Perception, an awarereness of impressions received through the senses. The process of gaining perception is called perceiving. An organism is said to perceive events, objects, or qualities in its environment, and many of its own bodily processes when it recognizes and gives meaning to them.

Perceiving is an ability possessed only by those animals—such as mammals, birds, and reptiles—that have a highly developed nervous system. It involves the cortex of the brain, where complex responses to sensory stimulation are made. When an earthworm turns aside from an obstacle in its path, the action is said to result not from perception, but from a simple nervous reflex.

In the past, some psychologists believed that sensory impressions and perceptions are identical. However, most modern psychologists and physiologists distinguish between the two experiences. If a person perceives a light moving through the darkness, he may be seeing a lamp carried along a path, or a series of stationary lamps being turned on and off in rapid succession. Thus, two kinds of sensory stimuli may produce one kind of perception. Other experiments show that one sensory impression can produce two or more perceptions.

The Gestalt school of psychology advances the theory that the whole is greater than the sum of its parts. According to this theory, a perception is more than the sum of a number of sensory impressions, and is an organized and meaningful experience having a configuration, or pattern, of its own.

For example, if a light is attached to the rim of a rolling wheel, the light is perceived—in darkness—as moving in a pattern of scallops. If the light is placed on the wheel's hub, it is perceived as moving in a straight line. But if one light is placed on the rim and another on the hub, the resulting perception is not of a straight line of light moving through scallops. Instead, the rim light is perceived as revolving around the hub light.

Principles of Perception

A Two-way Process

Perception is determined by the qualities of both the thing perceived and the perceiver. For example, every living organism constantly receives great numbers of sensory impressions, but perceives relatively few objects and events at any given time. Selection is based on attraction, and only a few things attract attention. Generally speaking, whatever contrasts with its background is apt to be perceived. Bright, large, or moving objects, loud sounds, or sudden silence after continuous noise, usually attract attention, as do phenomena that are repeated.

However, selection is also based in part on the personality of the perceiver, his needs, motives, interests, and expectations. For example, a person who expects to be awakened by an alarm clock at a certain time will probably sleep through an earlier, perhaps louder, sound.

Selection and all other aspects of perception are affected also by the perceiver's previous experiences, learning, and maturity. The odor of baking bread is recognized and given meaning only by those whose previous experiences help them to identify the odor. A photographer who uses a view camera, which projects upside-down images onto the viewing screen, soon learns to perceive objects on the screen as being right side up. Young animals tend to develop depth perception at about the time they first crawl or walk, thereby avoiding falls from high places.

Perceptual Constancy

One of the characteristics of perception is the constancy of form, size, and—to lesser extent—brightness of objects. An observer watching a car moving past him into the distance does not perceive it as getting smaller, although the image of the car on his retina does grow smaller. A square table top is perceived as square even when the observer's eyes are almost on a level with it, and the retinal image is a rhombus.

Brightness constancy is incomplete. Objects that are perceived as white in a bright light continue to look white in dim light. Black objects appear black whether in dim or bright light. However, if two objects of an identical gray are placed so that one is in shadow, the shadowed object will appear to be slightly darker than the one in light.

Sense Cooperation

A perception rarely depends upon only one class of sensory impressions. Usually, several classes of impressions join in contributing to the formation of one perception. For example, the perception of one's own position in space depends upon both visual impressions and kinesthetic (relating to the muscle sense) impressions. If a perception derived from one class of sensory impressions does not agree with that derived from other sensory impressions, it is called an illusion. An imaginary perception is called a hallucination.