Split Brain

Abstract

A ‘split brain,’ usually the brain of a mammal, is one in which all direct, ‘one-neuron’ connections between the two forebrain cerebral cortices have been cut. The largest interhemispheric bridge, or commissure, is the corpus callosum (Bogen 1985, Innocenti 1986). The first split-brain operation was performed on a cat in 1952, combining commissurotomy (sectioning of the corpus callosum and anterior commissure) with division of visual crossover fibers, the optic chiasm, at the base of the brain. Split-brain cats see and learn with each eye connected to a different hemisphere. Transfer of visual learning between the hemispheres is abolished. In monkeys commissurotomy separates two eye–hand coordination and learning systems, each hemisphere controlling movements of the limb on the other side of the body within the visual half field on that side. An intention to move one hand determines which cortical visual system will learn. Perception around the body in the periphery of the visual field, including ‘ambient’ pre-attentive awareness of space and motion properties of objects, is not divided in the split brain, indicating that subhemispheric (brainstem) systems, which remain unified, can integrate perceptuo-motor functions. In the 1960s, commissurotomy operations were performed on human patients to control life-threatening epilepsy. Psychological tests demonstrated a divided consciousness, and two different cognitive systems. Only one, in the left hemisphere, had normal control of speech, and it was superior in logical or ‘propositional’ thinking. Right hemisphere consciousness understood language at a simple level, and was superior in awareness of relationships and configurations of experience called ‘appositional.’ Roger Sperry, who initiated split-brain research and supervised the experiments on commisurotomy in humans, received a Nobel prize for this work in 1981 (Sperry 1982). Split-brain research has been enormously influential, clarifying varieties of consciousness in the brain, and the cerebral symmetry of human motives or cognitive strategies that guide different purposes in action.