Comparison of the N300 and N400 ERPs to picture stimuli incongruent and incongruent contexts

Jeff P. Hamm*, Blake W. Johnson, Ian J. Kirk

Department of Psychology, Centre for Cognitive Neuroscience, University of Auckland, Private Bag 92019, Auckland, New Zealand

Accepted 15 May 2002

Abstract

Objectives: The aim of this study was to examine the N300 and N400 effect to pictures that were semantically incongruous to a prior

object name. Based upon theories of object identification, the semantic incongruity was manipulated to occur early or late in the object

processing stream.

Methods: High-density visual event-related potentials were measured in response to passively viewed black and white line drawings of

common objects. Pictures were preceded with an object name at either the basic (categorical) or subordinate (specific) level. The object either

matched or mismatched with the name. With subordinate level names, mismatches could be within- or between-category.

Results: The N400 effect was found for both basic and subordinate level mismatches. The N400 was found for both the subordinate-within

and subordinate-between. Comparison of the scalp distributions between these N400 effects suggested a common effect was found for all

conditions. The N300 effect, however, was only found for between-category mismatches, and only when semantic expectations were high in

the match baseline (subordinate matches).

Conclusions: The findings are consistent with theories of object identification that suggest that objects are initially categorized prior to

being identified at more specific levels. The N300 appears to reflect the categorisation while the N400 effect appears to be responsive to all

semantic mismatches. Comparison of scalp topographies, functional differences, and different estimated cortical source locations suggest that

the N300 and N400 are two distinct semantic effects that reflect aspects of object identification. q 2002 Elsevier Science Ireland Ltd. All

rights reserved.

Keywords: ERP; Object identification; N300; N400; Semantics

1. Introduction

When a picture violates a semantic context the event-

related potential (ERP) is negative relative to that elicited

by pictures that do not violate the semantic context. The

scalp distribution of the difference is initially seen as a

frontally distributed negativity (Barrett and Rugg, 1990;

McPherson and Holcomb, 1999; Pratarelli, 1994), followed

by a later central–parietal distribution. Time windows that

have been analysed for the initial (frontal) difference have

varied, including 150–250 ms (Pratarelli, 1994), 225–

325 ms (McPherson and Holcomb, 1999), and 250–

350 ms (Barrett and Rugg, 1990). The later central–parietal

negativity has been observed at time windows of 300–

500 ms (Pratarelli, 1994), 325–500 ms (McPherson and

Holcomb, 1999), or 350–550 ms (Barrett and Rugg,

1990). In the following, we refer to the early frontal differ-

ence ERP as the ‘d-N300’ and the later central–parietal

difference as the ‘d-N400’. This nomenclature serves to

emphasize that the effects are seen in the difference wave,

the result of a subtraction of a semantically congruent ERP

from a semantically incongruent ERP. Additionally, this

helps to distinguish the effect from corresponding compo-

nents in the unsubtracted ERPs.

The temporal and topographic differences of the d-N300

and the d-N400 negativities suggest that they are generated

by (at least partially) different brain systems. These brain

systems may process critically different aspects of the

picture-identification task. It is also possible that the differ-

ent scalp topographies are the result of a common network

of cortical regions, but that the relative activity of these

regions changes over time. The properties of the picture

d-N400 strongly resemble those of the d-N400 obtained

with words that violate a semantic context, an ERP whose

properties have been characterised extensively since first

reported by Kutas and Hillyard (1980a,b).

The d-N400 has been shown for words that violate the

Clinical Neurophysiology 113 (2002) 1339–1350

1388-2457/02/$ - see front matter q 2002 Elsevier Science Ireland Ltd. All rights reserved.

PII: S1388-2457(02)00161-X

www.elsevier.com/locate/clinph

CLINPH 2001174

* Corresponding author.

E-mail address: j.hamm@auckland.ac.nz (J.P. Hamm).

semantic context of a sentence presented either visually

(e.g. Kutas and Hillyard, 1980a,b) or auditorily (Connolly

and Phillips, 1994). Additionally, the d-N400 has been

shown for words that are not semantically related to

previous words in a list (Polich, 1985) or to a single word

(Pratarelli, 1994) or a picture prime (McPherson and

Holcomb, 1999; Byrne et al., 1995), pictures of objects

that do not complete a sentence (Ganis et al., 1996;

Nigam et al., 1992; Federmeier and Kutas, 2001), pictures

that are not related to an olfactory prime (Sarfarazi et al.,

1999), and pictures that are semantically unrelated to a

previous picture (Barrett and Rugg, 1990; McPherson and

Holcomb, 1999). Although it is not yet clear if this family of

d-N400s is essentially the same effect, these findings

support the notion that the d-N400 indexes aspects of

semantic processing. If these are all the same d-N400,

then this would indicate the existence of a semantic system

that is independent of the modality of either the target stimu-

lus or its context.

In contrast to the d-N400, the d-N300 appears to be speci-

fic to the processing of picture stimuli (Barrett and Rugg,

1990; McPherson and Holcomb, 1999). However, it appears

to have similar characteristics to the d-N400 in that it is

sensitive to the semantic, rather than physical properties

of the picture-identification task. Barrett and Rugg (1990)

reported that the d-N300 is not found when comparing

physically similar vs. physically dissimilar picture pairs,

but reported a d-N300 when semantically unrelated pairs

are compared to semantically related pairs. McPherson

and Holcomb (1992, 1999) report that the d-N300 differ-

entiates related from unrelated picture pairs but not moder-

ately related from highly related, while the later d-N400

differentiates highly related, moderately related, and unre-

lated pairs.

Given that the d-N300 and the d-N400 both appear to

index semantic aspects of a picture-identification task, it is

possible that the two components represent the same proces-

sing events, which are simply initiated earlier for pictures

than for words. The fact that the two components have

different scalp distributions (Barrett and Rugg, 1990;

Holcomb and McPherson, 1994; McPherson and Holcomb,

1999; Pratarelli, 1994) may represent a change in the acti-

vation pattern over a common cortical network. However,

the fact that the amplitudes of the two components can be

manipulated independently (McPherson and Holcomb,

1992, 1999) suggests otherwise. Even if the change in

scalp topography reflects a shift in activation patterns of a

common network, it seems that the two effects index differ-

ent aspects of the semantic identification of pictures, one of

which is initiated at a measurably earlier latency than the

other. This notion fits very well with how the processes of

object identification are currently conceptualised by cogni-

tive psychologists.

Objects are thought to be identified as a member of a

category prior to determining their specific identity

(Hamm and McMullen, 1998; Jolicoeur et al., 1984; Marr

and Nishihara, 1978; Rosch et al., 1976). The general cate-

gory (e.g. ‘dog’) that an object belongs to, is referred to as

the ‘basic’ level (Rosch et al., 1976). After additional

processing, a more specific identity (e.g. ‘collie’) is obtained

from this general categorisation and is referred to as the

‘subordinate’ level (Rosch et al., 1976).

A complication to the basic/subordinate distinction is

that not all objects that fit within a basic-level category

are equally representative of that category. In fact Jolicoeur

et al. (1984) have proposed that atypical exemplars of a

category tend to be identified at a more specific level,

normally considered subordinate. Jolicoeur et al. (1984)

have suggested that the initial ‘categorical’ identification

be referred to as the ‘entry level’ because not all objects

will necessarily be initially identified with a ‘basic level’

name.

However, the categorical representation is not always

described in terms of a categorical name. Some theories

are based upon the notion of a structural description that

may be shared by objects with different basic names but

nonetheless contains semantic information (Biederman,

1987; Hamm, 1997; Marr and Nishihara, 1978). For exam-

ple, 4-legged animals can be meaningfully represented by a

common structural description, which would comprise a

‘quadruped’ category. On this view, it is not even necessary

that the structural representation maps to a name, (i.e. a

basic- or entry-level name) in order for the representation

to be meaningful. For example, objects such as shirts, jack-

ets, sweaters, coats, and smocks would share a common

structural representation, and despite the fact that there is

no name (at least in English) for the structural category that

comprises ‘torso covers’, this representation provides

semantic information pertaining to the object1. In an object

naming task, objects such as whales and dolphins might be

mis-named as ‘fish’ because the shared structural represen-

tation is most commonly associated with the name fish.

Such a mis-labelling would not reflect a true misidentifica-

tion. Properly classifying whales and dolphins as mammals

requires an adjustment of information that can only occur

after the fact that these ‘fish-like’ objects have been more

specifically identified by differentiation from other structu-

rally similar objects. This would be similar to deciding that a

bird cannot fly only after identifying the specific exemplar

as an ostrich.

Despite differences concerning the specific form of infor-

mation that defines a category, all of these theories of object

identification have in common the notion that objects are

initially identified in a fairly general manner. Subsequently,

more specific identity information becomes available.

J.P. Hamm et al. / Clinical Neurophysiology 113 (2002) 1339–13501340

1 Clothes is not a suitable term for the structural category that comprises

shirts, jackets, and sweaters because it also encompasses objects that are not

structurally similar with shirts, jackets, and sweaters, such as socks, pants,

hats, etc. There is no word, in English, that encompasses the structural

category of ‘torso covers’ in the way that ‘quadruped’ encompasses dogs,

cats, horses, deer, etc.

Since previous studies have shown that the d-N300 and

the d-N400 index different aspects of object semantics, we

reasoned that these two components may be logical candi-

dates in the search for neurophysiological evidence of the

categorical/specific distinction in object recognition. Speci-

fically, we wished to examine the possibility that these two

effects are differentially sensitive to semantic mismatches of

word–picture pairs at either the categorical or specific

levels.

2. Methods

2.1. Subjects

Thirty-two subjects (19 males, 13 females; mean age 26)

were divided into two groups. Group 1 comprised 7 males

and 9 females (mean age 23) and group 2 comprised 12

males and 4 females (mean age 29). All subjects were

recruited from the student population at the University of

Auckland and were paid for their participation in the current

experiment. Handedness was assessed via the Edinburgh

handedness inventory (Oldfield, 1971; EHI). Subject hand-

edness was divided into 31 right handers ðEHI . 0Þ, and 1

left hander ðEHI , 0Þ.

2.2. Electroencephalogram (EEG) acquisition

Electrical Geodesics Inc. 128 channel Ag/AgCl electrode

nets (Tucker, 1993) were used. Fig. 1 shows a schematic of

the electrode locations, with the filled circles corresponding

to the electrodes presented in Fig. 2a and b. Electrodes

closest to the location of the standard 10–20 system are

marked with an asterisk for convenience, however, it should

be noted that due to the design of the electrode net, actual

electrode placement may be as far as 1–2 cm away for any

given subject (see Johnson and Hamm, 2000 for detailed

description of electrode placement relative to gross head

and brain anatomy). Placement of Cz, Nz, and the mastoids

are not marked with asterisks because their positions corre-

spond with the actual 10–20 positions. EEG was recorded

continuously (250 Hz sampling rate; 0.1–39.2 Hz analogue

band pass) during experiments with Electrical Geodesics

Inc. amplifiers (200 MV input impedance) and acquisition

software running on a Power Macintosh 9600/200 computer

with a National Instruments PCI-1200 12 bit analogue to

digital conversion card. Electrode impedances ranged from

10 to 50 kV, which is an acceptable range for the high

impedance amplifiers of the system. EEG was initially

acquired using a common vertex (Cz) reference and re-refer-

enced in off-line analyses to the average reference (Bertrand

et al., 1985). Use of the average reference recovers Cz as an

active electrode, resulting in 129 channels of data. Eye arte-

fact was removed from individual trial epochs using proce-

dures from Jervis et al. (1985). Trials in which any of the

electro-oculogram (EOG) channels were marked bad were

dropped from the averaging process.

2.3. Stimuli

Objects consisted of 72 black and white line drawings

depicting 12 exemplars from each of the categories ‘bird’,

‘dog’, ‘bug’, ‘car’, ‘boat’, and ‘aircraft’. In light of those

theories that suggest the description of the initial represen-

tation to be of a structural nature (Biederman, 1987; Hamm,

1997; Marr and Nishihara, 1978), structurally dissimilar

categories were chosen.

The objects were selected from Snodgrass and Vander-

wort (1980) or Hamm (1997). Stimuli were presented on a

15 inches VGA monitor with a resolution of 640 £

480 pixels. All stimuli were viewed at approximately

57 cm and designed to fit within a square subtending 6.38

of visual angle. Word stimuli were presented in upper case

and comprised the basic category labels (e.g. ‘dog’) given

above or a specific exemplar of a category (e.g. ‘collie’).

Millisecond timing routines and trigger synchronisation

with picture onsets (rather than simply with the onset of

the raster scan) were obtained as described by Hamm

(2001).

2.4. Procedure

A trial began with the presentation of an object name at

either the categorical or specific level. Names were

presented for a duration of 1000 ms. After a 500 ms interval

where the screen was left blank, a picture of an object in the

upright orientation was shown for a duration of 1500 ms.

The next trial was presented 2500 ms later. Subjects were

instructed to restrain from blinking, moving their eyes, and

swallowing during the presentation of the object stimulus.

Subjects were not required to make any decisions concern-

ing the relationship between the presented name and the

object, but were instructed to silently read the name and

silently identify the object. All subjects were presented

with a block of subordinate pairs and basic pairs, with the

order counterbalanced. Additionally, half the subjects,

counterbalanced with order, received ‘within category’

mismatch pairs during the subordinate block (Group 1)

J.P. Hamm et al. / Clinical Neurophysiology 113 (2002) 1339–1350 1341

Fig. 1. Electrode montage employed in ERP recordings. Filled circles refer

to the electrode positions shown in Fig. 2.

and half received ‘between category’ mismatch pairs

(Group 2). This between groups manipulation was neces-

sary due to the limited number of stimuli available and to

minimize the number of times stimuli had to be repeated to

obtain reliable ERPs. All 72 pictures’ stimuli were

presented twice during each block of presentations, once

as a member of a matching pair and once as a member of

a mismatching pair. No more than 4 presentations in a row

could be of the same condition in terms of matching or

mismatching. Mismatch objects at the basic level and for

subordinate-between category mismatches were randomly

selected from the other categories, with no more than 3

pairs between any two categories. For example, 10 dogs

were paired with two separate names from each of the cate-

gories bird, bug, car, boat, and aircraft, and the remaining

two dogs were randomly paired with ‘non-dog’ names from

separate categories.

2.5. Overview of statistical analysis

Four difference waves were calculated from the grand

average epochs and will be the focus of the analysis. All

differences were calculated by subtracting the relevant

match from the relevant mismatch epoch. To reiterate, all

epochs were collected from the picture stimuli and the

condition labels ‘basic’ and ‘subordinate’ are used in refer-

ence to the level of specificity of the preceding word. For the

subordinate conditions the terms within and between refer to

the categorical relationship between the object name and the

J.P. Hamm et al. / Clinical Neurophysiology 113 (2002) 1339–13501342

Fig. 2. Overlay of mismatch (grey) and match (black) waveforms plotted with negative up at a selection of electrode positions for each of the 4 semantic

conditions.

presented picture stimulus. The following gives examples

for the different subtractions.

2.6. Subordinate-between

Match ERPs are in response to picture stimuli that were

preceded by their correct subordinate name. For example,

the ERP in response to a picture of a collie that was

preceded by the name collie. Mismatch ERPs are in

response to picture stimuli that were preceded by an incor-

rect subordinate name of an object from a different basic

category than the picture. An example of this is the ERP in

response to a picture of a collie that was preceded by the

name duck. This condition involved 16 of the 32 subjects

(Group 1).

2.7. Subordinate-within

The match condition corresponds to that described for the

subordinate-between condition previously. Mismatching

ERPs, however, are in response to picture stimuli that

were preceded by an incorrect subordinate name of an

object from the same basic category than the picture. An

example of this is the ERP in response to a picture of a collie

that was preceded by the name poodle. This condition

involved 16 of the 32 subjects (Group 2).

2.8. Basic

Match ERPs are in response to picture stimuli that were

preceded by their correct basic category name. An example

of this is the ERP in response to a picture of a collie that was

preceded by the name dog. Mismatch ERPs are in response

to picture stimuli that were preceded by an inappropriate

basic name. An example of this is the ERP in response to

a picture of a collie that was preceded by the name bird. This

condition involved all 32 subjects.

2.9. Basic/subordinate combination

In this comparison, the match ERP from the subordinate

condition is subtracted from the mismatch ERP from the

basic condition. This condition involved all 32 subjects.

2.10. Time windows

Time windows for analysis of the d-N300 and the d-N400

were determined by the following procedure. First, the

global field power (GFP; Skrandies, 1995) of the 4 differ-

ence waves was calculated and visually examined. Peaks in

the GFP within the time frames associated with the d-N300

and d-N400 were then selected.

Fairly narrow time windows (^20 ms centred on the GFP

peaks) were analysed in order to minimise overlap between

the d-N300 and d-N400. The mean voltage over the time

windows were calculated for each subject at each electrode

for both the mismatch and match conditions. A t test was

then performed at each of the 129 electrodes comparing the

match with the mismatch condition. To correct for multiple

comparisons, we applied a Bonferroni correction factor,

derived using the PCA approach described by Hopf and

Mangun (2000). For all time windows, this correction factor

ranged between 7 and 10. For consistency, and conservati-

vism, the maximum-derived correction factor was

employed for all analysis (although none of the interpreta-

tions change if the specific correction factor is used for each

analysis).

The final step in comparison between conditions is to

filter out any comparisons that may simply reflect statistical

error. After the Bonferroni correction factor is applied to the

t tests, we can still expect on average 6.45 electrodes to

produce t values larger than the critical value. Therefore,

unless significantly more than 6.45 electrodes show t values

larger than the critical value as tested by chi-square, the

conditions will not be considered reliably different and the

‘significant’ electrodes will be assumed to reflect statistical

error.

2.11. Comparison of scalp distributions

It was expected that the experimental conditions in this

study would produce both d-N300 and d-N400 effects across

a number of comparisons. Additionally, these two effects

were expected to differ in their scalp distributions. Differ-

ences in scalp distribution may be tested by analysis of

variance (ANOVA), and predict a significant interaction

between electrode and condition. As pointed out by

McCarthy and Wood (1985), the data from each condition

must be normalised prior to this analysis. The results from

all ANOVAs reported are derived from the normalised data

using Greenhouse–Geisser corrected degrees of freedom

due to violations of sphericity (Greenhouse and Geisser,

1959).

It was also expected that the distributions of d-N300

effects across conditions would be similar, as would the

distributions of the d-N400s. If we attempt to test this ques-

tion via the interaction term between condition and elec-

trode with a two-way ANOVA, then we are predicting a

non-significant interaction. To avoid testing an important

prediction by expecting a null result, a test for scalp simi-

larity is required. McCarthy and Wood (1985) point out that

if an effect size increases, the increase is not additive across

electrodes, rather voltages are multiplied by the change in

effect size. What this describes is a linear relationship in

voltage values across electrodes. Therefore, if two condi-

tions produce the same scalp distribution, this predicts a

significant correlation between the condition voltages over

electrodes. Additionally, this correlation is predicted in the

non-normalised data. The critical value for this test is deter-

mined by converting t critical to r2. For this test, t critical is

based upon the Greenhouse–Geisser corrected degrees of

freedom for electrodes minus 1. In summary, scalp differ-

ences are determined by a significant interaction between

condition and electrode after normalisation. Scalp similarity

J.P. Hamm et al. / Clinical Neurophysiology 113 (2002) 1339–1350 1343

is determined by a significant correlation across electrodes

between conditions without normalisation. Due to the non-

independent nature of the electrodes, degrees of freedom for

both tests were corrected using the Greenhouse–Geisser

correction factor.

Source localisations of the N300 and N400 were

performed using low-resolution electromagnetic tomogra-

phy (LORETA; Pascual-Marqui et al., 1994). This method

is preferred over single dipole modelling because it is unli-

kely that either of these difference components will reflect

differences arising from a single brain area.

3. Results

Fig. 2 shows match and mismatch waveforms at each of

the semantic levels studied (subordinate-between category,

subordinate-within category, basic, and basic/subordinate

combination). The overlaid waveforms show that for all

conditions except basic, divergences between waveforms

(mismatch ERPs more negative) are apparent at a number

of midline electrodes from Fz to Pz, during a time window of

approximately 250–650 ms. For the basic condition, the

match/mismatch divergence occurs over a more restricted

region of space (Cz) and time (approximately 350–650 ms).

Fig. 3 shows the data for each condition in a more

compact form as the GFP of the match–mismatch difference

waves. We consider experimental effects for each semantic

condition separately.

3.1. Subordinate-between

Fig. 3a shows that GFP differences are concentrated in a

latency window of approximately 200–500 ms. We chose

the largest 4 peaks (240, 264, 336, and 468 ms) in this time

region for statistical analyses. At the 468 ms peak, 8 elec-

trodes showed significant differences, a number that is not

larger than expected by chance (x 2ð1Þ ¼ 0:39, P . 0:50).

This peak is therefore considered unreliable, and will not

be discussed further. Peaks at 240, 264, and 336 ms

produced significant differences at 33, 37, and 27 electro-

des, respectively (all x2ð1Þ . 68:9, P , 0:001).

For the first two peaks there was no significant electrode

by time–window interaction ðFð2:6;39:4Þ , 1:0Þ and they

showed a significant correlation ðr2 ¼ 0:98 . r2ðcritÞ ¼

0:95Þ with each other. From this analysis, we considered

the 240 and 264 ms peaks to represent the same effect,

and employed the later peak (which is larger) for subsequent

analysis.

Comparing the 264 ms peak with the 336 ms peak in a

similar fashion showed a significant electrode by time–

window interaction (F(6.0,89.9) ¼ 4.08, P , 0.01), and they

were not significantly correlated (r2¼0.09 , r2(crit) ¼ 0.57)

These analyses indicated that the two peaks represent inde-

pendent effects. We therefore designated the 264 ms peak as

the d-N300, and the 336 ms peak as the d-N400.

Scalp distributions of the two effects are shown in Fig. 4a.

The d-N300 manifest as negative with a fronto-central

distribution, with a reversed-polarity field distributed over

posterior scalp. In contrast, the d-N400 is distributed

centrally and lateralised to the left hemisphere. These

scalp distributions are consistent with those previously

reported for the N300 (Barrett and Rugg, 1990) and the

N400 (Johnson and Hamm, 2000).

From the foregoing analyses, we conclude that semantic

mismatches at the subordinate level, between categories,

elicited both a d-N300 and a later d-N400.

3.2. Subordinate-within

Large GFP peaks are seen at 356 and 396 ms latency (Fig.

3b), showing 30 and 28 electrodes, respectively, with signif-

icant differences between the mismatch and match wave-

forms (both x2ð1Þ . 75:7, P , 0:01). There was no

significant electrode by time–window interaction between

the two peaks ðFð3:2;47:3Þ , 1:0Þ, and they were highly corre-

lated ðr2 ¼ 0:94 . r2ðcritÞ ¼ 0:88Þ. From this we considered

the two peaks to reflect the same effect, and used the larger

(396 ms) peak for subsequent analyses. The latency and

scalp topography (Fig. 4b) of the 396 ms peak suggests

that it is a d-N400.

Finally, we analysed a time window during the largest

GFP peak in the d-N300 latency range (248 ^ 20 ms).

J.P. Hamm et al. / Clinical Neurophysiology 113 (2002) 1339–13501344

Fig. 3. Global field power of the mismatch–match difference waves for

each of the 4 semantic conditions. Arrows and vertical lines indicate the

centre of the time windows for analysis. Lines indicate windows where

no reliable difference was found between the conditions. Open arrows

indicate windows where reliable d-N300s were found. Filled arrows indi-

cate windows where reliable d-N400s were found. See text for exact

latencies.

Only one electrode produced a significant t value, which is

considered a statistical error. From these analyses, we

conclude that semantic mismatches at the subordinate

level, within categories, elicited a d-N400 but there is no

evidence of an earlier d-N300.

3.3. Basic

Time windows centred at 296, 324, and 388 ms were

chosen from the GFP difference plots (Fig. 3c). Analyses

of these windows produced significant t values at 12, 19, and

J.P. Hamm et al. / Clinical Neurophysiology 113 (2002) 1339–1350 1345

Fig. 4. Distribution of mean voltage difference between mismatch and match ERPs for all conditions. The histograms show the mean voltage difference at all

electrodes for the tested time windows (negative plotted up). The electrodes showing significant t values are plotted in black, with the electrodes showing non-

significant differences plotted in grey.

50 electrodes, respectively, all of which are greater numbers

than would be expected by chance (all x2ð1Þ . 5:02,

P , 0:05).

Analysis of the first and second peaks neither showed a

significant interaction between electrode and time-window

ðFð4:6;141:6Þ , 1:0Þ nor did comparison of the second and

third peaks ðFð4:8;147:3Þ , 1:0Þ. Significant correlations

between peaks were also obtained (296 with 324 ms,

r2 ¼ 0:92 . r2ðcritÞ ¼ 0:70; 324 with 388 ms,

r2 ¼ 0:84 . r2ðcritÞ ¼ 0:68). We concluded that the 3 peaks

reflect the same effect, and used the largest (388 ms) for

further analyses. Based upon the latency and scalp topogra-

phy (Fig. 4c), this peak is considered to be a d-N400. We

conclude, therefore, that semantic mismatches at the basic

level elicited a d-N400 but there is no evidence for an earlier

d-N300. Additionally, analysis of the two groups separately

did not reveal a d-N300 effect for either set of subjects.

3.4. Basic/subordinate combination

Time windows centred at 240, 332, and 384 ms were

chosen from the GFP difference plots (Fig. 3d). Analyses

of these windows produced significant t values at 58, 51, and

53 electrodes, respectively, all of which are greater numbers

than would be expected by chance (all x2ð1Þ . 322,

P , 0:001).

Examination of electrode by time–window interactions

showed a significant interaction between the 240 and

332 ms peaks (Fð3:7;113:3Þ ¼ 14:14, P , 0:01) and a non-

significant correlation ðr2 ¼ 0:05 , r2ðcritÞ ¼ 0:81Þ, indicat-

ing they reflect different processes. However, there was no

significant interaction between the 332 and the 384 ms

peaks ðFð4:8;147:3Þ , 1:0Þ, which were highly correlated

ðr2 ¼ 0:95 . r2ðcritÞ ¼ 0:68Þ. The larger peak (332 ms) was

therefore used in subsequent analyses.

On the basis of latencies and scalp distributions (Fig. 4d),

we designated the 240 ms peak as a d-N300, and the 332 ms

peak as a d-N400. From these analyses, we conclude that

semantic mismatches in the basic/subordinate combination

elicited both a d-N300 and a d-N400.

3.5. Comparison of the d-N300 topography across different

semantic conditions

As seen in Fig. 4a and d, the d-N300 exhibits very similar

scalp distributions in both the conditions (subordinate-

between and basic/subordinate combination) in which it

was elicited. This visual similarity can be analysed more

formally using the time–window and correlation methods

described in the preceding sections.

Because the basic/subordinate combination included all

32 subjects, while the subordinate-between condition

involved only 16 of them, two comparisons were made: a

within-subjects comparison involving only the subjects in

the subordinate-between condition; and a between-subjects

comparison, comparing the subordinate-between subjects

with the subordinate-within subjects.

For the within-subjects comparison, there was no inter-

action between condition and electrode ðFð5:02;75:3Þ , 1:0Þ

and the two effects were correlated across electrodes

ðr2 ¼ 0:67 . r2ðcritÞ ¼ 0:66Þ. This was replicated in the

between-subjects comparison, which showed no significant

group by electrode interaction ðFð5:7;171:2Þ , 1:0Þ and highly

related distributions ðr2 ¼ 0:81 . r2ðcritÞ ¼ 0:59Þ. These

analyses support the inference that d-N300 potential fields

exhibit similar scalp distributions in both semantic condi-

tions, suggesting they result from activation of similar brain

regions. Additionally, this demonstrates that the d-N300

shown in basic/subordinate combination was not only due

to the subjects who showed a d-N300 effect in the subordi-

nate-between conditions but also due to the subordinate-

within subjects who had not shown a d-N300 effect (both

groups individually show more significant electrodes than

expected by chance; 30 and 32 electrodes for subordinate-

within and subordinate-between groups, respectively, both

x2ð1Þ . 90:00, P , 0:001).

3.6. Comparison of the d-N400 topography across different

semantic conditions

We began with a between-groups comparison of the d-

N400 for the subordinate-within group with that for the

subordinate-between group. There was no significant

group by electrode interaction (Fð6:9;208:2Þ ¼ 1:27,

P . 0:20) and the effects were correlated over electrodes

ðr2 ¼ 0:66 . r2ðcritÞ ¼ 0:51Þ. As this analysis indicated simi-

lar topographies between groups, the data were combined

(adjusting for the temporal offsets between the time

windows used in each group) to form a ‘Subordinate d-

N400’ condition. This allowed subsequent analyses to be

within-subjects, with an N of 32.

The scalp distribution of the subordinate d-N400 did not

differ from the d-N400 shown for the basic condition

(Fð6:6;203:1Þ ¼ 1:18, P . 0:3) and was highly correlated

ðr2 ¼ 0:78 . r2ðcritÞ ¼ 0:53Þ. The scalp distribution of the

subordinate d-N400 did not differ from the d-N400 from

the basic/subordinate combination (Fð4:0;125:1Þ ¼ 1:18,

P . 0:3) and was also highly correlated

(r2 ¼ 0.91 . r2(crit) ¼ 0.77). Finally, the scalp distribution of

the d-N400 from the basic condition did not differ from that

of the d-N400 from the basic/subordinate combination condi-

tion (Fð5:9;182:3Þ ¼ 1:18, P . 0:3) and was found to be corre-

lated ðr2 ¼ 0:89 . r2ðcritÞ ¼ 0:58Þ. These analyses support the

inference that d-N400 potential fields exhibit similar scalp

distributions in all semantic conditions, suggesting that they

result from a similar configuration of brain generators.

Finally, the scalp distributions of the d-N400 for pictures

was compared with previously published data of the d-N400

effect obtained from semantically incongruous words at the

end of sentences (Johnson and Hamm, 2000). From this data,

a ^20 ms time window was selected about the peak differ-

ence at 448 ms. Because some, but not all, of the subjects in

the current experiment had participated in the previous study,

J.P. Hamm et al. / Clinical Neurophysiology 113 (2002) 1339–13501346

ANOVA could not be performed. However, the question of

interest is whether or not the d-N400 effects found for

pictures are similar to that found for words. The correlation

analysis, which tests for similarity, was performed using the

most conservative r2(crit) from the current comparisons of the

d-N400 effects, namely r2ðcritÞ ¼ 0:77. The correlation

between the subordinate d-N400 and the word d-N400 just

failed to meet this criterion ðr2 ¼ 0:76Þ, while both the d-

N400 for the basic and basic/subordinate combination were

found to be similar (r2 ¼ 0:81 and 0:80, respectively).

3.7. Source estimates

LORETA source estimates were performed after

temporal alignment and averaging of the d-N300s and d-

N400s reported previously. Estimated brain sources are

shown in Fig. 5. Note that because these sources are based

on the difference-waves, these reflect areas of differential

activation between the mismatching and matching condi-

tions. As shown in Fig. 5a, the d-N300 estimated sources

are in bilateral frontal areas and bilateral occipital/parietal

areas. Sources for the d-N400 include a right-frontal and

bilateral–temporal areas and are shown in Fig. 5b.

Because the source analysis is based on grand-averaged

data, using grand-averaged electrode positions, we do not

attempt to translate this output to Talairach coordinates

(Talairach and Tounoux, 1988), which would imply an

unwarranted degree of accuracy. In this case the accuracy

of the sources estimates is likely on the order of the average

size of a lobe. Our goal in this analysis was rather more

modest than exact localisation of sources: we simply wished

to demonstrate that the sources, like the surface potential

fields, for the two components are dissimilar.

4. Discussion

In the present study, we measured brain potentials while

subjects viewed word–picture pairs that were either seman-

tically congruent or semantically incongruent, in a paradigm

where semantic mismatches could occur at either a catego-

rical or a specific level (Hamm and McMullen, 1998). Our

data show that two differences in the ERPs known to index

semantic aspects of object processing (the d-N300 and the d-

N400) are differentially sensitive to the level at which

mismatches occur.

The d-N400 was elicited in all 4 semantic conditions, as

expected for an effect that is known to be robustly produced

by all manner of semantic incongruencies, regardless of

modality of presentation. The current d-N400 exhibited a

characteristic central–parietal spatial distribution that was

visually and statistically similar across conditions. Addition-

ally, the scalp distribution of the d-N400s corresponded with

the d-N400 distribution obtained with incongruent sentence

endings that do not involve pictorial stimuli (e.g. Johnson and

Hamm, 2000). The distribution of this d-N400 effect was not

shown to be different between conditions, and was signifi-

cantly similar as determined by the spatial correlation. This

suggests that the all conditions produced semantic expecta-

tions that were more consistent with the match stimuli than

the mismatch.

In contrast, the d-N300 exhibited the fronto-central scalp

distribution reported in previous studies using pictorial

stimuli (Barrett and Rugg, 1990; McPherson and Holcomb,

1999) and was obtained in only two of our 4 experimental

conditions. First, and most tellingly, when mismatches

occurred at a subordinate semantic level, the d-N300 was

elicited when the mismatch was between categories, but not

when it was within a category. This result provides good

evidence that the d-N300 is sensitive to categorical-level

mismatches, and suggests that d-N300 may reflect the

semantic categorisation of object stimuli, an interpretation

similar to that offered by Federmeier and Kutas (2001). This

finding is also consistent with McPherson and Holcomb’s

(1992, 1999) finding that the d-N300 does not differentiate

between moderately and highly semantically related pairs2

J.P. Hamm et al. / Clinical Neurophysiology 113 (2002) 1339–1350 1347

Fig. 5. LORETA source estimations showing areas that are differentially activated between mismatching and matching conditions. Series A shows the d-N300.

Series B shows the d-N400. Arrows indicate the approximate depth of the source activation centre. Z-coordinates correspond to depth, with z ¼ 0:02

approximately equal to slice 0 mm in Talairach space and z ¼ 0:10 approximately equal to slice 66 mm. Scale bars indicate strength of activation, with

lighter areas indicating increased activity.

2 The subordinate match condition would be considered the highly related

pair, with the subordinate-within mismatch waveform representing the

moderately related pair, although the difference in ‘relatedness’ is probably

not as extreme as in McPherson and Holcomb’s (1992, 1999) studies.

although the d-N400 does. This provides physiological

support for the notion that objects are initially classified as

members of a basic-level category (Rosch et al., 1976) or as

a member of a structural group (Marr and Nishihara, 1978).

Further, the fact that the d-N300 occurs at a measurably

earlier latency than the d-N400 is consistent with the idea

that categorisation of a given object occurs prior to the

determination of more specific identity information. These

ERP results parallel the observation that between-category

mismatches result in faster RTs than within-category

mismatches (Hamm and McMullen, 1998), and with the

observation that it takes longer to differentiate visually simi-

lar items than it does to differentiate dissimilar items

(Murray, 1998).

When object names were at the basic-level, only the d-

N400 effect was elicited. On the face of it, this observation

does not fit well with the argument outlined previously,

because mismatches in this condition crossed the same cate-

gorical boundary as they did in the subordinate-between

condition. However, by their nature basic-level names

provide less semantic information than subordinate names.

This would result in a less specific semantic expectation for

the following object as compared to that elicited by a subor-

dinate name.

Reduction in semantic expectations is known to reduce

the amplitude of the d-N400 (Kutas et al., 1984). The

current results suggest that the d-N300 may also be modu-

lated by semantic expectations and that the basic-match

baseline produced insufficient expectations. This explana-

tion is supported by the fact that the d-N300 was produced

during the basic/subordinate combination analysis. Addi-

tionally, the basic/subordinate combination analysis

produced a d-N300 effect for both groups of subjects and

it should be noted that for the subordinate-within group, a d-

N300 was not found for either their basic or subordinate

analysis individually. This shows that by increasing the

semantic expectations in the match ERP baseline, a d-

N300 effect is shown with the basic mismatch. This suggests

that the d-N300 effect represents suppression as a function

of meeting semantic expectations, rather than increased

negativity as a function of the degree of semantic violation.

This is the same explanation as is offered for the more

widely studied d-N400 effect (Kutas and Hillyard, 1984;

Kutas et al., 1984; Van Petten and Kutas, 1991a,b). If so,

the lack of a d-N300 for the subordinate-within condition

would suggest that mismatching stimuli produced as much

suppression as the matching stimuli. This is consistent with

the notion that the stimuli are initially identified as being

members of the expected category, and only after additional

processing is the mismatch detected (Hamm and McMullen,

1998).

The scalp topographies of the d-N400 effect from the

current experiments were reliably correlated with the distri-

bution of the d-N400 effect for semantically incongruous

words that terminate sentences. Additionally, source estima-

tions of the d-N400 effect for pictures were consistent with

those proposed for the d-N400 effect for semantically incon-

gruous words that terminate sentences (Johnson and Hamm,

2000). Overall, these data suggest that pictures and words

may at some point contact a common semantic network,

possibly a result of when object identification contacts

object names.

Finally, source estimations of the d-N300 effect did not

correspond to source estimations for the d-N400 effect. This

suggests that the d-N300 is produced by a different under-

lying network of cortical areas than that responsible for the

d-N400 effect. Both the d-N300 and the d-N400 suggested

sources in the frontal lobes. The frontal lobes have been

suggested to be involved with the processing of object

stimuli (Kelly et al., 1998; Koutstaal et al., 2001), and so

this is not an unreasonable source location. The temporal

sources of the d-N400 are consistent with the notion that the

temporal lobes are involved in the processing of identity

information (Ungerleider and Mishkin, 1982). The occipi-

tal–parietal sources of the d-N300 are suggestive of a visual

process. The visual source for the d-N300 is more consistent

with the notion that objects are initially classified as a

member of a structural group (Marr and Nishihara, 1978)

than it is with classification as members of a semantic group

with a common name (Rosch et al., 1976). Contacting a

meaningful structural representation also logically ties the

early processes of object identification with the visual infor-

mation available rather than with the availability of an

entry-level name.

The finding of a common scalp distribution for the d-

N400 between pictures and words is not, however, univer-

sal. Ganis et al. (1996) found that the d-N400 was more

frontally distributed for picture stimuli than for word

stimuli. Additionally, Federmeier and Kutas (2001) found

a more frontally distributed d-N400 for between-category

mismatches than for within-category mismatches. Both of

these findings involve the comparison of a condition that is

likely to produce a d-N300 with a condition that does not

produce a d-N300. Additionally, the time windows in the

previous studies were larger (300–400 ms post-stimulus:

Federmeier and Kutas, 2001; 325–475 ms post-stimulus:

Ganis et al., 1996) than that of the current experiment,

and more importantly include times when the d-N300 effect

may influence the mean voltage calculation, and therefore

the scalp topography.

It should also be noted that in both Ganis et al’s (1996)

and Federmeier and Kutas’s (2001) experiments the seman-

tic context was created using sentences, which could

produce much larger expectations than a single object

name as employed here. Based on the current finding that

the d-N300 is modulated by semantic expectations,

sentences would be expected to produce a larger d-N300

effect by inducing stronger semantic expectations.

Additionally, the reference electrodes employed in the

previous studies were the arithmetic sum of the mastoids

(Ganis et al., 1996) and the average mastoids (Federmeier

and Kutas, 2001). The mastoids are located in the positive

J.P. Hamm et al. / Clinical Neurophysiology 113 (2002) 1339–13501348

end of the dipole for both the d-N300 and the d-N400 effect

when the average reference is employed. The mastoids

would project the negative end of the d-N300 over a larger

scalp area than shown with the average reference here, and

possibly increase the amount of overlap between the two

effects. For these reasons, the more frontally distributed d-

N400s found (Ganis et al., 1996; Federmeier and Kutas,

2001) may reflect the involvement of the d-N300, and as

such the findings are not at odds with each other.

It is thought that increasing the semantic expectations

will produce a larger d-N300 effect. If this also results in

a longer lasting d-N300, this would increase the influence on

the mean-voltage over the selected time windows of the

previous studies. Conditions that do not produce a d-N300

effect, or produce a much smaller d-N300, will be less

affected by this overlap, and as such the scalp distribution

will be less frontally influenced. Because the d-N300 is

much less understood than the d-N400 effect, these possibi-

lities need to be explored before it can be determined if the

family of d-N400 effects are indicative of a truly amodal

semantic system. Even if it is determined that the d-N400

reflects a common and amodal semantic network, the fact

that the d-N300 occurs with pictorial stimuli and not words,

indicates that pictures, at least, also access a separate seman-

tic network (Ganis et al., 1996; Pavio, 1975, 1990). There-

fore, when comparing d-N400 effects between stimuli of

different types, the possibility that a common d-N400 effect

is partially co-occurring with an additional semantic effect

for one or both stimuli, should be considered. The current

and previous studies all indicate the importance of under-

standing the parameters that influence the d-N300 effect

before attempting to determine if the d-N400 effects for

pictures and words reflect a common semantic network.

Finally, a few methodological issues of the current results

should be mentioned. First, each stimulus was repeated 4

times in total over the course of the experiment. Repetition

of the stimuli may reduce the magnitude of the d-N400

(Federmeier and Kutas, 2001) and may likewise reduce

the magnitude of the d-N300. However, even if the current

effects are reduced relative to a theoretical maximum, the

fact that the d-N300 effect, if present for subordinate-within

violations, has been reduced to zero while the d-N400 effect

has not is more evidence that these two effects are not one

and the same.

Additionally, due to the block design, the semantic expec-

tations are not equated between conditions. Reduced seman-

tic expectations is thought to be why the d-N300 is not

apparent in the basic conditions, but does appear in the

basic/combination comparisons for both groups indepen-

dently and in total. It is possible that different expectancies

were generated during the subordinate-within and subordi-

nate-between groups. Grouping the conditions into a within-

subjects design where basic, subordinate-within, and subor-

dinate-between conditions are mixed does not change the

possibilities that subjects expectancies will shift when given

a basic or subordinate prime. Although such a design would

equate expectancies on subordinate primes, it may reduce

the overall level of expectancy. However, it is important that

the interpretations offered here hold even if subordinate-

within violations produce a d-N300 provided that the effect

is smaller than subordinate-between violations.

In conclusion it is proposed that the d-N300 effect and the

d-N400 effect are generated by distinct underlying networks

of cortical activity that are activated at different points in the

overall process of object identification. Both effects are indi-

cative of object semantics, with the d-N300 effect reflecting

early categorisation of objects, most likely as members of a

semantically meaningful structural group (Marr and Nishi-

hara, 1978). The d-N400 may be sensitive to information

extracted after this initial categorisation, and also may

represent the same d-N400 effect produced by words.

However, until the parameters that influence the d-N300

effect are better understood, such conclusions, both for

and against a common semantic network, should be viewed

with caution. These findings offer new possibilities for the

study of the processes underlying the identification of object

stimuli and additional considerations when comparing

semantic ERPs between pictures and words specifically,

and between stimulus modalities in general.

Acknowledgements

This research was supported by the Royal Society of New

Zealand Marsden Fund Grant UOA813. The authors wish to

thank Professor M.C. Corballis, Editor Dr M. Hallett, and 4

anonymous reviewers for their suggestions and comments

during the preparation of this manuscript.

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