Neural site of the redundant target effect electrophysiological evidence.

The present study represents an attempt to find an electrophysiological correlate of the redundant targets effect, or RTE (i.e., the speeding up of reaction time, or RT, for redundant vs. single targets). Subjects made a speeded response either to one small checkerboard presented to the left or right of fixation or to a pair of identical checkerboards presented simultaneously to both hemifields. Both single and double targets could appear either in the upper or lower visual hemifield. The task required detection but not discrimination of the stimuli. During task performance, we recorded the event-related potentials (ERPs) elicited by the checkerboard targets. As in previous studies, we found that manual RTs to bilateral stimuli were faster than those to unilateral stimuli. This effect was more marked for lower- than for upper-field stimuli and could not be ascribed to probability summation. In addition, we found that the P1 and N1 components of the visual ERP had a shorter latency for bilateral than for summed unilateral stimuli presented to the two hemifields. In parallel with the behavioral findings, the latency values for the above components showed a larger RTE for lower-field stimuli. These findings indicate that the RTE occurs at the level of early visual processing, probably in the extrastriate visual cortex, rather than at late decisional or pre-motor stages.