Mind: A Brief Introduction (John R. Searle) / quote 103 of 138
Split-brain patients and two conscious fields
One of the ways to study consciousness is to study the pathological or degenerate forms of consciousness, and this is a method that I will use several times in this book. The split-brain patients suffered from terrible forms of epilepsy that could not be treated by the normal methods. In desperation, the doctors cut the corpus callosum, the body of tissue that connects the two brain hemispheres. This did, in fact, cure many of the patients of epilepsy, but it had some other interesting effects. Most strikingly, it caused the patient to behave in certain circumstances as if he had two independent centers of consciousness. In a typical experiment the following occurs: the patient is shown a spoon, but the spoon is put in a portion of his left visual field so that the visual stimulus only goes to the right side of his brain. Language is on the left side of his brain. He is then asked, “What do you see?” Since he has no visual perception of the spoon on the left side of the brain where he has language; and since, because of the severed corpus callosum, there is only very imperfect communication between the two hemispheres, the patient in such a case says, “I do not see anything.” However, he then reaches out with his left hand, which is controlled by his right hemisphere, where the visual experience of the spoon does occur, and grabs the spoon. There is a large number of experiments of this kind conducted by Roger Sperry and Michael Gazzaniga.2 Does the patient have one or two centers of consciousness? At present, we just do not know for sure. But, at least, we have to consider the possibility that there are, in fact, two conscious fields inside one brain, one corresponding to each hemisphere and that in the normal case the two fields of consciousness coalesce into a single unified conscious field.