The Gestalt Laws of Organization
Basic principles by which we organize bits and pieces of information (stimuli) are known as the Gestalt laws of organization. (In German, Gestalt means, roughly, "a pattern perceived as a whole." It's often capitalized because that's the rule for nouns in German.) They include closure, simplicity, proximity, and similarity. The closure principle applies when we assume we're looking at an octagonal stop sign even if part of it is concealed by tree branches. The proximity principle kicks in when we see a group of people close together and assume they're a group, even if they
aren't. When things we see are similar in appearance, like geese in flight, we tend to perceive them as grouped together. The simplicity principle is the most basic Gestalt principle.
It holds, for example, that in a observing a complex design, we'll tend to perceive the simplest form it could represent.
If it could be a design for a circuit board or a "Y" shape, we'll tend to see the "Y" shape.
Top-Down and Bottom-Up Processing
Top-down processing is guided by experience, expectations, and motivations that are part of higher-level processing. It's typically guided by understanding or being aware of a con- text. For example, if we're familiar with what goes on at a Jewish Passover Seder or a Polish wedding, we'll pick up the clues as to what's going on when we come across such a social scene because we're familiar with the contexts.
Bottom- up processing complements top-down processing. Even if we've been to a Passover Seder or a Polish wedding, we may not be sure what's going on until we pick up clues and "con- nect the dots." In the first instance, the first clues might be men wearing yarmulkes and people speaking what sounds like Hebrew. In the second instance, we may recognize that we're observing a wedding before we pick up other clues and figure out that we're at an Eastern Orthodox Church, people are speaking in a strange tongue, and polka music starts up. The main thing to remember here is that we use both top- down and bottom-up processing to determine the context of a situation and how we should behave.
Perceptual Constancy
Sensory stimuli are subject to the brain's organization. Your brain tends to group visual information together into familiar shapes or objects. For example, when you see a car pass down the street, you don't imagine the car itself is shrinking as it moves away from you. The ability to discern size at different distances is possible because your brain maintains size constancy. Your brain can also maintain shape constancy.
For example, your brain knows that a globe is spherical although it looks like a circle, even when you're looking at
only one part of it. The same phenomenon applies to bright- ness. At the beach, you'll notice that a yellow bathing suit is just as eye-catching in the sunlight as it is on a cloudy day. This experience is known as brightness constancy.
Size constancy, shape constancy, and brightness constancy are all examples of perceptual constancy. Perceptual constancy means that we tend to see what we expect to see. However, perceptual constancy requires that we be familiar with our environments. Here's an interesting illustration of that fact: An anthropologist recorded a strange event when he led some African forest people out of their normal environment onto a broad, grassy plain. The people began to point and laugh at grazing African buffalo in the distance. To them, the crea- tures seemed tiny and strange. The forest people lived in an environment where it was unusual to see ahead more than a few feet. Since their brains couldn't interpret large animals from a distance, the people couldn't maintain a sense of size constancy.
Depth Perception: Translating 2-D to 3-D
There's an "angle shift" between what we see with the right eye and what we see with the left eye. You've probably noticed this. If you haven't, just hold a pencil in front of your face and look at it with first one eye than the other eye. We call this perspective shift binocular disparity. Monocular cues also help us see depth, even with just one eye. In paintings, for example, we can get a sense of depth perception by the relative size of objects or by receding and converging vertical lines like those of a railroad track. We can also detect texture gradients as "closer" objects are more detailed, while more apparently remote objects are less detailed. An interesting monocular cue is called motion parallax. For example, observing the countryside from a moving car, we see objects closest to us passing by quickly while more remote objects pass by more slowly.
Motion Perception
Motion perception depends on a cooperative alliance between information we've internalized about the world to adapt to life as we know it, plus external motion cues. For example, when someone tosses you a basketball, the retina detects the sphere getting larger. In the context of motion perception, we won't see the basketball in terms of size constancy, but in terms of an approaching object in motion. Even more fascinating, we actually catch the ball.
Perceptual Illusions
This final section is a fascinating discussion of visual (percep- tual) illusions. The best way to make sense of the discussion is by thinking creatively about the illustrations of these kinds of "perceptual tricks."