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The corpus collosum is the structure in the human brain that connects the left and right hemispheres, and allows them to communicate. Sometimes disease or a stroke will impact the collosum, damaging it. At other times, it may be sugically severed as an extreme measure to cure some types of drug-resistant epilepsy. As a result of the collotomy, split brain syndrome may develop in the patient.

Collotomized patients usually behave much like other people, but there are a few ways in which they are markedly different. Ask a patient to hold an object in their right hand and they will be able to name it. Tell them to pick it up with their left hand, and they will be at a complete loss for words. The same thing happened when the patient was asked to cover one eye and read words off of a card. As long as the right eye was covered, they were unable to do this. This is because the right side of the body is connected to the left hemisphere of the brain, which is the side responsible for language skills. With the pathway to the right hemisphere disabled, it is unable to communicate with the sensory hardware handled by that side, and thus unable to identify the object.

Once you realize this, you have no trouble at all understanding the next part of the test, where subjects were asked to fondle ten items behind a screen, and match them up to the image projected in front of their left eye. They had no trouble doing this, as the right hemisphere is better with abstract concepts and spatial relationships. They could even select the best match, i.e. if a pack of cigarettes were shown but not placed behind the screen, the right brain would select an ashtray.

The weird part is that after the collotomy, each hemisphere may develop a different personality with different emotions, so that an argument with a collotomized person might involve them giving you sober, well-reasoned arguments while their mute right brain expresses itself by bringing the left arm up to slap you.

Collotomized people eventually learn to live normal lives, as the two hemispheres learn to communicate using the relatively low-bandwidth spinal column. The hero of Stanislaw Lem's book Peace on Earth is a man who has had a collotomy.

Thanks to www.indiana.edu/~pietsch/split-brain.html

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