Visual integration in the split brain.

A review of evidence suggests that subjects with forebrain commissurotomy can integrate information about location, orientation, and movement between visual hemifields, albeit with some loss of spatial and temporal resolution, even when it is flashed briefly in parafoveal vision. The classic disconnection syndrome for visual stimuli is largely explained by the lack of subcortical transfer of information about form. Voluntary location-based ('spotlight') attention is integrated interhemispherically in these subjects but object-based attention and controlled visual search can function concurrently and independently in the two hemispheres. The data support the distinction between two visual systems, each with its own perceptual and attentional subsystems: (1) a relatively precise cortical system dedicated primarily to the analysis of form and the fine-grained exploration of visual scenes; and (2) a 'second' subcortical system dedicated to a relatively low-resolution processing of movement and location, including 'spotlight' attention.