The brain is perhaps the most complex machine in the Universe. It consists of two cerebral hemispheres, each with many different modules. Fortunately, all these separate parts are not autonomous agents. They are highly interconnected, all working in harmony to create one unique being: you.
But what would happen if we destroyed this harmony? What if some modules start operating independently from the rest? Interestingly, this is not just a thought experiment; for some people, it is reality.
In so-called ‘split-brain’ patients, the corpus callosum – the highway for communication between the left and the right cerebral hemispheres – is surgically severed to halt otherwise intractable epilepsy.
The operation is effective in stopping epilepsy; if a neural firestorm starts in one hemisphere, the isolation ensures that it does not spread to the other half. But without the corpus callosum the hemispheres have virtually no means of exchanging information.
What, then, happens to the person? If the parts are no longer synchronised, does the brain still produce one person? The neuroscientists Roger Sperry and Michael Gazzaniga set out to investigate this issue in the 1960s and ’70s, and found astonishing data suggesting that when you split the brain, you split the person as well. Sperry won the Nobel prize in medicine for his split-brain work in 1981.
How did the researchers prove that splitting the brain produces two persons, one per hemisphere? Through a clever set-up controlling the flow of visual information to the brain.
They already knew that both eyes sent information to both brain hemispheres – and that the relationship was complex. If you fixated on one point, then everything to the left of that point (the left visual field) was processed by the right hemisphere, and everything to the right of your fixation point (the right visual field) was processed by the left hemisphere. Moreover, the left hemisphere controlled the right side of the body and language output, while the right hemisphere controlled the left side of the body.
When Sperry and Gazzaniga presented stimuli to the right visual field (processed by the speaking left hemisphere), the patient responded normally. However, when stimuli were presented to the left visual field (processed by the mute right hemisphere), the patient said he saw nothing. Yet his left hand would draw the image shown. When asked why his left hand did that, the patient looked baffled, and responded that he had no idea.
What was going on here? The left hemisphere could not…
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