Split brains

For the purposes of this entry, a split-brain patient is one who has had a complete forebrain commissurotomy. In this operation (which has been replaced by less radical procedures), the corpus callosum and the other neural links (‘commissures’) between the two cerebral hemispheres were completely severed.

Patients underwent the operation for the relief of otherwise uncontrollable epilepsy, and it was considered a therapeutic success. Epileptic attacks disappeared, became less frequent or were confined to one hemisphere. Once the patients recovered from the operation, they were able to resume their normal lives; people who knew these patients before the operation would not notice any dramatic changes in their personality, intellect or everyday behaviour.

Observation under controlled conditions discloses a different picture. When input is limited to one cerebral hemisphere and response demanded of it, the behaviour of split-brain patients is decidedly abnormal, as the following simple example illustrates. ‘Key ring’ is flashed on a screen for a tenth of a second so that ‘key’ appears in the left visual field and ‘ring’ in the right. If split-brain subjects are asked to say what they saw, they respond that they saw ‘ring’ and show no sign of seeing ‘key’. But, if they are asked, instead, to retrieve with their left hands the object named on the screen from an array of items concealed from sight, they will pick out a key while rejecting a ring. Asked to point with the left hand to the object named on the screen, they point to a key or a picture of a key and not to a ring or a picture of a ring. If they are allowed to use both hands to pick out the object named from an array of items hidden from sight, their left and right hands work independently, the right settling on a ring while rejecting a key and the left doing the opposite. Someone seems to have seen ‘key’. Someone else seems to have seen ‘ring’. No one seems to have seen ‘key ring’. With suitable controls, input from the other sensory modalities, except taste, can also be confined exclusively to one hemisphere. When a response depends upon it, split-brain patients behave in similar abnormal ways.

The standard explanation of such behaviour is roughly as follows. The structure of the visual system assures that the left half of the field of vision is conveyed to the right hemisphere and vice versa. Normally, information about the contralateral visual field is supplied to each hemisphere by neural communication across the commissures and by subsequent eye movement. Since the commissures of split-brain patients are severed and the short exposure time serves as a control for eye movement, their right hemispheres see only the word ‘key’ and their left only the word ‘ring’. In most people, speech production is localized in the left hemisphere; and so the oral response to the question reports only what the left hemisphere saw: the word ‘ring’. The left hand is primarily controlled by the right hemisphere; so it retrieves the object the right hemisphere saw named – a key – and points to a key or a picture of a key. (Notice that this explanation presupposes speech comprehension in the mute right hemisphere.) Similarly, the right hand is primarily controlled by the left hemisphere, thus accounting for the independent search of items concealed from sight. The failure to elicit any response suggesting that ‘key ring’ was seen is that the contents of the visual field available to each hemisphere are not the same and, because of the severing of the commissures and the experimental controls, not communicated to the opposite hemisphere.

Behaviour of the sort illustrated in the ‘key ring’ example and its explanation fuel a natural, tantalizing line of inference.

The line of inference just sketched can be used to define philosophical positions on split brains. For a variety of reasons, conservative challenges to Sperry (see above) hold that it should stop short of its third step. Eccles (1970) once claimed that there are no conscious mental phenomena known to be associated with the nonverbal right hemisphere and, later, that whatever conscious processes might be associated with it are subhuman in character. Others have argued that the disunity in consciousness split-brain patients sometimes exhibit is not a standing condition and does not imply two minds; yet others that, although split-brain patients have two minds, they constitute a single person because a single control structure governs both. Sperry (1968) has consistently held the middle ground, endorsing the line of inference as far as its third step. He has refused to join Bogen (1985) and Puccetti (1973) in taking the radical fourth step because, he claims, intact cerebral commissures are the physical basis of unity of consciousness and mind. Many other positions have been taken besides those mentioned. Thomas Nagel’s is particularly striking. He claims that there is no answer to the question of how many minds or persons split-brain patients contain, and that this shows that our ordinary concept of the unity of a person ‘may resist the sort of coordination with the understanding of humans as physical systems, that would be necessary to anything describable as an understanding of the physical basis of mind’ (1971).

The wide diversity of opinion has several sources. The data are unexpected and sometimes messy. Any attempt to deal with them faces a special version of the mind–body problem. One must decide, on some principled grounds, the relation of various anatomic, neurological and behavioural data to mentalistic descriptions; and the proper account of the mentalistic notions of prime concern – consciousness, mind and person – is, to put it mildly, controversial. We also still lack a detailed understanding of how brain structures are responsible for the psychological distinctions involved, for example, what specific role the corpus callosum plays, what a control structure is, and how to count centres of consciousness.

Besides raising the philosophical issues discussed above, research on split brains has provided much insight into problems of deep physiological and psychological interest, for example, hemispheric specialization, which are not immediately tied to them. It has also provided a launching pad for a variety of ‘thought experiments’ in philosophical discussions of personal identity.