Effect of task decision on P300

Previous studies have shown that the structure of an experimental task may modulate P300 (P3) responsiveness to task-related test stimuli. In this study, four tasks were compared to study possible effects of decision type on P3s to rare and frequent visual word stimuli. One or three oddballs were designated in two separate experiments. Subjects in each experiment viewed randomized and repeated presentations of nine 5- to 8-letter words, and performed each of four target detection and choice discrimination tasks: (1) silent counting of oddball(s), (2) button-press to oddball(s), (3) choice response (‘yes’ or ‘no’) to oddball(s) and frequents, and (4) choice button-press to oddball(s) and frequents. Although oddball stimuli elicited characteristic ‘target’ P3s over the parietaL scalp, there was no significant effect of task type on the oddball-evoked P3. Frequents tended to elicit larger P3s in the choice discrimination tasks (yes/no, go/go) than in target-detection tasks (counting, go/no-go), indicating a selective effect of decision type on P3s to frequent-category events.