Psychologists, including G. T. Fechner and William McDougall, have wondered what would happen to consciousness if the brain were divided in this way. McDougall, it is said, even tried to persuade the physiologist C. S. Sherrington to undertake to divide his, McDougall's, corpus callosum if he became incurably ill. Nerve connectionists tended to think conscious experience would be destroyed or divided, but McDougall, a mentalist, believed consciousness would remain unified. It was frustrating that, for a long time, studies of animals, and a few human cases, with corpus callosum sectioned gave no interesting evidence. Consciousness appeared to be slightly depressed and there were transitory lapses in voluntary coordination, but that was all. In irony, Karl Lashley suggested that the corpus callosum might serve simply to hold the hemispheres together. Warren McCulloch said its only known function was to spread epilepsy.
Cerebral commissurotomy, the split-brain operation, has been performed with varying completeness on a small number of human beings since the mid-1940s, always in hopes of checking crippling epilepsy, to stop the non-functional neural discharges reverberating between the hemispheres and severely damaging the cortical tissues. The breakthrough in estimation of the mental effects of this operation came from investigations in Roger Sperry's laboratory at the California Institute of Technology, following the first effective experiments on the consequences of commissurotomy in cats and monkeys. The animal studies had established new methods. They revealed simple explanations for why all previous research had observed only trivial and uninformative consequences of so great a change in brain structure. With control for orienting, and of information exchange between the hemispheres through transactions with the external world, the split-brain animals were found to have totally divided perception and learning. When free, their movements, alertness, and general motivation were entirely normal.
In Los Angeles, the neurosurgeons Philip Vogel and Joseph Bogen concluded that selected epileptic patients would benefit from the surgery and suffer no serious mental loss. Between 1962 and 1968, nine complete operations were performed with success in reducing fits. Psychological tests performed by Michael Gazzaniga, Sperry, and Bogen at the California Institute soon revealed that, while the general psychological state and behaviour was, in most cases, little affected, there was a profound change in mental activities. Other studies with commissurotomy patients carried out since, in the USA, France, and Australia, have produced similar findings.
For the commissurotomy subject (Fig. 2), direct awareness is no longer whole. An object felt in the left hand out of sight cannot be matched to the same kind of object felt separately and unseen in the right hand. As long as the eyes are stationary, something seen just to the left of the fixation point cannot be compared to something seen on the right side. Comparable divisions in olfactory and auditory awareness may be demonstrated. Furthermore, although sight and touch communicate normally on each side, left visual field to left hand or right visual field to right hand, the crossed two-hemisphere combinations fail, as if experiences of eye and hand were obtained by separate persons. There is no evidence that perceptual information needed to identify an object can cross the midline of the visual field, or between the hands, to unify the patient's awareness. While the division of sight for detail is extremely sharp at the centre of the field, as long as the patient keeps his eyes still, with freedom to look to left and right and to see in both halves of vision what both hands are holding, the division of awareness ceases to be apparent. Indeed, the subject himself seems unaware of anything amiss, except when evidence is presented to him of an inconsistency in his conscious judgement. Then it would appear he feels some lapse of concentration, or absent-mindedness.
With stimuli on arms, legs, face, or trunk, there is some transfer of feeling between the sides. These less discriminatory parts of the body are represented in duplicate, with both sides in each hemisphere of the brain, and their functions, sensory and motor, are cross-integrated at levels of the brain below the hemispheres. Interesting results have been obtained with large, long-lasting stimuli moving in the periphery of vision. Seeing the spatial layout in surroundings at large, called 'ambient vision', is vital in steering on a confined or irregular route or in a cluttered environment, and even in maintaining the balance of standing or walking. It also functions to give approximate location to off-centre targets of attention before the eyes move to fixate. Evidently the semi-conscious appreciation of the location and orientation of major features in outside space, mainly picked up from dynamic transformations of the visual image, is not divided by commissurotomy. Indeed, the general background or context of body coordination and orienting must be intact for commissurotomy subjects to retain the freedom of action and coherence of awareness they ordinarily exhibit. To this degree the operation does not divide the agency of the subject, or the experience of whole-body action. The two halves of the neocortex are kept in functional relationship, coordinated through ascending and descending links with the sub-hemispheric regions of the brain stem (see Fig. 1).
By far the most dramatic finding of the early tests was the total failure of the right cerebral cortex on its own to express itself in speech. It could not utter words to explain its awareness or knowledge. In contrast, when stimuli were given to the left cortex the subject could say perfectly normally what the experience had been like. Objects were named, compared, and described, and the occurrence or non-occurrence of stimulus events was correctly reported. Yet similar tests of the right half of the brain, with stimuli in the left visual field or left hand, totally failed. The subjects often gave no response. If urged to reply, they said that there might have been some weak and ill-defined event, or else they confabulated experiences, as if unable to apply a test of truth or falsity to spontaneously imagined answers to questions.
These events not only confirm a division of awareness, but they raise important questions which have been debated in clinical neurology since the discovery, over a century ago, that muteness or disturbance of language comprehension can result from brain injury confined to the left hemisphere. Could the right hemisphere comprehend spoken or written language at all? Could it express itself to any degree in signs, by writing, or by gesture? Could it make any utterance? Could it reason and think? Was it really conscious? The commissurotomy patients offered a wonderfully direct approach to these questions, and ingenious experiments were designed by Sperry and his students Jerre Levy, Robert Nebes, Harold Gordon, and Dahlia and Eran Zaidel to interrogate the unspeaking right hemisphere.
Some comprehension of spoken and written language was certainly present in the mute side of the brain. Information about how the right hemisphere should perform a test could be conveyed by telling it what to do, and if the name of a common object was projected to the right cortex only, the patient could retrieve a correct example by hand, or identify a picture of it by pointing. The right hemisphere could solve very simple arithmetic problems, giving its answer by arranging plastic digits out of sight with the left hand. Nevertheless, it was clear that both the vocabulary as understood and the powers of calculation of the right hemisphere were distinctly inferior to these abilities in the left hemisphere of the same patient. Rarely, a patient was able to start an utterance or begin to write a word with the right hemisphere, but, in these tests, the vigilance of the more competent left hemisphere blocked all such initiatives after the first syllable or letter. In general only the left hemisphere could speak, write, or calculate.
When Levy applied non-verbal intelligence tests, the results indicated that there were some functions for which the left hemisphere did not dominate: for some modes of thinking the right hemisphere was superior. All these right brain tasks involved visual or touch perception of difficult configurations, judgements involving exploration of shapes by hand, or manipulative construction of geometric assemblies or patterns. It appeared that the right hemisphere was able to notice the shape of things more completely than the left. Taken with evidence that systematic calculation and forming logical propositions with words were better performed by the left hemisphere, these results favoured the idea that the right hemisphere is better at taking in the structure of things synthetically, without analysis, assimilating all components at once in an ensemble, figure, or Gestalt. Nebes discovered that the right hemisphere may have a clearer memory of the appearance of things, in the sense that it was better able to recognize familiar objects with incomplete pictorial data, and better able to perceive whole shapes from parts seen or felt in the hand.
The way hands are normally used hints at differences in awareness of the hemispheres. (See handedness.) In normal manipulation, a right-hander supports and orients an object in the grasp of the left hand, to facilitate discrete moves of the right fingers that are more finely controlled. Consider such a simple act as taking the last drop of soup from a bowl with a spoon. To grasp, support, and orient objects, the left hand must 'understand' the dimensions and distribution of matter in an object, and usually this kind of judgement does not need visual inspection. In contrast, the discrete and precise acts of the right hand require a succession of decisions that are aimed or guided by a sequence of brief visual fixations. Writing is a cultivated skill that uses the right-hand endowment for rapid repeated cycles of action in the service of language. It may be a learned adaptation of the brain mechanism for gestural communication, the right hand of most persons being dominant for expressive gesticulation. In many tests, the left hand of the commissurotomy patients was more efficient than the right at feeling shapes that resist analysis into highly familiar elements. The left hand palpated complex raised patterns as wholes, as if sensing shape directly. In contrast, the right hand tended to feel the contours, corners, etc. one by one, as if trying to build up an inventory of discrete experiences along a line in time. Recently, experiments with the same subjects have demonstrated that the right hemisphere tends to be superior at metaphorical rather than literal perceptions, and that it perceives the emotions or moods in facial expressions or vocalizations better than the left.
To explore further the processes that direct awareness in the hemispheres, Levy, Trevarthen, and Sperry (1972) gave split-brain subjects a free choice of which hemisphere to use to control responses in tests. Halves of two different pictures were joined together down the vertical midline to make a double picture called a stimulus chimera. When this is presented to the split-brain patient with the join on the fixation point, information about each half is received in a different hemisphere. The tasks are designed so that in every trial the correct choice may be obtained by using the experience of either the left or right hemisphere. Preference for one half of the chimera depends on one-sided mental strategies that arise in response to the test instructions. With this kind of test, preferred modes of understanding of the hemispheres can be sensitively determined, as well as the cerebral functions that allocate attention between the two hemispheres. (See Fig. 3.)
The choices of the commissurotomy patients with chimeric stimuli confirm that thought in words favours the left hemisphere. Single words can be read by the right hemisphere, but the left is always preferred if the meaning of the words must be understood and not just their visual appearance or pattern. Further tests show that the right hemisphere is virtually unable to imagine the sound of a word for an object seen, even a very common one like an 'eye', so it cannot solve a test requiring silent rhyming 'in the head' (for example, 'eye' matches 'pie', 'key' matches 'bee'). It seems as if the habitual, and inherently favoured, dominance of the left hemisphere for speaking is tied in with a one-sided ability to predict how words will sound. The right hemisphere can know the meaning of a word from its sound, but it cannot make a sound image for itself from sight of the word, or from sight of the object the word stands for.
Preference for the right hemisphere in matching things by their appearance becomes strong when meaningless or unanalysable shapes are used, especially if these are not representing familiar objects, with a simple name. An extraordinary superiority of the right hemisphere for knowing a face, especially when it lacks bold distinctive features such as glasses, moustache, hat, or birthmark, relates to a rare consequence of damage to the posterior part of this hemisphere. This inability to recognize even the most familiar faces, called prosopagnosia, can greatly embarrass social life. With split-brain persons and stimuli restricted to the left hemisphere, face recognition is poor and identification is achieved by a laborious checklist of distinctive semantic elements to be memorized and searched for. There is obviously a stark contrast in hemisphere cognitive style, reminiscent of differences described in the way the two hands go about knowing or using objects. In addition to these apparently fixed differences in the organization of hemispheric cognitive structures, commissurotomy patients show a varying activation of the hemispheres under brain-stem control that can favour one or other side independently of task requirements. Sometimes the 'wrong' hemisphere is active in doing a task, and performance suffers. This 'metacontrol' may cause differences in the way normal individuals process cognitive problems; i.e. it may determine differences in mental abilities — for example, making one person skilled at visuoconstructive tasks while another is gifted at verbal rationalizations.
Eran Zaidel has developed a method for blocking off half of the visual field of one eye of a commissurotomy patient. He attaches to the eye a contact lens which carries a small optical system and a screen. The patient can cast his eye over a test array in a normal way while picking up visual information by only one hemisphere. The subject has to interpret a story or picture or solve puzzles, many involving choice of the one picture from a group that will identify a concept to which he or she has been cued by a preceding stimulus. These tests prove that both hemispheres have elaborate awareness of the meanings of words and pictures. Metaphorical relationships form an important component of consciousness of meaning in both of them. Objects may be linked in awareness by their abstract properties or customary usefulness and social importance as well as by more obvious features. The usual names, colours, temperatures, and many other properties of things may be correctly identified when each thing is represented by a simple black and white picture. The tests of Zaidel and Sperry have shown that both hemispheres of commissurotomy patients have awareness of themselves as persons and a strong sense of the social and political value, or meaning, of pictures or objects.
Comprehension of words, spoken or written, is surprisingly rich in the right hemisphere, and all grammatical classes of words may be comprehended, but its consciousness does fail with relatively difficult, abstract, or rare words. When words are combined in a proposition, the comprehension of the right hemisphere falls drastically. When simplified items of no particular identity, such as plastic chips of differing size, form, and colour, are used as tokens for arbitrary grouping defined by short descriptions (for example, 'Point to a small red circle and a large yellow triangle'), this too proves difficult for the right hemisphere. A token test of this description was discovered by the Italian neuropsychologists Di Renzi and Vignolo to be extremely sensitive to left-hemisphere lesions. The linguistic abilities of the right hemisphere thus resemble those of a nursery-school child who understands language best when it is fitted into the world of objects, interpersonal acts, and events, all of which sustain the meaning of what is said. Disembedded or context-free propositions lacking interpersonal force require concentration of the mind on categories, critical formulae, or rules for action. These processes of thought may be developed by transformation of inherent human skills for establishing precise identity or harmony of purpose between thinking agents. Such propositions are difficult alike for young children and the disconnected right hemisphere of an adult.
Commissurotomy patients have helped us understand how consciousness, intention, and feelings are generated in activity at different levels of the brain. Thus separated cortices may experience and learn separately, but each may command coherent activity of the whole body. Feelings of dismay, embarrassment, or amusement, generated in one hemisphere by perceptions of threat, or risk, or teasing, invade the brain stem to cause expressions and emotions of the whole person, in spite of the operation. Levels of attentiveness and the shifting aim of orientation and purpose are also patterned within brain-stem regions, which can transmit no detailed evidence of experience. The precautions needed to reveal divided awareness after brain bisection emphasize how, in normal active life, information about the world is constantly reflected to all parts of the brain as it and the body engage in changing relations with the external world. It does not appear necessary to imagine that the 'self', which has to maintain a unity, is destroyed when the forebrain commissures are cut, although some of its activities and memories are depleted after the operation.
Fig. 1. The corpus callosum (CC) is the principal integrator of mental processes which are carried out differently in the two halves of the cortex. It complements connections through the brain stem.
Fig. 2. Commissurotomy, division of the corpus callosum to relieve epilepsy, causes separate awarenesses in left and right halves of the visual field and for objects in left and right hand. Integrations through the brain stem keep the behaviour of the person coherent, but do not permit unification of consciousness. Testing a split-brain patient involves control of orienting movements, with one eye covered and stimuli flashed for 1/10th second on a screen, or objects felt out of sight on a carpet so no tell-tale sounds are fed back to the subject's ears. Using conflicting auditory stimuli in earphones, division of hearing may be demonstrated.
Fig. 3. a. Stimulus chimeras, joining left and right halves of different pictures, permit study of preferences in cognitive processes of the hemispheres. The stimuli are flashed in the precise centre of the visual field in a tachistoscope. If asked to say what was seen, the split-brain subject identifies the right half of the chimera, signifying preferential use of the left hemisphere. If pointing to match the picture that looks most like the stimulus, the left half is selected, indicating that the right hemisphere assumes control of this response. b.By varying instructions one can obtain three different kinds of response from one stimulus. Saying the name or pointing silently to a picture of an object with a name that rhymes with that of the stimulus causes the left hemisphere to take charge. Visual matching engages the right hemisphere.
Note that in both these diagrams, the awareness of a half stimulus is shown as completed. Indeed experiments indicate that the subject imagines freely over the mid-line, presumably because there is no information at variance with imagined parts in the hemisphere doing the imagining.
(Published 1987)
— Colwyn Trevarthen
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