Crashing Through (27 page)

Wire-stretching device similar to the type found in some hardware stores.

Corneal epithelial stem cells of the kind transplanted into May’s eye (as seen under electron microscope)

To the infant, who lacks knowledge of the world and its objects, the universe must seem a vast collection of these colorful, meaningless shapes, a panorama of fossils and strange tools. One can imagine that it must feel overwhelming to the infant, this overflow of visual information he cannot even begin to sort.

How is the very young child to make sense of this jumble of visual data? How is he to translate these shapes into three dimensions and give them meaning, to make them more than just a collection of colorful blobs? How is he to build the knowledge of the world and its objects that is so essential to vision? It’s not as if anyone can explain it to him.

There is only one way for the very young child to do this. He must interact with the things he sees. He must experiment with them, investigate them, explore them, probe them, play with them, touch, taste, smell, and hear them. He must handle everything, manipulate everything, go to and reach for everything. He must make his nursery his laboratory, a place in which his endless tests and trials with things—especially by touch—lead to a knowledge of their textures, shapes, purposes, and functions, to an understanding of their natures. Without that constant and direct interaction and experimentation with things, he cannot begin to form his set of assumptions about the world and its objects. Without touching a glass of water—tipping it, dropping it, breaking it, spilling it, shaking it, hearing its sounds, watching its water levels, observing the change of light and shade on its side as it’s lifted, seeing it used to take a drink—he can’t know a glass of water as anything but just a shape, a random fossil, and he might not even interpret it as a shape in three dimensions. In the laboratory of the young child, it makes perfect sense to eat carpet because that’s yet another way to know the carpet’s visual nature.

Interaction with the world and its objects is so critical during early development that without it a person might never see properly. In a landmark experiment, Richard Held and Alan Hein at MIT raised two kittens in total darkness. For a short period during each day the kittens were placed in baskets that hung just above the ground from opposite ends of a pole. Holes were cut in one basket to allow one kitten’s paws to reach the ground, but not in the other. The apparatus was constructed such that when one basket moved, the other moved identically.

When the lights were turned on, the kitten in the basket with holes was allowed to run along the ground, causing its basket and its mate’s basket to move identically, and giving each of them an identical visual experience.

At the end of the experiment, only the kitten that had been allowed to actively move the basket could move around the world using vision. The passive kitten—though it had exactly the same visual experience—was functionally blind. Seeing the world passively was not enough; interaction with the world was essential for vision to be useful.

Other studies show similar results. There is evidence that people brought up in iron lungs—in which their interaction with objects is severely restricted—cannot see things properly beyond the range of their body movements, beyond what they could touch. In one strange case, a boy was raised in a pawnshop and surrounded by all manner of objects. To ensure that he didn’t touch the items or meddle with their tags, he was kept in a playpen. When removed, he simply could not judge distances beyond what he’d experienced by touch in the playpen.

The lesson in all this is clear: interaction with the world and its objects is critical to building the knowledge necessary for useful vision.

Mike May had that crucial interaction with the world. He had built his knowledge and formed his set of assumptions. By the time of his accident, at age three, he could see nearly as well as an adult.

Yet after his stem cell transplant surgery, he found himself with a strange and different kind of vision. He could perceive motion and color almost perfectly, but he could not make sense of faces, perceive things in depth (unless they were moving), or readily recognize objects. What explains this dichotomy? Is there something about perceiving motion and color that’s different from perceiving faces, depth, and objects? And how does that relate to knowledge?

Ione Fine set out to answer that question by examining how people learn to perceive these things. She believed the answer might go a long way toward explaining May’s vision. And maybe toward helping him improve.

FACE PERCEPTION

To most people, human faces appear distinctly unique, the most personal and nuanced objects in the world. In reality, they are very similar to one another. Differences of just a millimeter or two in symmetry or space between the eyes or in the eyebrow curvature or cheekbone angle or forehead height can make two quite similar faces look vastly different.

Animal faces are distinguished by the same kinds of tiny variations. Yet to us, chimpanzee faces look alike and sheep faces seem identical. Why do we see such profound difference in human faces but not in animal faces?

The answer lies in learning. Through intense practice that begins in early childhood, we make ourselves into experts on human faces.

Practice and learning are everything. That’s why shepherds can identify their sheep by faces—they’ve practiced all their lives with sheep faces, and now they’re experts. And that’s why people sometimes struggle to distinguish among faces from different ethnic or age groups—they haven’t sufficiently practiced and interacted with them.

Practice with human faces doesn’t just help a person identify and recognize faces. It also makes it possible to judge a person’s gender, read her expressions, assess her interest in us, predict her mood. Often, the difference between a smile and a frown is just a tiny change in the angle of—or even the shadow on—the corner of the mouth. A one-millimeter shift in the pupils of a person standing across the room can tell us whether that person is looking at us or just over our shoulder; a one-millimeter shift in her eyebrow can tell us if she’s interested or angry—all this at a distance of thirty feet. People would never be able to attach meaning to those minuscule differences without the benefit of massive practice—and massive learning.

That kind of learning takes years of intense practice; children are still developing their face-perception skills at five or six years of age.

DEPTH PERCEPTION

When we open our eyes, a two-dimensional image falls on our retinas. Yet we perceive the world robustly and in three dimensions; its depth feels absolutely real to us, not at all a trick of the brain. How does that happen? How do we translate our flat retinal images into the majestic three-dimensional world in which we move and interact so confidently?

There seem to be three kinds of clues that the visual system uses to perceive depth:

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Pictorial cues

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Motion cues

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Stereopsis

Pictorial Cues to Depth

Pictorial cues are features in a photograph or painting or other two-dimensional representation that can produce the impression of depth. They are the ones Italian painters discovered in the early Renaissance. The most important are:

Occlusion

When an object hides another object, that object is seen as being closer.

Relative Height

The closer an object is to the horizon, the farther away the object appears.

Note that this is true both for the ships in the illustration (which are below the horizon) and for the balloons (which are above the horizon).

Cast Shadows

Shadows can indicate an object’s depth. (The two photos are identical but for the addition of shadows in the photo on the right.)

Relative Size

The farther away an equal-sized object is, the less room it will occupy on one’s field of view.