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You possess a prodigious brain. It contains around one hundred thousand million nerve cells, with perhaps a thousand times as many interconnections between them. It is three times the size that would be predicted for an ape of our proportions. The striking process of encephalisation that has occurred since our ancestral line departed from that of the chimp, five million years ago, is undoubtedly one of the cornerstones of our humanity. 

But while size matters, a line of research originally launched by 19th-century neurologists, including Pierre Broca in France and Carl Wernicke in Germany, has pointed to a second key principle of human brain evolution — the lateralisation of brain function. 

The apparent symmetry of the brain's paired hemispheres is deceptive: our preference for using our "dominant" hand, usually of course the right, for skilled tasks, turns out to reflect a profound division of labour between the two sides of the brain. 

For most of us, the left hemisphere, controlling the right hand, takes the lead in comprehending and expressing language, and in praxis, the control of skilled action, while the right hemisphere leads in perception and spatial awareness. This specialisation of function within the hemispheres has allowed an increase in the efficiency of neural processing which has greatly augmented our brain power — or such is the textbook view. 

Iain McGilchrist's remarkable book extends this received wisdom with a hugely ambitious, absorbing and questionable thesis: the two hemispheres have radically contrasting personalities; that they live in a state of creative tension, sometimes declining into open war; and that their struggle for supremacy provides the key to understanding the major cultural movements of human history. 

Every so often in neurology, one encounters cases that point beyond the standard view of hemisphere differences in the direction McGilchrist pursues. 

A young man with a stroke near the front of the left hemisphere plunges abruptly into suicidal depression — despite the near complete loss of language, he manages to convey his state of mind with graphic intensity. His shift of mood seems to reflect some loss of balance, an unopposed force. Following surgery to the right hemisphere, a second patient loses the ability to "get" the gist of conversation — there is no problem with language, but there seems to be a grave problem with sense. An elderly man with a form of dementia that affects the left hemisphere develops a passion for art: it is as if a suppressed capacity has been released. Clinical experience indeed sometimes hints that the two sides of the brain are in tension, though hints falls short of any kind of proof.

Some of these hints are being more systematically explored, using the novel techniques, like functional Magnetic Resonance Imaging (fMRI), that allow us to visualise the brain in action — this exploration is work in progress. McGilchrist builds up his image of the two hemispheres by way of a series of contrasts. While he claims impartiality, it is clear where his sympathies lie. But he emphasises, surely rightly, that most human functioning, and all major human achievement, require a collaborative effort between the hemispheres. Here are some of the contrasts in question. The right hemisphere, the Master, is engaged, empathetic, receptive, intuitive, metaphorical, humorous, particular, musical, holistic; the left, the Emissary, is detached, rational, acquisitive, conceptual, literal, straight-laced, abstract, verbal, analytic.

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