Our brain has a left half and a right half. A thick bridge of nerves connects them. In the 1960s, some patients had this bridge cut to stop serious seizures.

In one test, a picture was shown only to the right half of the brain. The patient said, "I saw nothing," because the left half controls speech. But the patient's left hand picked the correct object. So the two halves can work separately.

The two halves of the brain are joined by a thick bundle of nerve fibers called the corpus callosum. In the 1960s, some people with severe epilepsy had it cut so that seizures could not spread from one side to the other. The neuroscientist Sperry and his graduate student Gazzaniga then tested these patients.

When a picture was flashed only to the right hemisphere, patients said they saw nothing, yet their left hand chose the correct object. In another test, the left hemisphere saw a chicken foot and the right saw a snowy scene. The left hand picked a shovel, and the patient explained that it was for cleaning the chicken house. Gazzaniga called this storytelling system the "interpreter."

Split-brain research began in the 1960s, when the corpus callosum, the bundle of fibers linking the two hemispheres, was cut in patients with severe epilepsy. Sperry and Gazzaniga showed that information presented to only one hemisphere stayed there: patients denied seeing an object flashed to the right hemisphere, while their left hand could still identify it.

The left hemisphere also invented plausible reasons for actions it had not planned, a process Gazzaniga named the "interpreter." This work earned Sperry a share of the 1981 Nobel Prize and encouraged the popular claim that such patients have two minds. A 2017 study, however, challenged that view. Re-examining two patients, Pinto and colleagues found that both could respond to stimuli in either visual field far above chance, whether they answered with the left hand, the right hand or speech.

But they could not judge whether two stimuli on opposite sides were the same. The authors concluded that perception may be divided while consciousness remains unified. Because the evidence comes from very few patients who differ in surgery and history, neither conclusion is final, and the findings certainly do not support simple "right-brain" or "left-brain" personality types.