Stereognosis in the chronic split brain: hemispheric differences, ipsilateral control and sensory integration across the midline.

Benton's test of Stereognosis was administered separately to the left and right hands of six patients with complete cerebral commissurotomy and two patients with partial commissurotomy sparing the splenium. Experiment 1 displayed the multiple choice card in free vision for all patients whereas Experiment 2 displayed it for ocular exploration in one visual field at a time, either the same or opposite to the exploring hand, in two complete commissurotomy patients fitted with special contact lens systems. There were deficits in stereognosis without primary somatosensory impairment in both disconnected hemispheres and the deficit was more marked in the hemisphere with predominant extracallosal damage. The two disconnected hemispheres appeared to use different strategies for recognizing shapes. As can be expected, naming of stimuli explored with the left hand was worse than of stimuli explored by the right hand, but it was above chance and variable across patients. Left hand naming was not affected by restricting stimulus exploration to distal joints. Naming of stimuli explored in the left visual hemifield was worse than of stimuli explored with the left hand, suggesting that it reflects left rather than right hemisphere speech. There was surprisingly good performance in the cross-hemisphere condition where one hand explored the stimulus and the multiple choice card was explored in the opposite visual field. The left hand-right hemifield combination was more effective than the right hand-left field combination, suggesting better "ipsilateral" tactile/kinesthetic projections in the left than right hemisphere.