What happens when the two halves of the brain are cut apart?

When the bridge between the two halves of the brain is cut, each half can know things the other half cannot report. The bridge is the corpus callosum, and cutting it is an operation usually used as a last resort to treat refractory epilepsy. Afterwards, each hemisphere has its own separate perception, concepts and impulses to act. Conflicts between them are very rare, and when one arises, one hemisphere usually overrides the other.

Roger Sperry and Michael Gazzaniga tested these patients. When lights flashed on both sides of a screen, patients described only the lights in the right visual field, yet they could accurately point to lights on both sides. Each hemisphere had received the information, but only the left hemisphere could verbally report it. Their 1967 report, "The Split Brain in Man", found distinct cognitive patterns, yet the patients' personalities and emotions were still intact. Sperry earned the 1981 Nobel Prize in Physiology or Medicine for his contributions.

The popular idea of left-brained logic and right-brained creativity goes much further than the evidence. Language is lateralized to the left hemisphere in 96% of right-handers and 60% of left-handers, but the idea of "right-brained" or "left-brained" people is considered a widespread myth that oversimplifies the true nature of the cerebral hemispheres. A 2013 University of Utah study found that brain scans revealed similarity on both sides of the brain.