The difference in findings for silent-counting and button-press responses could stem from movement-related potentials (MRPs) in the latter task. In oddball tasks where a right button press was the predominant mode of response (e.g.,20,21), lateralized MRPs, which are negative in polarity and larger over frontocentral sites contralateral to the hand movement, might have interfered with demonstration of the P3 asymmetry difference between schizophrenic patients and controls. However, studies comparing P3 findings for silent-counting and button-pressing responses have yielded conflicting findings.26,27 Ford et al.26 measured ERPs of 17 men with schizophrenia and 11 healthy controls in three tasks in which subjects counted oddball tones or pressed a button with either the right or left hand. While button presses significantly affected P3 amplitude and asymmetry over left and right motor cortices (C3/4), schizophrenic patients did not show smaller P3 over left than right temporal lobe sites (T7/8) in either silent-counting or button-pressing tasks. In contrast, Salisbury et al.27 found that overall P3 amplitude was smaller and P3 asymmetry was reduced when healthy controls (n = 46) responded with the right finger as
