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Dual Consciousness in Split-Brain Patients

Gazzaniga (1998), reviewing 30 years of split-brain research in humans, concluded that his first three patients were quite unusual in possessing language abilities in both hemispheres. The majority of split-brain patients show no language comprehension in the right hemisphere.

How were Gazzaniga's first three patients unusual?

Split-brain patients were studied with an apparatus that took advantage of the fact that each half of the visual field projects to the opposite side of the brain. If a split-brain patient is seated in front of a screen and told to look at an "X" in the middle of the screen, and if the picture is flashed on the screen too quickly for an eye movement to occur, then everything to the left of the X will go to the right hemisphere. Everything to the right of the X will go to the left hemisphere. Using this apparatus, researchers Sperry and his student Gazzaniga were able to send messages to one side of the brain or the other.

The split-brain apparatus showed different scenes to the two hemispheres

Suppose the right brain sees a snowy scene, while the left-brain sees a chicken leg. The patient is given a multiple-choice test consisting of four pictures that might relate to the scene. One of the four alternatives is a shovel; another is a chicken. Asked to point to a symbol corresponding to the image on the screen, each hand points to a different symbol. The right hand points to a chicken, because the pattern on the right side of the screen (therefore going to the left hemisphere, which controls the right hand) is a chicken leg. However, the left hand points to a shovel, because the right hemisphere sees only what is left of center: the snow scene.

To repeat: the left hemisphere, which controls speech, sees only the chicken leg. If you ask the patient why the left hand pointed to a shovel, the talking hemisphere does not know! But it quickly makes up an answer, such as, "Well, you have to clean up after those chickens."

What form of "instant rationalization" occurs in split-brain patients?

This is an example of a rationalization - a rational but false explanation. It occurs so smoothly and quickly that it suggests rationalization of mysterious brain activity is a well-developed talent in humans. Perhaps we often have no idea why a thought or feeling occurs to us. If we have to explain our behavior, we simply make up something that sounds reasonable.

In his 1998 30 year review article, Gazzaniga affirmed the tendency of the left hemisphere to confabulate or rationalize. The left brain (the talker) quickly makes up reasons for unexplained activity controlled by the right hemisphere, in all the split-brain patients he studied.

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