To replicate the findings of a sensory gating deficit in schizophrenia, and to determine if normals and schizophrenics are equally affected by cognitively mediated task-relevant factors, 10 healthy and 10 schizophrenic young adults were tested in two experimental conditions. In the first condition a pair of identical auditory clicks (conditioning stimulus and testing stimulus) was administered at an inter-stimulus interval fixed at 500 msec. In the second condition, the second stimulus could have one of two possible frequencies, and subjects were required to count the presentations of one and ignore the other. Subjects also completed two matched blocks of single stimulus (i.e. testing only) presentations to provide a baseline for assessing the effect of the warning stimulus on the evoked response. We found, in disagreement with previous results, that in schizophrenics, passive exposure to the paired stimuli produced a suppression of P50 amplitude to the second stimulus, that did not differ from that found in normals. However, under paired presentation conditions, testing P50 amplitude in normals, but not in schizophrenics, was enhanced by the count/no-count task introduction. We suggest that both groups are equally susceptible to the task independent (possibly "hard-wired") suppressing effect of a conditioning stimulus presentation. However, only normals seem to exhibit a task-dependent effect, whose action at the P50 range demands the presence of a warning (conditioning) stimulus. Inspection of the full epoch data showed an apparently lesser task-related, warning dependent modulation of early ERP activity in schizophrenics, and a normal (or even supernormal) modulation of late activity. We consider this to support a notion of an early processing deficit in schizophrenia.
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