N400 ABNORMALITIES IN UNMEDICATED SCHIZOPHRENIA DURING A LEXICAL DECISION TASK REVEALED BY LORETA (LOW RESOLUTION ELECTRO-MAGNETIC TOMOGRAPHY)

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Background: Event-related potential (ERP) measures of language processing have been reported to be abnormal in schizophrenia. However, few studies have evaluated unmedicated patients, nor are the cortical systems involved in these deficits well defined.Methods: ERPs were used to investigate lexical priming in eighteen unmedicated patients with schizophrenia and eighteen age-matched control subjects. Pairs of stimuli were visually presented. The first stimulus was a word. The second stimulus was either semantically related, unrelated to the initial word, or a non-word. The subject decided whether the second stimulus was a word or non-word. The second stimulus elicited the P200, N3530, and the late positive components (LPC). Low resolution electro-magnetic tomography (LORETA) was applied to the N400 difference waveforms to assess the brain cortical distribution of N400 neural activity for both groups.Results: Both N350 and LPC latency were prolonged in schizophrenia, consistent with slowed semantic processing. The N400 priming effect, measured from the difference ERPs in each condition, was reduced in patients. The N400 current density was prominent over the bilateral prefrontal and bilateral temporal in the control group. Patients, in contrast, showed decreased N400 current density, primarily over bilateral posterior temporal areas.Conclusion: The diminished N400 effect suggested reduced sensitivity to semantic relationships in schizophrenia, perhaps reflecting abnormal semantic network connectivity. Reduced temporal region N400 activation in schizophrenic patients supported the temporal lobe structures as the neural substrates subserving language and verbal disassociations in schizophrenia.