Cerebral Hemispheres and Consciousness

Epilepsy patients who have undergone cerebral commissurotomy exhibit striking behavior patterns, about which many data have been gathered. Philosophers have drawn certain metaphysical and epistemological conclusions from these data, including: That commissurotomy patients possess two streams of consciousness; that they possess two minds; that they are two persons; that all normal human organisms are actually composites of two persons; and that the commonsense concept of the mind must be discarded. I argue that a few such patients possess two stream of (...) consciousness and two minds, yet all are single persons, and that the commonsense concept of mind is so vague that it is pointless to attempt a systematic evaluation of it. (shrink)

This thesis discusses the philosophical positions that have arisen out of the research with cerebral commissurotomy patients. Following this discussion we develop and defend a position of our own. The first chapter discusses the operation and the postoperative tests that were conducted. Evidence is set out that makes it prima facie plausible that these patients have two minds. This evidence is basically evidence that the two hemispheres are not capable of pooling their informational content in special situations. Evidence for this (...) non integration is found in all the senses except taste. This chapter also discusses hemispheric specialization, language in the minor hemisphere and the cuing mechanisms of the split hemispheres. The second chapter is devoted to Puccetti and Bogen who argue that normal humans have two minds. Their evidence for this comes from the evidence for two minds in the cerebral commissurotomy patients. Puccetti argues that if two minds were not present to start with, a simple operation would not elicit two minds from one. Puccetti also discusses tests on normal humans that are to support his position. These arguments are discussed and rejected. The third chapter discusses additional evidence for only one mind in normal humans. Puccetti's arguments are not only inadequate in that they do not prove their point but they ignore a great deal of evidence that there is only one mind in normal humans. This evidence comes from interference that two different tasks cause for each other. There is also evidence that the two hemispheres work together on one task and that their specialization is complimentary, resulting in functional dependence, rather than being duplicative. The fourth chapter is devoted to Eccles' attempt to reduce the minor hemisphere to an unconscious, computer like entity. He argues from what he calls, 'the unity of consciousness' and on epistomological grounds concerning evidence for a mind other than one's own. The argument from the unity of consciousness, however, begs the question and his epistomological considerations leave him in grave danger of solipsism. Chapter five discusses Dewitt's attempt to mediate between Eccles and Puccetti. Dewitt believes that Puccetti is right in asserting that these patients have two minds. On the other hand he is impressed with Eccles theory that the lack of language in the minor hemisphere causes a great disparity between the minor hemisphere and the major hemisphere. He therefore concludes that the patients have two minds, one of which does not qualify for personhood. Dewitt, however, has gravely underestimated the ability of the minor hemisphere. It has more language capacity than he believes and gives every evidence of being a person. It is also questionable whether or not language is a necessary condition for the self-awareness that goes with personhood. Chapter six looks at Robinson's attempts to undermine the two mind interpretation of the evidence. He does this mainly by trying to find tiimilar counter examples that resist a two mind interpretation. His program fails due to the dissimilarity of the counter examples and the cerebral commissurotomy results. Chapter seven considers Nagel's contention that the patients have an uncountable number of minds. Nagel does not believe that there is a strange uncountable number of minds that these patients actually have. Instead he believes that it is impossible to say how many minds they do have because we do not know how to count them. Nagel reasons that we have good evidence to believe that the patients have one mine and good evidence that they have two. Since both cannot be true we do not know what to say about these patients. Nagel believes that the above considerations make it difficult for us to understand these patients' mental lives. We agree with Nagel that there are times when it is difficult if not impossible to say how many minds the commissurotomy patient has. We disagree that this is always the case. Our problem with counting is not, however, a failure to understand something about the patients' mental lives, but is due to a counting problem when two hemispheres are only partially integrated. We agree with a suggestion from Nagel that an unusual connection between the hemispheres does not settle the question of how many minds the patient might be said to have. The hemispheres do seem to be able to integrate or pool their information using sophisticated muing mechanisms. Our position is that the patients usually have one mind that integrates the two hemispheres of the brain by cuing. The testing situations, however, interfere with this cuing and thereby cause a temporary, partial nonintegration between the hemispheres. During this time we, therefore do not know how many minds the patient can be said to have. (shrink)

Half a Brain is Enough is the extraordinary story of Nico, a three-year-old boy who was given a right hemispherectomy to control his severe intractable epilepsy...

What do the relationships observed in the occurrence of various limb, facial, and speech apraxias following left hemisphere damage mean for Corballis's theory? What does the right hemisphere's role in nonpropositional and automatic speech production tell us about the coevolution of right hand preference and speech; how could the possibility that the right hemisphere may be “dominant” for some aspects of speech be accommodated by his theory?

Unlike all other lateral specializations, the necessity for unilateral dominance is clear only for the case of the motor control of the speech organs lying on the midline of the body and innervated from both hemispheres. All functional asymmetries are likely to be a consequence of this asymmetry of executive control.

Chicks reared in the absence of light and rat pups reared without extra stimulation fail to exhibit behavioral laterality, implying that a threshold amount of environmental stimulation is necessary for the brain to follow an asymmetry pathway. Reverse asymmetry has been reported in the chick, but not the rat, though a sex difference resembling reverse asymmetry has been found in the rat.