Processing capacity and the frontal P3.

In the present study, unexpected, task-irrelevant, visual stimuli (probes) were presented against the background of one of two visual tasks. Subjects either performed an easy (low task load) or hard (high task load) visual task. During task performance probes were presented consisting of standard (80%), slightly deviating (deviants, 10%) and highly deviating stimuli (rares, 10%). The rare stimuli induced both a frontal (peak latency 448 ms) and a parietal P3 (peak latency 502 ms). The latter P3 was smaller during the high load condition, but in contrast, the frontal P3 was not influenced by task-load. The results of the present study suggest that the frontal and the parietal P3 are generated by two different neural sources, of which only the latter is capacity-limited.