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 fi ndings that are indicative of an interruption of interhemispheric comm unication and integration in individuals lacking the corpus callosum. Several groups of patients with callosal pathology (acallosals, patien ts with commissurotomy or callosotomy, either complete or partial) wer e compared to matched controls. Interhemispheric transfer was tested i n two different experiments involving pointing to a light source while maintaining central fixation. In the first experiment, a learning par adigm was used to measure transfer of a motor skill from the trained t o the untrained hand. In the second experiment, subjects pointed to vi sual 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 th e peri-central and lateral fields. Subjects with callosal pathology we re impaired on all tasks involving transfer of motor and visuo-spatial skills and on some of the tasks requiring sensory integration of visu al and tactile information across the body midline. We conclude that t hese functions require an intact corpus callosum since none of these d eficits were seen in controls equated for IQ.