Studies of event-related brain potentials (ERPs) have shown that attributes of the ERP can be used as dependent variables in the study of human information processing. These variables can complement the information gained from the study of overt, skeletal responses. The manner in which the P300 component of the EPR can be used to study human information processing is illustrated in this report. Specifically, we show that through an analysis of the covariation of the latency of P300 component and reaction time, it is possible to examine the relation between the probability of a stimulus and the speed of response to that stimulus. Our data indicate that increased in the probability of a stimulus reduce reaction time by decreasing both stimulus-evaluation and response-production times. We also examine changes in reaction time and P300 latency induced by the match or mismatch between two stimuli presented consecutively, again as a function of probability. Models of the effects of stimulus matching on reaction time are evaluated.
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