Extent and limits of callosal plasticity: presence of disconnection symptoms in callosal agenesis.

Although earlier studies have emphasized the absence of 'split-brain' symptoms in callosal agenesis patients, the notion of an 'asymptomatic' acallosal brain has lately been challenged. We report a number of findings that are indicative of an interruption of interhemispheric communication and integration in individuals lacking the corpus callosum. Several groups of patients with callosal pathology (acallosals, patients with commissurotomy or callosotomy, either complete or partial) were compared to matched controls. Interhemispheric transfer was tested in two different experiments involving pointing to a light source while maintaining central fixation. In the first experiment, a learning paradigm was used to measure transfer of a motor skill from the trained to the untrained hand. In the second experiment, subjects pointed to visual targets at different locations on a perimeter. Midline fusion, a recurrent theme when describing callosal function, was assessed using tasks which included depth perception with binocular and/or monocular cues, two-point discrimination thresholds and sound localization in the peri-central and lateral fields. Subjects with callosal pathology were impaired on all tasks involving transfer of motor and visuo-spatial skills and on some of the tasks requiring sensory integration of visual and tactile information across the body midline. We conclude that these functions require an intact corpus callosum since none of these deficits were seen in controls equated for IQ.