The term split brain refers to a neurological condition when the main COMMISSURES connecting the two CEREBRAL HEMISPHERES have been surgically disconnected (see COMMISSURE; HEMISPHERE). The term initially described an animal preparation used in the classic work of Ronald Myers and Roger Sperry. It was subsequently applied to a small group of patients—split-brain patients—in whom, as a last resort, the CORPUS CALLOSUM (and in some cases also the ANTERIOR COMMISSURE) was sectioned in order to reduce the spread of EPILEPSY from one cerebral hemisphere into the other. Myers’ studies on cats and monkeys were important because they established new and hitherto unexploited methods. There had been previous attempts to cut these main pathways between the two cerebral hemispheres but the outcome, whether in humans or animals, had seemed to be trivial with seemingly few consequences from such a major change in brain structure. With carefully controlled studies, maintaining orienting and preventing information exchange between the hemispheres through transactions with the external world, Myers & Sperry found that the split-brain animals appeared to have totally divided perception and learning. At the same time, their movements and alertness when moving about freely and their general motivation seemed to be unchanged.
The split-brain patients showed profound changes in some of their mental activities. Direct awareness was no longer whole. An object placed in the left hand out of sight could not be matched to the same object felt separately and unseen in the right hand. As long as the eyes remained stationary, something seen just to the left of the fixation point could not be accurately compared to something seen on the right side.
Similar divisions in olfactory and auditory awareness were also demonstrated. By far the most dramatic finding of the early tests was the total failure of the RIGHT HEMISPHERE on its own to express itself in speech. It was unable to produce words to explain that of which it was aware or which it thought it knew. By contrast, when stimuli were given to the LEFT HEMISPHERE, the subject could say perfectly well what the experience had been like. Subsequent, more detailed testing has shown that some transfer of information is possible through subcortical pathways but the basic dramatic findings of the surgical splitbrain remain well established. In the case of human commissurotomy patients, Sperry argued that there were ‘two minds in one brain’.
