An operation (formerly used to relieve epileptic fits) in which the corpus callosum is severed, rendering the two hemispheres of the brain unable to communicate with each other. Although this operation does not have much impact on the ability of the patient to lead a normal life, the split brain will occasionally manifest itself in odd and rather intriguing ways. For example and angry split-brain patient in the midst of of a temper tantrum might find that, as his right hand tries to throw things around, his left hand grabs his arm. Or while dressing, one hand might be putting on clothes while the other is taking clothes off.

To researchers, these patients are a valuable means of discovering differences between the brain's two hemispheres. The scientists can flash images in front of one of the patient's eyes and see how one hemisphere interprets visual information as compared to the other. It turns out that the two halves of the brain interpret reality in relatively different ways. For example, the right brain groups items by physical appearance, whereas the left brain groups items by function. When shown shapes, the left brain breaks them down into their constituent parts, while the right brain sees the object as a whole.

The left brain analyzes and interprets the world by breaking it down into things it can understand. The right brain sees the world as a whole and accepts what it sees without much interpretation. So while we might always use both sides simultaneously, a commissurotomy patient can wink and suddenly see the world in a whole new way.

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