As a last resort, people suffering from severe form of epilepsy are advised to go through corpus callosotomy, a surgical procedure to partially or fully cut the corpus callosum, the bundle of nerves that connects left hemisphere to the right hemisphere of the brain, to stop their epileptic fits spreading from one hemisphere to the other. They are called split-brain patients. The surgery results in broken communication between left and right hemispheres, affecting the ability for their VP and LB to communicate.

Professor Roger Sperry, along with Professor Michael Gazzaniga, were first to study the effects of a split brain in humans in the 1960s. They found that both hemispheres have distinctive characteristics and can operate practically independently (a discovery which earned Roger Sperry Nobel prize). In other words, they have two opinions, decisions, feelings, etc., emerging from a single brain, one from each hemisphere.

It is not easy to know the effects resulting out of split brain in such patients by verbally communicating with them. The reason for the same is that, as language in most humans is processed in the left hemisphere, their verbal responses only reveal what's going on in the left hemisphere. Even then, if you let their right hemisphere communicate through non language based activities, like drawing, gesturing, etc., allowing it to express its desires, interests, opinions, etc., differences between the way they "think" are revealed (which match predictions of the DOS model).

For the same reason, such differences can be exposed in a laboratory, where specially carried out experiments are used to communicate with each hemisphere separately.

Patient Paul S.

In cases when a person is in "two minds", i.e., has equally strong, but differing opinions about a decision to make, the arguments between VP and LB resolve the matter (more under title How does CFM Work?), but not in the case of split-brain patients.

Sperry and Gazzaniga were involved in a case which demonstrated this in the best possible manner. The patient, Paul S, was a rare case of having speech areas in both his hemispheres (i.e., his right hemisphere additionally had language capabilities), while at the same time, his corpus callosum required to be cut to cure his epilepsy. This enabled them to interview both hemispheres of his brain separately after the surgery. When they asked him what he wanted to be, his left hemisphere (LB) "wanted to become" a draftsman, as it was programmed to be so, but his desire of becoming a race car driver was voiced by his right hemisphere (VP), which is, as per DOS model, where desires are formed.

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