Cortical and Subcortical Interhemispheric Interactions Following Partial and Complete Callosotomy

The corpus callosum is the largest fiber tract in the human brain, and plays a critical role in many aspects of interhemispheric integration. Much has been learned about the structure and connectivity of this fiber tract through anatomical studies on a variety of species. The functional implications of these anatomical findings are supported by behavioral studies on monkeys and humans with lesions of the corpus callosum. Much of the behavioral data in humans is derived from studies on patients who have undergone callosotomy for the control of intractableepilepsy.Surgicalsectionofthecorpus callosum results in a classic disconnectionsyndromecharacterizedbyabreakdown in communication between the 2 hemispheres. In the early stages of recovery from callosotomy,thiscommunicationfailurecan bemanifestedinstartlingways.Forexample, some patients have exhibited the “alien hand” syndrome after surgery, in which the left hand appears to behave as if it were controlled by some other person. As recovery progresses, the overt symptoms tend to abate, and these “split-brain” patients are remarkably unaffected in daily living. Despitetheappearanceofnormalcerebralfunction, many signs of hemispheric disconnection persist and can be revealed through specialized testing procedures. The chronic disconnection of the cerebral hemispheres has made patients with callosotomy an invaluable population for the study of the specialized capabilities of each hemisphere in isolation. Comparisons between patients with callosotomy and those with localized lesionsinvolvingdifferentportionsofthecorpuscallosumalsoprovideapowerfulmethod of investigating the functional capacities of callosal regions.