Suppose your brain were cut in half, separating the left and right sides. Would your mind also be divided? This question has been of philosophical interest since René Descartes, the seventeenth-century French philosopher, formulated mind–body dualism—the notion that the mind is distinct from the body, and therefore from the brain. Early in the twentieth century William McDougall, a prominent psychologist, wanted to test this doctrine by requesting that his own brain be split should he ever become incurably ill. He was a dualist, believing that his over-arching mind would unite the physically divided halves of his brain. But the surgery was never performed, and McDougall died in 1938. One might still wonder, I suppose, whether his mind has soldiered on. Split-brain surgery was actually performed, though, in the 1960s, when two Californian neurosurgeons, Philip J. Vogel and Joseph E. Bogen, attempted to provide relief to patients with intractable epilepsy. To do so, they decided to cut each patient’s corpus callosum, the main band of fibres connecting the two sides of the brain. The idea was to prevent the spread of seizures, and in this respect the operation was more successful than anticipated. The seizures were either greatly reduced or readily brought under control with drug therapy. At first, the operation seemed to confirm dualism. Bogen remarked that these split-brain patients showed what he called ‘social ordinariness’, meaning that their everyday behaviour seemed entirely normal. One patient, asked how he felt after the operation, joked that he felt fine—except for a splitting headache.
Dualism, though, was effectively discounted when psychologist Roger W. Sperry undertook more subtle experiments to test each side of the brain separately, and came to the conclusion that each side dwelt in its own individual consciousness, with its own thoughts, feelings and memories. He found, moreover, that each side seemed to exhibit a different kind of consciousness. Only the left side could produce speech, or do calculations, while the right side seemed more proficient at spatial or emotional processing. Sperry received the Nobel Prize for his work in 1981.
The split brain also seemed to capture the spirit of the divisive 1970s. Bogen, along with journalist Robert E. Ornstein, set about popularising the idea of the brain having two sides, declaring the left to be the side of rationality, logic and masculine thought, and the right the side of creativity, intuition and femininity. The distinction rapidly became part of folklore, leading to what might be termed a right-brain cult, fed in part by feminism and resistance to the Vietnam War, with the left brain embodying the military-industrial establishment of the West and the right brain the supposedly peace-loving East. The 1970s slogan ‘Make love, not war’ was a call to arms—embracing ones, not military ones—to the right brain.
All this was a gross exaggeration of the neurological facts, but the myth persists. A Google search for ‘right brain’ still produces some eighty million entries, many of them calling on techniques to encourage right-brain activity in art, education, therapy, business and even literature, despite the left brain being responsible for language. The right brain has become not so much an emblem of peace and love as a means of profit-making. Beware of sellers of right-brain potions.
In the meantime, split-brain research has dwindled, in part because the operation is now seldom performed, except in some extreme cases in which psychological testing is difficult, if not impossible. Drug therapies for epilepsy have improved to the point that surgery is either unnecessary, or is directed more precisely to the locus of the problem. Research with the few remaining testable patients has also shown that the split brain is not quite so split as originally thought.
Although the corpus callosum connects the two cerebral cortices, which house higher mental processes, there are lower connections that maintain a unity of emotion and probably also consciousness. Much of the early work was based on the fact that vision is neatly divided down the middle, with the right side of space projecting to the left side of the brain and the left side of space to the right side of the brain. It now transpires that the visual world is at least partially connected via subcortical pathways, and one split-brained patient for years drove his ute on the highways of the northeastern United States without incident.
Split-brain research has nevertheless taught us a lot about how the mind works, and now takes its place alongside brain imaging and evidence from other kinds of neurological interventions. It may never be possible to entirely prove or disprove the theory that the mind is separate from the brain—opinion remains, you might say, split—but the more we understand the brain and observe its influence on how we think, feel and behave, the more unlikely that theory becomes.