Iain McGilchrist

The Skeptical Brain

About the division of the brain

How the structure of the brain affects our world

Posted Dec 03, 2010

This is my first ever blog, so let me introduce myself. 

I trained in medicine after pursuing an academic career in the humanities, mainly because of my interest in the relationship between mind and body, and between mind and brain.  The philosophers and psychologists I studied on the topic were themselves too disembodied in their approach, and I thought I’d better go off and find out for myself at first hand (or as near first hand as I could manage), in a more embodied way, what happened when things went wrong with a person’s mind or body, and how that affected their body or mind.  I have spent the last 20 years practising as a psychiatrist, researching the brain, and trying to understand what it was I was seeing before my eyes.  During that time I have also been gestating a book.

This book, called The Master and his Emissary: the Divided Brain and the Making of the Western World, was published recently by Yale University Press.  Its thesis is that our divided brains give us two fundamentally incompatible, but equally necessary, ‘takes’ on the world.

I am, of course, all too aware that there is a prejudice amongst neuroscientists against any attempt to discuss the hemispheres, as though anyone doing so could not be ‘serious’.  But it seems to me that this is too quick a reaction – a sort of kneejerk reaction, based on a part-truth. 

First of all, there is, whether one likes it or not, a divide there, which is not just a meaningless fact.  Why do the cerebral hemispheres, whose entire purpose is to make connections, have to be divided – unlike the lower parts of the brain – at all?  Why has the extent of the divide got more marked, rather than less so?  The ratio of the size of the corpus callosum (the band of fibres at the base of the brain which connects the hemispheres) to the volume of the cerebral hemispheres has decreased, not increased, in size with evolution.  For that matter, why is one of the main functions of the corpus callosum, if not its main function, to inhibit the other hemisphere? 

Why is the brain asymmetrical, rather than just symmetrically expanded, if there is really no essential difference here?  After all, the box in which it sits is symmetrical: why take trouble to expand one side of the brain in one area, and the other in another, if each half does essentially the same thing?  Why are there differences not just in the size, weight and shape of the hemispheres, but in their structure (both the gyral conformation and cell architecture), and their function (their comparative reliance on specific neurotransmitters, and their neuroendocrine sensitivities), if they are not doing different things?  And the neuropsychology evidence certainly suggests that they are.  So why on earth should we not be trying to work out what that is?

I said there was a part-truth behind the dismissive reaction: we did indeed get it wrong in the past.  The popular misconception, originating in the 60s and 70s, which has gone into mass culture, that the left hemisphere alone deals with language and reason, and that the right hemisphere alone deals with emotion and visual imagery, that the left hemisphere alone is involved in thinking, and the right hemisphere alone in creativity, is the culprit here.  It has become abundantly clear over the last 30 years or so that each of these aspects of mental life, along with every other aspect (so far without exception), is served by both hemispheres, not one. 

But that is not a reason to give up trying to understand what the difference between the hemispheres might be.  It just means we need to sophisticate our thinking.  And it turns out that if we stop thinking of the brain as a machine, and therefore about what ‘function’ it is carrying out in any one part, and instead think of it as part of a person, as having a way of carrying out whatever it does, an entirely consistent difference between the hemispheres, which has meaning in terms of evolution, reveals itself.  

The other reason that people commonly react against any treatment of hemisphere difference is that it suggests too simple a dichotomy.  But this too, though I understand what lies behind the reaction, is, in my view, mistaken.  First of all, we didn’t make the dichotomy.  It’s not a dichotomy that is being imported: it’s a given that needs to be explained.  This is the fallacy of shooting the messenger.  Secondly, it is possible to make something very sophisticated by different combinations of binary positions: you do not have to stay with the dichotomy, as if it were the end of everything.  It may be the beginning.  In all human situations in the real world we use both of these options, the left and right hemispheres.  To talk about a dichotomy, as though that ended the matter, would be like objecting to the ‘on/off’ choices of the binary code that operates a computer, as being too simple: from these basic choices many complex structures can spring.  But the ‘on’ and the ‘off’ are, nonetheless, quite different in their effect.

Most of those who have bothered to study the area in any depth – Tim Crow, VS Ramachandran, Joseph Hellige, John Cutting, Marcel Kinsbourne, Robert Ornstein, Claude Braun and Elkhonon Goldberg, to name a few – agree that there is something here of central importance.  So, after all, did Roger Sperry, who won a Nobel prize for his work on split brains.  So what is it?  And what, if anything, follows from it?

I think plenty follows from it.  If the two halves of the brain have a different take on the world, they offer competing versions of what anything is – anything at all, including ourselves, our brains and our minds, and the world in which we live.  In fact the problem is recursive, because they influence what we see when we look at the very issue in question – the issue of the divided brain.  

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