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It is often helpful when thinking about biological processes to consider some apparently similar yet better understood non-biological process. In the case of visual perception an obvious choice would be color photography. Since in many respects eyes resemble cameras, and percepts photographs, is it not reasonable to assume that perception is a sort of photographic process whereby samples of the external world become spontaneously and accurately reproduced somewhere inside our heads Unfortunately, the answer must be no. The best that can be said of the photographic analogy is that it points up what perception is not. Beyond this it is superficial and misleading. Four simple experiments should make the matter plain.

In the first a person is asked to match a pair of black and white discs, which are rotating at such a speed as to make them appear uniformly grey. One disc is standing in shadow, the other in bright illumination. By adjusting the ratio of black to white in one of the discs the subject tries to make it look the same as the other. The results show him to be remarkably accurate, for it seems he has made the proportion of black to white in the brightly illuminated disc almost identical with that in the disc which stood in shadow. But there is nothing photographic about his perception, for when the matched discs, still spinning, are photographed, the resulting print shows them to be quite dissimilar in appearance.The disc in shadow is obviously very much darker than the other one. What has happened Both the camera and the person were accurate, but their criteria differed. One might say that the camera recorded things as they look and the person things as they are. But the situation is manifestly (明白地) more complex than this, for the person also recorded things as they look. He did better than the camera because he made them look as they really are. He was not misled by the differences in illumination. He showed perceptual constancy. By reason of an extremely rapid, wholly unconscious piece of computation he received a more accurate record of the external world than could the camera.

In the second experiment a person is asked to match with a color card the colors of two pictures in dim illumination.One is of a leaf, the other of a donkey. Both are colored an equal shade of green. In making his match he chooses a much stronger green for the leaf than for the donkey. The leaf evidently looks greener than the donkey. The percipient (有感知力的人) makes a perceptual world compatible with his own experience. It hardly needs saying that cameras lack this versatility.

In the third experiment hungry, thirsty and satiated (充分满足的) people are asked to equalize the brightness of pictures depicting food, water and other objects unrelated to hunger or thirst. When the intensities at which they set the pictures are measured it is found that hungry people see pictures relating to food as brighter than the rest (i.e. to equalize the pictures they make the food ones less intense), and thirsty people do likewise with “drink” pictures. For the satiated group no differences are obtained between the different objects. In other words, perception serves to satisfy needs, not to enrich subjective experience. Unlike a photograph the percept is determined by more than just the stimulus.

The fourth experiment is of a rather different kind. With ears plugged, their eyes beneath translucent(透亮的)goggles (转动眼珠) and their bodies either encased (包装) in cotton or wool, or floating naked in water at body temperature, people are deprived for considerable periods of external stimulation. Contrary to what one might expect,however, such circumstances result not in a lack of perceptual experience but rather a surprising change in what is perceived. The subjects in such an experiment begin to see, feel and hear things which bear no more relationship to the immediate external world than does a dream in someone who is asleep. These people are not asleep yet their hallucinations, or so-called autistic‘ perceptions, may be as vivid, if not more so, than any normal percept.

The second experiment shows that ________.