Story of Psychology - Morton Hunt [304]
The argument that depth perception is the product of our associating cues with our experiences of depth began with Locke and Berkeley. From their time to the present, psychologists in the associationist behaviorist tradition have maintained that unconsciously or consciously we link the cues in the two-dimensional retinal image with our experiences of how far away the objects are that produce those cues.
The alternative notion, that we perceive depth as a result of a kind of logical reasoning about what we see, was first voiced in 1843 by J. S. Mill, who said of perception that what we observe is one-tenth observation and nine-tenths inference. Later in the century, Helmholtz argued, in more detail, that we unconsciously infer three-dimensional reality from the two-dimensional retinal data. From then until now, a number of cognitively oriented psychologists have held that perception, including that of depth, is partially or even largely a product of higher mental functions—“thoughtlike processes,” Irvin Rock terms them—of which inference from cues is only one.57
Whichever view one prefers, the cues to depth are familiar enough in everyday life, and their role in perception has been demonstrated in many hundreds of experiments. Here are the principal cues and a few representative experiments:58
—Apparent size: The farther away any object is, the smaller it seems, but if we already know how big it is—a person, for instance—we judge how far away it is from its apparent size even if it is on a featureless plain that gives no cue. In a 1951 experiment, one researcher made up playing cards ranging from half the normal size to twice the normal size and showed them to subjects under laboratory conditions in which there were no cues to distance. The subjects thought the double-size cards were close to them and the half-size cards far from them. All were at the same distance.59 Everyone, moreover, has experienced the moon illusion—the full moon looks remarkably larger when it is on the horizon than when overhead. Of the explanations currently offered, the most persuasive is that when the moon is close to objects on the horizon, they affect our judgment of its size; when it is overhead, away from all such clues, we judge it differently.
—Perspective: Parallel lines running away from the viewer, such as railroad tracks or the edges of walls, converge with distance. How powerfully we are influenced—or, one should say, informed—by this cue was shown earlier in Figure 13 on page 331: The perspective gradient enables us to perceive the farther figure as roughly the same size as the nearer one, although in fact, as shown, the image of the former is only a third the size of the latter.
—Interposition: When an object is partly concealed by another we realize that the concealed one is farther from us than the concealing one. In looking out over a cityscape, we easily sense the distance of a remote tall building from the fact that closer ones obscure its lower floors; at sea, on the other hand, the distance of a floating object is much harder to judge.
—The texture of a surface—a grassy field, a cement sidewalk—is constant, but the increasingly finer grain of the texture at greater distances makes it an important cue to the distance of anything on that surface.
—Faraway buildings or hills are pale and hazy compared with nearby ones, owing to the greater amount of atmosphere between them and us.
—Motion parallax—the changing relationship of things to each other as we move—is an important source of depth information, particularly when nearby objects are seen in relation to distant ones.
—Convergence and accommodation: When we look at something very close to us, our eyes angle inward and the muscles around each lens strive to keep it in focus. When we look at something far away, our eyes are parallel and the lenses relaxed. The concomitant visceral sensations are important cues to the distance of objects ten feet