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Split Brains

Those who have undergone split-brain surgery help us to understand hemispheric integration and our consequent sense of self. Split-brain surgery has been performed on some sufferers of severe epilepsy. The procedure involves cutting the corpus callosum—myelinated fibers connecting the hemispheres. Epileptic patients who have had it cut experience fewer seizures and don’t appear to change much in other respects—family and friends typically don’t notice anything. However, experimental studies reveal side effects. To understand these experiments, we must remind ourselves of how vision is wired. The leftmost part of our visual field is only seen by our right hemisphere, and the rightmost part is seen exclusively by the left hemisphere, because of optic nerve cross-over at the optic chiasm. With a split-brain patient, you can hold something up in the far left visual field so it is seen only by the right hemisphere. Let us say you hold up an apple in the far left visual field of a split-brain patient, A, and ask what A sees. A will be unable to tell you that it is an apple, because the left hemisphere, which is in charge of language abilities, never saw it. But A can grab an apple with his left hand (controlled by his right hemisphere) from things hidden beneath a blanket to show you what it was. A common interpretation is that consciousness has been split. Moreover, what is to say it is not like that, in some limited way, for everyone? We might live under the illusion of having a single, integrated consciousness because our language-oriented hemisphere dominates verbal life and consequently thought. Having examined research and ideas that don’t square easily with commonsense views of consciousness, we now examine how philosophers offer alternative perspectives.

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