The effect of intentional expectancy on mental processing: a chronopsychophysiological investigation.

Expected events are processed faster than unexpected ones. Previously, we have reported evidence that passive expectancies determined by the preceding event sequence mainly affected the duration of motoric processes. Here we examined the effects of instructed, intentional expectancies in a two-choice reaction time (RT) task. As predicted, RTs were shorter for expected than unexpected events. Onset of the lateralized readiness potential as an index of selective response activation, and the latency of the P300 component of the event-related potential, reflecting the duration of perceptual processing, indicated that intentional expectancy affected the time demands for central processing but did not influence the duration of early perceptual or motoric processes. Together the present and our previous results provide evidence that different kinds of expectancy can be distinguished by their locus of action within the information processing system.