Changes in auditory P3 event-related potential in schizophrenia and depression.

Event-related potentials during a two-tone discrimination task were recorded in 24 schizophrenic patients, 16 depressed patients and 59 control subjects. Recordings were made when patients were medication-free. Fourteen schizophrenic and 13 depressed patients were retested at 1 and 4 weeks after the start of treatment, and 13 schizophrenic patients were also tested between 6 and 24 months after the initial recordings. In the schizophrenic group, the P3 latency was significantly prolonged compared with that in the control and the depressed groups, and remained unchanged both after 4 weeks treatment with therapeutic doses of neuroleptic drugs and at long-term follow-up. In the depressed group, the P3 latency did not differ from that of controls. P3 amplitude by contrast was reduced in both the acutely depressed and schizophrenic groups and following treatment became normal in the depressed group but remained reduced in the schizophrenic group. It is suggested that a prolonged P3 latency and reduced P3 amplitude indicate an impairment of auditory information processing in some patients with schizophrenia which is independent of the presence of acute psychotic symptoms and is not influenced by neuroleptic medication.