Task redundancy and an enhancement of the late positive components in human visual evoked potentials

VEPs to four or six repetitive letter and checkerboard stimuli were measured. These stimuli were presented in a first triplet and a single stimulus or in first and second triplets. Three stimuli in a triplet were identical. The subject's task was either naming the letter stimulus or pattern matching the checkerboard. A degree of task-redundancy of the stimuli at intratrial stimulus positions 2 and 3 was varied by means of stimulus duration and indication of the task stimulus prior to the triplet. The stimulus duration was 300 msec and the prior information of the task stimulus was delivered by a letter or checkerboard warning stimulus (WS) in session 1. The stimulus duration was shorter (10 msec in session 2, 20 microseconds in session 3) and the WS of random pattern did not deliver the information of the task stimulus in session 2 and 3. At stimulus positions 2 and 3, the P260 of the occipital VEP enhanced in amplitude for the letter and checkerboard stimuli in session 1, whereas the P260 decreased in amplitude for both stimuli at the same positions in session 3.