Interhemispheric visual interaction in a patient with posterior callosectomy.

The role of anterior commissure (AC) and anterior parts of corpus callosum in visual interactions was investigated in a partial split-brain patient whose posterior and middle parts of the corpus callosum were resected surgically leaving intact only a thin portion of anterior corpus callosum. Although the primary visual areas of the two hemispheres are disconnected in the patient, we found that visual distracters presented to one hemisphere (in a crowding paradigm) impaired recognition of the target stimulus presented to the other hemisphere. The normal control group showed the same result. To rule out the possible contribution of subcortical areas to this interaction, we repeated the same crowding task with texture-defined stimuli. The patient again showed an interhemispheric interaction, even though subcortical structures respond poorly or do not respond at all to texture defined shapes. Despite the evidence for interhemispheric interaction, a classic match-to-sample task confirmed that the patient was unable to explicitly report when stimuli in left and right hemifields were the same or different. Similarly, in a search task, the patient's reaction time was unaffected by distracters in the hemifield opposite the target whereas normals' response time was affected. Considering the dissociation between these two tasks, we conclude that the anterior commissure and/or the anterior corpus callosum contribute to interhemispheric interactions in the attentional selection of location.