Depth-of-processing effects on semantic activation deficits in schizophrenia: an electrophysiological investigation

N400, an event-related brain potential (ERP) waveform elicited by meaningful stimuli, is normally reduced (made less negative) by relatedness between the eliciting stimulus and preceding ones (N400 semantic priming). Schizophrenia patients' N400 semantic priming deficits suggest impairment in using meaningful context to activate related concepts in semantic memory. We aimed to examine the degree to which this impairment can be ameliorated by task instructions that more explicitly require processing of stimulus meaning. We recorded ERPs from 16 schizophrenia patients and 16 controls who viewed prime words each followed at 750-ms stimulus-onset asynchrony by an unrelated or related target word, or a nonword, in a non-semantic task (indicating whether a letter occurred in the target) compared to an explicit semantic task (judging prime-target relatedness). Consistent with previous work, controls exhibited greater N400 semantic priming (larger amplitude reductions for related versus unrelated targets) in the semantic task than in the orthographic task. Schizophrenia patients showed this same pattern, although their N400 semantic priming effects were smaller than controls' across tasks. Nevertheless, patients' priming effects increased as much as did controls' from the orthographic to the semantic task. Thus, connections among related concepts in schizophrenia patients' semantic memory appear grossly intact, such that, given a meaningful stimulus, they can make use of explicit cues to activate related concepts at a neurophysiological level, although their ability to do so remains less than normal. These data provide support for further research on semantic-cueing strategies for cognitive remediation of verbal memory in schizophrenia.