Electrophysiological evidence for primary semantic memory functional organization deficits in schizophrenia
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N400, an event-related brain potential (ERP) waveform elicited by meaningful stimuli, is normally reduced by stimulus repetition (N400 repetition priming), and relatedness between the eliciting stimulus and preceding ones (relatedness priming). Schizophrenia patients' N400 relatedness priming deficits suggest impairment in using meaningful prime stimuli to facilitate processing of related concepts in semantic memory. To examine whether this deficiency arises from difficulty activating the prime concept per se, as indexed by reduced N400 repetition priming; or from impaired functional connections among concepts in semantic memory, as reflected by reduced relatedness priming but normal repetition priming; we recorded ERPs from 16 schizophrenia patients and 16 controls who viewed prime words each followed at 300- or 750-ms stimulus-onset asynchrony (SOA) by an unrelated, related or repeated target word, or a nonword, in a lexical-decision task. In both groups, N400s were largest (most negative) for unrelated, intermediate for related, and smallest for repeated targets. Schizophrenia patients exhibited subnormal N400 relatedness priming at the 300-ms SOA, but normal repetition priming at both SOAs, suggesting that their impairment in using prime words to activate related concepts results from abnormal functional connections among concepts within semantic memory, rather than inability to activate the prime concept itself.
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