Interhemispheric communication following unilateral cerebrovascular lesions

39 patients with a single small cerebrovascular lesion (20 in the right, 19 in the left hemisphere) were subjected to a simple reaction time (RT) task with visual stimuli flashed to the visual field either ipsilateral or contralateral to the cerebral lesion. The subject responded always with the ipsilateral hand. The crossed-uncrossed difference (CUD), i.e. the RT when both stimulus and response occur on the same side minus the RT when stimulus and response occur on opposite sides, is assumed to assess the transit time of information through callosal fibers, and in normal people is about 3-5 msec. In our patients the mean CUD, expressed as the difference between contralateral and ipsilateral responses, was 20 msec. Patients with parietal lesions had still longer CUDs, 37 msec on the average. There was no statistical difference in CUDs between right and left brain-damaged patients. The CUD in brain-damaged patients was of the same order of magnitude as that found in acallosal or split-brain patients. Nonetheless, the present findings are interpreted as reflecting the intrahemispheric rather than the interhemispheric delay in information transmission, with the possible additive effect of an asymmetrical orienting of attention.