The process of perception involves synthesizing, organizing, and interpreting sensory information in a meaningful way. Researchers often describe perceptual processing as occurring in two basic ways. The first is known as bottom-up processing, and it involves making sense of ambiguous information, kind of like assembling the individual pieces of a puzzle when you don't know what the final image will look like. The second type, top-down processing, involves drawing on your existing knowledge, experiences, and expectations to make sense of the information that you encounter in different situations.
Visual perception begins with specialized neurons in the visual cortex of the occipital lobe. Somehow, these neurons interact to give rise to mental images that include shapes, groupings, location, movement, and color. Perceptual principles that are based on properties of visual stimuli have been around for quite some time and are the focus here.
Gestalt Principles of Perception
The Gestalt psychologists naturally spent much of their time researching how you perceive “wholes” from partial and sometimes fragmentary visual information. One of the clearest illustrations of their idea that the whole is more than the sum of the parts is the principle of closure. For example, a line drawing of a familiar object such as a human face can omit many details but still easily be perceived as a face — even if parts of the outline are missing.
Another principle is proximity. Here, similar objects that are uniformly spaced tend to be seen as one group. If the objects are spread out in trios, however, the perception will be that there are several groups.
Similarity is the principle that you tend to perceive groups of objects in accord with the extent to which they are alike. A matrix of circles is seen as just that — a bunch of circles. But if you make some of the circles squares instead, the circles and squares are perceived as separate groups.
Finally, figure-ground is a basic but somewhat more complicated principle of visual perception in which you attend to part of a visual stimulus as the figure and ignore the rest of the image as the background. The classic “vase and faces” reversible figure provides an example. If you happen to attend first to the light area, you see a vase against a dark background. If you instead attend to the dark area, you see two faces in profile looking at each other.
You perceive the location of things and how far away they are in two basic ways. One is binocular vision, which is based on your normally having two eyes that are separated. When you cast your eyes on an object, each eye therefore has a different perspective — that is, the object is seen from two slightly different angles of view. At the same time, the object is projected to slightly different areas of each retina. The visual cortex then integrates the differences in neural impulses from the eyes and determines “where” including “near” or “far”
The other way is monocular cues. With only one functional eye, you can still use certain information to determine location. Or with both eyes, you use these monocular cues in conjunction with your binocular vision to determine location in a two-dimensional scene such as a television picture, a movie, or a cartoon. One such cue is interposition. If one object partially blocks the view of another, you perceive the first object as being closer.
Another is relative size. An object that is larger than another seems closer. Cartoonists in particular make extensive use of height in field. An object that is at the bottom of a cartoon panel appears closer; one toward the top appears farther away.
Among the many other monocular cues is linear perspective. Two lines that begin at the bottom of an illustration and then draw closer near the top appear to be receding into the distance. This perception corresponds to looking down a pair of railroad tracks.
Perceiving Movement and Motion
An obvious way that you perceive movement and motion is that retinal stimuli change as an object crosses our visual field. A less obvious one is that if our eyes move to “track” the object, the muscular changes generate neural impulses that the brain can also use in determining movement. You also perceive yourself as moving by using these cues in addition to bodily ones such as kinesthesis and equilibrium, discussed earlier in this chapter.
The human ability to detect very small movements in an environment is especially keen, and this may have an evolutionary basis. To our ancient ancestors who lived in the wild, any sort of movement signaled the possibility of danger and being killed and eaten, so those who were less alert to movement were less likely to pass along their genes.
However, cues for movement can be quite misleading. If you're flying in a plane, it may very well seem that the clouds are passing by and the plane is actually standing still. Similarly, while sitting in a car wash with your vision partly obscured by suds and water on the windows, you may get the impression that you and the car are moving and the car-wash machinery is standing still — especially if your car is being rocked a bit by the brushes. These kinds of effects occur because you lack an absolute reference point for what's moving and what isn't.