Interhemispheric visuo-motor integration in humans: the effect of redundant targets.

We used event-related functional magnetic resonance imaging (fMRI) to investigate the functional locus of response facilitation during parallel visuo-motor processing. In a simple reaction-time task, subjects typically respond faster to two copies of the same stimulus than to a single copy. This facilitation, called the redundant-target effect, can occur at three functional levels: perceptual, 'cognitive' or motor. Normal right handers were studied while performing a simple reaction-time task to unilateral (left or right) and bilateral light flashes. Subjects were instructed to respond with their right index finger. Reaction times were faster to bilateral light flashes than to unilateral ones, even right flashes. Greater fMRI signal for bilateral stimuli compared to unilateral ones was observed in the left precentral and postcentral gyrus, and in the right precentral gyrus. A greater fMRI signal for bilateral and for unilateral left stimuli, compared to unilateral right stimuli, was observed in an area of the right intraparietal sulcus. These results support the hypothesis that the functional locus of response facilitation during parallel visuo-motor processing is premotor.