N400 and lexical decisions: automatic or controlled processing?

Juan Silva-Pereyraa, b, ThalõÂa Harmonya, Gerardo Villanuevab, ThalõÂa FernaÂndezb,Mario RodrõÂguezb, LõÂdice GalaÂnc, Lourdes DõÂaz-Comasc, Jorge Bernalb,

Antonio FernaÂndez-Bouzasb, Erszebet Marosib, Alfonso Reyesb

aCentro de NeurobiologõÂa, Universidad Nacional AutoÂnoma de MeÂxico, Campus Juriquilla, QuereÂtaro, MexicobNeurociencias ENEP Iztacala, Universidad Nacional AutoÂnoma de MeÂxico, Apartado 314, 54090 Tlalnepantla, Estado de Mexico, Mexico

cCentro de Neurociencias, La Habana, Cuba

Accepted 30 December 1998

Abstract

Objective: To investigate whether the N400 effect is sensitive to automatic or controlled processes.

Methods: Two experiments were performed. In one experiment, directly related word pairs were used. In the other experiment, mediated-

related word pairs were used. In order to reduce controlled processes, each experiment consisted of 3 tasks: Low- and high-proportion of

related pairs, and single presentation lexical decision task.

Results: In the ®rst experiment, the amount of priming was equivalent for the 3 tasks. The N400 effect appeared in the high and low

proportion of directly related words, but not in the single presentation task. In the second experiment, behavioral priming was also found in

the 3 tasks. However, the N400 effect was observed only in the task with low proportion of related pairs.

Conclusion: These results suggest that the N400 effect may be related to controlled processes. q 1999 Elsevier Science Ireland Ltd. All

rights reserved.

Keywords: N400; Automatic and controlled processes; Lexical decision task; Semantic priming; Visual ERP

1. Introduction

1.1. Semantic priming

The lexical decision task requires subjects to indicate as

rapidly as possible, whether a letter string stimulus

presented on each trial, is or is not a word. In the lexical

decision paradigm used to evaluate the context effect or

semantic priming effect, a trial consists of two events.

First, a semantic context is provided by the presentation

of a single word (the prime), to which no overt response

is required. Second, the prime is followed by the presenta-

tion of a single letter string (the target). It has been reliably

demonstrated that words that are preceded by semantically

related words are recognized faster and more accurately

than words preceded by unrelated words (Meyer and Schva-

neveldt, 1971). This effect is known as semantic priming.

Several models have been proposed to explain semantic

priming, an example of which are the connectionist

approaches (Shallice, 1994). In this paper, however, the

latter will not be considered. We based our hypothesis on

3 theoretical mechanisms that have been proposed to

explain semantic priming: One which is automatic and

two which are controlled (Neely, 1977, 1991; Neely et al.,

1989; McNamara, 1992, 1994; Shelton and Martin, 1992).

Automatic processes are assumed to be fast, of short dura-

tion, not to require attention or awareness, and not to draw

from a common pool of resources. In contrast, controlled

processes are slower, involve resource capacity, are under

intentional control, and allow the subject's expectancies and

strategies to exert an effect (Posner and Snyder, 1975; Shif-

frin and Schneider, 1977).

Several theories of semantic memory have proposed that

memory traces are organized in networks and that retrieval

depends, at least partially, on an automatic, attention-free

process called `spreading activation' (Collins and Loftus,

1975). Presentation of a word activates the corresponding

node of this word in semantic memory. Part of this activa-

tion automatically spreads to words that are related in

memory. As a consequence, the activated nodes represent-

ing related words use less time for subsequent processing.

Manipulation of stimulus onset asynchrony (SOA) between

Clinical Neurophysiology 110 (1999) 813±824

CLINPH 981871388-2457/99/$ - see front matter q 1999 Elsevier Science Ireland Ltd. All rights reserved.

PII: S1388-2457(99)00009-7

E-mail address: jfsp@servidor.unam.mx (J. Silva-Pereyra)

primes and targets is a traditional method for assessing the

impact of spreading activation, because, under normal read-

ing conditions, this automatic process contributes to seman-

tic priming effects only within a restricted temporal window

of some 500 ms (Neely, 1977; De Groot, 1984). When the

interval is longer, enough time is allowed for controlled

processes to take place.

Expectancy is a second general mechanism used to

account for semantic priming effects. It is assumed that

subjects use the prime to generate expectancy for a set of

semantically related targets that would likely follow that

prime. Targets included in this expectancy set are recog-

nized more quickly than those that do not belong to this

set. This mechanism may be in¯uenced by instructions as

well as by the list structure of the materials presented. When

the stimulus list contains a large proportion of related word

pairs, it is highly probable that expectancy contributes to the

priming effect (Tweedy et al., 1977; den Heyer et al., 1983).

The third mechanism is known as semantic matching.

This mechanism is a speci®c instantiation of what others

have referred to as post-lexical priming mechanism (Seiden-

berg et al., 1984; Balota and Lorch, 1986; Neely et al.,

1989). After the prime's and target's lexical and semantic

representation have been activated, but before the subjects

have had time to complete their lexical decisions and/or

response to target, the subject may check whether the target

is semantically related or unrelated to the prime that

preceded it. If it is related, a `word' decision/response bias

ensues, thereby facilitating responses to related word

targets; if it is unrelated, a `non-word' decision/response

bias follows. Neely et al., (1989) have changed the propor-

tion of related words and the probability of non-words in

two conditions, namely high-dominance and low-domi-

nance exemplars as targets. They found that, if the prob-

ability of non-words increased, priming increased for low-

and high-dominance exemplars and for non-words. At the

same time, if the relatedness proportion increased only

priming for high-dominance increased. These results

suggest that relatedness proportion modi®ed expectancy

and that non-word proportion modulates the mechanism of

post-lexical priming or semantic matching.

Mediated-priming in lexical decision tasks is a procedure

used to reduce subject's strategies. Priming between pairs of

words that are connected only through a mediated associa-

tion is called mediated priming (e.g. from `lion' to `stripes'

via `tiger'; De Groot, 1984; Balota and Lorch, 1986).

McNamara and Altarriba (1988) have used a modi®ed

version of the original lexical decision task. In their task,

the stimuli were presented one by one (i.e. with no obvious

pairing between primes and targets), and subjects were

required to make a decision on the basis of the previous

letter string. With this procedure, subjects did not engage

in the generation of post-lexical strategies. Shelton and

Martin (1992) have replicated these results using the modi-

®ed procedure but with a very low proportion of related

words.

1.2. N400

A late event-related potential (ERP) negativity with a

peak latency near 400 ms (N400), has been observed in a

variety of circumstances associated with language func-

tions, including both semantic priming and sentence

comprehension contexts (Kutas and Hillyard, 1980, 1989;

Bentin et al., 1985, 1993; Rugg, 1985; Holcomb, 1988,

1993; Holcomb and Neville, 1990; Connolly and Phillips,

1994; Chwilla et al., 1995; Connolly et al., 1995; Osterhout

and Holcomb, 1995). In lexical decision tasks, this endo-

genous component has a larger amplitude for unrelated

targets, with respect to related ones. This effect is the so-

called N400 semantic priming effect. According to

Holcomb (1993), the N400 is small for related target

words, because the lexical detector for the target bene®ts

from the spread of activation associated with the processing

of the prime. Nevertheless, if the target is preceded by an

unrelated prime, there is no such bene®t and more resources

are required to detect the target. The use of these extra

resources may be what is re¯ected by the larger N400 for

these words (Holcomb and Neville, 1990).

Another candidate for the cognitive process underlying

the N400 is a controlled process of semantic integration

(Rugg et al., 1988; Rugg, 1990; Holcomb, 1993). Models

of word recognition assume a general division into lexical

access and lexical integration processes. The ®rst one can be

described as an automatic process whilst the latter would be

a controlled process. Lexical access involves the process of

computing a form representation onto corresponding entries

in the mental lexicon. This processing results in the activa-

tion of a subset of lexical elements together with their

semantic and syntactic attributes. Integration refers to the

process of speci®cally integrating a lexical element into a

higher order meaning representation of the entire sentence

or discourse (Marslen-Wilson, 1989; Zwitserlood, 1989).

According to the integration hypothesis, the amplitude of

the N400 re¯ects the ease which various knowledge sources

(e.g. lexical, syntactic and semantic) are used to form an

integrated discourse representation.

Few research papers have addressed the processing nature

of the N400 and the results are not yet conclusive. In this

paper, we analyze the N400 effect using semantic mediated

priming with the modi®ed version of McNamara and Altar-

riba (1988) to eliminate expectancy and post-lexical strate-

gies. Other authors, as we shall see below, have used

different procedures to reduce controlled processes: low

proportion relatedness (Holcomb, 1988), delayed letter

search task (Kutas and Hillyard, 1989; Besson et al.,

1992), stimuli degradation (Holcomb, 1993), variable

SOA (Anderson and Holcomb, 1995) and prime masking

(Brown and Hagoort, 1993).

Holcomb (1988) has manipulated the proportion of

related words and the instruction to either ignore or attend

to the semantic relationship within a pair in a lexical deci-

sion task. An N400 was observed for both high and low

J. Silva-Pereyra et al. / Clinical Neurophysiology 110 (1999) 813±824814

relatedness proportions. The amplitude was bigger in the

high proportion lists and when subjects attended to the

prime. He concluded that, apart from controlled processes,

the N400 also re¯ects automatic processes. However, the

manipulation of the low proportion of relatedness does not

exclude post-lexical priming. Thus, it is not clear whether

Automatic Spreading Activation in¯uences the amplitude of

the N400. Kutas and Hillyard (1989) study is also not

conclusive: they asked the subjects to read pairs of semantic

related or unrelated words followed by a single letter. The

objective of the task was to judge whether the letter was

present in either of the preceding words. The amplitude of

N400 was modulated by the degree of relatedness despite

the fact that the subject's task was not connected to the

meaning of the stimuli. However, the authors cannot

exclude that subjects were aware of the semantic relations

between the words.

Brown and Hagoort (1993) have compared the effect of

masked and unmasked presentation of a prime on the N400

to a following target. They claim that masking prevents

perceptual identi®cation, and as such, it rules out effects

from controlled processes (Marcel, 1983; Cheesman and

Merikle, 1984). Their results show a signi®cant N400 effect

for the unmasked presentation condition, but do not show

such effect for the masked presentation condition. The

authors have concluded that the N400 is not a manifestation

of the lexical access processes, but that it re¯ects aspects of

the semantic integration processes. However, the masking

technique is not universally accepted, and its most critical

aspect is the validity of the threshold setting procedures

(Merikle, 1982; Cheesman and Merikle, 1984; Holander,

1986). Using different kinds of judgement tasks to establish

near-threshold prime presentation conditions, Dagenbach et

al. (1989, 1990) have demonstrated that different threshold-

setting judgement tasks can indeed lead to different patterns

of masked semantic priming.

McCarthy and Nobre (1993) have analyzed the effect of

spatial selective attention on semantic categorization. The

N400 effect was only observed for those words presented at

the attended space, suggesting that the linguistic process

generating an N400 is not automatic.

Holcomb (1993) found a dissociation between behavioral

priming and N400 priming in lexical decision tasks, when

the target stimuli were intact and when they were degraded

(removing a random 33% of the elements that make up each

letter of the target or overlaying a matrix of dots on the

target). He found that subjects responded faster and more

accurately to related targets than to unrelated targets. More-

over, this priming effect was greater when the target was

degraded. However, although the N400 component was

larger in unrelated than related targets, there was no

evidence that this difference was larger in the degraded

blocks. These results suggest that the N400 effect is more

clearly associated with a mechanism that is not directly

altered by degradation, such as the lexical integration

process.

Chwilla et al. (1995) have also observed that the N400

effect was absent in a physical task where subjects had to

discriminate between uppercase and lowercase letters. A

lexicallity test on reaction times demonstrated that the

physical task was performed non-lexically. Their results

indicate that N400 priming effect is only evoked when the

task performance induces the semantic aspects of words.

Anderson and Holcomb (1995) have extended Holcomb

and Neville (1990) ®ndings by comparing visual and audi-

tory semantic priming effects using different SOAs. The

comparison of priming effects at short and long SOAs

would reveal the sensitivity of the N400 to automatic versus

controlled processing. N400 effects were found in both short

and long SOAs, suggesting that the N400 is sensitive to both

types of processing in the visual modality. However,

although a short SOA condition has been shown to be effec-

tive in excluding expectancy-induced priming effects

(Neely, 1977), it does not rule out the in¯uence of semantic

matching processes (Neely, 1991).

To summarize, the experiments performed in relation

with the mechanism of N400 priming are not conclusive,

since they show that the N400 effect may re¯ect an auto-

matic and/or a controlled process, depending on the experi-

mental maneuvers. As psycholinguistics have shown, the

use of low relatedness proportion or short SOA is useful

but not enough to reduce controlled processes, since post-

lexical priming still remains. The use of semantic mediated-

priming with the single presentation lexical decision task

(McNamara and Altarriba, 1988), with subjects required

to make a decision for each stimulus, may reduce expec-

tancy and postlexical strategies. In this paper, this procedure

was used in order to explore whether an N400 persists.

2. Experiment 1

2.1. Methods and materials

In this experiment, directly related priming was analyzed.

Three tasks were studied: (1) high proportion (HP) of related

pairs (62%) in a lexical decision task; (2) low proportion

(LP) of related pairs (20%) in a lexical decision task and (3)

single presentation (SP) lexical decision task with low

proportion (20%) of related pairs, using the modi®ed

version of the original lexical decision task (McNamara

and Altarriba, 1988).

In all tasks, 25% for non-words and 21% for total non-

words were used. These non-word percentages were

selected in an attempt to reduce the semantic matching

effect (Neely et al., 1989).

2.1.1. Subjects

Subjects were 18 undergraduate male students between

20 and 30 years of age, with a mean age of 25.6 years. All 18

subjects were right-handed and had no history of familial

sinistrality, except for one subject, whose mother was left-

J. Silva-Pereyra et al. / Clinical Neurophysiology 110 (1999) 813±824 815

handed. Subjects had normal vision and no history of neuro-

logical or psychiatric disorders.

2.1.2. Stimuli

In the construction of the list of word pairs and non-

words, the primes consisted of 400 nouns from a large list

of 5000 words in Spanish including adjectives, adverbs,

articles, conjunctions, nouns, prepositions and verbs,

ordered according to their frequency of occurrence in Mexi-

can Spanish (Lara et al., 1980).

The list of primes was read to a sample of 30 undergrad-

uates and graduates, at a rate of one word every 4 s. Instruc-

tions were given to the subjects, asking them to write down

the ®rst word that came to their minds after hearing the word

read to them. Examples were given as pairs of concrete

words (`chair' ± `table').

All 400 words produced by each of the 30 subjects were

stored and analyzed for their frequency of occurrence with

its corresponding percentage. The highest frequency word

was selected. In this way, from the initial 400 words, we

obtained a list of 256, which were not repeated and were not

verbs or generalized nouns (i.e. those which contain many

elements like `fruits,' `¯owers' and `animals'). This new list

served as the list of mediators.

Another 520 nouns from the same dictionary were used to

construct a list of 260 unrelated words. Random pairs were

formed. To verify their unrelatedness, the list of paired

words was shown to 10 other subjects, who had to rate

them from 0 (no association) to 5 (strongest association).

Those pairs with a score equal to 3 or more were again

paired randomly. Three hundred and twenty words of the

520 words used for the unrelated pairs were selected. By

replacing the position of vowels and consonants from the

list of unrelated pairs, we formed the non-word list.

All primes and mediators were both words and non-words

between 3 and 10 letters long (median � 5, SD � 1:56).

Words and non-words were also between two and 3 sylla-

bles long (median � 2, SD � 0:65). Appendix A shows the

words used as primes, mediators and targets.

The words from the list of primes and the words from the

list of mediators were used in the present experiment as

prime-target pairs. One hundred and nineteen prime-target

pairs and 280 prime-target pairs were used in the HP task

and the LP task, respectively. These pairs were distributed

as shown in Table 1. In the SP task, a new list of 280 prime-

target pairs with the same distribution of the pairs as in the

low proportion condition was used.

2.1.3. Procedure

The following sequence of events occurred in each trial in

the HP and LP tasks: (1) a ®xation point (a plus sign) was

presented in the center of the screen for 1000 ms; (2) a dark

screen was presented for 500 ms; (3) a prime letter string

was presented in the center of the screen (for 200 ms); (4) a

dark screen was presented for 100 ms; (5) a target letter

string was presented in the center of the screen for 500

ms; (6) a 2 s intertrial interval was included before the

next trial.

For the SP task, the stimuli were presented in a contin-

uous sequence, without a ®xation point. Each stimulus was

presented for 500 ms followed by 1000 ms dark screen

intervals.

Subjects were instructed to respond rapidly and accu-

rately to each target stimulus. If the letter string was a

word, they had to press the letter B on the keyboard with

the index ®nger. If the letter string was a non-word, they had

to press the letter M with the middle ®nger. The assignation

of B for yes was counterbalanced across subjects.

The stimuli were delivered by a MindTracer system

synchronized to the Medicid 03E-acquisition system.

Correct and incorrect responses were automatically marked

in the recording. For the HP and LP tasks, primes were

displayed in 1 cm lowercase letters, and target in 1 cm

uppercase letters in the center of a 14 inch computer monitor

(white letters on a black screen). For the SP task, all stimuli

were displayed in 1 cm uppercase letters. The stimuli were

presented on a display window of a PC monitor. At the

viewing distance employed, each letter subtended a visual

angle of approximately 0:573 £ 0:5738.

2.1.4. Recordings

The electroencephalogram (EEG) was recorded with Ag/

AgCl electrodes and referenced to linked earlobes from Fp1,

Fp2, F3, F4, C3, C4, P3, P4, O1, O2, F7, F8, T3, T4, T5, T6,

Fz, Cz, Pz and Oz of the 10/20 international system. The

ampli®er bandwidth was set between 0.5 and 30 Hz (60 Hz

notch in). All electrode impedances were at or below 5 kVand ampli®ed with a gain of 20 000. The EEG was sampled

every 5 ms and edited off-line by visual inspection. Epochs

of 1025 ms with a pre-stimulus time of 100 ms were

analyzed. The electro-oculogram (EOG) was recorded

using the same speci®cations as the EEG. Horizontal eye

movements were recorded with electrodes placed over the

outer canthi of both eyes, and vertical movements and blinks

were recorded with supra and infraorbital electrodes on the

right eye. Special care was taken to reject segments with eye

movements or other artifacts. Trials on which EEG or EOG

activity exceeded ^ 75 mV were rejected. Baseline correc-

J. Silva-Pereyra et al. / Clinical Neurophysiology 110 (1999) 813±824816

Table 1

Distribution of high proportion and low proportion tasks

High

proportion

task

Low proportion task

and single

presentation lexical

decision task

Word pairs (prime-target) Number of pairs Number of pairs

Word-word (related pair) 50 30

Word-word (unrelated pair) 30 120

Non-word ± word 30 30

Word ± non-word 10 30

Non-word ± non-word 20 20

tion was performed in relation to the pre-stimulus time (100

ms).

2.1.5. Data analysis

Conventional mean reaction time (RT) was computed for

both related and unrelated words. Only trials with correct

responses and with latencies between 300 and 1500 ms were

included in the mean RT.

A two-way rm-ANOVA was performed on the mean RT.

Variables included target type (related, unrelated) and task

(HP, LP and SP tasks). Few errors were observed in all tasks

and for this reason they were not considered in the data

analysis.

Average ERPs to related and unrelated words from

correct response trials were obtained. The N400 was de®ned

as the maximum amplitude of a negative wave between 400

and 450 ms after target onset. The voltages for each point

within this time window were added up and averaged in

order to obtain the mean amplitude.

A 3 way rm-ANOVA was performed on the mean ampli-

tude based on data from all subjects. Variables included

target type (related, unrelated), task (HP, LP and SP tasks)

and anterior-posterior localization (Fz, Cz, Pz and Oz). This

analysis was designed to assess the signi®cance of target

type across the tasks on the anterior-posterior localization.

A 4 way rm-ANOVA (target type £ task £ left-right

hemisphere £ electrode localization, e.g. F3, C3, P3, O1,

F4, C4, P4 and O2) was performed to assess the scalp distri-

bution of ERP effects and the signi®cance of differences

between left- and right-hemisphere scalp locations.

The Greenhouse±Geisser correction was applied to those

analyses with more than one degree of freedom in the

numerator; the epsilon correction factor is provided (Green-

house and Geisser, 1959). The Tukey honest signi®cant

difference (HSD) for post-hoc pairwise comparisons in the

repeated-measures analysis was used.

2.2. Results

2.2.1. Behavioral ®ndings

Fig. 1 shows the mean RT results in each task. Across task

condition, subjects responded faster to related targets than to

unrelated targets (main effect target type: F�1; 17� � 48:6,

P � 0:0001). There was no signi®cant Target type £ Task

interaction.

2.2.2. ERPs

Less than 20% of trials was rejected for artifacts or incor-

rect responses in order to compute the average ERP of both

related and unrelated conditions. Although there was a

strong difference between the number of related and unre-

lated pairs within tasks, a random selection was performed

in order that the number of segments accepted was approxi-

mately equal in both conditions (from 22 to 24 in all cases).

The grand-average ERPs for related and unrelated word

pairs are plotted in Fig. 2. One of the ®rst visible compo-

nents in all ERPs is an anterior negativity peaking around

100 ms (N100), which is more apparent in the low and in the

high proportion tasks. Following the N100, an anterior posi-

tivity peaking around 200 ms was elicited (P200). Over

posterior sites, a positivity around 100 ms and a negativity

around 180 ms can be seen (N180). After the P200, there

was a negative-going wave that peaked at about 400 ms

(N400) with a broad scalp distribution.

J. Silva-Pereyra et al. / Clinical Neurophysiology 110 (1999) 813±824 817

Fig. 1. This ®gure shows the reaction times for the 3 tasks on experiments 1 and 2. The thicker line shows the responses to unrelated pairs and the thinner line

shows the responses to related pairs. The priming effect was observed equally in the 3 tasks, but in the second experiment, with mediated-related words the

priming effect was smaller than in ®rst experiment with directly related pairs.

2.2.3. Three way rm-ANOVA

Across tasks, the mean amplitude of the N400 was larger

for unrelated than for related word pairs (main effect of

target type: F�1; 17� � 23:9, P � 0:0001). Effects of Target

type £ Task interaction are plotted in Fig. 3. The N400

effect was larger in the HP and LP tasks than in the SP

task (F�2; 34� � 4:7, P � 0:023, e � 0:8240). There was

no signi®cant N400 effect in the SP task. There was also a

signi®cant Target type £ electrode localization interaction

(F�3; 51� � 5:82, P � 0:01, e � 0:5481). These results indi-

cate that the N400 effect is larger over Cz and Pz than over

Fz and Oz.

2.2.4. Four way rm-ANOVA

In the same way as for the 3 way rm-ANOVA for the mid-

line localizations, there was a signi®cant main effect of

Target type in 4 way rm-ANOVA (F�1; 17� � 21:4,

P � 0:0002), indicating an N400 effect across tasks and

electrode localization. N400s to both related and unrelated

pairs were signi®cantly larger at localizations over the left

hemisphere than the right hemisphere (F�1; 17� � 5:8,

P � 0:023). Effects of Target type £ Task are plotted in

Fig. 3. This ®gure shows that the N400 effect was larger

in the HP and LP tasks than in the SP task (F�2; 34� � 6:38,

P � 0:009, e � 0:7731). In fact, there was no N400 effect in

the SP task. There was also a signi®cant Target type £electrode localization, Target type £ Hemisphere and

Target type £ electrode localization £ Hemisphere interac-

tions. These results indicate that the N400 effect was larger

over Centro-parietal regions than over other recording sites

across tasks. The N400 effect was also larger over the right

hemisphere than over the left hemisphere. The N400 effect

was larger over the right hemisphere only in centro-parietal

regions.

3. Experiment 2

In this experiment, the effect of mediated priming on the

N400 was analyzed. The same 3 tasks as in experiment 1

were studied, but the word pairs differed. Using the list of

mediators (direct priming target) of experiment 1, a new list

(mediated priming targets) was created. The percentage of

non-words was 20% and the total percentage of non-words

was 22%.

3.1. Methods

3.1.1. Subjects

Subjects were 20 undergraduate male students between

20 and 30 years of age, with a mean age of 25.4 years.

Eighteen subjects were dextral and had no history of famil-

ial sinistrality. Two subjects had a left-handed mother or

father. Subjects had normal vision and no history of neuro-

logical or psychiatric disorders.

3.1.2. Stimuli

In the construction of the list of mediated-priming targets,

a list of triplets was created. This list contained an initial

word (prime). The prime was associated with a second word

(mediator). And the mediator was associated with a third

word (target). Thus, the ®rst and the third word comprise a

J. Silva-Pereyra et al. / Clinical Neurophysiology 110 (1999) 813±824818

Fig. 2. ERPs in experiment 1: (a) high proportion task (b) low proportion

task, and (c) single presentation lexical decision task. The thicker line

shows the responses to unrelated pairs and the thinner line shows the

responses to related pairs. There was no signi®cant N400 effect in (c).

mediated pair. These mediated pairs did not show direct

associations (Balota and Lorch, 1986). The list of 256

mediators of experiment 1 was read to a new group of 20

undergraduates to obtain the related pairs for these media-

tors. The same procedure as in experiment 1 was performed

to obtain the list of target words. 64 repeated triplets were

excluded.

For triplets validation (prime-mediator-target), the proce-

dure described by Balota and Lorch (1986) was used. In a

new sample of 20 undergraduates and graduates, the relat-

edness between the prime and target was evaluated. The

students were asked to rate the degree of association for

word pairs constructed from the triads. Each subject was

presented a list of 192 word pairs to rate on a 6 point

scale, ranging from strongest association (5) to no associa-

tion (0). Those triplets with values equal or higher to 3 were

disregarded and a list of 110 triplets without repetition was

obtained.

The number of pairs for each experimental condition was

equal to Experiment 1.

3.1.3. Procedure

The same procedure as in experiment 1 was used.

3.1.4. Recordings and analysis data

The same recordings and analysis data as in Experiment 1

were used.

J. Silva-Pereyra et al. / Clinical Neurophysiology 110 (1999) 813±824 819

Fig. 3. N400 amplitude on the left hand side: the effects of Target type £ Task interaction on experiment 1, using the midline electrodes (top) and F3, F4, C3,

C4, P3, P4, O1 and O2 (bottom). On the right hand side: the effects of target type £ task interaction on experiment 2, using the mid-line electrodes (top) and F3,

F4, C3, C4, P3, P4, O1 and O2 (bottom). There was an N400 effect using directly related pairs (experiment 1) in the high and low proportion tasks but not in the

single presentation task. Using mediated related pairs (experiment 2) the effect was observed only in the low proportion task.

3.2. Results

3.2.1. Behavioral ®ndings

Fig. 1 shows the RTs for the 3 tasks. As shown in experi-

ment 1, subjects responded faster to mediated word pairs

than to unrelated word pairs across tasks (main effect of

Target type: F�1; 19� � 58, P � 0:0001). There was no

signi®cant Target type £ Task interaction.

3.2.2. ERPs

Fewer than 20% of the trials was rejected for artifact or

incorrect responses, in order to calculate the average ERP of

both related and unrelated conditions. Although there was a

strong difference between the number of related and unre-

lated pairs in tasks, the number of segments accepted was

approximately equal in both conditions (22 to 24 in all cases).

The grand average ERPs for mediated and unrelated word

pairs are plotted in Fig. 4. In a similar way to the previous

experiment, one of the ®rst visible components in all ®gures

is an anterior negativity peaking around 100 ms (N100),

which is more apparent in the LP than in the HP task.

Following the N100, an anterior positivity peaking around

200 ms was elicited (P200). Over the posterior sites, a posi-

tivity around 100 ms and a negativity around 170 ms can be

observed (N170). After the P200, there was a negative-

going wave that peaked at about 400 ms (N400) and had a

broad scalp distribution.

3.2.3. Three way rm-ANOVA

There was a main effect of electrode localization

(F�3; 57� � 3:47, P � 0:0357, e � 0:7447). Oz was more

negative than any other site.

The effects of Target type £ Task interaction are plotted

in Fig. 3. The N400 effect was only observed in LP task

(F�2; 38� � 4:1, P � 0:0362, e � 0:7799).

3.2.4. Four way rm-ANOVA

There were electrode localization and hemisphere main

effects (F�3; 57� � 9:58, P � 0:0004, e � 0:6892 and

F�1; 19� � 8:51, P � 0:0089). Occipital regions were

more negative than other regions and the left hemisphere

was more negative than the right hemisphere.

The effects of Target type £ Task are plotted in Fig. 3.

The N400 effect was only observed for the LP task

(F�2; 38� � 3:79, P � 0:05, e � 0:6908). There were signif-

icant electrode localization £ hemisphere and Task £electrode localization £ hemisphere interactions (F(3,57) �11:1, P � 0:0002, e � 0:6308 and F(6,114) � 7:54,

P � 0:0001, e � 0:5514, respectively). These results indi-

cate that the left hemisphere was more negative than the

right hemisphere over frontal and central region, in the

HP and LP tasks.

3.2.5. Inter-experiments analysis

Using the pooled data from the two experiments, 3 way

ANOVAs were performed to assess priming and N400

effects using relatedness type as a factor in the analysis.

The variables included for the ANOVA with behavioral

J. Silva-Pereyra et al. / Clinical Neurophysiology 110 (1999) 813±824820

Fig. 4. ERPs in experiment 2: (a) high proportion task (b) low proportion

task, and (c) single presentation lexical decision task. The thicker line

shows the responses to unrelated pairs and the thinner line shows the

responses to related pairs. A signi®cant N400 effect was only observed in

(b).

data were Relatedness type (directly related pairs and

mediated related pairs), Target type (related and unrelated),

and Task (HP, LP and SP). There was a signi®cant Target

type £ Relatedness type interaction (F�1; 36� � 7:54,

P � 0:0093). This result means that the priming effect

was smaller in mediated related pairs than in directly related

pairs (Fig. 5).

The variables included for the analyses with N400 data

were Relatedness type, Target type, Task and Electrode loca-

lization (for the mid-line Fz, Cz, Pz and Oz were used, and

when Hemisphere was employed as another factor, F3, F4,

C3, C4, P3, P4, O1 and O2 were used). Fig. 5 shows the

signi®cant Target type £ Relatedness type interactions

(F�1; 36� � 4:6, P , 0:036 for the mid-line leads and

F�1; 36� � 4:5, P , 0:041 with Hemisphere factor).

Three-way ANOVAs were also performed for each task

to assess the N400 effect using relatedness type. Variables

included Target type, Relatedness type and electrode loca-

lization (Fz, Cz, Pz and Oz). Only on the HP task, there were

main effects of Target type and Relatedness type

(F�1; 36� � 8:5, P � 0:006 and F�1; 36� � 4:3, P � 0:04,

respectively). This indicates that there was an N400 effect

and that the directly related pairs were more negative than

mediated related pairs. There was a signi®cant Target

type £ Relatedness type interaction (F�1; 36� � 5:06,

P � 0:0307). This means that the N400 effect was larger

when directly related pairs were used than when mediated

pairs were used.

4. Discussion

Behavioral studies that used low proportion of related

pairs as a procedure to reduce subject's strategies, showed

a signi®cant reduction of the priming effect (Tweedy et al.,

1977; den Heyer et al., 1983; Shelton and Martin, 1992).

This procedure possibly reduces the use of an expect-

ancy strategy. In the same way, other behavioral studies

that used a single-presentation lexical decision task,

showed a great reduction (but still signi®cant) priming

effect (McNamara and Altarriba, 1988; Shelton and

Martin, 1992). This ®nding supports the idea that semantic

priming involves both automatic as well as controlled

processes.

Shelton and Martin (1992) showed that the priming effect

with mediated related pairs is smaller than with tasks with

directly related pairs. According to the spreading activation

model, the greater distance between two concepts or nodes

(relatedness level), the lesser the amount of priming effects.

According to them, the decision is also more automatic

because the subject may not be conscious of the relationship

between word pairs.

In the present study, behavioral priming effects were

found in the 3 tasks used in the two experiments. However,

there were no differences between high and low proportion

of related pairs and single presentation tasks. There was a

smaller amount of priming effect during mediated related

pairs than during directly related words. This ®nding

supports the fact that semantic priming effects decrease

with less relatedness between prime and target.

The ERP results are quite different from the behavioral

ones given that they were sensitive to the experimental

maneuvers in reducing subject strategies. A signi®cant

N400 effect was observed in experiment 1. There was also

a signi®cant Target type £ Task interaction. This result indi-

cates an N400 effect in HP and LP tasks, but not on the SP

J. Silva-Pereyra et al. / Clinical Neurophysiology 110 (1999) 813±824 821

Fig. 5. This ®gure shows the Relatedness type £ Target type interaction for behavioral and N400 data. The thicker line shows the responses to unrelated pairs

and the thinner line shows the responses to related pairs. It is possible to observed a decrease in the RT as well as in the N400 effect during the second

experiment (mediated-related pairs).

task. This N400 effect showed a central-parietal topography,

which is larger over the right hemisphere, as has been

previously described (Holcomb, 1988; Kutas and Van

Petten, 1988; Curran et al., 1993). There were no signi®cant

differences between the N400 effect on the HP and LP tasks.

In contrast to the ®ndings of Holcomb (1988), this result

suggests that the so-called low proportion of related-pairs

procedure does not have an important role in the size of the

N400 effect. A possible explanation for the present ®nding

is that the level of relatedness was perhaps not equal on the

HP and LP tasks.

There is an important difference between the HP and LP

tasks with respect to the SP task, because there was no

evidence of an N400 effect on the SP task neither on experi-

ment 1 nor on experiment 2. The case seems to be that single

presentation and continuous responses (SP task) indeed

reduce the possibility of usage of strategies of any type

and appearance of the N400 effect independent of related-

ness. To our knowledge, there are no papers describing a

single presentation lexical decision task during ERP record-

ing. For this very reason we are unable to compare our

results with anybody else's. The morphology of the N400

during the SP task is in accordance with any other N400

previously described. However, there was no difference

between related and unrelated pairs (Figs. 2 and 4), suggest-

ing that controlled processes are the speci®c mechanisms

responsible for the N400 effect.

With respect to the mediated-related pairs during the HP

task, the N400 effect seems to disappear when compared

with the results obtained on experiment 1. High proportion

of related pairs enhance priming due to expectancy (Tweedy

et al., 1977), however, the presentation of mediated-related

pairs decrease subject strategies (Balota and Lorch, 1986).

The absence of an N400 effect in this case strongly supports

the hypothesis that the N400 effect is related with controlled

process.

However, LP results are not so clear. Theoretically, LP

reduces expectancy but not semantic matching to the

presentation of directly related pairs (den Heyer et al.,

1983). It is not clear if mediated-related pairs reduce both

expectancy and semantic matching strategies. For the case

of HP, the most important strategy is expectancy, and

mediated pairs produce a complete reduction of the N400

effect. However, the SP task reduces all type of strategies

and N400 effect was observed either with directly or

mediated-related pairs, supporting our hypothesis that the

N400 is related with controlled processes. Thus, it seems

possible that mediated-pairs modulate expectancy but not

semantic matching. If this is the case, our N400 results for

the LP task would be explained.

In conclusion, it seems that the N400 effect mirrors a

controlled process. This process could be manipulated

with the procedures suggested to reduce decision strategies.

It is possible that the strongest procedure to reduce the

controlled processes is the single presentation lexical deci-

sion task.

Acknowledgements

We wish to thank Dra. Maritza Rivera Gaxiola for her

invaluable suggestions and her contribution in improving

the manuscript. This work was partially supported by Grants

DGAPA IN214295 and IN208596 and CONACyT 1028P-

H.

Appendix A

Prime Mediator Target

Single presentation lexical decision taskMantel mesa silla

Tenedor cuchara sopa

Placa carro llanta

Pincel pintura casa

Salvavidas mar peces

Taxi viaje avioÂn

Hotel cama almohada

Apagador luz sol

Fuego agua vaso

Falda mujer hombre

Calceta zapato suela

Campana iglesia religioÂn

BuÂho noche estrella

Trompa elefante zooloÂgico

Tomate rojo sangre

Pastor ovejas lana

obrero faÂbrica humo

mago conejo zanahoria

Ladrillo casa hogar

Enchiladas mole ®esta

Aceite grasa cerdo

Hormiga trabajo dinero

Bufanda frõÂo hielo

Bombero fuego estufa

Crayola cera vela

Vendedor mercado verduras

Tobillo pie zapato

Mariposa colores arcoiris

AÂ rbol hojas libro

Al®ler aguja hilo

Low proportion taskMosco piquete dolor

Hacha aÂrbol hojas

Canario jaula barrotes

Capullo mariposa alas

ratoÂn gato perro

Guajolote pavo navidad

Horno pastel cumpleanÄos

Lagartija sol calor

Tocino huevos gallina

AlacraÂn veneno muerte

Carretilla arena playa

J. Silva-Pereyra et al. / Clinical Neurophysiology 110 (1999) 813±824822

(continued)

Prime Mediator Target

Corbata camisa botones

Melena leoÂn tigre

DiaÂmetro cõÂrculo cuadrado

Tinta pluma laÂpiz

Semana dõÂa noche

Ostra perla collar

Torero toro vaca

Toro vaca leche

Gato raton queso

DõÂa noche obscuro

Verano invierno nieve

Mano pie rodilla

AlgodoÂn alcohol borracho

Guitarra piano tecla

Luna sol calor

Boda anillo dedo

Ojos nariz olor

Pantalones camisa cuello

Boca trompa elefante

High proportion taskNinÄo ninÄa vestido

TeleÂfono nuÂmeros letras

PerioÂdico noticia televisioÂn

Techo piso alfombra

Dientes cepillo pelo

Cuchillo pistola disparo

Motor gasolina petroÂleo

MamõÂfero animal vegetal

Domador circo payaso

Maestro doctor enfermera

Carne cerdo sucio

Caracol baboso tonto

ChicharroÂn puerco mugre

Cuerpo hombre trabajador

Abrazo amor corazoÂn

Remo lancha motor

Espina rosa ¯or

Cola perro hueso

Listo inteligente sabio

Pellizco dolor medicina

lima limoÂn agrio

Dado juego pelota

Siembra maõÂz tortilla

Pasta dientes boca

Faja gordo grasa

ChõÂcharos verde pasto

Batalla guerra paz

Pupila ojo nariz

SateÂlite luna craÂter

Payaso diversioÂn juego

Antena televisioÂn programa

(continued)

Prime Mediator Target

funda almohada cama

pinzas depilar vello

Sardina lata refresco

Moneda peso baÂscula

Lombriz tierra planta

AzadoÂn siembra maõÂz

Servilleta limpio jaboÂn

Plumero plumas paÂjaro

Pimienta sal mar

Recreo descanso silloÂn

Cerveza vino uva

Almeja concha pan

Resortera piedra tierra

Alma cuerpo brazo

Actriz teatro butaca

Demonio in®erno cielo

Gis blanco negro

Enojo alegrõÂa sonrisa

Manzana plaÂtano penca

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