Story of Psychology - Morton Hunt [289]
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.
FIGURE 25
The triangle that does not exist
As we proceed, we will learn the explanations for some of these illusions; for now, the point is that perception in human beings is not simply a physiological process that transmits representations of outside stimuli to the central nervous system; it often involves higher mental processes that make sense (and sometimes nonsense) of the impulses arriving via the optic nerves.
A third interesting question—Edwin Boring, in his monumental