Cut the link between the brain's halves and each side knows different things.
To treat severe epilepsy, surgeons sometimes sever the corpus callosum, the bridge between the brain's two hemispheres. In the 1960s Roger Sperry and Michael Gazzaniga found that in these patients the right hemisphere could recognise an object perfectly well yet be unable to name it. The speaking half simply didn't know.
The corpus callosum is a thick band of millions of nerve fibres linking the left and right hemispheres. Cutting it, an operation called a corpus callosotomy, is a last-resort treatment for epilepsy that resists drugs, because it can reduce the severity of seizures. Remarkably, patients often seem entirely normal afterwards. Their personalities and emotions stay intact, and in daily life they can be hard to tell apart from anyone else.
Careful tests tell a different story. Each hemisphere mainly receives vision from the opposite side of the visual field and touch from the opposite hand, and in most people speech is controlled by the left hemisphere. In work Sperry and Gazzaniga summarised in 1967, patients staring at a central point could describe lights flashed to their right but not their left, though they could point to both. An object held out of sight in the left hand could be recognised and matched, but not named. The right hemisphere knew; it just had no access to words.
Sometimes the speaking hemisphere fills the gap with a story. In one well-known test, a patient's two hemispheres were shown different pictures, a chicken's foot and a snowy field, and asked to choose associated words. The patient picked 'chicken' and 'shovel'. Asked why, the talking left hemisphere, unaware of the snow, explained that the shovel was for cleaning out the chicken coop. Occasionally the two sides seem to conflict, as with one man who pulled his trousers up with one hand and down with the other, though such cases are rare.
Sperry received the 1981 Nobel Prize in Physiology or Medicine for this work on how the hemispheres specialise. It is often boiled down to an 'analytic' left brain and an 'intuitive' right brain, but later researchers such as Jerre Levy argued that the hemispheres normally work as an integrated whole and that no mental task relies on one side alone. The deepest question, whether cutting the connection also splits consciousness, remains open.
Source: Wikipedia — Split-brain · Text summarised from Wikipedia (CC BY-SA 4.0)
