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From the 1960s, a radical treatment for intractable epilepsy was to cut the corpus callosum, the main commissure connecting the two sides of the brain. In 1981, Roger W. Sperry won the Nobel Prize for his work on the apparent division of consciousness between the two cerebral hemispheres in these patients. Our own work has focused on vision. Each half of the visual world projects to the opposite side of the brain, yet split-brained patients show surprising integrity of perception across the visual midline, especially in elementary perceptual processes. This work has shown greater unity of consciousness in the patients than implied by the earlier work, and shown how mental life depends on both cortical and subcortical processes. It also provides a glimpse of the evolution of the human mind.
In the 1960s, two neurosurgeons in Los Angeles, Phillip J. Vogel and Joseph E. Bogen, decided to treat intractable multifocal epilepsy by cutting the main fibre tracts linking the two cerebral hemispheres, with the primary intent of preventing the development of full-blown seizures. By far the largest and most important fibre tract to be cut was the corpus callosum, but in early operations other tracts were also sectioned, including the anterior commissure and the hippocampal commissure. Later operations were typically less radical, restricting surgery to the corpus callosum, and in some cases involving only selected regions of the corpus callosum. Subcortical connections were left intact. The operations succeeded better than expected in treating epilepsy, and in some cases, with the help of medication, seizures were effectively eliminated. Moreover, so-called "split-brained" patients seemed on the surface to be largely mentally and emotionally unaffected by the operation.
The idea of splitting the brain had long been of interest from psychological and philosophical perspectives, especially as a crucial test of Cartesian dualism. Gustav Fechner, the 19thcentury experimental psychologist, and William McDougall, a prominent British psychologist of the early 20th century, both wondered what would happen to consciousness if the brain were split. Indeed McDougall is said to have tried to persuade the esteemed physiologist, Charles S. Sherrington, to sever his, McDougall's, corpus callosum if he should become incurably ill. McDougall was a convinced dualist who believed his mind would remain intact if his brain were divided,...