Impact of instructed relevance on the visual ERP.

Event-related potentials have been used to study the distinction between attention to novelty and effortful orienting to task-relevant items. However, effects due to stimulus frequency, response production, and instructed task-relevance have not been fully separated. The current study used a visual oddball design in which two stimuli were presented, one frequently (P = 0.8) and one infrequently (P = 0.2). Subjects (n = 21) participated in 4 blocks, two in which the infrequent stimulus was the instructed target and two in which the frequent stimulus was the instructed target. This was crossed with two blocks in which subjects were instructed to respond to the targets with a keypress and two blocks in which subjects were instructed to press a key to the ignored non-targets and respond to the targets by withholding a keypress. The results showed N1 effects for instruction, suggesting enhancement due to an attentional set established prior to stimulus delivery, and for frequency, suggesting orienting to the infrequent stimuli. A prefrontal positivity (P2a) was enhanced to instructed targets, but instruction only entered as an interactive factor on the posterior N2 at the same latency, suggesting interaction between frontal areas of evaluation and posterior areas of perceptual representation in identification of task-relevant stimuli. The P300 was enhanced to the infrequent stimuli, but there was no main effect for target instruction, although instruction did impact a higher-order interaction.