Perceiving the World
[Cont.]

PICTORIAL DEPTH CUES. When combined, they can create a powerful illusion of depth.

l. Linear perspective. This cue is based on the apparent convergence of parallel lines in the environment. If you stand between two railroad tracks, they appear to meet near the horizon. Since you know they are parallel, their convergence implies great distance.

2. Relative size. If an artist wishes to depict two objects of the same size at different distances, the artist makes the more distant object smaller. Films such as Star Wars and Return of the Jedi created sensational illusions of depth by rapidly changing the image size of planets, space stations, and starships.

3. Light and shadow. Most objects in the environment are lighted in ways that create clear patterns of light and shadow. Copying such patterns of light and shadow can give a two dimensional design a three-dimensional appearance.

4. Overlap. (Also known as interposition ) . It is a depth cue that occurs when one object partially blocks another object. Hold your hands up and have a friend try to tell from across the room which is nearer. Relative size will give the answer if one hand is much nearer to your friend than the other. But if one hand is only slightly closer than the other, your friend may have difficulty --until you slide one hand in front of the other. Overlap then removes any doubt.

5. Texture gradients. Changes in texture also contribute to depth perception. If you stand in the middle of a cobblestone street, the street will look coarse near your feet. However, its texture will get smaller and finer if you look into the distance.

6. Aerial perspective. Smog, fog, dust, and haze add to the apparent distance of an object. Because of aerial perspective, objects seen at great distance tend to be hazy, washed-out in color, and lacking in detail. This is true even in clear air, but it is increasingly the case in our mechanized society. As a matter of fact, aerial haze is often most noticeable when it is missing. If you have traveled the wide open spaces of states such as Colorado or Wyoming, you may have seen mountain ranges that looked only a few miles away, and then were shocked to find that you were actually viewing them through 50 miles of crystal-clear air.

7. Relative motion. relative motion, also known as motion parallax , can be seen by looking out a window and moving your head from side to side. Notice that objects near you appear to move a sizable distance as your head moves. In comparison, trees, houses, and telephone poles at a greater distance appear to move slightly in relation to the background. Distant objects like hills, mountains, or clouds don't seem to move at all. Not really a pictorial cue except in movies, television, or animated cartoons. However, wen it is present, depth is almost always perceived. Much of the apparent depth of a good movie comes from the relative motion of objects captured by the camera. People who can only see with one eye depend heavily on motion parallax. Often, they make frequent head movements to exaggerate parallax and improve depth perception.

Pictorial depth cues are not entirely universal. Some cultures use only selected pictorial cues to represent depth. People in these cultures may not easily recognize other cues (Deregowski, 1972). For example, researcher William Hudson tested members of remote tribes who do not use relative size to show depth in drawings. These people perceive simplified drawings as two-dimensional designs. They do not assume, as we do, that a larger image means that an object is closer.

How do the depth perception cues relate to daily experience? Like the bodily depth cues, we constantly use the pictorial cues to gauge depth and judge distances. Cues of both types also combine to produce an intriguing illusion. When the moon is on the horizon, it tends to look as large as a silver dollar. When it is directly overhead, it looks like a dime, very much smaller than it did earlier the same evening. Contrary to what some people believe, the moon's image is not magnified by the atmosphere. If you take a photograph of the moon and measure its image, you will find that it is not larger on the horizon. But the moon looks larger when it's low in the sky. This is because the apparent distance of the moon is greater when it is near the horizon than when it is overhead. When the moon is overhead there are few depth cues around it. In contrast, when you see the moon on the horizon, it is behind houses, trees, telephone poles, and mountains. These objects add numerous depth cues, which cause the horizon to seem more distant than the sky overhead (Dember & Warm, 1980).

The apparent distance hypothesis. The moon will immediately appear to shrink when depth cues are eliminated. It is directly related to changes in accommodation (Iavecchia et al., 1983; Roscoe, 1985). Extra depth cues near the horizon cause the eyes to focus on a more distant point than they do when you look overhead. Such changes in accommodation appear to provide the brain with a "yardstick" for judging the size of images, including that of the moon.

V. PERCEPTUAL LEARNING
Learning has a powerful impact on perception, something we have already seen in other ways. How does learning affect perception?

A. Perceptual habits. One way learning affects perception is through ingrained patters of organization and attention, referred to as perceptual habits. Knowing what to expect and where to look. Viewing a face upside down, on the other hand, is not a familiar nor a habitual experience, and one is not apt to be good at it. Magicians make use of perceptual habits to distract observers while performing tricks. Another kind of "magic" is related to consistency in the environment. It is usually safe to assume that a room is shaped roughly like a box. This need not be true, however. When viewed from a certain point, a lopsided room can be made to appear square. This is done by carefully distorting the proportions of the walls, floor, ceiling, and windows. Most people choose shape constancy and see people "shrink" and "grow" before their eyes when the figure moves in the Ames room.

The brain is especially sensitive to perceptual features in the environment. At least some of this sensitivity appears to be learned. Cats raised with vertical stripes are "blind" to horizontal lines, the cats raised with "horizontal" stripes acted as if vertical lines were invisible (Lewin, 1974). Other experiments show that there is an actual decrease in brain cells tuned to the missing features (Grobstein & Chow, 1975). Would it be possible, then, for an adult to adapt to a completely new perceptual world?

B. Inverted Vision. An answer comes from experiments in which people wore lenses that invert visual images. In one experiment, a subject wore goggles that turned the world upside down and reversed objects from right to left. At first, even the simplest tasks --walking, eating, and so forth --were incredibly difficult. Imagine trying to reach for a door handle and watching your hand shoot off in the wrong direction. Subjects also reported that head movements made the world swing violently through space, causing severe headaches and nausea. Yet, after several days they began to adapt to inverted vision. Their success, while not complete, was impressive. Such a high degree of adaptation is related to superior human learning abilities. If the eyes of goldfish are surgically turned upside down, the fish swim in circles and rarely adapt (Sperry, 1956). Did everything turn upright again for the humans? No. While they wore the goggles, their visual images remained upside down. But subjects learned to perform most routine activities, and their inverted world began to seem relatively normal. IN later experiments, subjects wearing inverting lenses were able to successfully drive cars. One subject even flew an airplane after a few weeks of adaptation. Active movement in a new visual world seems to be a key to rapid adaptation. In one experiment, people wore glasses that grossly distorted vision. Those who walked on their own adapted more quickly than subjects pushed around in a wheeled cart (Held, 1971). Why does movement help? Probably because commands sent to the muscles can be related to sensory feedback (McBurney & Collings, 1984). Remaining immobile would be like watching a weird movie over which you have no control. There would be little reason for any perceptual learning to occur.

C. Adaptation Level. An important factor affecting perception is the external context in which a stimulus is judged. For example. a man 6 feet in height will look "tall" when surrounded by others of average height, and "short" among a group of professional basketball players. Context alters apparent size. In addition to external contexts, we all have internal frames of reference, or standards by which stimuli are judged. If you were asked to lift a 10-pound weight, would you label it light, medium, or heavy? The answer to this question depends on your adaptation level (Helson, 1964). This is your own personal "medium point," or frame of reference. Each person's adaptation level is constantly modified by experience. If most of the weights you lift in day-to-day life average around 10 pounds, you will call a 10-pound weight medium. If you are a watcdhmaker and spend your days lifting tiny watch parts, you will probably call a 10-pound weight heavy. Etc.

D. Illusions. Perceptual learning is responsible for a number of illusions. In an illusion, length, position, motion, curvature, or direction is consistently misjudged (Gillam, 1980). Illusions differ from hallucinations in that illusions distort stimuli that actually exist. People who are hallucinating perceive objects or events that have no external reality (for example, they hear voices that are not there). If you think you see a 3-foot-tall butterfly, you can confirm you are hallucinating by trying to touch its wings. To detect an illusion, it is often necessary to measure a drawing or apply a straightedge to it.

Illusions are a fascinating challenge to our understanding of perception. On occasion, they also have practical uses. An illusion called stroboscopic movement puts the "motion" in motion pictures. The strobe lights sometimes used on dance floors reverse this illusion. Each time the strobe flashes, it "freezes" dancers in particular positions. However, if the flashes are speeded up sufficiently, normal motion is seen. In a similar way, movies project a rapid series of "snapshots" on the screen, so the gaps in motion are imperceptible.

Generally speaking, size and shape constancy, habitual eye movements, continuity, and perceptual habits combine in various ways to produce illusions (such as: Which of the horizontal lines is longer? Is the horizontal or vertical line longer?, etc.)

(A group of people in South Africa, the Zulus, live in a "round" culture. In their daily lives, Zulus rarely encounter a straight line: their huts are shaped like rounded mounds and arranged in a circle, tools and toys are curved, and there are no straight roads or square buildings. The typical Zulu does not experience the illusion of one line longer than the other in (a))

VI. MOTIVES AND PERCEPTION
You are surrounded by sights, sounds, odors, tastes, and touch sensations. Which are you aware of? The first stage of perception is attention, the selection of incoming messages. There is little doubt about the importance of attention.

A. Attention. Selective attention refers to the fact that we give some messages priority and put others on hold (Johnston & Dark, 1986). Psychologists have found it helpful to think of selective attention as a sort of bottleneck, or narrowing in the information channel linking the senses to perception (Reed, 1988). When one message enters the bottleneck, it seems to prevent others from passing through. This may be why it is very difficult to listen to two people speaking at once. Typically, you can "tune in" one person or the other, but not both. Divided attention often arises from our limited capacity for storing and thinking about information. At any moment, you must divide your mental effort among tasks, each of which requires more or less attention. However, as skill becomes more automatic, it requires less attention. Are some stimuli more attention-getting than others? Yes. Very intense stimuli usually command attention. Stimuli that are brighter, louder, or larger tend to capture attention: A gunshot in a library would be hard to ignore. Big, bright cars probably get more tickets than small, dull ones. Loud, irritating comedian Don Rickles has made a career out of the first principle of attention. Repetitious stimuli are also attention-getting. A dripping faucet at night makes little noise by normal standards, but because of repetition, it may become as attention-getting as a single sound many times louder. This effect is used repeatedly, so to speak, in television and radio commercials. ATTENTION IS ALSO FREQUENTLY RELATED TO contrast OR change IN STIMULATION. The contrasting type styles draw attention because they are unexpected. Norman Mackworth and Geoffrey Loftus (1978) found that people focus first and longest on unexpected objects. Change, contrast, and incongruity are perhaps the most basic sources of attention. We quickly habituate (respond less) to predictable and unchanging stimuli.

B. Habituation. Adaptation decreases the actual number of sensory messages sent to the brain. When messages do reach the brain, the body makes a sort of "What is it?" reaction known as the orientation response (OR). An OR is characterized by enlarged pupils, brain wave changes, a short pause in breathing, increased blood flow to the head, and turning toward the stimulus (Dember & Warm, 1979). Have you ever seen someone do a double take? If so, you have observed an orientation response. At first a new album holds your attention all the way through. But when the album becomes "old," a whole side may play without your really attending to it. When a stimulus is repeated without change, the OR habituates, or decreases.

C. Motives. Motives also play a role in attention. If you are riding in a car and are hungry, you will notice restaurants and billboards picturing food. If you are running low on gas, your attention will shift to gas stations. Advertisers, of course, know that their pitch will be more effective if it gets your attention. Ads are therefore loud, repetitious, and intentionally irritating. They are also designed to take advantage of two motives that are widespread in our society: anxiety and sex --to advertise everything from mouthwash to automobile tires. In addition to directing attention, motives may alter what is perceived. And, an emotional stimulus can shift attention away from other information.

D. Perceptual Expectancies. Perception seems to proceed in two major ways. In bottom-up processing, we analyze information starting at the "bottom" with small units (features) and build upward into a complete perception (Goldstein, 1984). The reverse also seems to occur. Many experiences are organized using one's knowledge of the world. This is called top-down processing. In this case, pre-existing knowledge is used to rapidly organize features into a meaningful whole.

Another good example of top-down processing is found in perceptual expectancies. A runner in the starting blocks at a track meet is set to respond in a certain way. Likewise, past experience, motives, context, or suggestion may create a perceptual expectancy that sets you to perceive in a certain way. If a car backfires, runners at a track meet may jump the gun. As a matter of fact, we all frequently jump the gun when perceiving. In essence, an expectancy is a perceptual hypothesis we are very likely to apply to a stimulus --even if applying it is inappropriate. Perceptual sets often lead us to see what we expect to see. For example, let's say you are driving across the desert. You are very low on gas. Finally, you see a sign approaching. On it are the worlds FUEL AHEAD. You relax, knowing you will not be stranded. But as you draw nearer, the words on the sign become FOOD AHEAD. Most people have had similar experiences in which expectation altered their perceptions. Perceptual expectancies are frequently created by suggestion. This is especially true of perceiving other people. For example, a psychology professor once arranged for a guest lecturer to teach his class. Half the students in the class were given a page of notes that described the lecturer as a "rather cold person, industrious, critical, practical, and determined." The other students got notes describing him as a "rather warm person, industrious, critical, practical, and determined" (Kelley, 1950; italics added). Students who received the "cold" description perceived the lecturer as unhappy and irritable and didn't volunteer in class discussion. Those who got the "warm" description saw the lecturer as happy and good-natured, and they actively took part in discussion with him.

E. Categories. Psychologist Jerome Bruner used a tachistoscope --a device for projecting pictures for very short periods) to flash pictures of cards on a screen. He found that observers misperceived cards that did not fit their knowledge and expectations. For instance, a red six of spades would be misperceived as a normal six of hearts (Bruner & Postman, 1949). Bruner believes that perceptual learning builds up mental categories. Experiences are then "sorted" into these categories. Since observers had no category for a red six of spades, they saw it as a six of hearts. Categories such as "punk," "mental patient," "queer," "honky," "bitch," and so on, are particularly likely to distort perception. Perceptual categories, especially those defined by labels, do made a difference, This is especially true in perceiving people, where even trained observers may be influenced. Fore example, in one study, psychotherapists were shown a videotaped interview. Half of the therapists were told that the man being interviewed was applying for a job. The rest were told that the man was a mental patient. Therapists who thought the man was a job applicant perceived him as "realistic," "sincere," and "pleasant." Those who thought he was a patient perceived him as "defensive," "dependent," and "impulsive" (Langer & Abelson, 1974).