MONITORING THE VISUAL WORLD - HEMISPHERIC ASYMMETRIES AND SUBCORTICALPROCESSES IN ATTENTION

Hemispheric specialization and subcortical processes in visual attenti on were investigated in callosotomy (split-brain) patients by measurin g reaction times to lateralized stimuli in a spatial cuing paradigm. C uing effects were obtained for targets presented to the right hemisphe re (left visual hemifield) but not for those presented to the left hem isphere. These cuing effects were manifest as faster reaction times wh en the cue correctly indicated the location of the subsequent target ( valid trials), as compared to trials in which the cue and target appea red in opposite hemifields (invalid trials). This pattern suggests tha t the right hemisphere allocated attention to cued locations in either visual hemifield, whereas the left hemisphere allocated attention pre dominantly to the right hemifield. This finding is consistent with a b ody of evidence from studies in patients with cortical lesions who dis play different attentional deficits for right versus left hemisphere d amage. Because the present pattern occurs in patients whose cerebral h emispheres are separated at the cortical level, it suggests that right hemisphere attentional allocation to events in the ipsilateral visual half-field is mediated in part via intact subcortical systems.