Hemispheric interactions in temporal judgments about spatially separated stimuli

Human observers judged whether pairs of spatially separated lights when briefly flashed were successive or simultaneous. When the lights were in opposite hemifields, normal participants were better at detecting leftward than rightward succession, consistent with a left-hemispheric locus and a delay in transfer from right to left hemisphere. However, there was no hemifield difference with unilateral presentation, suggesting that neither hemisphere is uniquely specialized for the task. A participant with forebrain commissurotomy (L.B.) proved unable to make the discrimination when the lights were in opposite hemifields, implying that subcortical mechanisms for interhemispheric integration Lack the necessary temporal resolution. However, L.B. performed at better than chance when the disks were presented unilaterally to either hemifield.