In the 1960s, a surgeon named Joseph Bogen began performing a radical procedure on patients with severe epilepsy. He cut the corpus callosum — the thick bundle of around 200 million nerve fibres connecting the brain’s two hemispheres. The procedure stopped the seizures. But it also, inadvertently, created some of the most philosophically challenging patients in the history of neuroscience.
What the corpus callosum does
Under normal circumstances, your brain’s two hemispheres communicate constantly. Information processed in the right hemisphere (which receives input from the left visual field and controls the left side of the body) is immediately shared with the left (which processes language, controls the right side, and receives right visual field input). The integration is so fast and seamless that we experience a unified perception of the world.
Cut the connection, and the hemispheres stop communicating. Each continues to function — but independently.
What researchers found
Roger Sperry and Michael Gazzaniga designed ingenious experiments to probe these patients. Because each hemisphere only processes the opposite visual field, they could present information to one hemisphere without the other seeing it — by flashing an image very briefly to one side.
When a word like “apple” was flashed to the left visual field (processed by the right hemisphere), a patient could not say what they had seen — language production is primarily in the left hemisphere, which hadn’t received the information. But if asked to pick the object with their left hand from a bag of items, they chose the apple. The right hemisphere knew what it had seen. It simply couldn’t talk about it.
Even stranger: when asked why they had chosen the apple, patients would confabulate — inventing a plausible reason on the spot. “I just fancied something to eat.” The left hemisphere, unaware of the actual reason, generated a narrative anyway.
Two minds in one skull
What Sperry and Gazzaniga’s work suggested — and what remains philosophically unresolved — is that the split-brain patient may have two separate streams of consciousness coexisting in one body. The left hemisphere and right hemisphere can hold different beliefs, different preferences, and different knowledge simultaneously, with no awareness of the other.
This challenges some basic assumptions about personal identity. What makes you one person? If the two halves of your brain were not talking to each other, would you still be a single self?
Gazzaniga later argued that the left hemisphere acts as an “interpreter” — constantly weaving incoming information into a coherent narrative, whether or not that narrative is accurate. The confabulation isn’t a bug; it may be a feature of how consciousness works. Your sense of a unified self may be a story your brain tells, constructed in real time from information that is itself fragmented.