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By Root 1361 0

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.†

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.”1

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.2

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