We tested whether the N400 event-related potential (ERP) indexes the integration of semantic knowledge in the context or whether it indexes the inhibition of activated, but inappropriate, knowledge. A distractor-prime-target word sequence was presented in each trial. Subjects had to make semantic relatedness judgments on prime-target pairs. In the first experiment, subjects had an additional task. They either had to ignore or to attend to distractors. In critical conditions, that is, when distractors were related to targets, the times to make the prime-target semantic relatedness judgments were longer when subjects had to attend to distractors than when they had to ignore them. In accordance with the inhibition hypothesis, the amplitudes of the N400 elicited by distractors were larger in the ignore than in the attend task. In the second experiment, the same distractor-prime-target triplets were used. However, there was no additional task. Subjects only had to make the prime-target semantic-relatedness judgment. They were then split in two subgroups: the good ignorers, who did not take much longer to make the judgment in critical than in control conditions, and the poor ignorers, that is, those who did take much longer. Results were again consistent with the inhibition idea. The amplitudes of the N400s evoked by distractors were larger in the good than in the poor ignorers. The results of these two studies are taken together to support the idea that N400 index a semantic inhibition rather than an integration effort. IntroductionThe N400 is a brain potential that is elicited by the presentation of potentially meaningful stimuli in tasks that do not focus on the elementary physical features of these stimuli. Several hypotheses as to the nature of the computations performed by the brain process that are responsible for this potential have been proposed. Recently, some consideration has been given to the possibility that the N400 potential indexes not only activation but also inhibition processes (Barber et al., 2004;

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