The attention-related somatosensory evoked potential late positive wave in psychiatric patients.

Somatosensory evoked potential (SEP) late waves (P400) associated with selective attention were elicited by having subjects count electrical stimuli to one of four randomly stimulated fingers. P400 amplitude was lower in a diagnostically heterogeneous group of 20 psychiatric patients than in 16 nonpatients. Among patients, a general psychopathology factor derived from the Millon Clinical Multiaxial Inventory was significantly correlated with a factor accounting for most of the P400 amplitude variance. In psychiatric patients, attention-related late activity amplitude is apparently reduced from normal in evoked potentials of all modalities; degree of reduction appears quantitatively related to level of psychopathology.