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The Mind's Eye is about how our "mind's eye" - that mysterious theatre within, upon which we stage our private visual experience - works, and what goes wrong when it doesn't. But how to separate the mind's eye from the distracting input from the actual eye? Oliver Sacks is our greatest chronicler of people with unusual neurological and sensory disabilities and experiences, and the remarkable adaptations they people have made. Now, sadly, we learn that Sacks has begun to go blind due to eye cancer. The Mind's Eye includes his brilliant, poignant observations of this process, his terror and grief, culminating in his awe-inspiring, resilient use of that blindness to begin to better "dissect" out the mind's eye, now that his is no longer pounded by visual input from without.

Sacks has been writing about the adaptive capacities of the brain long before the brain's "neuroplasticity" - its ability to change its structure and function in response to experience - was generally appreciated. In his 1995 volume An Anthropologist on Mars, he observed that his subjects, in response to neurological illness, seemed to "reach out to life" and reorganize because "of the brain's remarkable plasticity." His writing at that time was generally about how understanding of plasticity might be used to compensate for, or accommodate to, these conditions, as opposed to "cure" certain brain disorders, something that wasn't even considered a possibility except in a few renegade quarters as late as the mid-1990s.

The book begins with stories about people with unusual vision problems, or adaptations. A musician who loses the ability to recognize objects and sight-read develops a remarkable musical memory, so she no longer needs the written music.

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A Man of Letters describes how Canadian novelist Howard Engel one day woke unable to recognize words in his morning newspaper (which now appeared to be written in a unfamiliar language), because of a stroke that rendered him alexic and how he went on reorganize his brain to read again. Engel learned to trace words with his fingers, and thus use motor memory to take words in. In essence, he learned "reading by writing," He also learned to "read" by moving his tongue, in a "metamodal sensory-motor alchemy." What Engel did was develop a neuroplastic compensation, strengthening a motor circuit in his brain that developed for another purpose.

Sometimes "compensations" can lead to jaw-dropping improvements, simply by strengthening circuits, as in the case of a woman who, in response to becoming paralyzed, learned to memorize the entire New York Times crossword - its configuration, all its clues - in a single intense inspection, and then solve it at her leisure, without effort, later in the day.

In other cases, Sacks's subjects use their plasticity to rise to a "normal" level of functioning. Stereo Sue is the story of Sue Barry, a scientist who grew up with no depth vision. She gained it in adulthood, with training, even though most believed the adult brain was not plastic enough to change. This result is a neuroplastic cure because she, rather than learning to "accommodate" or "compensate" for her problem, actually fixed it with the help of a developmental optometrist.

Apart from Barry herself (who has written a wonderful book about her cure, Fixing My Gaze), Sacks, as one who, since childhood, was passionately interested in stereoscopic vision and playing with building stereoscopes (he is a member of the New York Stereoscopic Society), is remarkably able to explain the difference between looking at a scene with one eye or two, and how the perception of depth deepens our appreciation of life.

In Face-Blind, we learn that Sacks, not unlike his most famous case, The Man who Mistook his Wife for a Hat, has had a lifelong brain condition rendering him unable to recognize faces. His patient, Dr. P., had a stroke, affecting the fusiform face area of the brain, giving rise to a condition called prosopagnosia. Sacks has a congenital version of this condition which has never improved, despite "a lifetime of trying to compensate," showing, perhaps the limits to plasticity (or perhaps the limits of compensation.) After many years, he's been able to recognize his closest friends, Eric Korn and Jonathan Miller, but usually by identifying individual features just off the face, such as Korn's thick spectacles or Miller's mop of red hair.

Sometimes his descriptions feel comical, but the condition is devastating. "I am thrown particularly when I see people out of context, even if I have been with them five minutes before. This happened one morning just after my appointment with my psychiatrist (I had been seeing him twice weekly for several years at this point). A few minutes after I left his office, a soberly dressed man greeted me in the lobby of the building. I was puzzled as to why this stranger seem to know me, until the doorman addressed him by name - it was, of course, my own analyst."

The deficit is so specific that he is better at recognizing his neighbours' dogs than the neighbours themselves. On several occasions, he apologized for almost bumping into a large bearded man, only to realize that "that large bearded man was myself in the mirror." Once, grooming himself in the reflection of a restaurant window, he realized he was not looking at his reflection of himself self-grooming, but at a man looking at him oddly. Imagine the challenge posed the time he met another face-blind colleague for the first time in a restaurant.

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Two per cent of the population have this poorly understood disorder. Needless to say, face-blindness makes a child's life at school very problematic, and one can imagine the teasing, not to mention all the armchair psychiatry people with rare conditions must endure. "I think that a significant part of what is variously called my 'shyness,' my 'reclusiveness,' my 'social ineptutude,' my 'eccentricity,' even my 'Asperger's syndrome,' is a consequence and a misinterpretation of my difficulty recognizing faces," Sacks writes.

In 2005, while at the movies, Sacks began to see a burst of spectral colours and noticed a blind spot within the flare. His doctor discovered a malignant melanoma in Sacks's right eye on his retina a millimetre from where central vision is processed. To avoid removing it (Sacks's left eye was already damaged from a blow in youth), his doctor fashioned a radioactive plaque, which he surgically inserted into the eye to kill the tumour. In the recovery room, the doctor asked his groggy patient if he knew where he was. He should have known better. Immediately Sacks (whose autobiography is subtitled Memories of a Chemical Boyhood) found himself saying that he is "in the recovery room," and he just had attached to his eye "a plaque containing radionioiodine (I-125, to be precise). … I was sorry it was not radioactive ruthenium instead of iodine (I have a thing for the platinum metals) but that 125, at least, was memorable for being the smallest number that was the sum of two squares in two different ways." Sacks, his uncommon ardour for the periodic table renewed, had been recalled to life.

The tumour and the treatment destroy enough of his retina to create a circular blind spot in his right eye that he calls "bagel vision." A flock of birds flies by, disappears into the blind spot, only to re-emerge on the other side. But this visual hole is not empty. It is a window filled with involuntary images: faces, figures, landscapes. With the loss of central vision, Sacks begins having hallucinations, which he thinks "show me the background activity, the idling of my visual system." Hallucinations may also occur because his visual cortex, platform for his mind's eye, is no longer receiving normal amounts of input from his retina, and is becoming neuroplastically altered and supersensitive.

Often, his blind spot "fills in" what is missing. If he is looking at a pattern, and part of it disappears into his blind spot, his brain soon creates that pattern in the hole. (All our blind spots do this, which is why we don't know we have them; we are blind to our blind spots.) At Times Square, Sacks begins to play with his blind spot, seeing how far it will go "filling in" patterns. He speculates that the visual cortex is not so much a duplicating device as "an averaging device, capable of sampling what was presented to it and making a statistically plausible … representation of it." By casting his blind spot on his foot, his brain slowly fills the emptiness with a phantom foot, airier than his "normal" foot, but complete with toes.

Though he had been weak at mental visualization, now when he looks at something, then closes his eyes, he continues to experience not just an afterimage lasting seconds, but a super-clear "persistence of vision" lasting many minutes, as if he hadn't closed his eyes at all. This signals his brain's slowness to erase, but also that he is developing a powerful ability to visualize. When he closes his eyes and imagines his arms, he now sees them in much more vivid visual detail than before.

But, alas, his tumour returns; then he loses depth perception, his precious "stereo vision," so that his reflection in the mirror no longer seems behind the mirror. He loses peripheral vision on his right, too, so that he is, for practical purposes, blind in his right eye. As of this writing, though not totally blind, if he wishes to go for a walk in his New York neighbourhood, it must be with the help of friends.

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This is a deeply moving book, and one can only imagine the duress under which it was written, and yet, as it rises to the concluding essay, The Mind's Eye, one begins to forget that duress as Sacks outlines the surprising range of neuroplastic reorganizations that occur in blindness. There are those, such as U.K. religion professor John Hull, who went blind at 48, and eventually lost not only his visual memories, but also "the very idea of seeing," so that the sense of objects as having appearances vanished. He surrendered to this change "with acquiescence and joy," thinking he had entered a new order. His writing became stronger. His case reminds us that visual imagery is not essential to thought, as many argue.

At another extreme, blind Zoltan Torey, an Australian psychologist, declined switching to a more auditory mode, and uses his freed-up visual cortex to expand his mental eye, making it so vivid that he is able to climb a ladder and replace his entire roof guttering single-handedly. Similarly, Jacques Lusseyran, a French Resistance fighter, though he forgot the face of his mother and stopped caring what people looked like, developed (along with an infallible "nose" for detecting traitors) a vast stage on his mind's eye, filled not with darkness but all the colours of the rainbow. He was capable of giving his walking companions a visual inventory of all they would see that far exceeded what they were able to observe with the naked eye. Which of these developments will occur will depend in part on one's history, and how one trains oneself.

The New York Times has called Sacks "the poet laureate of medicine." Blind poets, such as Homer and Milton, have long been symbols of a special power conferred by blindness, allowing them to see what the sighted can not, and develop superior oral memories that enable them to compose hundreds of pages of verse by recitation. Perhaps, in addition to crediting their talent, we can also credit these powers to their brains' neuroplasticity. When input to the visual cortex - almost half our cortex - is blocked with an absolute blindfold, all that cortical real estate is reallocated, within hours, to processing touch and sound. But now we learn, it can also be neuroplastically reassigned to the mind's eye.

We tend to think of vision as a primary experience that connects us directly with reality, and the mind's eye, and language, as merely reflecting, or describing, reality. But if seeing is our only direct access, then how do we account for the scientific report and EEG study by Helder Bértolo that found congenitally blind people actually see visions in their dreams and can draw them out? And some congenitally blind people, such as Helen Keller, write in a way that "startles one with its brilliantly visual quality."

Sacks concludes with a profound paradoxical question: "If there is a fundamental difference between experience and description, between direct and mediated knowledge of the world, how it is that language can be so powerful? Language, that most human invention, can enable what, in principle should not be possible. It can allow all of us, even the congenitally blind, to see with another person's eyes." Thus can blind poets see, and make us see.

Norman Doidge is a psychiatrist and author of The Brain That Changes Itself. His website is Copyright © 2010 by Norman Doidge

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