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The wonder of our visual sense, revealed through its pathologies
on September 22, 2010
If I were to ask you to descrbe the differences were between what your eyes see, and what you see, you'd probably think it an odd question. After all what you see is what your eyes see, right? Curiously enough, what you see when you perceive the world around you is very different from what your eyes "see."
Consider this: The human eye can detect fine detail over an angle of about 2 degrees. That's not much; it's roughly the area of a dime held at arm's length. Your first instinct is probably to say nonsense; after all, you can easily perceive the entire scene before you, over an angle of at least 90 and as much as 180 degrees. You're right, at least in part. You perceive the wide expanse of the world before you, but what you perceive and what your eyes take in are two very different things. The world you perceive is not the raw input from your eyes, but rather something constructed by your brain, using input from your eyes as well as a lifetime's experience and memory of the world around you.
Here's another example. You've probably, at one time or another in your childhood, placed a finger in front of your face, and then viewed it through each eye in turn, noticing how it appears to jump back and forth and you switched eyes. Obviously, your eyes see slightly different pictures of the world. Yet when you look at the world, you don't see two different pictures. You see a single picture of the world, with a sense of depth and dimensionality not apparent when viewing with either eye alone. That third dimension isn't there in the pictures coming from your eyes- it has to be added by the brain.
Neurologist Oliver Sacks has made a second career for himself writing about neurological affectations, and how they affect the people who suffer them. In this book, he examines how vision works, and what happens when it doesn't. Sacks has a particular insight into the problems of those whose vision differs from that of the population at large, as he himself suffers from prosopagnosia- the inability to recognize faces. For years, this was assumed to be a purely psychological problem. How could someone with excellent vision fail to recognize a face- even that of a family member? But for severe prosopagnosiacs, even the face of a parent or child is a nondescript set of features, no different from any other. This can and does affect recognition of things as well as people. Sacks, for example, tells how how he many times walked past his own house many times until a neighbor or family member spotted him and guided him home again. Prosopagnosia can range from the slight to the severe. Perhaps as many as 2.5% of the population carry a gene that predisposes them to the condition, and most mild prosopagnodiacs are probably unaware that they have the condition, thinking instead that they simply have a "bad memory for faces." Sacks speculates if many instances of social shyness may in fact be due to the difficulties brought on by prosopagnosia; his own mother was painfully shy, and he suspect, given the genetic component, that he may have inherited his condition from her.
A related condition Sacks discusses at length is alexia, the inability to recognize letters.Usually brought on my injury, disease, or stroke, alexics can see letters, but the letters make no sense to them. One subject, a writer by trade, describes his post-stroke perception of English language as looking like "Serbo Croation (cyrillic) characters." Curiously enough, most sufferers have no difficulty writing, a condition known as "alexia sin agraphia"- alexia without agraphia. They can write, but they cannot recognize their own handwriting after they write. To a neuroscientist, this is strong evidence for very different areas of the brain being involved in the production of text and the perception of it; to a writer, or a voracious reader, it can be a devastating condition. Some found they can switch to audio books and dictation, and a very few have managed to teach themselves new strategies to read, if slowly.
Midway through the book Sacks describes the discovery of a tumor in his dominant eye. Though the tumor is treated, successfully, he loses a part of the visual field in the affected eye, and eventually, most sight. This leads to a number of very curious things. At one point, Sacks describes closing his eye- and continuing to see the scene about him, as if his eyes were still wide open. The brain, Sacks notes, is predisposed towards receiving information from the senses, and if deprived of that information, will fill in as best it can. There is a rare condition in which the sufferers are objectively blind, yet maintain that they can see, even as they find themselves bumping into objects, and many older people with visual impairment suffer from Charles Bonnet syndrome, a condition in which the mind creates objects (and occasionally people) to fill in for missing visual stimuli. (Charles Bonnet syndrome is rarely reported, as the sufferers are often afraid it will be taken as a sign of senility.)
Sacks also discusses stereo vision, and those who have lost and gained it, and the loss and recovery of vision in general. Interesting, although most sighted people who lose vision eventually lose their visual imagery as well, some gain an enhanced sense of visual imagery. One subject Sacks discusses became so good at integrating the information from his other senses into his visual imagery that he could confidently walk down the street without a cane or dog. Another repaired the roof of his garage- at night (terrifying his neighbor!), since the presence or absence of light made no difference to him.
As with all Sacks' books, "The Mind's Eye" is a superb synthesis of science, medicine, and insight into the human experience. His obvious empathy, and even affection, for the people he meets and consults with come through in his writing, and help the reader to see the person behind the affliction, and to give each of us greater appreciation for the wonder and the mystery of the senses we possess.