syllables as units in infantspeech perception Infant Behavior and Development 4247ndash260
Brown C amp Matthews J (1997) The role of feature geometryin the development of phonemic contrasts In S J Hannahsamp M Young-Scholten (Eds) Focus on phonologicalacquisition (pp 67ndash112) Amsterdam Benjamins
Brown C M Hagoort P amp Kutas M (2000) Postlexicalintegration processes in language comprehension Evidencefrom brain-imaging research In M Gazzaniga (Ed) The newcognitive neurosciences (2nd ed pp 881ndash895)Cambridge MIT Press
Conboy B (2003) Patterns of language processing and growthin early EnglishndashSpanish bilingualism (Doctoral dissertationUniversity of California San Diego and San Diego StateUniversity 2000) Dissertation Abstracts InternationalB The Sciences and Engineering 63 (UMI No 5193)
Conboy B amp Mills D L (2000) Cerebral specialization forlanguage comprehension in bilingual toddlers CognitiveNeuroscience Society 2000 Annual Meeting AbstractsSupplement of the Journal of Cognitive Neuroscience 17
Connolly J F Service E DrsquoArcy R C N Kujala A Alho K(2001) Phonological aspects of word recognition as revealedby high-resolution spatio-temporal brain mappingNeuroReport 12 237ndash243
Dehaene-Lambertz G (2000) Cerebral specialization forspeech and non-speech stimuli in infants Journal ofCognitive Neuroscience 12 449ndash460
Dehaene-Lambertz G amp Dehaene S (1994) Speed andcerebral correlates of syllable discrimination in infantsNature 370 292ndash294
Eimas P D Siqueland E R Jusczyk P amp Vigorito J (1971)Speech perception in infants Science 171 303ndash306
Fennell C E amp Werker J F (2003) Early word learnersrsquoability to access phonetic detail in well-known wordsLanguage and Speech 46 245ndash264
Fenson L Dale P S Reznick J S Bates E Thal D Jamp Pethick S J (1994) Variability in early communicativedevelopment Monographs of the Society for Research inChild Development 59 1ndash173
Fernald A Swingley D amp Pinto J P (2001) When half aword is enough Infants can recognize spoken words usingpartial phonetic information Child Development 721003ndash1015
Fischler I S (1990) Comprehending language withevent-related potentials In J W Rohrbaugh R Parasuramanamp R Johnson (Eds) Event-related brain potentials(pp 165ndash170) New York Oxford University Press
Halle P amp de Boysson-Bardies B (1994) Emergence of anearly receptive lexicon Infantsrsquo recognition of wordsInfant Behavior and Development 17 119ndash129
Halle P amp de Boysson-Bardies B (1996) The format ofrepresentation of recognized words in infantsrsquo earlyreceptive lexicon Infant Behavior and Development 19463ndash481
Holcomb P J amp Neville H J (1991) The electrophysiologyof spoken sentence processing Psychobiology 19286ndash300
Holcomb P J Coffey S A amp Neville H J (1992) Visual andauditory sentence processing A developmental analysisusing event-related brain potentials DevelopmentalNeuropsychology 8 203ndash241
Jusczyk P W (1997) The discovery of spoken languageCambridge MIT Press
Jusczyk P W amp Aslin R N (1995) Infantsrsquo detection ofsound patterns of words in fluent speech CognitivePsychology 29 1ndash23
Kutas M amp Hillyard S (1980) Reading senseless sentencesBrain potentials ref lect semantic incongruity Science 207203ndash205
Luck S amp Hillyard S (2000) The operation of selectiveattention at multiple stages of processing Evidence fromhuman and monkey electrophysiology In M Gazzaniga(Ed) The new cognitive neurosciences (2nd edpp 687ndash700) Cambridge MIT Press
Mills D L Coffey-Corina S A amp Neville H J (1997)Language comprehension and cerebral specialization from13ndash20 months [(Special issue on origins of languagedisorders)] In D Thal amp J Reilly (Eds) DevelopmentalNeuropsychology 13 397ndash446
Mills D M Coffey-Corina S A amp Neville H J (1994)Variability in cerebral organization during primary languageacquisition In G Dawson amp K Fischer (Eds) Humanbehavior and the developing brain (pp 427ndash455) NewYork Guilford Publications
Mills D M Coffey-Corina S A amp Neville H J (1993)Language acquisition and cerebral specialization in20-month-old infants Journal of Cognitive Neuroscience 5326ndash342
Mills et al 1463
Mills D L Conboy B amp Paton C (in press) Do changes inbrain organization reflect shifts in symbolic functioning InL Namy (Ed) Symbol use and symbolic representationMahwah NJ Erlbaum
Mills D L Plunkett K Prat C amp Schafer G (in press)Watching the infant brain learn words Effects ofvocabulary size and experience Cognitive Development
Molfese D L (1989) Electrophysiological correlates of wordmeanings in 14-month-old human infants DevelopmentalNeuropsychology 5 79ndash103
Molfese D L (1990) Auditory evoked responses recordedfrom 16-month-old human infants to words they did anddid not know Brain and Language 38 345ndash363
Molfese D L Burger-Judisch L M amp Hans L L (1991)Consonant discrimination by newborn infantsElectrophysiological differences DevelopmentalNeuropsychology 7 177ndash195
Molfese D L amp Molfese V J (1988) Right hemisphereresponses from preschool children to temporal cuescontained in speech and nonspeech materialsElectrophysiological correlates Brain and Language33 245ndash259
Molfese D L Morse P A amp Peters C J (1990) Auditoryevoked responses to names for different objectsCross-modal processing as a basis for infant languageacquisition Developmental Psychology 26 780ndash795
Molfese D L Wetzel W F amp Gill L A (1993) Known versusunknown word discriminations in 12-month-old humaninfants Electrophysiological correlates DevelopmentalNeuropsychology 9 241ndash258
Naigles L R (2002) Form is easy meaning is hard Resolvinga paradox in early child language Cognition 8 157ndash199
Nazzi T amp Bertoncini J (2003) Before and after thevocabulary spurt Two modes of word acquisitionDevelopmental Science 6 136ndash142
Osterhout L amp Holcomb P J (1995) Event-related potentialsand language comprehension In M D Rugg amp M G HColes (Eds) Electrophyisology of mind (pp 171ndash215)Oxford Oxford University Press
Richards J E amp Hunter S K (2001) Testing neural models ofthe development of infant attention DevelopmentalPsychobiology 40 226ndash236
Sereno S C Brewer C C amp OrsquoDonnell P J (2003)Context effects in word recognition Evidence for earlyinteractive processing Psychological Science 14328ndash333
Simos P G amp Molfese D L (1997) Electrophysiologicalresponses from a temporal order continuum in the newborninfant Neuropsychologia 35 89ndash98
Simos P G Molfese D L amp Brenden R A (1997) Behavioraland electrophysiological indices of voicing-cuediscrimination Laterality patterns and developmentBrain and Language 57 122ndash150
St George M amp Mills D L (2001) Electrophysiologicalstudies of language development In J Weissenborn ampB Hoehle (Eds) Language acquisition and languagedisorders (pp 247ndash259) Amsterdam John BenjaminsPublishing
Stager C L amp Werker J F (1997) Infants listen for morephonetic detail in speech perception than in word-learningtasks Nature 388 381ndash382
Swingley D (2003) Phonetic detail in the developing lexiconLanguage and Speech 46 265ndash294
Swingley D amp Aslin R N (2000) Spoken word recognitionand lexical representation in very young children Cognition76 147ndash166
Swingley D amp Aslin R N (2002) Lexical neighborhoodsand the wordndashform representations of 14-month-oldsPsychological Science 13 480ndash484
Werker J F Cohen L B Lloyd V L Casasola M amp StagerC L (1998) Acquisition of wordndashobject associations by14-month-old infants Developmental Psychology 341289ndash1309
Werker J F amp Fennell C E (2004) From listening to soundsto listening to words Early steps in word learning In G Hallamp S Waxman (Eds) Weaving a lexicon (pp 79ndash109)Cambridge MA MIT Press
Werker J F Fennell C E T Corcoran K M amp Stager C L(2002) Infantsrsquo ability to learn phonetically similar wordsEffects of age and vocabulary size Infancy 3 1ndash30
Werker J F amp Tees R C (1984) Cross-language speechperception Evidence for perceptual reorganization duringthe first year of life Infant Behavior and Development 749ndash63
1464 Journal of Cognitive Neuroscience Volume 16 Number 8
their close phonological neighbors from the beginningof the word
Procedure
Language Assessment
Within one week prior to testing parents were asked tocomplete the MacArthur CDI (Fenson et al 1994)which provided an estimate of the childrsquos vocabularysize and percentile ranking relative to other children ofthe same age To ensure that the words to be used in thestudy as lsquolsquoknownrsquorsquo were comprehended by the childparents also completed a vocabulary checklist ratingscale indicating how sure they were that their childunderstood andor produced each word on a scale of1 (very sure they did not know that word) to 4 (very suretheir child understoodproduced a given word in avariety of different contexts and with different exem-plars) Additionally the children were asked to identify apicture of each word to be used as a known word from atwo-choice picture book All lsquolsquoknownrsquorsquo words used hadreceived a rating of 4 and were correctly identified in thepicture-pointing task
Electrophysiological Recording
The EEG was recorded using tin electrodes affixed to anelastic cap (Electro-Cap International) from sites overfrontal (F7 and F8) anterior temporal (50 of thedistance from F78 and T34) temporal (33 of thedistance from T34 to C34) parietal (50 of the dis-tance between T34 and P34) and occipital (O1 andO2) regions of the left and right hemispheres Addition-ally the electrooculogram was recorded from electrodesplaced over and under the eye to reject trials on whichblinks and vertical eye movement occurred and fromleft and right frontal electrodes to reject trials on whichhorizontal eye movement occurred Impedances werekept below 5 k and were balanced (within 1 k) acrossthe left and right hemispheres at any given position TheEEG was amplified by SA Instruments amplifiers with abandpass of 01 to 100 Hz and sampled continuouslyevery 4 msec All electrodes were referenced to linkedmastoids3 Averages of the EEG were conducted using2-sec epochs (ie 100 msec prestimulus and 1900 msecpoststimulus) The averaged ERPs were also digitallyfiltered off-line with a 60-Hz low-pass filter
Testing
Ten known 10 phonetically similar nonsense words(referred to as phonemic contrasts) and 10 phoneticallydissimilar nonsense (referred to as nonsense words)words were each presented six times in random orderfor a total of 60 trials per condition During testingchildren sat on their parentrsquos lap and listened to words
presented from a speaker located behind a movingpuppet in a puppet theater Words were presented ata variable rate between 1800 and 3000 msec SOA
Artifact Rejection
Artifact rejection was conducted off-line using a com-puter program to reject blinks and horizontal eye move-ment and amplifier blocking Individual thresholds wereset for each child based on visual inspection of the EEGepochs time-locked to each stimulus A mean of 50 ofthe trials were rejected due to eye and movementartifact The number of artifact-free trials per word-typeretained for analysis ranged from 12 to 53 (SD = 113)out of a possible 60 trials per condition There were nosignificant differences in the percentage of trials rejectedfor the different experimental conditions age groupsor sex
Measurement of ERP Components
Peak latencies and amplitudes were quantified by com-puter with reference to the 100-msec prestimulus base-line for the maximum negative or positive point in aspecified time window The time windows for eachcomponent were set according to the criteria used inour previous studies the first positive componentcalled the P100 was defined as the most positivedeflection between 50 and 175 msec The P100 indexesauditory sensory processing Moreover the lateral dis-tribution of the P100 has been shown to vary withpercentile ranking on the MacArthur CDI score Chil-dren who score at the 50th percentile or higher for theirage show a P100 left greater than right asymmetry Themean amplitude within the time window 200ndash400 afterword onset was quantified by computer with referenceto the 100-msec prestimulus baseline This time windowwas chosen because it had been shown to differ forknown and unknown words in our previous studies
Acknowledgments
This research was conducted while the first author was at theProject in Cognitive and Neural Development at the Universityof California at San Diego The research was supported by NIHgrant DC0048111 to H Neville from NIDCD NSERC (NaturalSciences and Engineering Research Council of Canada) grantto J Werker and HFSP (Human Frontierrsquos Science Program)grant to J Werker and H Neville The third author RenateZangl was funded in part by two postdoctoral grants (J-1646J-1817) from the Austrian Science Foundation
We thank Amy Adamson Greg Appelbaum Teresa Mitchelland Lisa Sanders (in alphabetical order not necessarily in orderof magnitude of contribution) for their help in acquiring andanalyzing the data and Chris Fennell and Liz Lewis for theircomments on an earlier version of the manuscript We areespecially indebted to the children and their parents whoparticipated in these studies and their parents without whomthe research would not be possible
1462 Journal of Cognitive Neuroscience Volume 16 Number 8
Reprint requests should be sent to Debra L Mills who is nowat the Department of Psychology 532 Kilgo Circle EmoryUniversity Atlanta GA 30322 or via e-mail dmills2emoryedu
Notes
1 Unlike the switch task the visual fixation task is an on-linetask with two simultaneously presented choices The twoindicators of success are shorter latency to look away from theincorrect object and longer looking times overall to the correctmatch The habituation phase in the lsquolsquoswitchrsquorsquo task leads to theprediction of a novelty preference in the test phase (ie longerlooking to the incorrect pairing)2 In previous studies Mills and colleagues typically findapproximately 10ndash15 of children show a reversed ERPresponse with a larger amplitude ERP response to unknownthan to known words In previous work Mills and colleagueshave treated those children as noise in the data and have keptthem in for the data analyses Ultimately we want tounderstand the nature of lexical representations in this groupof children showing an aberrant ERP response For the currentstudy however in order to compare the ERP response tophonemic contrasts to known words we felt it necessary toinclude in our sample only those children who showed themuch more typical pattern of response resulting in the ex-clusion of three children All effects reported here show thesame pattern with these children included but the variabilityfrom their inclusion did reduce the differences in somecomparisons3 We are aware of the controversies surrounding the use oflinked mastoids Linked mastoids were used here to increasethe number of active cites given the number of amplifiersavailable and to provide consistency with previous studies Apilot study using similar auditory stimuli was conducted toexamine possible distortions in the distribution of scalp activityresulting from forced linkage This was determined by re-cording from one mastoid using the other as a reference andlinking the mastoids off-line These pilot data were comparedwith the data recorded using linked mastoids and did not yieldsignificant differences
REFERENCES
Bailey T M amp Plunkett K (2002) Phonological specificity inearly words Cognitive Development 17 1265ndash1282
Bertoncini J amp Mehler J (1981) Syllables as units in infantspeech perception Infant Behavior and Development 4247ndash260
Brown C amp Matthews J (1997) The role of feature geometryin the development of phonemic contrasts In S J Hannahsamp M Young-Scholten (Eds) Focus on phonologicalacquisition (pp 67ndash112) Amsterdam Benjamins
Brown C M Hagoort P amp Kutas M (2000) Postlexicalintegration processes in language comprehension Evidencefrom brain-imaging research In M Gazzaniga (Ed) The newcognitive neurosciences (2nd ed pp 881ndash895)Cambridge MIT Press
Conboy B (2003) Patterns of language processing and growthin early EnglishndashSpanish bilingualism (Doctoral dissertationUniversity of California San Diego and San Diego StateUniversity 2000) Dissertation Abstracts InternationalB The Sciences and Engineering 63 (UMI No 5193)
Conboy B amp Mills D L (2000) Cerebral specialization forlanguage comprehension in bilingual toddlers CognitiveNeuroscience Society 2000 Annual Meeting AbstractsSupplement of the Journal of Cognitive Neuroscience 17
Connolly J F Service E DrsquoArcy R C N Kujala A Alho K(2001) Phonological aspects of word recognition as revealedby high-resolution spatio-temporal brain mappingNeuroReport 12 237ndash243
Dehaene-Lambertz G (2000) Cerebral specialization forspeech and non-speech stimuli in infants Journal ofCognitive Neuroscience 12 449ndash460
Dehaene-Lambertz G amp Dehaene S (1994) Speed andcerebral correlates of syllable discrimination in infantsNature 370 292ndash294
Eimas P D Siqueland E R Jusczyk P amp Vigorito J (1971)Speech perception in infants Science 171 303ndash306
Fennell C E amp Werker J F (2003) Early word learnersrsquoability to access phonetic detail in well-known wordsLanguage and Speech 46 245ndash264
Fenson L Dale P S Reznick J S Bates E Thal D Jamp Pethick S J (1994) Variability in early communicativedevelopment Monographs of the Society for Research inChild Development 59 1ndash173
Fernald A Swingley D amp Pinto J P (2001) When half aword is enough Infants can recognize spoken words usingpartial phonetic information Child Development 721003ndash1015
Fischler I S (1990) Comprehending language withevent-related potentials In J W Rohrbaugh R Parasuramanamp R Johnson (Eds) Event-related brain potentials(pp 165ndash170) New York Oxford University Press
Halle P amp de Boysson-Bardies B (1994) Emergence of anearly receptive lexicon Infantsrsquo recognition of wordsInfant Behavior and Development 17 119ndash129
Halle P amp de Boysson-Bardies B (1996) The format ofrepresentation of recognized words in infantsrsquo earlyreceptive lexicon Infant Behavior and Development 19463ndash481
Holcomb P J amp Neville H J (1991) The electrophysiologyof spoken sentence processing Psychobiology 19286ndash300
Holcomb P J Coffey S A amp Neville H J (1992) Visual andauditory sentence processing A developmental analysisusing event-related brain potentials DevelopmentalNeuropsychology 8 203ndash241
Jusczyk P W (1997) The discovery of spoken languageCambridge MIT Press
Jusczyk P W amp Aslin R N (1995) Infantsrsquo detection ofsound patterns of words in fluent speech CognitivePsychology 29 1ndash23
Kutas M amp Hillyard S (1980) Reading senseless sentencesBrain potentials ref lect semantic incongruity Science 207203ndash205
Luck S amp Hillyard S (2000) The operation of selectiveattention at multiple stages of processing Evidence fromhuman and monkey electrophysiology In M Gazzaniga(Ed) The new cognitive neurosciences (2nd edpp 687ndash700) Cambridge MIT Press
Mills D L Coffey-Corina S A amp Neville H J (1997)Language comprehension and cerebral specialization from13ndash20 months [(Special issue on origins of languagedisorders)] In D Thal amp J Reilly (Eds) DevelopmentalNeuropsychology 13 397ndash446
Mills D M Coffey-Corina S A amp Neville H J (1994)Variability in cerebral organization during primary languageacquisition In G Dawson amp K Fischer (Eds) Humanbehavior and the developing brain (pp 427ndash455) NewYork Guilford Publications
Mills D M Coffey-Corina S A amp Neville H J (1993)Language acquisition and cerebral specialization in20-month-old infants Journal of Cognitive Neuroscience 5326ndash342
Mills et al 1463
Mills D L Conboy B amp Paton C (in press) Do changes inbrain organization reflect shifts in symbolic functioning InL Namy (Ed) Symbol use and symbolic representationMahwah NJ Erlbaum
Mills D L Plunkett K Prat C amp Schafer G (in press)Watching the infant brain learn words Effects ofvocabulary size and experience Cognitive Development
Molfese D L (1989) Electrophysiological correlates of wordmeanings in 14-month-old human infants DevelopmentalNeuropsychology 5 79ndash103
Molfese D L (1990) Auditory evoked responses recordedfrom 16-month-old human infants to words they did anddid not know Brain and Language 38 345ndash363
Molfese D L Burger-Judisch L M amp Hans L L (1991)Consonant discrimination by newborn infantsElectrophysiological differences DevelopmentalNeuropsychology 7 177ndash195
Molfese D L amp Molfese V J (1988) Right hemisphereresponses from preschool children to temporal cuescontained in speech and nonspeech materialsElectrophysiological correlates Brain and Language33 245ndash259
Molfese D L Morse P A amp Peters C J (1990) Auditoryevoked responses to names for different objectsCross-modal processing as a basis for infant languageacquisition Developmental Psychology 26 780ndash795
Molfese D L Wetzel W F amp Gill L A (1993) Known versusunknown word discriminations in 12-month-old humaninfants Electrophysiological correlates DevelopmentalNeuropsychology 9 241ndash258
Naigles L R (2002) Form is easy meaning is hard Resolvinga paradox in early child language Cognition 8 157ndash199
Nazzi T amp Bertoncini J (2003) Before and after thevocabulary spurt Two modes of word acquisitionDevelopmental Science 6 136ndash142
Osterhout L amp Holcomb P J (1995) Event-related potentialsand language comprehension In M D Rugg amp M G HColes (Eds) Electrophyisology of mind (pp 171ndash215)Oxford Oxford University Press
Richards J E amp Hunter S K (2001) Testing neural models ofthe development of infant attention DevelopmentalPsychobiology 40 226ndash236
Sereno S C Brewer C C amp OrsquoDonnell P J (2003)Context effects in word recognition Evidence for earlyinteractive processing Psychological Science 14328ndash333
Simos P G amp Molfese D L (1997) Electrophysiologicalresponses from a temporal order continuum in the newborninfant Neuropsychologia 35 89ndash98
Simos P G Molfese D L amp Brenden R A (1997) Behavioraland electrophysiological indices of voicing-cuediscrimination Laterality patterns and developmentBrain and Language 57 122ndash150
St George M amp Mills D L (2001) Electrophysiologicalstudies of language development In J Weissenborn ampB Hoehle (Eds) Language acquisition and languagedisorders (pp 247ndash259) Amsterdam John BenjaminsPublishing
Stager C L amp Werker J F (1997) Infants listen for morephonetic detail in speech perception than in word-learningtasks Nature 388 381ndash382
Swingley D (2003) Phonetic detail in the developing lexiconLanguage and Speech 46 265ndash294
Swingley D amp Aslin R N (2000) Spoken word recognitionand lexical representation in very young children Cognition76 147ndash166
Swingley D amp Aslin R N (2002) Lexical neighborhoodsand the wordndashform representations of 14-month-oldsPsychological Science 13 480ndash484
Werker J F Cohen L B Lloyd V L Casasola M amp StagerC L (1998) Acquisition of wordndashobject associations by14-month-old infants Developmental Psychology 341289ndash1309
Werker J F amp Fennell C E (2004) From listening to soundsto listening to words Early steps in word learning In G Hallamp S Waxman (Eds) Weaving a lexicon (pp 79ndash109)Cambridge MA MIT Press
Werker J F Fennell C E T Corcoran K M amp Stager C L(2002) Infantsrsquo ability to learn phonetically similar wordsEffects of age and vocabulary size Infancy 3 1ndash30
Werker J F amp Tees R C (1984) Cross-language speechperception Evidence for perceptual reorganization duringthe first year of life Infant Behavior and Development 749ndash63
1464 Journal of Cognitive Neuroscience Volume 16 Number 8
Reprint requests should be sent to Debra L Mills who is nowat the Department of Psychology 532 Kilgo Circle EmoryUniversity Atlanta GA 30322 or via e-mail dmills2emoryedu
Notes
1 Unlike the switch task the visual fixation task is an on-linetask with two simultaneously presented choices The twoindicators of success are shorter latency to look away from theincorrect object and longer looking times overall to the correctmatch The habituation phase in the lsquolsquoswitchrsquorsquo task leads to theprediction of a novelty preference in the test phase (ie longerlooking to the incorrect pairing)2 In previous studies Mills and colleagues typically findapproximately 10ndash15 of children show a reversed ERPresponse with a larger amplitude ERP response to unknownthan to known words In previous work Mills and colleagueshave treated those children as noise in the data and have keptthem in for the data analyses Ultimately we want tounderstand the nature of lexical representations in this groupof children showing an aberrant ERP response For the currentstudy however in order to compare the ERP response tophonemic contrasts to known words we felt it necessary toinclude in our sample only those children who showed themuch more typical pattern of response resulting in the ex-clusion of three children All effects reported here show thesame pattern with these children included but the variabilityfrom their inclusion did reduce the differences in somecomparisons3 We are aware of the controversies surrounding the use oflinked mastoids Linked mastoids were used here to increasethe number of active cites given the number of amplifiersavailable and to provide consistency with previous studies Apilot study using similar auditory stimuli was conducted toexamine possible distortions in the distribution of scalp activityresulting from forced linkage This was determined by re-cording from one mastoid using the other as a reference andlinking the mastoids off-line These pilot data were comparedwith the data recorded using linked mastoids and did not yieldsignificant differences
REFERENCES
Bailey T M amp Plunkett K (2002) Phonological specificity inearly words Cognitive Development 17 1265ndash1282
Bertoncini J amp Mehler J (1981) Syllables as units in infantspeech perception Infant Behavior and Development 4247ndash260
Brown C amp Matthews J (1997) The role of feature geometryin the development of phonemic contrasts In S J Hannahsamp M Young-Scholten (Eds) Focus on phonologicalacquisition (pp 67ndash112) Amsterdam Benjamins
Brown C M Hagoort P amp Kutas M (2000) Postlexicalintegration processes in language comprehension Evidencefrom brain-imaging research In M Gazzaniga (Ed) The newcognitive neurosciences (2nd ed pp 881ndash895)Cambridge MIT Press
Conboy B (2003) Patterns of language processing and growthin early EnglishndashSpanish bilingualism (Doctoral dissertationUniversity of California San Diego and San Diego StateUniversity 2000) Dissertation Abstracts InternationalB The Sciences and Engineering 63 (UMI No 5193)
Conboy B amp Mills D L (2000) Cerebral specialization forlanguage comprehension in bilingual toddlers CognitiveNeuroscience Society 2000 Annual Meeting AbstractsSupplement of the Journal of Cognitive Neuroscience 17
Connolly J F Service E DrsquoArcy R C N Kujala A Alho K(2001) Phonological aspects of word recognition as revealedby high-resolution spatio-temporal brain mappingNeuroReport 12 237ndash243
Dehaene-Lambertz G (2000) Cerebral specialization forspeech and non-speech stimuli in infants Journal ofCognitive Neuroscience 12 449ndash460
Dehaene-Lambertz G amp Dehaene S (1994) Speed andcerebral correlates of syllable discrimination in infantsNature 370 292ndash294
Eimas P D Siqueland E R Jusczyk P amp Vigorito J (1971)Speech perception in infants Science 171 303ndash306
Fennell C E amp Werker J F (2003) Early word learnersrsquoability to access phonetic detail in well-known wordsLanguage and Speech 46 245ndash264
Fenson L Dale P S Reznick J S Bates E Thal D Jamp Pethick S J (1994) Variability in early communicativedevelopment Monographs of the Society for Research inChild Development 59 1ndash173
Fernald A Swingley D amp Pinto J P (2001) When half aword is enough Infants can recognize spoken words usingpartial phonetic information Child Development 721003ndash1015
Fischler I S (1990) Comprehending language withevent-related potentials In J W Rohrbaugh R Parasuramanamp R Johnson (Eds) Event-related brain potentials(pp 165ndash170) New York Oxford University Press
Halle P amp de Boysson-Bardies B (1994) Emergence of anearly receptive lexicon Infantsrsquo recognition of wordsInfant Behavior and Development 17 119ndash129
Halle P amp de Boysson-Bardies B (1996) The format ofrepresentation of recognized words in infantsrsquo earlyreceptive lexicon Infant Behavior and Development 19463ndash481
Holcomb P J amp Neville H J (1991) The electrophysiologyof spoken sentence processing Psychobiology 19286ndash300
Holcomb P J Coffey S A amp Neville H J (1992) Visual andauditory sentence processing A developmental analysisusing event-related brain potentials DevelopmentalNeuropsychology 8 203ndash241
Jusczyk P W (1997) The discovery of spoken languageCambridge MIT Press
Jusczyk P W amp Aslin R N (1995) Infantsrsquo detection ofsound patterns of words in fluent speech CognitivePsychology 29 1ndash23
Kutas M amp Hillyard S (1980) Reading senseless sentencesBrain potentials ref lect semantic incongruity Science 207203ndash205
Luck S amp Hillyard S (2000) The operation of selectiveattention at multiple stages of processing Evidence fromhuman and monkey electrophysiology In M Gazzaniga(Ed) The new cognitive neurosciences (2nd edpp 687ndash700) Cambridge MIT Press
Mills D L Coffey-Corina S A amp Neville H J (1997)Language comprehension and cerebral specialization from13ndash20 months [(Special issue on origins of languagedisorders)] In D Thal amp J Reilly (Eds) DevelopmentalNeuropsychology 13 397ndash446
Mills D M Coffey-Corina S A amp Neville H J (1994)Variability in cerebral organization during primary languageacquisition In G Dawson amp K Fischer (Eds) Humanbehavior and the developing brain (pp 427ndash455) NewYork Guilford Publications
Mills D M Coffey-Corina S A amp Neville H J (1993)Language acquisition and cerebral specialization in20-month-old infants Journal of Cognitive Neuroscience 5326ndash342
Mills et al 1463
Mills D L Conboy B amp Paton C (in press) Do changes inbrain organization reflect shifts in symbolic functioning InL Namy (Ed) Symbol use and symbolic representationMahwah NJ Erlbaum
Mills D L Plunkett K Prat C amp Schafer G (in press)Watching the infant brain learn words Effects ofvocabulary size and experience Cognitive Development
Molfese D L (1989) Electrophysiological correlates of wordmeanings in 14-month-old human infants DevelopmentalNeuropsychology 5 79ndash103
Molfese D L (1990) Auditory evoked responses recordedfrom 16-month-old human infants to words they did anddid not know Brain and Language 38 345ndash363
Molfese D L Burger-Judisch L M amp Hans L L (1991)Consonant discrimination by newborn infantsElectrophysiological differences DevelopmentalNeuropsychology 7 177ndash195
Molfese D L amp Molfese V J (1988) Right hemisphereresponses from preschool children to temporal cuescontained in speech and nonspeech materialsElectrophysiological correlates Brain and Language33 245ndash259
Molfese D L Morse P A amp Peters C J (1990) Auditoryevoked responses to names for different objectsCross-modal processing as a basis for infant languageacquisition Developmental Psychology 26 780ndash795
Molfese D L Wetzel W F amp Gill L A (1993) Known versusunknown word discriminations in 12-month-old humaninfants Electrophysiological correlates DevelopmentalNeuropsychology 9 241ndash258
Naigles L R (2002) Form is easy meaning is hard Resolvinga paradox in early child language Cognition 8 157ndash199
Nazzi T amp Bertoncini J (2003) Before and after thevocabulary spurt Two modes of word acquisitionDevelopmental Science 6 136ndash142
Osterhout L amp Holcomb P J (1995) Event-related potentialsand language comprehension In M D Rugg amp M G HColes (Eds) Electrophyisology of mind (pp 171ndash215)Oxford Oxford University Press
Richards J E amp Hunter S K (2001) Testing neural models ofthe development of infant attention DevelopmentalPsychobiology 40 226ndash236
Sereno S C Brewer C C amp OrsquoDonnell P J (2003)Context effects in word recognition Evidence for earlyinteractive processing Psychological Science 14328ndash333
Simos P G amp Molfese D L (1997) Electrophysiologicalresponses from a temporal order continuum in the newborninfant Neuropsychologia 35 89ndash98
Simos P G Molfese D L amp Brenden R A (1997) Behavioraland electrophysiological indices of voicing-cuediscrimination Laterality patterns and developmentBrain and Language 57 122ndash150
St George M amp Mills D L (2001) Electrophysiologicalstudies of language development In J Weissenborn ampB Hoehle (Eds) Language acquisition and languagedisorders (pp 247ndash259) Amsterdam John BenjaminsPublishing
Stager C L amp Werker J F (1997) Infants listen for morephonetic detail in speech perception than in word-learningtasks Nature 388 381ndash382
Swingley D (2003) Phonetic detail in the developing lexiconLanguage and Speech 46 265ndash294
Swingley D amp Aslin R N (2000) Spoken word recognitionand lexical representation in very young children Cognition76 147ndash166
Swingley D amp Aslin R N (2002) Lexical neighborhoodsand the wordndashform representations of 14-month-oldsPsychological Science 13 480ndash484
Werker J F Cohen L B Lloyd V L Casasola M amp StagerC L (1998) Acquisition of wordndashobject associations by14-month-old infants Developmental Psychology 341289ndash1309
Werker J F amp Fennell C E (2004) From listening to soundsto listening to words Early steps in word learning In G Hallamp S Waxman (Eds) Weaving a lexicon (pp 79ndash109)Cambridge MA MIT Press
Werker J F Fennell C E T Corcoran K M amp Stager C L(2002) Infantsrsquo ability to learn phonetically similar wordsEffects of age and vocabulary size Infancy 3 1ndash30
Werker J F amp Tees R C (1984) Cross-language speechperception Evidence for perceptual reorganization duringthe first year of life Infant Behavior and Development 749ndash63
1464 Journal of Cognitive Neuroscience Volume 16 Number 8
Mills D L Conboy B amp Paton C (in press) Do changes inbrain organization reflect shifts in symbolic functioning InL Namy (Ed) Symbol use and symbolic representationMahwah NJ Erlbaum
Mills D L Plunkett K Prat C amp Schafer G (in press)Watching the infant brain learn words Effects ofvocabulary size and experience Cognitive Development
Molfese D L (1989) Electrophysiological correlates of wordmeanings in 14-month-old human infants DevelopmentalNeuropsychology 5 79ndash103
Molfese D L (1990) Auditory evoked responses recordedfrom 16-month-old human infants to words they did anddid not know Brain and Language 38 345ndash363
Molfese D L Burger-Judisch L M amp Hans L L (1991)Consonant discrimination by newborn infantsElectrophysiological differences DevelopmentalNeuropsychology 7 177ndash195
Molfese D L amp Molfese V J (1988) Right hemisphereresponses from preschool children to temporal cuescontained in speech and nonspeech materialsElectrophysiological correlates Brain and Language33 245ndash259
Molfese D L Morse P A amp Peters C J (1990) Auditoryevoked responses to names for different objectsCross-modal processing as a basis for infant languageacquisition Developmental Psychology 26 780ndash795
Molfese D L Wetzel W F amp Gill L A (1993) Known versusunknown word discriminations in 12-month-old humaninfants Electrophysiological correlates DevelopmentalNeuropsychology 9 241ndash258
Naigles L R (2002) Form is easy meaning is hard Resolvinga paradox in early child language Cognition 8 157ndash199
Nazzi T amp Bertoncini J (2003) Before and after thevocabulary spurt Two modes of word acquisitionDevelopmental Science 6 136ndash142
Osterhout L amp Holcomb P J (1995) Event-related potentialsand language comprehension In M D Rugg amp M G HColes (Eds) Electrophyisology of mind (pp 171ndash215)Oxford Oxford University Press
Richards J E amp Hunter S K (2001) Testing neural models ofthe development of infant attention DevelopmentalPsychobiology 40 226ndash236
Sereno S C Brewer C C amp OrsquoDonnell P J (2003)Context effects in word recognition Evidence for earlyinteractive processing Psychological Science 14328ndash333
Simos P G amp Molfese D L (1997) Electrophysiologicalresponses from a temporal order continuum in the newborninfant Neuropsychologia 35 89ndash98
Simos P G Molfese D L amp Brenden R A (1997) Behavioraland electrophysiological indices of voicing-cuediscrimination Laterality patterns and developmentBrain and Language 57 122ndash150
St George M amp Mills D L (2001) Electrophysiologicalstudies of language development In J Weissenborn ampB Hoehle (Eds) Language acquisition and languagedisorders (pp 247ndash259) Amsterdam John BenjaminsPublishing
Stager C L amp Werker J F (1997) Infants listen for morephonetic detail in speech perception than in word-learningtasks Nature 388 381ndash382
Swingley D (2003) Phonetic detail in the developing lexiconLanguage and Speech 46 265ndash294
Swingley D amp Aslin R N (2000) Spoken word recognitionand lexical representation in very young children Cognition76 147ndash166
Swingley D amp Aslin R N (2002) Lexical neighborhoodsand the wordndashform representations of 14-month-oldsPsychological Science 13 480ndash484
Werker J F Cohen L B Lloyd V L Casasola M amp StagerC L (1998) Acquisition of wordndashobject associations by14-month-old infants Developmental Psychology 341289ndash1309
Werker J F amp Fennell C E (2004) From listening to soundsto listening to words Early steps in word learning In G Hallamp S Waxman (Eds) Weaving a lexicon (pp 79ndash109)Cambridge MA MIT Press
Werker J F Fennell C E T Corcoran K M amp Stager C L(2002) Infantsrsquo ability to learn phonetically similar wordsEffects of age and vocabulary size Infancy 3 1ndash30
Werker J F amp Tees R C (1984) Cross-language speechperception Evidence for perceptual reorganization duringthe first year of life Infant Behavior and Development 749ndash63
1464 Journal of Cognitive Neuroscience Volume 16 Number 8
![Multilayered Assembly of Poly(Vinylidene Fluoride) and ...€¦ · quadruple-shape memory effect.[12] Based on this principle, the intermediate shapes of a MSMP can be finely tuned](https://static.documents.pub/doc/80x56/600ebd09034e3b7c5e1d27ff/multilayered-assembly-of-polyvinylidene-fluoride-and-quadruple-shape-memory.jpg)
