A large positive ERP component (P300, or P3; see figure) that peaks between 300 and 500 ms after stimulus presentation appears to be related to the “significance” of a stimulus. A stimulus can be significant by being relevant to a task (e.g., the subject must press a button whenever the stimulus occurs), by motivational factors (e.g., the subject wins money after responding to the stimulus correctly), or by occurring rarely or unpredictably. Significance is unrelated to the physical features of the stimulus itself (for a review, see Regan 1989).
