ASYMMETRY AND THE BRAIN - THE DIVIDED BRAIN - The Master and His Emissary: The Divided Brain and the Making of the Western World - Iain McGilchrist

The Master and His Emissary: The Divided Brain and the Making of the Western World - Iain McGilchrist (2009)



THE TOPIC OF THE DIFFERENCE BETWEEN THE HEMISPHERES, THEIR fundamental asymmetry, has fascinated people for a very long time indeed. In fact speculation on the subject goes back more than two millennia: Greek phys-icians in the third century BC held that the right hemisphere was specialised for perception, and the left hemisphere for understanding - which, if nothing else, shows a remarkably interesting train of thought.1

In more modern times, the physician Arthur Wigan published his thoughtful study, The Duality of the Mind, in 1844, prompted by his fascination with a handful of cases he stumbled across where an individual who had remained apparently unremarkable in life was found at post mortem to have one cerebral hemisphere destroyed by disease. Over a period of 20 years Wigan collected further instances, concluding that each hemisphere on its own could support human consciousness, and that therefore we ‘must have two minds with two brains’, with mental disease resulting when they are in conflict.2 But he did not make any suggestions as to how they differed, and appears to have assumed that they are largely interchangeable - a sort of ‘belt and braces’ approach by evolution to the possibility of one hemisphere being irremediably damaged.


That leads us to a good first question: why are there two cerebral hemispheres at all? After all, there is no necessity for an organ whose entire function, as it is commonly understood, is to make connections, to have this almost wholly divided structure. Over the course of the long evolution of homo sapiens sapiens there could have been developments towards a unified brain, which might on the face of it offer enormous advantages. It is true that the brain's embryological origins lie in two distinct halves. But this cannot be the answer, not only because, earlier still, the primitive hemispheres themselves arise from a single midline structure, the prosencephalon, at about five weeks' gestation (see Figure 1.1), but because midline structures and connections between the halves of the brain do develop later in fetal development at some levels, even though the hemispheres themselves remain deeply divided.

The Master and his Emissary

Fig. 1.1 Embryonic origins of the cerebral hemispheres and other brain regions

And the cranium which encases the brain starts off, embryologically speaking, in several pieces on either side of the brain, but ends as a fused whole - so why not the brain itself? Instead what we see is a tendency positively to enhance the anatomical separation.

For a long time the function of the corpus callosum, the main band of neural tissue that connects the two hemispheres at their base (see Figure 1.2), was unknown. At one stage it was believed to be no more than a kind of bolster, a supportive structure that stopped the two hemispheres from sagging. Now we know that it is there to allow the hemispheres to communicate. But in what sense? What is the communication like?

The corpus callosum contains an estimated 300-800 million fibres connecting topologically similar areas in either hemisphere. Yet only 2 per cent of cortical neurones are connected by this tract.3 What is more, the main purpose of a large number of these connections is actually to inhibit - in other words to stop the other hemisphere interfering. Neurones can have an excitatory or inhibitory action, excitatory neurones causing further neuronal activity downstream, while inhibitory neurones suppress it. Although the majority of cells projecting to the corpus callosum use the facilitatory neurotransmitter glutamate, and are excitatory, there are significant populations of nerve cells (those that use the neurotransmitter gamma-amino butyric acid, or GABA for short) whose function is inhibitory. Even the excitatory fibres often terminate on intermediary neurones, or ‘interneurones’, whose function is inhibitory.4 Inhibition is, of course, not a straightforward concept. Inhibition at the neurophysiological level does not necessarily equate with inhibition at the functional level, any more than letting your foot off the brake pedal causes the car to halt: neural inhibition may set in train a sequence of activity, so that the net result is functionally permissive. But the evidence is that the primary effect of callosal transmission is to produce functional inhibition.5 So much is this the case that a number of neuroscientists have proposed that the whole point of the corpus callosum is to allow one hemisphere to inhibit the other.6 Stimulation of neurones in one hemisphere commonly results in an initial brief excitatory response, followed by a prolonged inhibitory arousal in the other, contralateral, hemisphere. Such inhibition can be widespread, and can be seen on imaging.7

The Master and his Emissary

Fig. 1.2 The brain viewed from above, showing the corpus callosum

Clearly the corpus callosum does also have excitatory functions - the transfer of information, not just prevention of confusion, is important - and both this and the inhibitory role are necessary for normal human functioning.8 But it sets one thinking about the virtues of division, and the degree to which each hemisphere can deal with reality on its own. Severing the corpus callosum altogether produces surprisingly little effect. The surgeons who performed the first so-called ‘split-brain’ procedures for the treatment of epilepsy, in which the corpus callosum is severed, were amazed to discover quite how normally their recovering patients functioned in everyday life, almost (with some interesting exceptions that I will explore later) as if nothing had happened.

You might think that as brains evolve to become larger, the interhemispheric connections would increase in tandem. But not at all: they actually decrease relative to brain size.9 The bigger the brain, the less interconnected it is. Rather than taking the opportunity to increase connectedness, evolution appears to be moving in the opposite direction. And there is a close relationship between the separation of the hemispheres on the one hand and the development of something that keeps cropping up in this unfolding story: the asymmetry of the hemispheres. Because it turns out that the greater the brain asymmetry, too, the smaller the corpus callosum, suggesting that the evolution both of brain size and of hemisphere asymmetry went hand in hand with a reduction in interhemispheric connectivity.10 And, in the ultimate case of the modern human brain, its twin hemispheres have been characterised as two autonomous systems.11

So is there actually some purpose in the division of neuronal, and therefore, mental processes? If so, what could that be?

I have mentioned the view of Kinsbourne that, following the physiological principle of opponent processors, duality refines control. I believe that is right, as far as it goes. But the story goes a long, long way further than that, because the brain is not just a tool for grappling with the world. It's what brings the world about.

The mind-brain question is not the subject of this book, and it is not one I have the skill or the space to address at any length. The argument of the book does not depend on holding one view or another. But it is nonetheless legitimate to ask where the author of a book like this stands on it. Hence this very brief diversion.

One could call the mind the brain's experience of itself.12 Such a formulation is immediately problematic, since the brain is involved in constituting the world in which, alone, there can be such a thing as experience - it helps to ground experience, for which mind is already needed. But let's accept such a phrase at face value. Brain then necessarily gives structure to mind. That would not, however, equate mind and brain. It is sometimes assumed so, because of the tendency when using a phrase such as ‘the brain's experience of itself’ to focus on the word ‘brain’, which we think we understand, rather than on the troublesome word ‘experience’, which we don't.

All attempts at explanation depend, whether explicitly or implicitly, on drawing parallels between the thing to be explained and some other thing that we believe we already understand better. But the fundamental problem in explaining the experience of consciousness is that there is nothing else remotely like it to compare it with: it is itself the ground of all experience. There is nothing else which has the ‘inwardness’ that consciousness has. Phenomenologically, and ontologically, it is unique. As I will try to show, the analytic process cannot deal with uniqueness: there is an irresistible temptation for it to move from the uniqueness of something to its assumed non-existence, since the reality of the unique would have to be captured by idioms that apply to nothing else.13

Is consciousness a product of the brain? The only certainty here is that anyone who thinks they can answer this question with certainty has to be wrong. We have only our conceptions of consciousness and of the brain to go on; and the one thing we do know for certain is that everything we know of the brain is a product of consciousness. That is, scientifically speaking, far more certain than that consciousness itself is a product of the brain. It may be or it may not; but what is an undeniable fact is the idea that there is a universe of things, in which there is one thing called the brain, and another thing called the mind, together with the scientific principles that would allow the one to emerge from the other - these are all ideas, products of consciousness, and therefore only as good as the particular models used by that consciousness to understand the world. We do not know if mind depends on matter, because everything we know about matter is itself a mental creation. In that sense, Descartes was right: the one undeniable fact is our consciousness. He was wrong, however, most would now agree, to think of mind and body as two separate substances (two ‘whats’). This was, I believe, a typical product of a certain way of thinking which I suggest is characteristic of the brain's left hemisphere, a concern with the ‘whatness’ of things. Where it was so obviously a matter of two ‘hownesses’ in the same thing, two different modes of being (as the right hemisphere would see it), he could formulate this only as two whatnesses, two different things. Equally it is a misplaced concern with the whatness of things that leads to the apparently anti-Cartesian, materialist, idea that the mind and body are the same thing. We are not sure, and could never be sure, if mind, or even body, is a thing at all. Mind has the characteristics of a process more than of a thing; a becoming, a way of being, more than an entity. Every individual mind is a process of interaction with whatever it is that exists apart from ourselves according to its own private history.

The type of monism represented by the scientific materialism most often espoused by neuroscientists is not radically distinct from the Cartesian dualism to which it is often thought to be opposed. Its solution to the problem has been simply to ‘explain away’ one part of the duality, by claiming to reduce one to the other. Instead of two whatnesses, there is just one: matter. But Descartes was honest enough to acknowledge that there is a real problem here, one he wrestled with, as is clear from the passage in Meditation VI where he writes:

… I am not merely present in my body as a sailor is present in a ship, but … am very closely joined and, as it were, intermingled with it, so that I form with it a single entity.14

Phenomenologically speaking, there is here both a unity, a ‘single entity’, and the most profound disparity; and any account that fails to do full justice to both the unity and disparity cannot be taken seriously. There may be just one whatness here, but it has more than one howness, and that matters. Though (according to the left hemisphere) a thing, a quantity, a whatness, can be reduced to another - that is to say, accounted for in terms of its constituents - one way of being, a quality, a howness, cannot be reduced to another.15


Let's leave the divided nature of the brain for a moment and take a slightly closer look at the brain as a whole (see Figure 1.3). The next thing one notices, after the interhemispheric divide, is the extraordinary expansion of the human frontal lobes, the most lately evolved part of the brain.

The Master and his Emissary

Fig. 1.3 The brain viewed from the left side

Whereas the frontal lobes represent about 7 per cent of the total brain volume of a relatively intelligent animal such as the dog, and take up about 17 per cent of the brain in the lesser apes, they represent as much as 35 per cent of the human brain. In fact it's much the same with the great apes, but the difference between our frontal lobes and those of the great apes lies in the proportion of white matter.16 White matter looks white because of the sheath of myelin, a phospholipid layer which in some neurones surrounds the axons, the long processes of the nerve cell whereby outgoing messages are communicated. This myelin sheath greatly speeds transmission: the implication of the larger amount in human frontal lobes is that the regions are more profusely interconnected in humans. Incidentally, there's also more white matter in the human right hemisphere than in the left, a point I will return to.17

The defining features of the human condition can all be traced to our ability to stand back from the world, from our selves and from the immediacy of experience. This enables us to plan, to think flexibly and inventively, and, in brief, to take control of the world around us rather than simply respond to it passively. This distance, this ability to rise above the world in which we live, has been made possible by the evolution of the frontal lobes.

Clearly we have to inhabit the world of immediate bodily experience, the actual terrain in which we live, and where our engagement with the world takes place alongside our fellow human beings, and we need to inhabit it fully. Yet at the same time we need to rise above the landscape in which we move, so that we can see what one might call the territory. To understand the landscape we need both to go out into the felt, lived world of experience as far as possible, along what one might think of as the horizontal axis, but also to rise above it, on the vertical axis. To live headlong, at ground level, without being able to pause (stand outside the immediate push of time) and rise (in space) is to be like an animal; yet to float off up into the air is not to live at all - just to be a detached observing eye. One needs to bring what one has learned from one's ascent back into the world where life is going on, and incorporate it in such a way that it enriches experience and enables more of whatever it is that ‘discloses itself’ to us (in Heidegger's phrase) to do just that. But it is still only on the ground that it will do so, not up in the air.

There is an optimal degree of separation between our selves and the world we perceive, if we are to understand it, much as there is between the reader's eye and the page: too much and we cannot make out what is written, but, equally, too little and we cannot read the letters at all.18 This ‘necessary distance’, as we might call it (it turns out to be crucial to the story unfolding in this book), is not the same as detachment. Distance can yield detachment, as when we coldly calculate how to outwit our opponent, by imagining what he believes will be our next move. It enables us to exploit and use. But what is less often remarked is that, in total contrast, it also has the opposite effect. By standing back from the animal immediacy of our experience we are able to be more empathic with others, who we come to see, for the first time, as beings like ourselves.

The frontal lobes not only teach us to betray, but to trust. Through them we learn to take another's perspective and to control our own immediate needs and desires. If this necessary distance is midwife to the world of Machiavelli, it also delivers the world of Erasmus. The evolution of the frontal lobes prepares us at the same time to be exploiters of the world and of one another, and to be citizens one with another and guardians of the world. If it has made us the most powerful and destructive of animals, it has also turned us, famously, into the ‘social animal’, and into an animal with a spiritual dimension.

Immediately we can see the problem here. In order to stay in touch with the complexity and immediacy of experience, especially if we are to empathise with, and create bonds with, others, we need to maintain the broadest experience of the world as it comes to us. We need to be going out into the experiential world along the horizontal axis, if you like. By contrast, in order to control or manipulate we need to be able to remove ourselves from certain aspects of experience, and in fact to map the world from the vertical axis - like the strategy map in a general's HQ - in order to plan our campaigns. Might this in itself give us a clue to the question of why the brain is divided?

Yes and no. For one thing the explanation cannot simply have to do with human brains, for the obvious reason that the brains of animals and birds are also divided. But it might very well give a clue as to a way in which the already divided brain might become useful to its human possessor. Before going on to consider that further, let us move a step closer in our look at the overall structure of the brain.


When most people think of differences in the structure of the hemispheres, the first thing that springs to mind is the now familiar fact that the brain is asymmetrically larger on the left side. In fact this difference is not so obvious as it probably sounds, though the difference is there all right. It had been known since the middle of the nineteenth century that the faculty of speech was associated with the left frontal area, a region now named, probably unjustly, after Paul Broca, a French physician whose observations were anticipated some quarter of a century earlier by his compatriot, Marc Dax.19 They had both noticed that those who suffered a stroke or other damage to this part of the brain tended to lose their faculty of speech. Later the Prussian neurologist Carl Wernicke discovered, through similar observations, that the comprehension of language was distinct from that of speech, and was located further back in the left hemisphere, in the posterior superior temporal gyrus, a region that now bears his name.20 It was the association with language which led to the left hemisphere being referred to as ‘dominant’, since it did all the talking.

Not long after, two Austrian anatomists, Richard Heschl and Oscar Eberstaller, independently observed that there are visible asymmetries in this region, Heschl lending his name to the transverse gyri in the left superior temporal lobe where incoming auditory information is processed.21 After that things went quiet for a while until, in the 1930s, Richard Pfeifer found that the planum temporale, a region just posterior to Heschl's gyrus within the Sylvian fissure, and again involved with language and auditory function, was larger on the left. This finding was confirmed and expanded by Geschwind and Levitsky in the 1960s, who reported that in 65 per cent of cases the planum temporale is on average some 30 per cent larger on the left than on the right.22 Subsequently analysis of skulls and brain scans revealed that there is a generalised enlargement of the posterior part of the left hemisphere in the region of the parietal lobe, known as the left petalia (the term petalia was originally applied to the impression left on the inner surface of the skull by protrusions of one hemisphere relative to the other, but is now applied to the protrusion itself).23

But that is not all. It is not just the left hemisphere that has its area of expansion. The normal brain appears to have been twisted about its central axis, the fissure between the cerebral hemispheres. The brain is not only wider on the left towards the back, but also wider on the right towards the front; as well as extending further back on the left, even a little under the right hemisphere, it extends further forward on the right, even a little overlapping the left. It is as though someone had got hold of the brain from below and given it a fairly sharp tweak clockwise. The effect is subtle, but highly consistent, and is referred to by neuroscientists as Yakovlevian torque (see Figure 1.4).24

What on earth is this about? Why is the brain asymmetrical in this way? If the higher brain functions were just distributed in the brain according to the dictates of space, there would be no reason for local deformities of this kind, rather than an overall diffuse and symmetrical expansion of brain capacity, especially given that the skull that contains it starts out symmetrical.

It has been accepted since the days of the great anatomist John Hunter that structure is at some level an expression of function, an idea reinforced in the early twentieth century by the work of D'Arcy Thompson.25 The relationship of anatomical asymmetries to functional ones is of great theoretical interest.26 Although larger size does not always equate to greater functional capacity, it most commonly does so.27

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Fig. 1.4 Yakovlevian torque

Function is reflected in volume throughout the central nervous system, in cerebrum, cerebellum and spinal cord.28 A nice example, which not only illustrates the point, but suggests that brain areas in individuals may actually grow in response to use, is the fact that the right posterior hippocampus, the area of the brain which stores complex three-dimensional maps in space, is larger in London cabbies, taxi drivers with extensive navigational experience.29 Another vivid demonstration of the principle comes from the left hemisphere of songbirds, which expands during the mating season, and then shrinks again once the mating season is over.30 And there is specific evidence that these particular asymmetrical expansions of the cerebral cortex in which we are interested are likely to be related to function.31

The conventional explanation of the best-known anatomical asymmetry in the brain has been that, since, in Aristotle's famous phrase, man is the social animal, he needs language, and language is a complicated system, which requires a lot of brain space. Since it makes sense that language should be housed in one place, one hemisphere or the other was going to have to specialise in language, displacing other functions, and this just happened to be the left hemisphere, which has, appropriately, expanded in the ‘language region’ of the posterior left hemisphere to accommodate this function. Language is what separates us from the other animals: it is what gives us the power to communicate and to think. Surely it is obvious that it must have been the drive to language that caused this expansion of the left hemisphere?

As I hope to show in due course, I believe every part of this proposition is wrong, though the reasons why, as well as the reasons we make the assumptions we do, are profoundly revealing of the nature of the brain itself. And obviously it goes no way to account for an expansion in the right frontal lobe.


These questions about the meaning of structure have answers, but in order to understand them, we need to take a closer look at function.

In fact the phenomenon of functional differences between the hemispheres goes a long way down the tree of phylogeny, far further than anything like language or handedness. And that is what we would expect from the fact that the structurally divided, bihemispheric brain, is not a new invention: bihemispheric structure must have offered possibilities that were adaptive. Lateralisation of function is widespread in vertebrates.32 It is even true that some of the same neuroendocrine differences that characterise the human brain - differences in neurotransmitters or neurohormonal receptors between the hemispheres - are already present in the brains of rats.33 We have merely taken this whole process much further. So what is the advantage to birds and animals?

Animals and birds may not have the problems posed by our frontal lobes to deal with, but they do already experience competing needs. This can be seen at one level in terms of the types of attention they are required to bring to bear on the world. There is a need to focus attention narrowly and with precision, as a bird, for example, needs to focus on a grain of corn that it must eat, in order to pick it out from, say, the pieces of grit on which it lies. At the same time there is a need for open attention, as wide as possible, to guard against a possible predator. That requires some doing. It's like a particularly bad case of trying to rub your tummy and pat your head at the same time - only worse, because it's an impossibility. Not only are these two different exercises that need to be carried on simultaneously, they are two quite different kinds of exercise, requiring not just that attention should be divided, but that it should be of two distinct types at once.

If we pull back a bit from this same distinction between focussed attention and open attention, we could see it as part of a broader conflict, expressed as a difference in context, in what world we are inhabiting. On the one hand, there is the context, the world, of ‘me’ - just me and my needs, as an individual competing with other individuals, my ability to peck that seed, pursue that rabbit, or grab that fruit. I need to use, or to manipulate, the world for my ends, and for that I need narrow-focus attention. On the other hand, I need to see myself in the broader context of the world at large, and in relation to others, whether they be friend or foe: I have a need to take account of myself as a member of my social group, to see potential allies, and beyond that to see potential mates and potential enemies. Here I may feel myself to be part of something much bigger than myself, and even existing in and through that ‘something’ that is bigger than myself - the flight or flock with which I scavenge, breed and roam, the pack with which I hunt, the mate and offspring that I also feed, and ultimately everything that goes on in my purview. This requires less of a wilfully directed, narrowly focussed attention, and more of an open, receptive, widely diffused alertness to whatever exists, with allegiances outside of the self.

These basic incompatibilities suggest the need to keep parts of the brain distinct, in case they interfere with one another. There are already hints here as to why the brain may need to segregate its workings into two hemispheres. If you are a bird, in fact, you solve the conundrum of how to eat and stay alive by employing different strategies with either eye: the right eye (left hemisphere) for getting and feeding, the left eye (right hemisphere) for vigilant awareness of the environment. More generally, chicks prioritise local information with the right eye (left hemisphere), and global information with the left eye (right hemisphere). And it turns out, not surprisingly, that chicks that are properly lateralised in this way are able to use these two types of attention more effectively than are those in whom, experimentally, lateralisation has not been permitted to develop.34 Many types of bird show more alarm behaviour when viewing a predator with the left eye (right hemisphere),35 are better at detecting predators with the left eye,36 and will choose to examine predators with their left eye,37 to the extent that if they have detected a predator with their right eye, they will actually turn their head so as to examine it further with the left.38 Hand-raised ravens will even follow the direction of gaze of a human experimenter looking upwards, using their left eye.39 For many animals there are biases at the population level towards, again, watching out for predators with the left eye (right hemisphere).40 In marmosets, individual animals with more strongly lateralised brains are better able, because of hemisphere specialisation, to forage and remain aware of predators.41 There are shorter reaction times in cats that have a lateralised paw preference.42 Lateralised chimps are more efficient at fishing for termites than unlateralised chimps.43 Even individual human brains that are, for one reason or another, less ‘lateralised’ than the norm appear to show global deficits.44 In a word, lateralisation brings evolutionary advantages, particularly in carrying out dual-attention tasks.45 As one researcher has put it succinctly: asymmetry pays.46

In predatory birds and animals, it is the left hemisphere that latches on, through the right eye and the right foot, to the prey.47 It is certainly true of familiar prey: in toads, a novel or unusual choice of prey may activate the right hemisphere, until it becomes familiar as an object of prey, when it once again activates the left.48 In general, toads attend to their prey with the left hemisphere, but interact with their fellow toads using the right hemisphere.49

The advantages accrue not only to the individual: being a more lateralised species at the population level carries advantages in social cohesion.50 That may be because the right hemisphere appears to be deeply involved in social functioning, not just in primates, where it is specialised in the expression of social feelings, but in lower animals and birds as well.51 For example, chicks preferentially use the left eye (right hemisphere) for differentiating familiar members of the species from one another, and from those who are not familiar, and in general for gathering social information.52 Chicks approach their parents or an object on which they have imprinted using their left eye (right hemisphere),53 as do Australian magpies.54 Though black-winged stilts peck more, and more successfully, at prey using the right eye (left hemisphere), males are more likely to direct courtship displays to females that are seen with their left eye (right hemisphere).55 The right hemisphere is the main locus of early social experience in rats.56 In most animal species, intense emotional responses are related to the right hemisphere and inhibited by the left.57

Perhaps it is just a nice coincidence that the wry-billed plover, a native of New Zealand, which uses its beak to search for food under stones, has a bill which is curved to the right, so that it will be of most use to its manipulative left hemisphere.58 No doubt there may be counterexamples. But there does seem to be a consistent thread running all the way through. Speech is in the left hemisphere in humans: what then about the instrumental vocalisations of other species? They arise also in the left hemisphere, in such diverse creatures as frogs, passerine birds, mice, rats, gerbils, and marmosets.59 Similarly there is a strong right eye (left hemisphere) bias for tool manufacture in crows, even where using the right eye makes the task more difficult.60 This has, as we will see when we come to consider the human situation, some important resonances for the nature of our own world. But when it comes to mediating new experience and information it is already the right hemisphere, in animals as in humans, not the left, that is of crucial importance.61

The consistent differences go further than this, differences that again foreshadow differences in humans. Look at the more subtle discriminatory functions. The right hemisphere in birds, as in humans, is associated with detailed discrimination and with topography;62 while the left hemisphere of many vertebrate animals, again as in humans, is specialised in categorisation of stimuli and fine control of motor response.63 Pigeons can, remarkably enough, categorise pictures of everyday scenes depending on the content. Still more remarkable, however, is the fact that each hemisphere apparently adopts its own strategy, with the pigeon's left hemisphere using a ‘local’ strategy - grouping the images according to particular features that must be invariably present - whereas its right hemisphere relies more on a ‘global’ strategy, taking account of the thing as a whole and comparing it with an ideal exemplar.64 The full significance of that finding will become apparent only when we come to look at the human brain.

In general terms, then, the left hemisphere yields narrow, focussed attention, mainly for the purpose of getting and feeding. The right hemisphere yields a broad, vigilant attention, the purpose of which appears to be awareness of signals from the surroundings, especially of other creatures, who are potential predators or potential mates, foes or friends; and it is involved in bonding in social animals. It might then be that the division of the human brain is also the result of the need to bring to bear two incompatible types of attention on the world at the same time, one narrow, focussed, and directed by our needs, and the other broad, open, and directed towards whatever else is going on in the world apart from ourselves.

In humans, just as in animals and birds, it turns out that each hemisphere attends to the world in a different way - and the ways are consistent. The right hemisphere underwrites breadth and flexibility of attention, where the left hemisphere brings to bear focussed attention. This has the related consequence that the right hemisphere sees things whole, and in their context, where the left hemisphere sees things abstracted from context, and broken into parts, from which it then reconstructs a ‘whole’: something very different. And it also turns out that the capacities that help us, as humans, form bonds with others - empathy, emotional understanding, and so on - which involve a quite different kind of attention paid to the world, are largely right-hemisphere functions.


Attention is not just another ‘function’ alongside other cognitive functions. Its ontological status is of something prior to functions and even to things. The kind of attention we bring to bear on the world changes the nature of the world we attend to, the very nature of the world in which those ‘functions’ would be carried out, and in which those ‘things’ would exist. Attention changes what kind of a thing comes into being for us: in that way it changes the world. If you are my friend, the way in which I attend to you will be different from the way in which I would attend to you if you were my employer, my patient, the suspect in a crime I am investigating, my lover, my aunt, a body waiting to be dissected. In all these circumstances, except the last, you will also have a quite different experience not just of me, but of yourself: you would feel changed if I changed the type of my attention. And yet nothing objectively has changed.

So it is, not just with the human world, but with everything with which we come into contact. A mountain that is a landmark to a navigator, a source of wealth to the prospector, a many-textured form to a painter, or to another the dwelling place of the gods, is changed by the attention given to it. There is no ‘real’ mountain which can be distinguished from these, no one way of thinking which reveals the true mountain.

Science, however, purports to be uncovering such a reality. Its apparently value-free descriptions are assumed to deliver the truth about the object, onto which our feelings and desires are later painted. Yet this highly objective stance, this ‘view from nowhere’, to use Nagel's phrase, is itself value-laden. It is just one particular way of looking at things, a way which privileges detachment, a lack of commitment of the viewer to the object viewed. For some purposes this can be undeniably useful. But its use in such causes does not make it truer or more real, closer to the nature of things.

Attention also changes who we are, we who are doing the attending. Our knowledge of neurobiology (for example, of mirror neurones and their function, which I will touch on later) and of neuropsychology (for example, from experiments in association-priming, which again we will have time to consider in due course) shows that by attending to someone else performing an action, and even by thinking about them doing so - even, in fact, by thinking about certain sorts of people at all - we become objectively, measurably, more like them, in how we behave, think and feel. Through the direction and nature of our attention, we prove ourselves to be partners in creation, both of the world and of ourselves. In keeping with this, attention is inescapably bound up with value - unlike what we conceive as ‘cognitive functions’, which are neutral in this respect. Values enter through the way in which those functions are exercised: they can be used in different ways for different purposes to different ends. Attention, however, intrinsically is a way in which, not a thing: it is intrinsically a relationship, not a brute fact. It is a ‘howness’, a something between, an aspect of consciousness itself, not a ‘whatness’, a thing in itself, an object of consciousness. It brings into being a world and, with it, depending on its nature, a set of values.


This leads to a fundamental point about any attempt to understand the brain. It is a particularly acute case of the problems encountered in understanding anything. The nature of the attention one brings to bear on anything alters what one finds; what we aim to understand changes its nature with the context in which it lies; and we can only ever understand anything as a something.

There is no way round these problems - if they are problems. To attempt to detach oneself entirely is just to bring a special kind of attention to bear which will have important consequences for what we find. Similarly we cannot see something without there being a context, even if the context appears to be that of ‘no context’, a thing ripped free of its moorings in the lived world. That is just a special, highly value-laden kind of context in itself, and it certainly alters what we find, too. Nor can we say that we do not see things as anything at all - that we just see them, full stop. There is always a model by which we are understanding, an exemplar with which we are comparing, what we see, and where it is not identified it usually means that we have tacitly adopted the model of the machine.

Does that mean that all attempts to approach truth - other than to say everything has its truth in its own way - are doomed, that every version of reality has equal value? Certainly not. I will explore these issues later, as they are central to this book. That needs to wait until we have had a look at what the hemispheres actually ‘do’.

Such considerations apply to the attempt to understand anything at all. But when we come to look at what we refer to as brain functions, there is a problem of a wholly different order. We are not ‘just’ looking at things in the world - a lump of rock, or even a person - but the processes whereby the world itself, together with the rock or the person, might be brought into being for us at all, the very foundations of the fact of our experience, including any idea we might have about the nature of the world, and the nature of the brain, and even the idea that this is so. If it is true that attention changes the nature of what we find, how do we decide the most appropriate attention for that? One that tries to ignore the inwardness of experience? What possible context is there in which to place the foundations of experience of all contexts whatever? And what kind of a thing are we to see it ‘as’? The answer is far from obvious, but in the absence of an attempt to address the question we do not give no answer. We answer with the model we understand - the only kind of thing we can ever fully understand, for the simple reason that we made it: the machine.

We cannot look at the world coming into being within the brain, without that qualifying the world in which the brain itself exists; our understanding of the brain's ways of understanding alters our understanding of the brain itself - the process is not unidirectional, but reciprocal. If it turns out that the hemispheres have different ways of construing the world, this is not just an interesting fact about an efficient information-processing system; it tells us something about the nature of reality, about the nature of our experience of the world, and needs to be allowed to qualify our understanding of the brain as well.

For physicians like myself, this is manifested in the astonishing and moving experiences of our patients, both those with discrete neurological lesions and those with what are thought of as more ordinary psychiatric conditions. For them it is not a matter of ‘data loss’, but of nothing less than the world itself truly having changed. This is why trying to persuade them of an alternative reality is of limited value, unless they have already managed to regain the world in which we are living.


In this chapter I have raised a number of questions arising from the structure of the human brain, and done little as yet to answer them. Why are the hemispheres separate? The separation of the hemispheres seems not accidental, but positively conserved, and the degree of separation carefully controlled by the band of tissue that connects them. This suggests that the mind, and the world of experience that it creates, may have a similar need to keep things apart. Why?

Birds and animals, like us, have divided hemispheres. In them the difference seems to have to do with the necessity of attending to the world in two ways at once. Does it in humans? The frontal lobes are particularly highly developed in humans. Their function is to yield distance - necessary for our most characteristically human qualities, whether that be foresight or empathy. As a result we need to be able to be open to whatever there is, and yet, at the same time, to provide a ‘map’, a version of the world which is simpler, clearer and therefore more useful. This does not, of course, in itself account for the existence of two hemispheres, but could it give a clue as to a way in which the separation of the hemispheres might become particularly useful?

The brain is structurally asymmetrical, which probably indicates asymmetry of function. This has always been thought to be because of language - which after all is a sort of ‘map’, or version of the world. Is that not, surely, the reason that there is an expansion in the posterior part of the left hemisphere? This account cannot be right for a number of reasons which I will consider in Chapter 3, quite apart from the fact that it does nothing to explain the expansion in the anterior part of the right hemisphere. The answer to the questions I have raised will have to wait until we reach that chapter. But there is something we should consider, as we approach the next chapter, in which we will take a much closer look at what actually goes on in the two hemispheres of the human brain.

Experience is forever in motion, ramifying and unpredictable. In order for us to know anything at all, that thing must have enduring properties. If all things flow, and one can never step into the same river twice - Heraclitus's phrase is, I believe, a brilliant evocation of the core reality of the right hemisphere's world - one will always be taken unawares by experience, since nothing being ever repeated, nothing can ever be known. We have to find a way of fixing it as it flies, stepping back from the immediacy of experience, stepping outside the flow. Hence the brain has to attend to the world in two completely different ways, and in so doing to bring two different worlds into being. In the one, we experience - the live, complex, embodied, world of individual, always unique beings, forever in flux, a net of interdependencies, forming and reforming wholes, a world with which we are deeply connected. In the other we ‘experience’ our experience in a special way: a ‘re-presented’ version of it, containing now static, separable, bounded, but essentially fragmented entities, grouped into classes, on which predictions can be based. This kind of attention isolates, fixes and makes each thing explicit by bringing it under the spotlight of attention. In doing so it renders things inert, mechanical, lifeless. But it also enables us for the first time to know, and consequently to learn and to make things. This gives us power.

These two aspects of the world are not symmetrically opposed. They are not equivalent, for example, to the ‘subjective’ and ‘objective’ points of view, concepts which are themselves a product of, and already reflect, one particular way of being in the world - which in fact, importantly, already reflect a ‘view’ of the world. The distinction I am trying to make is between, on the one hand, the way in which we experience the world pre-reflectively, before we have had a chance to ‘view’ it at all, or divide it up into bits - a world in which what later has come to be thought of as subjective and objective are held in a suspension which embraces each potential ‘pole’, and their togetherness, together; and, on the other hand, the world we are more used to thinking of, in which subjective and objective appear as separate poles. At its simplest, a world where there is ‘betweenness’, and one where there is not. These are not different ways of thinking about the world: they are different ways of being in the world. And their difference is not symmetrical, but fundamentally asymmetrical.

With that in mind, let's turn to the hemispheres for a closer look at what they ‘do’.