N400 abnormalities in unmedicated patients with schizophrenia during a lexical decision task.

A lexical decision task was used to investigate semantic processing in schizophrenia. Eighteen unmedicated schizophrenics and 18 gender/age-matched controls were tested. Subjects were visually presented with pairs of words. The target word (S2) was either a non-word, semantically related, or unrelated to preceding word (S1). Subjects decided whether the S2 was a word or non-word. Event related potentials (ERPs) elicited by the S2, including N350, and the late positive component (LPC) were measured. The latency of the N350 was prolonged in schizophrenia. The N400 effect, measured by the mean amplitude (300-500 ms) from difference waves (unrelated word-related word, non-word-related word), was smaller in patients. Peak amplitude of the LPC was reduced and latency of the LPC was delayed in the schizophrenics. Behaviorally, control subjects responded much faster to related words compared to unrelated or non-words, while patients showed little difference in processing speed between word categories. A reduced N400 effect suggested inefficient utilization of the context, while prolonged latency of the ERP components suggested a general delay of semantic information processing in schizophrenia.