Exchange of views

Top Brain, Bottom Brain: A reply to Stephen Kosslyn & Wayne Miller

Reply by Iain McGilchrist to publicity for Top Brain, Bottom Brain, suggesting that the authors have ‘debunked’ the ‘old left/right theory’:

Kosslyn and Miller are researchers I admire. They are quite right to take issue with what they call dated and crude ideas of hemisphere difference. So do I. ‘Dated’ and ‘crude’ are by definition bad. Agreed, management jargon about almost entirely fictional differences between the hemispheres are a hurdle one has to get over: I wrote a book taking such ideas to task. But that is quite different from suggesting we would be wrong to think in terms of hemisphere difference at all.

They write, as if it is some revelation, that ‘the brain doesn’t work one part at a time, but rather as a single interactive system, with all parts contributing in concert, as neuroscientists have long known’. Who, one feels like asking, do they think their readers are?

It is not that they are wrong to point to ‘top-bottom’ differences in the brain: in fact the idea is so plainly right, and has been known to be the case for so long, that I cannot imagine who it is they think might disagree with them. As was pointed out some 30 years ago by Nobel prize-winner Roger Sperry, ‘qualitative shifts in mental control may involve up-down, front-back, or various other organisational changes as well as left-right differences’: and as the distinguished neuroscientist Marcel Kinsbourne has reiterated, the brain as a whole is a dynamic system with these same three main, anatomically obvious, functional equilibria. The existence of one doesn’t in any way suggest the absence of the others: they are not independent, but interconnected in such a way that each is, in fact, implied in the others.

The brain is not only deeply divided down the middle (an odd fact, given that both its purpose and its power lies precisely in making connections), but clearly asymmetrical: the hemispheres reliably differ in size, weight, shape, surface structure, cell architecture in some areas, grey to white matter ratio, response to endocrine hormones and in neurotransmitter profile. Not only that, but much of the neural traffic between hemispheres has an inhibitory function. Such asymmetry is observable in most, if not all, animals: animal ethologists have been describing reliable differences in laterality for decades. Animals whose brains are not properly lateralised have poorer rates of survival, an observation that gives rise to the familiar adage, ‘asymmetry pays’. Humans are no exception to this rule. Any clinician could tell you that reliably different, and obviously different, changes occur in a patient’s experiential world depending on the side of the brain that is affected. To suppose that there are not significant differences between the hemispheres is a deeply irrational position, a dogmatic response in an area which demands a more thoughtful and subtle approach.

The crude, old ideas that logic and language are in the left, and images and emotions in the right, were exploded long ago. Each hemisphere is involved in absolutely everything we do. But it is hardly a scientific response to throw one’s hands up in despair as a result, and dismiss the topic of hemisphere difference. One needs to examine one’s thinking and see what it is one is missing. As soon as one stops asking the question appropriate to a machine – ‘what does it do?’ – and asks the question appropriate to part of a person – ‘in what manner does it do what it does?’ – the answer starts to become clearer. Differences between the hemispheres in birds, animals and humans ultimately relate to differences in attention, which have evolved for clear reasons of survival. But since the nature of the attention we bring to bear on the world changes what it is we find there, and since what we find there influences the kind of attention we pay in future, differences of attention are not just technical, mechanical, issues, but have significant human experiential and philosophical consequences. They change the world we inhabit.

Who could disagree with Kosslyn and Miller for a moment that the brain needs to function seamlessly from the point of view of individual experience? Whatever the hemispheres deliver is synthesised in consciousness in such a way as not to impair, but positively to facilitate, our immediate responses: we are not, while living from moment to moment, aware of the different ‘takes’ on the world that each hemisphere makes possible. But on reflection we are aware that many aspects of experience present incompatibilities and ‘paradoxes’. The advantage of having extensive neuroscientific data about hemisphere difference is precisely that it enables us to see hemisphere differences as if ‘from the outside’.

The problem of hemisphere conflict is not primarily about the individual’s day to day experience, but about the way individuals conceive – and in the end a culture comes to conceive – the nature of the world in which we live. It is about two ‘takes’ on the world, one of which, to put it simply and briefly, is concerned with closing down to a certainty and the other concerned with opening up to a possibility. One, therefore (the left), aims to reach one correct answer (‘either/or’): the other (the right) is more able to live with ambivalence and the possibility of two apparently incompatible possibilities being true (‘both/and’). In an era which prizes consistency within a system of thinking above fidelity to the sometimes irresoluble complexities of the real world, one of these ‘takes’ can become comparatively neglected.

It is admittedly hard to judge an argument on the basis of a short newspaper article. But let’s hope Kosslyn and Miller have more nuanced ideas about hemisphere difference than this piece suggests. Rather than dismiss something which evolution seems to find so valuable, despite its apparent costs, we should be bending our minds to what such differences really mean about us as human beings and why they came about in the first place. It is one of the most important, and fascinating, questions in neuroscience. We may get the answer wrong, but one way to be certainly wrong is not to attempt it at all.

Reply by Wayne Miller & Steve Kosslyn:

We thank … Iain for his thoughtful commentary. Iain raises important points, which we do in fact address in the book: in Chapter Five, “Sweeping Claims.” This is an excerpt from that chapter:

Researchers have known for decades that none of the sweeping assertions about left brain/right brain differences are supported by solid science. Although they were not shouting from the mountaintops, these scientists had unimpeachable evidence that the popular culture versions of the left brain/right brain theory do not capture how the brain really works.

For example, the left hemisphere is often described as verbal and the right as perceptual—but this distinction doesn’t hold up as a generalization. In reality, both hemispheres typically contribute to both sorts of activities—but do so, often subtly, in different ways.

Consider language: Typically, the left hemisphere produces correct word order—to say, for instance, “I have two left feet” instead of “I two left feet have.” (Yoda’s fractured English may indicate that his alien brain didn’t include a human-standard left hemisphere.) But the right hemisphere also is crucial in language: It extracts the implied meaning—that the speaker doesn’t literally have two left feet but has trouble with physical coordination, much as a person would if she were cursed with actually having two feet shaped like the left one (each with the big toe on the right and the smallest toe on the far left).

And although it is true that the left hemisphere controls speech and plays a major role in grammar and comprehension, the right hemisphere plays a key role not only in our comprehending implied meaning but also in our understanding and producing verbal metaphors and humor, and it is largely responsible for helping us to decipher the meaning of changes in speaking tone, such as the rising tone at the end of a spoken question. And both hemispheres play critical roles in extracting meaning in general. Indeed, neuroimaging studies have conclusively shown that many aspects of language processing are distributed over both hemispheres.

Similarly, consider perception: For example, if you look at a house, the left hemisphere will allow you to pick up on the shapes of the doors, windows, and other parts, while the right will allow you to take in the overall contours of the building. At the same time, the left hemisphere will specify the relative locations of the parts in terms of categories, such as “the window is left of the front door,” while the right hemisphere will specify locations in terms of specific distances, such as by indicating the precise distance the window is from the door. Again, brain imaging studies have conclusively shown that many aspects of perceptual processing are distributed over both hemispheres.

The larger issue is not just that people are being classified as “right-brained” or “left-brained” by so-called experts. It’s that the hemispheres are being classified in terms of simple overreaching dichotomies—such as the left’s being verbal, analytic, and logical, and the right’s being perceptual, intuitive, and emotional. It just doesn’t work that way.

Here are the two fundamental problems:

First, it is true that small areas of the brain are specialized in different ways in the two cerebral hemispheres, but these specializations are very specific. For example, a region near the front of the left hemisphere is adept at controlling the movements of the tongue, lips, and vocal cords during speech—but the corresponding part of the right hemisphere plays a crucial role in controlling such movements during singing. Similarly, a region of the left hemisphere under the temples classifies details of visually perceived objects, whereas the corresponding region of the right hemisphere classifies the overall shape of visually perceived objects. In addition, another region under the temples of the left hemisphere organizes speech sounds into the units of a familiar language, whereas the corresponding region in the right hemisphere organizes environmental sounds (such as the sound of rushing water or animal calls). And so on and so forth.

Although small brain areas sometimes do function differently in the two hemispheres, there may not be anything in common that characterizes how they function differently. For instance, what does the difference between controlling speech versus controlling singing have to do with the difference between classifying parts versus classifying overall shapes? So, when you start to group such small areas together into a larger area, any common thread soon breaks—and a simple dichotomy cannot characterize the larger area. The sorts of documented differences between left-brain and right-brain functioning are hardly the stuff of popular generalizations, but they are fundamentally important to a genuine understanding of brain functioning. The fine print matters.

The second fundamental problem is that each of the specialized brain areas does not work alone but rather works as part of a system that includes many other brain areas—including areas on the opposite side of the brain.

To understand language fully, for example, you need to understand the syntax (the structure of sentences, which is better accomplished by the left hemisphere), the meaning of changes in tone (which is better accomplished by the right hemisphere), and how meaning is deciphered (which is accomplished by both hemispheres working together). In other words, the two hemispheres are part of a single system. Let’s return to our example of a bicycle: It has handlebars, a seat, pedals, gears, a chain, and wheels. All of the parts are designed to work together to accomplish a specific goal (helping a person get from place to place quickly and easily). No one part alone would accomplish much; the power of the machine lies in how the parts all work together. The same is true of the brain.

So the hemispheres do differ, but at a more specific and detailed level than is claimed in the popular press and on the Internet. One half-brain is not “logical” and the other “intuitive,” nor is one more “analytical” and the other more “creative.” Both halves play important roles in logical and intuitive thinking, in analytical and creative thinking, and so forth. All of the popular distinctions involve complex functions, which are accomplished by multiple processes—some of which may operate better in the left hemisphere and some of which may operate better in the right hemisphere—but the overall functions cannot be said to be entirely the province of one or the other hemisphere.

And far from having separate lives, the two halves work together, as [Nobel Prize-winning Caltech neuroscientist Roger W.] Sperry himself noted. They are not isolated systems that compete or engage in some kind of cerebral tug-of-war; one is not an undisciplined child, the other a spoilsport that throws schoolyard tantrums. Rather, as we have stressed, the brain is a single, marvelously complicated, and deeply integrated system. Like those of a well-maintained bicycle, the parts of the brain do have different functions—but, like the parts of a bike, they are designed to work together.

Reply by Iain McGilchrist:

Many thanks to Wayne Miller for his response. I think there is common ground here. We certainly agree that the old idea that the left hemisphere alone dealt with language and reason, while the right hemisphere alone dealt with images and emotions, does not hold, and that it has been known not to hold for a long while. The Master and his Emissary takes this as its starting point.

But I do not think it is true that ‘there may not be anything in common that characterizes how [the hemispheres] function differently’. The idea that the division, and furthermore the asymmetry, of the brain are without coherent meaning, despite such asymmetry being necessary for the proper functioning of birds, animals and humans, is unconvincing.

There are clear, well-validated and reliable differences in the nature of the attention offered by the hemispheres, which are consistent across species and genera, including humans, and this leads to differing ‘takes’ on the world. The nature of attention alters what it finds. Thus brain lesions lead not just to loss of a function, as if we were dealing with an aberrant component in the programme of a machine, but to a different mode of experiencing the world. Although this may sometimes be subtle, and require patient observation and investigation to reveal itself, it can prove none the less important for that, and it is often very striking indeed.

I also agree, of course, that the hemispheres work together – just as the ‘top’ and ‘bottom’ of the brain, in Kosslyn & Miller’s model, need to. We could not function without this being the case. But there are several points here. First, precisely because the brain is massively interconnected, rather than composed of isolated modules, lesions at different points in a hemisphere can have effects that have what Wittgenstein called a ‘family resemblance’. This will be more marked within the hemisphere, because the two hemispheres are the most markedly distinct neuronal masses in the brain, each capable of sustaining consciousness on its own, and each neurone has more intra- than inter- hemispheric connections. Second, although broadly speaking the hemispheres need to cooperate, they also need to be able to function to a degree independently: that is, from an evolutionary point of view, the raison d’être of their separation. In fact proper co-operation itself implies proper differentiation: it is not teamwork for the surgeon and the scrub nurse both to try to make the incision. Although the majority of fibres crossing the cerebral commissures are glutamatergic and excitatory, a very substantial number of these terminate on GABA-ergic interneurones whose function is inhibitory. The way in which the hemispheres balance mutual excitation and inhibition is complex, and is again dealt with at some length in The Master and his Emissary (especially chapter 6). Third, when we stop living headlong, and reflect on the nature of ourselves, the world, and our relationship with it, we become aware, for the first time, of the essentially incompatible models of the world each hemisphere offers. Each reveals some incontrovertible part of experience that the other does not. We therefore have to choose between them, or to accept the paradoxical nature of experience. How we resolve this aspect of the human condition is reflected in the debates that form the history of philosophy and inform the history of ideas, and has, I would contend, its correlates in the ‘world picture’ a society chooses to espouse.

I do not think that Kosslyn & Miller should be dismissed as making what they call ‘simple overreaching dichotomies’ just because they speak of ‘top brain’ and ‘bottom brain’. It is hard to make valid distinctions of a general nature without sounding simplistic, and yet sometimes we cannot and should not avoid attempting to understand and express the general principles of difference between complex entities. But I do regret that they feel the need to publicise their own, no doubt valid, dichotomies by setting up an Aunt Sally in the form of a long-discredited way of characterising the hemispheres, and in the process implying that all attempts to understand laterality are baseless: ‘The left brain/right brain story may be the mother of all urban legends: it sounds good and seems to make sense—but just isn’t true.’ Not so, by any manner of means. The old story deserves to be buried, but the real left brain/right brain story is one we are only just beginning to understand. It is a ‘brain story’, I submit, that will be ultimately of the most profound importance. We badly need to make better attempts to understand it, not to deny its existence.