Existing hypotheses propose that abnormal spread of activation in semantic long-term memory causes disorganized speech in schizophrenia. There is in fact behavioral and event-related brain potential (ERP) evidence that schizophrenia patients are deficient in using meaningful contextual stimuli to pre-activate related concepts and thereby facilitate, or prime, their processing. In contrast, results from other behavioral studies suggest that, following meaningful stimuli, schizophrenia patients activate weakly related concepts in particular to a greater than normal degree. In order to account for these different results, some researchers have proposed that the former abnormality may occur at shorter intervals and the latter at longer intervals following a meaningful stimulus.

The present studies examined in finer detail how relatedness and time course modulate activation of concepts in semantic memory in schizophrenia patients, and in healthy individuals as a function of schizotypal personality, which may be genetically linked to schizophrenia. As a measure of semantic activation, these studies used the amplitude of the N400 ERP component elicited by meaningful stimuli. N400 amplitude is normally reduced by factors thought to pre-activate the eliciting stimulus, such as its relatedness to preceding context.

We found evidence that schizophrenia patients have deficits in using meaningful contextual stimuli to pre-activate related concepts in general, over both a short and a long time course. The apparent discrepancy between these results and those of previous behavioral studies of schizophrenia patients suggesting increased spread of activation to weakly related items may stem from response-related factors affecting behavioral reaction time but not N400 amplitude. In addition, we found that patients' N400 abnormalities were specifically correlated not with disorganized language production, but with positive psychotic symptoms (i.e., delusions and hallucinations). Thus, semantic priming abnormalities may play a causal role in the development of delusions. Analogous to what we observed in schizophrenia patients, N400 data in healthy individuals were consistent with an association between higher schizotypy and decreased use of context to activate relate items and/or to inhibit unrelated items. This finding supports the view that decreased semantic priming may be one of a number of neurophysiological markers common to schizophrenia and schizotypal personality.