P3 in schizophrenia is affected by stimulus modality, response requirements, medication status, and negative symptoms.

Eighteen schizophrenics who were not taking medication, 13 schizophrenics who were taking medication, and 37 age-matched controls were tested with event-related potential paradigms designed to elicit P3 response automatically or effortfully (ie, with a choice reaction time task). Electroencephalograms were recorded from the 19 standard 10-20 electrode sites. Compared with controls, both groups of schizophrenics had reduced P3 amplitudes for both effortful and automatic paradigms. P3 latencies were delayed relative to controls for the medication-taking schizophrenics in the effortful paradigms. Negative symptoms derived from the Brief Psychiatric Rating Scale within 1 week of event-related potential testing correlated negatively with both auditory and visual P3 amplitude in the subjects who were not taking medication. There was no evidence that P3 is smaller over left temporal electrode sites in schizophrenics, as has been reported by others. P3 amplitude reduction in schizophrenia is a robust psychobiological phenomenon that is present regardless of medication status or task demands.