Monday, June 22, 2009

Why the Eyes Have it by Christopher Chabris

Why are we ­humans so good at seeing in color? Why do we have eyes on the front of our heads rather than on the sides, like horses? And how is it that we find it so easy to read when written language didn’t even exist until a few thousand years ago—a virtual millisecond in evolutionary time?
Most of us, understandably, have never given much thought to questions like these. What is surprising is that most cognitive scientists ­haven’t either. People who study the brain generally ask how it works the way it does, not why. But Mark Changizi, a professor at Rensselaer ­Polytechnic Institute and the author of “The Vision ­Revolution,” is indeed a man who asks why, and lucky for us: His ideas about the brain and mind are fascinating, and his explanations for our habits of seeing are, for the most part, persuasive.
Mr. Changizi takes care not to call himself a practitioner of evolutionary psychology. This is the one discipline of the mind sciences that focuses on why questions, but it often answers them by telling just-so stories that cannot be ­disproved. (Why do men have better spatial ability than women? Because a long time ago, in Africa, men needed spatial skills to track prey and to kill at a distance—a plausible theory but one that is difficult to test with experiments.) Instead Mr. Changizi calls ­himself a “theoretical neuroscientist,” seeking explanations for the design of the mind that are based on mathematical and physical analysis. He has his own stories, it is true, but they are grounded solidly in neuroscience, and they are backed up by data about a surprising range of human activities, from ­the colors found in historical ­costumes to the ­correspondence between the shapes found in written letters and the shapes found in ­nature.
Let’s start with the question of color. It is such a natural part of our visual experience that we don’t stop to wonder why we can see it at all. ­Without color television there would have been no “Miami Vice,” of course, but were we really missing out on so much when we had only black and white? The consensus explanation for our superior ability to perceive color is that primates evolved it to see fruit—you can’t order dinner if you can’t read the menu.
Mr. Changizi thinks otherwise. He proposes that color vision is useful for distinguishing the changes in other ­people’s skin color—changes that are caused by shifts in the volume and oxygenation levels of the blood. Such shifts, like blushing, often signal emotional states. The ability to see them is adaptive because it helps an observer to “read” states of mind and states of health in others, information that is in turn useful for predicting their behavior.
Our brains evolved in a time when people lived their entire lives without ever seeing someone with a skin color different from their own. Thus the skin color we grow up seeing, Mr. Changizi says, is “neutral” to us: It serves as a kind of baseline from which we notice even minor deviations in tint or hue. Almost every language has distinct words for some 11 basic colors, but none of them aptly describe the look of skin, which seems colorless (except in our recent multicultural societies, where skin color is newly prominent). As one might expect, primates without color vision tend to have furry faces and hands and thus less need to perceive skin color; ­primates with color vision are more “naked” in this respect, humans most of all.
James Steinberg
Conventional wisdom may be similarly misleading when it comes to binocular vision. It is said that we have two forward-facing eyes, which send our brains two separate images of almost everything in our field of view so that the brain can compare those images to estimate the distance of objects—a generally useful thing to know. But people who are blind in one eye, Mr. Changizi notes, can perform tasks like driving a car by using other cues that help them to judge distance. He offers a different explanation: that two eyes give us a sort of X-ray vision, allowing us to see “through” nearby objects to what is beyond.
You can experience this ability yourself by closing one eye and holding your forefinger near your face: It will appear in your field of vision, of course, and it will block what lies beyond or behind it. If you open both eyes, though, you will suddenly perceive your finger as transparent—that is, you will see it and see, ­unblocked, the full scene in front of you. Mr. Changizi observes that an animal in a leafy environment, with such an ability, gains an advantage: It can lurk in tall grass and still see what is “outside” its hiding place. He correlates the distance between the eyes and the density of vegetation in the habitats of animals and finds that animals with closer-set eyes do tend to live in ­forests rather than on plains.
As for reading, Mr. Changizi stops to observe how remarkable this ability is and how useful, giving us access to the minds of dead people (i.e., deceased writers) and permitting us to take in words much faster than we can by merely listening to them. He claims that we learn to read so easily because the symbols in our written alphabets have evolved, over many generations, to resemble the building blocks of natural scenes—­exactly what previous millennia of evolution adapted the brain to perceive quickly. A “T,” for example, appears in nature when one object overlaps ­another, like a stone lying on top of a stick. With statistical analysis, Mr. Changizi finds that the contour patterns most common in nature are also most common in letter shapes.
Mr. Changizi has more to say about our visual experience—about optical illusions, for instance, which he sees as artifacts of a trick the brain uses to cope with the one-tenth of a second it takes to process the light that hits our eyes and to determine what is actually in front of us. He calls for a new academic discipline of “visual linguistics,” and he tells us why there are no ­species with just one eye.
What does all this add up to? Provocative hypotheses but not settled truth—at least not yet. As a theoretician, Mr. Changizi leaves it to others to design experiments that might render a decisive ­verdict. Someone else will have to study how accurately people can perceive mental states from shifting skin tones, and someone else will have to ­determine whether, in most cases, looking at another ­person’s skin adds any useful information to what is easily known from facial expression, tone of voice and body ­language.
Still, the novel ideas that Mr. Changizi outlines in “The Vision Revolution”—together with the evidence he does present—may have a big effect on our understanding of the human brain. Their implication is that the environments we evolved in shaped the design of our visual system according to a set of deep principles. Our challenge now is to see them clearly.

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