Young Psychophysiologist Award Address, 1980

ABSTRACT The latency of the P300 component of the event‐related brain potential plays a unique role in mental chronometry. This is illustrated by presenting data from two experiments that addressed the following issues in information processing: a) the locus of the effect of stimulus probability on choice reaction time, and b) the locus of interference in performance on the Stroop color‐word test. Using P300 latency to index stimulus‐evaluation time, it is shown in the first experiment that both stimulus and response processing are affected by stimulus probability. The results suggest that more probable events are identified faster—an effect which is reflected in decreases in both P300 and response latency—and that the responses to them are emitted more rapidly—an effect which is reflected in reaction time only. In a second experiment, using the Stroop task, response time was found to vary with the congruence between the stimulus word and the color in which it was printed. However, the duration of stimulus processing, as indexed by P300 latency, remained constant. These results indicate that response competition is the primary source of Stroop interference. It is concluded that P300 latency can be viewed as an important addition to the set of dependent variables available to study human information processing.