Each chapter in this latest collection focuses on a person whose visual system is somehow compromised or enhanced. I say visual system rather than eye because most of what we experience as 'seeing' is accomplished by the central nervous system. The retina, which is the curved screen upon which our lens focuses the light entering our eye, and that forms the back wall of our eye, is actually part of the CNS. It's like a little piece of brain that hangs by a stalk (the optic nerve). The characters of The Mind's Eye include a pianist who loses the ability to read music, a mystery writer who looses the ability to read words (but not to write them), and several people who are selectively blind for faces but not necessarily for other classes of objects. There is a chapter devoted to stereoscopy, or how the brain combines the images from our two two-dimensional retinas to create an illusion of three-dimensional space. In this case, a woman with life-long strabismus, who could not see in three dimensions because she viewed the world through one eye at a time, gains the ability to see in 3-D. I found this section particularly charming, not so much for the case study but because of the fact that Dr. Sacks, having developed an interest in stereoscopes in his childhood, built himself several:
So when, at the age of ten, I developed a passion for photography, I wanted, of course, to make my own pairs of stereo photos. This was easy to do, by moving the camera horizontally about two-and-a-half inches between exposures, mimicking the distance between the two eyes. (I did not yet have a double-lens stereo camera, which would take simultaneous stereo pairs).We were able to have fun before television and the internet! Sacks's memoir, Uncle Tungsten, is full of such examples of himself or members of his family pursuing their curiosity about the world's flora, fauna, and physical phenomena, through active research. I delight in the fact that Sacks was a member of the New York Stereoscopic Society prior to his own eye troubles (not to mention the American Fern Society and the New York Mineralogical Club, both mentioned in his bio). The fact that Sacks pursues his curiosities so avidly, whether about people or phenomena, is reflected in the way he listens to and relates the stories of his patients.
After reading how Wheatstone explored stereoscopic effects by exaggerating or reversing the disparity between the two images, I began experimenting with this, too. I started taking pictures with greater and greater separations between them, and then I made a hyperstereoscope, using a cardboard tube about a yard long with four little mirrors. With this, I could turn myself, in effect, into a creature with eyes a yard apart. I could look through the hyperstereoscope at a very distant object, like the dome of St. Paul's Cathedral, which normally appeared as a flat semicircle on the horizon, and see it in its full rotundity, projecting towards me.
Through his case histories, Sacks also writes on how the brain evolved the skill of reading if it wasn't "designed to" do so, how our eyes take in surfaces and the boundaries between them but the brain constructs"objects" from them, the von Helmholz and Young theories of color vision, and how the brain is able to fill in holes in the visual field - both those naturally occurring in our blind spot (the place where the optic nerve exits from the retina) - and those created by injury or illness. Sacks's description of his own loss of vision is particularly evocative. He communicates through it not only how the brain constructs what we perceive from the cues the eyes collect, but also how the limits of the eye produce a different experience for the brain. When Sacks loses the peripheral visual field of one of his eyes to a tumor, he not only cannot see that portion of space, he seems to lose his awareness that it exists at all.
Kate and I finished our walk and headed back to my office. I walked ahead and got inot the elevator - but Kate had vanished. I presumed she was talking to the doorman or checking the mail, and waited for her to catch up. Then a voice to my right - her voice - said, " What are we waiting for?" I was dumbfounded - not just that I had failed to see her to my right, but that I had even failed to imagine her being there, because "there" did not exist for me.The last chapter, which lends its title to the book, was most interesting to me. Mental imagery is a particular interest of mine. Sacks writes of neural plasticity (the reassignment of new functions to neural real estate). For example the reallocation of parts of the visual cortex to either hearing or touch for those who are blind. Or the science-fiction like work of neuroscientist Paul Bach-y-Rita , who takes advantage of such flexibility by connecting the output of a video camera, point-by-point to a grid on one of the most sensitive part of the human body - the tongue. Such a device enables blind people to walk across a room avoiding obstacles, to catch a ball rolled toward them, and make other such perceptual judgments not previously afforded them. Are these people "seeing," Sacks asks? Whether exploring case studies, the evolution of neuroscience, or more recent avances, Sacks's writing is probing, accessible, and humane in The Mind's Eye.