| N400, a negative deflection peaking at about 400 ms after the target onset, is a well known ERP (event-related potential) component that was first observed when a semantically inappropriate word appeared unexpectedly at the end of a sentence. When asked to perform a lexical decision task, subjects typically respond faster and more accurately to the target with a related prime than to that with an unrelated prime, and the N400 amplitudes for the related target are smaller than those for the unrelated targets. This amplitude difference between related and unrelated targets is the N400 priming effect. Whether the N400 priming effect is based on semantic or associative relationship between lexical items, or both? Answer to this question is very important to research the neural mechanism underlying N400 priming effect. In the past three decades, a significant number of studies have exposed mechanisms underlying N400 priming effect. A popular mechanism proposed to explain the N400s is the word-level automatic process or lexical analytic process, and contrasts with equally popular postlexical mechanisms that are conscious and attentional (limited processing capacity). Postlexical mechanisms include context (semantic) integration, expectation process, and response strategies like postlexical checking. All these postlexical accounts assume that meaning has been accessed by the onset of N400. The present study based on the theories or mechanisms mentioned above, and we used ERP methodology to investigate electrophysiological correlates of cognitive representations underlying N400 priming effects for different types of prime-target relationships and highly blurred and consciously unidentifiable Chinese character targets.In experiment 1, We independently manipulated the presence of associative and semantic relationships while examining the N400 effect. Participants were asked to check if there is an animal had presented before during the presence of prime and target that shared an association (wind-mill), an association+semantic relationship (learn-study), a semantic relationship alone (road-street), or were unrelated (liver-desk). Modulation of the N400 (relative to unrelated word pairs) was observed for association and association+semantic word pairs but not for those that only shared a semantic relationship during 300-350 ms time window, and N400 effects were observed for three relationships during 350-500 ms time windows. However, N400 effects lasted during 500-900 ms time window only for association relationship. These findings suggest that associative and semantic relationships are neurally dissociable. Specifically, it is the associations between words, rather than the semantic relationship between them, that is critical to N400 priming effect.In experiment 2, We investigated the effects of stimulus degradation on ERP measures of semantic priming. The goal was to elucidate whether N400 effect is sensitive to lexical or postlexical process. ERPs of blurred Chinese characters were recorded while participants performed a delayed character-matching task. Results reveal that highly blurred and consciously unidentifiable targets that are semantically related to primes have elicited smaller N400 compared with highly blurred and unrelated targets. This N400 effect is greater, and its latency is shorter for slightly blurred and identifiable targets in contrast to highly blurred targets. Results have likewise revealed, for the first time, a significant interaction of the stimulus degradation with the N400 effect, as well as with behavioral effects. These findings support the view that the N400 component could index lexical processing, providing evidence against postlexical accounts.This study support the view that it is the associations between words, rather than the semantic relationship between them critical for N400 priming effect, what's more, the N400 component could index lexical processing, but not postlexical processing.
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