A qualitative limitation on visual transfer via the anterior commissure. Evidence from a case of callosal agenesis.

Interhemispheric transfer of visual information was investigated in a patient with agenesis of the corpus callosum. CT scanning, pneumoencephalography, and neuropsychological evaluation failed to reveal any evidence of extracallosal cortical damage. The patient's performance was within normal limits both in the left and right visual fields on tasks requiring identification of briefly presented material. In contrast, his performance was abnormally poor when required simply to detect or localize a dot presented to the right of fixation. Performance with presentations in the left visual field was normal. These findings suggested that the absence of the corpus callosum may result in a selective impairment of visuospatial, but not identification, processes for material initially presented to the left hemisphere. To document this possibility further, the patient was tested on tasks designed to assess these processes simultaneously by requiring him both to identify and judge the exact location of each stimulus presented. Performance was normal in both lateral fields for identification accuracy, but significantly worse in the right visual field for location judgements. A similar pattern of results was found on a task requiring the identification of a stimulus and specification of its spatial orientation. These findings suggest a qualitative limitation on the type of information which may be transferred via the anterior commissure, possibly due to the inability of temporal lobe neurons to encode spatially organized information. Furthermore, the findings suggest that in the normal brain the right hemisphere may be crucially involved in visual attention and spatial functions, regardless of the field of presentation.