Psychiatric drugs and the somatosensory P400 wave

The effects of antipsychotic and antidepressant medications on the somatosensory evoked potential (SEP) late wave (P400) were investigated in a diagnostically heterogeneous group of 51 psychiatric patients. SEPs were recorded from five scalp locations while subjects counted electrical stimuli to one of four randomly stimulated fingers. In seven subjects, tested both on and off medication, no significant drug effects were observed. P400 amplitudes of 14 patients being treated with antipsychotic medication were greater than those of unmedicated patients matched for age, sex, and diagnosis. P400 amplitudes did not differ between eight patients being treated with antidepressants and eight unmedicated matched patients. Accuracy in stimulus counting (index of task performance) was significantly correlated with P400 amplitude; more accurate counting was associated with higher P400 amplitude. When 14 medicated and 14 unmedicated subjects, matched for task performance, age, sex, and approximate diagnoses, were compared, P400 amplitude did not differ. These findings suggest that higher P400 amplitudes associated with medication probably reflected improved task performance with medication, rather than a direct action of the drugs on P400. The results suggest the conclusion that lower P400 amplitude in psychosis is not secondary to psychiatric drugs.