Differential-handed response to verbal and visual spatial stimuli: evidence of specialized hemispheric processing following callosotomy.

Left- and right-hand responses were studied in a patient who had undergone complete corpus callosotomy to examine selective hemispheric processing employing free-field stimuli. Tests of verbal and visual spatial processing were presented, and multiple choice performances were obtained using both left and right hands. A double dissociation emerged, with superior left-handed performance on visual spatial tasks, and better right-handed performance on verbal tasks. In addition, the patient displayed hemispatial dyscalculia in which he solved written arithmetic problems disregarding free-field numerical information presented toward his left. Evidence of multimodality left-sided extinction was also present. Further, despite unilateral callosal apraxia, the left hand displayed superiority during a continuous performance task. These results demonstrate that specialized functions of the cerebral hemispheres may be observed for response output in the absence of restricted hemispheric input, and that components of the neglect syndrome may be seen following corpus callosotomy.