The Master and His Emissary: The Divided Brain and the Making of the Western World

by Iain McGilchrist

It is not what each hemisphere does, but how it does it that matters.

The greatest division in the brain is that between the two hemispheric systems, which leads to their capacity for relatively independent function.

Nature got there before me, beginning with a remarkable physical division at the core of the brain, which she has since made more robust through

mechanisms of interhemispheric inhibition.

Why is the brain, an organ that exists only to make connections, divided? Why is it asymmetrical in so many measurable respects, both structural and functional, and why does its functioning seem to depend on its being

asymmetrical? And why is the major connection between the two cerebral hemispheres, the corpus callosum, getting proportionately smaller, and functionally more inhibitory, rather than larger, and functionally more facilitatory, with evolution?

But far more important than any of these, important as they nonetheless are, is that there is no alternative theory that makes sense as a whole of a large number of established hemisphere differences.

There are a number of answers to that point.

First, in place of a list of unconnected observations about a culture or society, and a series of problems requiring a comparable list of unconnected solutions, the recognition of hemispheric differences provides for the first time a way of seeing the picture as a coherent whole.

if I am right that we are currently in thrall to the left hemisphere’s view, one of the consequences would be, precisely, an inability to see the whole rather than a heap of disparate elements, together with a relative inability to understand what is happening, rather than simply document it, and attempt, as best one can, to find a series of ad hoc solutions.

Second, and following from this, it suggests that the best way to address such shortcomings as we identify will be not so much by piecemeal strategies, necessary as they are bound to become at some level, as by opening our eyes to the limitations of the view of the world which underlies them, the view which, as a society, we appear to adopt as our default. It is the aim of this book to do precisely that. We don’t need a lot more quick fixes. We need a shift in the paradigm.

Third, by showing that the left hemisphere, which underwrites the fragmented vision, is both literally more limited in what it can see, and less capable of understanding what it does see, than the right – and, to cap it all, is less aware of its own limitations – the book gives the reader good reason to re...

But there exists a fourth, most important, consideration. The book is not just a societal critique, but aims to achieve more: to add to an understanding of brain function, and to add thereby to our understanding of our own minds; to give us a means of e...

If a neuropsychologist had to choose three things to characterise most clearly the functional contribution of the right hemisphere, they would most probably be the capacity to read the human face, the capacity to sustain vigilant attention, and the capacity to empathise.

First, the left-hemisphere view is designed to aid you in grabbing stuff. Its purpose is utility and its evolutionary adaptation lies in the service of grasping and amassing ‘things’.

Second, the left-hemisphere view offers simple answers. Its mode of thinking prizes consistency above all, and claims to offer the same mechanistic models to explain everything that exists.

Third, the left hemisphere’s world view is easier to articulate. The left hemisphere is the speaking hemisphere: the right hemisphere has literally no voice.

The left hemisphere relies on concatenations of serial propositions and the literal aspects of language to make meaning explicit; by contrast, metaphor and narrative are often required to convey the implicit meanings available to the right hemisphere, and in a left-hemisphere-dominated culture, metaphors and narratives are disregarded as myths and fables or, at worst, downright lies.

The right hemisphere’s view is inclusive, ‘both/and’, synthetic, integrative; it realises the need for both. The left hemisphere’s view is exclusive, ‘either/or’, analytic and fragmentary – but, crucially, unaware of what it is missing. It therefore thinks it can go it alone.

My thesis is that for us as human beings there are two fundamentally opposed realities, two different modes of experience; that each is of ultimate importance in bringing about the recognisably human world; and that their difference is rooted in the bihemispheric structure of the brain. It follows that the hemispheres need to co-operate, but I believe they are in fact involved in a sort of power struggle, and that this explains many aspects of contemporary Western culture.

The Conclusion, therefore, is devoted to the world we now inhabit. Here I suggest that it is as if the left hemisphere, which creates a sort of self-reflexive virtual world, has blocked off the available exits, the ways out of the hall of mirrors, into a reality which the right hemisphere could enable us to understand.

An increasingly mechanistic, fragmented, decontextualised world, marked by unwarranted optimism mixed with paranoia and a feeling of emptiness, has come about, reflecting, I believe, the unopposed action of a dysfunctional left hemisphere.

I agree with Marcel Kinsbourne that the brain is, in one sense, a system of opponent processors. In other words, it contains mutually opposed elements whose contrary influence make possible finely calibrated responses to complex situations. Kinsbourne points to three such oppositional pairings within the brain that are likely to be of significance. These could be

loosely described as ‘up/down’ (the inhibiting effects of the cortex on the more basic automatic responses of the subcortical regions), ‘front/back’ (the inhibiting effects of the frontal lobes on the posterior cortex) and ‘right/left’ (the influence of the two hemispheres on one

The Duality of the Mind,

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.

But the evidence is that the primary effect

of callosal transmission is to produce functional inhibition.

But it sets one thinking about the virtues of division, and the degree to which each hemisphere can deal with reality on its own.

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.

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.

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.

The frontal lobes not only teach us to betray, but to trust.

Lateralisation of function is widespread in

Starting once again with the structure, most studies have found that the right hemisphere is longer, wider, and generally larger, as well as heavier, than the left.2 Interestingly this is true of social mammals in general.3 The right hemisphere is in fact wider than the left throughout most of its length, only the posterior parieto-occipital region being broader in the left

The cerebral hemispheres show a highly consistent right-greater-than-left asymmetry from childhood to adulthood, with the ventricles (spaces within the hemisphere that are filled with cerebrospinal fluid, and form effectively an inverse measure of brain volume) being larger

the expansion of the speech areas in the left hemisphere is also very early in origin and is detectable from 31 weeks’ gestation, being c...

the hemispheres differ in the number of neurones,8 neuronal size (the size of individual nerve cells),9 and the extent of dendritic branching (the number of connective processes put out by each nerve cell) within areas

There is greater dendritic overlap in cortical columns in the right hemisphere, which has been posited as a mechanism for greater interconnectivity compared with the

The finding that there is more white matter in the right hemisphere, facilitating transfer across

regions, also reflects its attention to the global picture, where the left hemisphere prioritises local communication, transfer of information within regions.

Neurochemically the hemispheres differ in their sensitivity to hormones (for example, the right hemisphere is more sensitive to testosterone),13 and to pharmacological agents;14 and they depend on preponderantly different neurotransmitters (the left hemisphere is more reliant on dopamine and the right hemisphere on noradrenaline).15 Such structural and functional differences16 at the brain level suggest there may indeed be basi...

Another is to use temporary experimental hemisphere inactivations. One way in which this

is achieved is by the Wada procedure, most commonly carried out prior to neurosurgery in order to discriminate which hemisphere is primarily responsible for speech. This involves injecting sodium amytal or a similar anaesthetic drug into the blood supply of one carotid artery at a time, thus anaesthetising one half of the brain at a time, while the other remains active.

people with higher IQs have lower cerebral metabolic rates during mentally active conditions;22 as do those with bigger brain size,23 which is also correlated with

Different aspects of the world come into being through the interaction of our brains with whatever it is that exists apart from ourselves, and precisely which aspects come into being depends on the nature of our attention.

The right hemisphere, as birds and animals show, is ‘on the look out’. It has to be open to whatever it is that exists apart from ourselves, as much as possible without preconceptions, not just focussing on what it already knows, or is interested in. This requires a mode of attention that is broader and more flexible than that of the left hemisphere.

Clearly within either hemisphere, and possibly between hemispheres, the system of control processes is complex. However, some broad consistent differences in hemisphere specialisation are striking when one comes to review the available evidence.

Without alertness, we are as if asleep, unresponsive to the world around us; without sustained attention, the world fragments; without vigilance, we cannot become aware of anything we do not already know.

Patients with right-hemisphere lesions also exhibit what is called perceptuomotor slowing, a sign of diminished alertness, associated with lapses of