The P3 complex as an index of information processing: the effects of response probability.

The amplitude and latency of the late positive (P3b) component of the cortical evoked potential were studied in a visual search task. The main independent variables were memory load, response type and relative response frequency. In the experiment we used the varied mapping procedure, a condition in which the target and distractor stimuli were thoroughly mixed across trials. The main goal of the study was to establish the locus of relative response frequency. In many models of choice reaction time the effect of this task variable is believed to occur in the response selection stage. If the latency of the P3b only indexes stimulus evaluation, independent of response selection and organization processes, relative response frequency should only affect reaction time and not the latency of P3b. The results indicated that the latency of P3b was very sensitive to the number of comparisons, response type and relative response frequency. In addition, only a subset of subjects selectively prepared for the most probable response. Only these subjects showed the effects of relative response frequency in their reaction times too. Response preparation, however, did not affect the latency of P3b. It was concluded that the latency of P3b is a very sensitive index of the search and comparison process, the binary decision process, and the probability of the type of the decision, but insensitive to motor preparation.