Selection within fixation: event-related potentials in a visual matching task.

In a visual matching task, suit (spades and/or hearts) target stimuli and irrelevant stimuli (either spade and/or heart suits or plus signs) were presented to a field of 1 deg. The irrelevant stimuli appeared either beside or between the targets. The 'same-different' reaction times (RT) and the event-related potentials were measured. 'Same' RTs were found to be longer in the presence of suit distractors that were different from the target, than in the case of plus-sign irrelevant stimuli and irrelevant stimuli identical to the targets. This effect was larger when the irrelevant stimuli appeared between the targets. 'Different' RTs were longer in the presence of suit distractors and when the irrelevant stimuli were between the targets. In the 'same' trials, the latency of the late positive complex (P300) was larger in the suit distractor conditions, but this effect was unrelated to the target localization factor. Thus, the P300 latency seems to be related to the complexity of the stimulus display and thus to the difficulty of input processing. Concerning the earlier latency range, in the conditions with short RT, a latent negative shift (with 170 ms peak latency) was detected when these responses were compared to conditions of longer RT.