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See Inside June/July 2008

Spheres of Influence

Split-brain patients—whose two hemispheres are separated surgically—provide fascinating clues to how a unitary sense of consciousness emerges from the furious activity of billions of brain cells

The human brain has approximately 100 billion neurons, and each, on average, connects to about 1,000 other neurons. A quick multiplication reveals that there are 100 trillion synaptical connections. So how is all this input getting spliced and integrated into a coherent package? How do we get order out of this chaos of connections? Even though it may not always seem so, our consciousness is rather kicked back and relaxed when you think about all the input with which the brain is being bombarded and all the processing that is going on. In fact, it is as if our consciousness is out on the golf course like the CEO of a big company while all the underlings are working. It occasionally listens to some chatter, makes a decision and then is out sunning itself.

We have gotten some clues about how consciousness emerges from studying “split brain” patients. The surgical procedure to cut the corpus callosum is a last ditch treatment effort for patients with severe intractable epilepsy for whom no other treatments have worked. Very few patients have had this surgery, and it is done even more rarely now because of improved medications and other modes of treatment. In fact, there have only been 10 split-brain patients who have been well tested. William Van Wagenen, a Rochester, N.Y., neurosurgeon, performed the procedure for the first time in 1940, following the observation that one of his patients with severe seizures got relief after developing a tumor in his corpus callosum. Epileptic seizures are caused by abnormal electrical discharges that in some people spread from one hemisphere to the other. It was thought that if the connection between the two sides of the brain were cut, then the electrical impulses causing the seizures would not spread from one side of the brain to the other. The great fear was what the side effects of the surgery might be. Would it create a split personality with two brains in one head?

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