Late positive components and stimulus evaluation time.

The amplitude and latency of late positive components were, together with reaction time (RT), studied in a task which combines visual and memory search. The visual display contained either one, two or four letters, as did the memory set. Six load combinations, resulting in one, four, eight and 16 comparisons, were examined. The reaction time data indicated a self-terminating search process. Three late positive components were present in the evoked potential: one at 375 msec after the onset of the display, one at 375 msec after the offset of the display and one around 600 msec. Only the latter component appeared to be sensitive to the number of comparisons. Adaptive averaging was applied to this latter component. The latency of this P300 suggested, in contrast to the RT data, an exhaustive search process. In addition there was a negligible correlation between the response latency and P300 latency at single trial level. Several hypotheses are suggested for what P300 could have to tell us.