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"This magazine is flush with tight smart writing."
Washington Post

Bryan Appleyard
The Mind's Eye
By Oliver Sacks (Picador 258pp 17.99)

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Oliver Sacks has been telling us some of the strangest stories in the world for forty years now. A neurologist, he writes of the ways in which the human brain both invents and perceives the world. He does so through endless anecdotes, told in an unadorned, attractive style. He draws no conclusions; his aim is to ask questions.

The big one in this book is: 'To what extent are we the authors, the creators, of our own experiences?'

The evidence for such a proposition has been mounting ever since the Seventies, when it became clear that the brain was not as hardwired as we thought it was. To a startling extent, it can remodel itself to cope with changes, especially those involving trauma.

This book is all about changes in the sense of sight. This time Sacks himself is one of the patients. In 2005 he found he had a melanoma in his right eye. The effects and the ensuing treatment are described in a way that is both clinical and harrowing.

'I cannot be sure what he said next,' he writes of the first visit to his doctor, 'for a voice had started up inside my head, shouting, "CANCER, CANCER, CANCER ..." and I could no longer hear him.' He had moved from what Bertrand Russell called 'knowledge by description' to 'knowledge by acquaintance': the cool but empathetic physician had become the suffering patient, seeing the mind's cataclysm from the inside.

He is still around - melanoma in the eye is treatable and tends not to spread - but the price of survival is a changed world. Of all the terrible symptoms he describes, perhaps the most intriguing is sudden, monocular vision. Sacks, it turns out, is very binocular by nature. He is even a member of the New York Stereoscopic Society, which exists 'to promote the art and practice of 3D imaging'.

For Sacks the loss of depth is traumatic, draining life out of the world. But even here he finds consolation, discovering in flatness that he can suddenly appreciate anew the painter's or photographer's art of composition. He sees a tree and its shadow on the same plane, 'as if both were painted on the wall - a vision both alarming and exquisite, for the 3-D reality had turned into a Japanese painting'.

His crisis forms a large part of the book but there is also the usual mix of philosophy and patient stories. The nature of the image is mysterious. Wittgenstein distinguished 'saying' and 'showing': the former is assertive and possessed of a logical structure; the latter presents information in a direct non-symbolic way with no formal structure. The brain, confusing as ever, uses both. Normally this is effortless, but disease can expose the confusion.

Lilian Kallir, for example, is struck down by alexia sine agraphia, in which the sufferer loses the ability to read but can, strangely, continue to write. Lilian is a brilliant pianist and the first sign of her condition is a sudden loss of her ability to read music. Nevertheless, she continues to be able to perform pieces she knows well flawlessly.

Or there is Patricia H: after a cerebral haemorrhage she is left aphasic (ie speechless) but her sight is unimpaired and she can mime, point and express herself with increasing fluency. Sight takes over where speech fails. Over a year under Sacks's care she learns to understand others through their gestures and facial expressions.

Always the plasticity of the brain amazes. Somehow, it manages to work around what appear to be utterly intractable problems. Vision is especially persistent. Sacks wrote an essay about a friend who went blind and, progressively, lost all visual imagery in his mind. Mistakenly he assumed this was normal. In fact, he was immediately contacted by blinded people who said it never happened to them and he had to revise his view.

People who are blinded retain a fully active visual cortex, though in congenitally blind people it tends to be 25 per cent smaller. But even they can, in some sense, 'see'. A Portuguese study showed that the normally sighted and the congenitally blind had 'equivalent visual activity'. This is the kind of information that gives one a kind of imaginative vertigo - how and what can they see?

One staggering development from this is the possibility of turning blind sight into actual sight. Since the visual cortex is still there and still active, it should be possible to stimulate it by means other than the eyes.

Sure enough, one scientist attached a stamp-sized sensor array to the tongues of blind people. This received information from a camera. A 'crude but nevertheless useful' picture was formed. Later innovations, using more advanced equipment, have enabled blind people to navigate a room or catch a ball.

As so often with Sacks, language is the key. Blinded in her forties, Arlene Gordon learned to 'see' through descriptions. She describes herself as having 'seen' places to which she travels. And the people who describe these places to her feel they are seeing these places for the first time. 'Too often,' she says, 'people with sight don't see anything!'

Sacks the doctor once again dramatises the most strange and thrilling scientific and cultural issue of our time - the nature of the human mind - through the simple act of telling stories. And he does so with avuncular good nature, even in the midst of his own agonies. Read him for endless consolation.

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Bryan Appleyard writes on everything for The Sunday Times and is writing a book about everything.