Decision making and the P300 component of the cortical evoked response

The amplitude of P300, and of other components of the evoked potentials, was examined during a task which required Ss to make a response appropriate to the conjoint properties of two rapidly successive visual patterns. In one set of conditions, the task was structured such that both stimuli were needed for the choice judgment. In another set of conditions, the task was changed from choice to simple RT by presenting the patterns in a predictable order. It was observed that P300 was enhanced during the choice RT conditions and that this enhancement was present only for the P300 following the stimulus permitting the choice and not the other, relevant but not decisive, stimulus appearing in close temporal proximity. An appreciable degree of independence between P300 and other components was indicated by the appearance of P300 under conditions in which other evoked potential components were entirely refractory. The data support an account for P300 in terms of poststimulus processes, such as decision making, and not in terms of preparatory adjustments.