The Story of Psychology (85 page)

A
minnow, with almost no brain to speak of, can see (more or less); so can an ant, whose entire nervous system consists of only a few hundred neurons; and so can many another creature that has nothing remotely akin to a mind. It might seem, therefore, that visual perception is a physiological function and, though it influences many psychological processes, is not itself one of them.
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Throughout the centuries, however, most philosophers and psychologists have considered perception, at least in human beings, a fundamentally psychological function; it is the mind’s link to external reality, of which we know only what our senses tell us. The derivation of knowledge from perceptions raises a host of interesting questions (interesting not in the lay sense of “absorbing” but in the scientific sense of “important” or “potentially illuminating”). But although philosophers have thought about perception for twenty-five hundred years and physiologists and psychologists have researched it for nearly four hundred, some of these questions remain moot and some have been answered in ways that raise new and equally puzzling questions. With the advent of cognitive neuroscience, however, a number of the most interesting questions have recently been, or are currently being, definitively answered.
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Consider one that the Greek philosophers were the first to ask: How do images of the outside world reach the intellect within?

Plato speculated that the eye actively seeks information by sending forth emanations of some kind that encompass objects—palpating them visually, so to speak. Democritus disagreed, arguing that perception works in the other direction: each object constantly imprints its likeness on the atoms of air, and these replicas, traveling to the viewer, interact with the atoms of the eye and re-create the likeness there, whence it passes to the mind. It was a better guess than Plato’s but wrong in all its details.

In 1604 the German astronomer Johannes Kepler made a leap forward in the understanding of vision. Recent developments in optics and optical instruments enabled him to recognize that the clear body in the front of the eye is a lens that bends rays of light coming from any object, casting an image of the object on the eye’s screenlike retina, from which the resulting nerve impulses are transmitted to the brain.

Ever since, the notion has prevailed that the eye is a kind of camera; the metaphor fits the facts of nearsightedness, farsightedness, and astigmatism, and their correction by eyeglasses. But while it is valid in some respects, it is seriously misleading in many others. Ralph N. Haber, long a leading figure in perception research, has called it “one of the most potent though misguided metaphors in psychology” and the source of much “mischief.”
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What sort of mischief? For one thing, in a camera the image projected by the lens is upside down, and in 1625 Christoph Scheiner, an astronomer, showed that this is also true of the eye. He carefully peeled away the outer coating of the back of an ox’s eye and through the semi-transparent retina saw an upside-down version of whatever he aimed the eye at. But if we see the image that is formed on the retina, why do we not see the world upside down? The question was to plague psychologists for three centuries.
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Another difficulty created by the eye-as-camera metaphor became evident with the advent of photography. To form a sharp image, a camera must be held still during the exposure or, in the case of a movie camera, open and close its shutter many times a second; our eyes, however, constantly jiggle back and forth, even when we look steadily at some point, yet do not produce blurred images. Although we are not aware of and do not normally experience these movements, we can see them by means of a simple procedure. Look steadily at the black dot in the center
of the diagram below for about twenty seconds, then quickly shift to the white dot and gaze at it fixedly. You will see an illusory pattern of black lines wavering slightly to and fro. The black lines are an afterimage, due to temporary fatigue of the retinal receptors on which the white lines fell for twenty seconds; the wavering is the never-ending movement in question.

FIGURE 21
Test pattern for perceiving constant eye movement

The meaning of the demonstration is that the eye may be somewhat like a camera, but seeing is nothing like taking pictures.

A second interesting question: Is what we see actually out there? A corollary question: Does it look like what we see? Folk wisdom has always held that we see what exists and what we see is a faithful account of what exists. We see a closed door before us, reach out to the doorknob, and it is where we expected it to be and does what we expected it to do; we lower ourselves onto a chair and it is real and solid, as it appeared to be; we raise a forkful of fettuccini Bolognese to our mouth and it is rich, meaty, and chewy, just as we anticipated. Common sense and philosophy agree that perception is contact with reality. Only a few rare birds, like Bishop Berkeley, have ever doubted that there is a world outside ourselves that corresponds to our perceptions.

But though nearly all of us reasonably assume our perceptions to be truthful, physicists now assure us that the colors we see do not exist as colors outside our heads. The red of a ripe apple, for instance, does not
exist as red in the apple; what does is a surface that absorbs all visible light except in the region of 650 nanometers wavelength, which it reflects. When that specific radiation reaches the human eye, the brain perceives it as what we call red. It may be disconcerting to think that the whole splendid colorful world we see on a spring day doesn’t really look like that outside of our own minds. But perhaps we should set aside this philosophic/metaphysical issue and consider a much more approachable problem of vision, namely, that we often have visual experiences we know are misleading or erroneous but cannot will ourselves to correct. The moon, on the horizon, looks huge; we are aware that it does not change size but cannot make ourselves see it as no larger than it is when overhead. We stare at a bright light and, looking away, see an after-image—a perception, but not of anything outside ourselves. We have dreams in which we see persons, places, and actions that are not before us, as they seem to be, or may not even exist.

There are, furthermore, the many illusions that psychologists have studied in the past and the present century. In the following diagram the gray tones of the inner areas look quite different from each other, but actually are identical, as you can determine by cutting a small hole in a piece of paper and centering it over first one and then the other. The mind, or at least the brain’s visual cortex, judges lightness in terms of contrast, not absolute intensity. What you see is not what exists.

FIGURE 22
Which central area is darker? Wrong!

Here are several other classical illusions, each named for its discoverer: (1) the Zöllner, (2), the Poggendorf, (3) the Jastrow, and (4) the Hering:
Contrary to what your eyes tell you (and as you can verify with a ruler), the straight lines in (1) are parallel to one another, the angled lines in (2) are aligned, not offset from each other, the figures in (3) are the same size, and the heavy lines in (4) are perfectly straight.

FIGURE 23
Four classic visual illusions

Another category of illusion consists of ambiguous figures that we can will ourselves to see as either one or the other of two different things. Two examples:

FIGURE 24
Two reversible figures

In (1) you can will yourself to see the familiar Necker cube as if you were looking down on it, with corner X closest to you, or as if you were looking up at it, with corner Y closest to you. In (2) you can see the handles attached inside the two white sides of the basket—or, if you choose, attached inside the gray sides.

Finally, in the following diagram there appears to be a triangle that is distinctly whiter than the surrounding area,
but it was you who created both the triangle and its brightness; no such figure is there, nor is the paper any whiter where the triangle seems to be than in the adjacent background.