CHAPTER 6
Perceiving the World
Theme: Perception is an active process; perceptual impressions are not always accurate representations of events.
Preview
Murder!
I was in a supermarket when an 8-year-old girl suddenly came running around a corner. She looked back and screamed, “Stop! Stop! You’re killing him! You’re killing my father!” Naturally I was interested! As I quickly retraced her path, I was greeted by a grisly scene. A man was stretched out on the floor with another man on top of him. The guy on top was huge and looked only half human. He had his victim by the throat and was beating his head against the floor.
There was blood everywhere. I decided to do the right thing.
I ran.
By the time the store manager and I returned to the “scene of the crime,” the police were just arriving. It took quite a while to sort things out, but here is what happened: The “guy on the bottom” had passed out and hit his head. That caused the cut (actually quite small), which explained the “blood everywhere.” The “guy on top” saw the first man fall and was trying to prevent him from further injuring himself. He was also loosening the man’s collar.
If I had never returned, I would have sworn in court that I had seen a murder. The girl’s description completely shaped my own perceptions. This perhaps is understandable. But I’ll never forget the shock I felt when I met the “murderer”—the man I had seen a few moments before as huge, vicious, and horriblelooking.
The man was not a stranger. He was a neighbor of mine. I had seen him dozens of times before. I know him by name. He is a rather small man.
In the last chapter we discussed sensation, the process of bringing information into the nervous system. This chapter is about perception, or how we assemble sensations into meaningful patterns. As we perceive events, the brain actively selects, organizes, and integrates sensory information to construct a “picture” or model of the world. This process is so automatic that it can take a drastic misperception like mine to call attention to it.
Perception creates faces, melodies, works of art, illusions, and on occasion, “murders” out of the raw material of sensation.
Let’s see how this takes place.
¡ Perception is an active process of assembling sensations into meaningful patterns that represent external events.
¡ Size, shape, and brightness constancies bring stability to our vision, which would otherwise seem distorted and erratic.
¡ We unconsciously use Gestalt principles to organize sensations into meaningful patterns.
¡ Our wondrous ability to perceive three-dimensional space is largely based on retinal disparity (a difference between what the right and left eyes see).
¡ Depth perception also depends on bodily cues and pictorial cues that provide added information about depth and distance.
¡ Perception is greatly affected by learning, motives, values, attention, and expectations. Private perceptual experiences don’t always accurately represent external events.
¡ Eyewitnesses frequently misperceive events—even important events such as crimes or accidents.
¡ You can improve the accuracy and objectivity of your perceptions through conscious effort and an awareness of factors that contribute to erroneous perceptions.
¡ Scientific evidence concerning the existence of extrasensory perception is mostly negative or inconclusive.
Gateways to Perception
What are perceptual constancies, and what is their role in perception?
What basic principles do we use to group sensations into meaningful patterns?
How is it possible to see depth and judge distance?
What effect does learning have on perception?
How is perception altered by attention, motives, values, and expectations?
How reliable are eyewitness reports?
Is extrasensory perception possible?
Key Questions
Perceptual Constancies— Taming an Unruly World
What would it be like to have your vision restored after a lifetime of blindness? Actually, a first look at the world can be disappointing.
Newly sighted persons must learn to identify objects; to read clocks, numbers, and letters; and to judge sizes and distances (Senden, 1960). Learning to “see” can be quite frustrating.
Richard Gregory (1990) describes a cataract patient named Mr. S. B. who had been blind since birth. After an operation restored Mr. S. B.’s sight at age 52, he struggled to use his vision. At first, he could judge distance only in familiar situations.
One day he was found crawling out of a hospital window to get a better look at traffic on the street. It’s easy to understand his curiosity, but he had to be restrained. His room was on the fourth floor!
Why would Mr. S. B. try to crawl out of a fourth-story window?
Couldn’t he at least tell distance from the size of the cars? No, because you must be familiar with objects to use their size to judge distance. Try holding your left hand a few inches in front of your nose and your right hand at arm’s length. Your right hand should appear to be about half the size of your left hand. Still, you know your right hand did not suddenly shrink because you have seen it many times at various distances. We call this
“Marilyn Numerisée #420,” © Yvaral 1990, courtesy Circle Gallery
Visual perception involves finding meaningful patterns in complex stimuli. If you look closely at this painting by the artist Yvaral, you will see that it is entirely made up of small, featureless squares. An infant or newly sighted person would see only a jumble of meaningless colors. But because the squares form a familiar pattern, you should easily see Marilyn Monroe’s face. (Or is that Madonna?)
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size constancy: The perceived size of an object remains the same, even though the size of its image on the retina changes.
Even newborn babies show some evidence of size constancy (Slater, Mattock, & Brown, 1990).
To perceive your hand accurately, you had to draw on past experience. Some perceptions are so basic they seem to be
native (inborn).An example is the ability to see a line on a piece of paper. However, much perception is empirical (based on prior experience). For instance, Colin Turnbull (1961) tells of the time he took a Pygmy from the dense rain forests of Africa to the vast African plains. The Pygmy had never before seen objects at a great distance. Hence, the first time he saw a herd of buffalo in the distance, he thought it was a swarm of insects.
Imagine his confusion when he was driven toward the animals.
He concluded that he was being fooled by witchcraft because the “insects” seemed to grow into buffalo before his eyes.
Perhaps you, too, have also experienced the failure of size constancy in unfamiliar situations. When viewed from an airplane or the top of a skyscraper, cars, houses, and people no longer seem normal in size; instead, they begin to look like toys.
Thus,we can summarize that size constancy,while being innate, is also affected by learning.
In shape constancy the perceived shape of an object remains stable, even though the shape of its retinal image changes. You can demonstrate shape constancy by looking at this page from directly overhead and then from an angle. Obviously, the page is rectangular, but most of the time the images that reach your eyes are distorted. Yet, while the book’s image changes, your perception of its shape remains constant. (For additional examples, see œFigure 6.1.) In a movie theater, preserving shape constancy is difficult if you sit in the front row or near the front and to the side. Nevertheless, most people are able to tolerate a fair amount of shape distortion, as long as all objects on the screen are similarly deformed. On the highway, alcohol intoxication impairs size and shape constancy, adding to the accident rate among drunk drivers (Farrimond, 1990).
Let’s say that you are outside in bright sunlight. Beside you, a friend is wearing a gray skirt and a white blouse. Suddenly a cloud shades the sun. It might seem that the blouse would grow dimmer, but it still appears to be bright white. This happens because the blouse continues to reflect a larger proportion of light
© Mark Richards/PhotoEdit
Almost everyone’s family album has at least one photo like this. Extreme viewing angles can make maintaining size constancy difficult, even for familiar objects.
(a) (b)
œ Figure 6.1 Shape constancy. (a) When a door is open, its image actually forms a trapezoid. Shape constancy is indicated by the fact that it is still perceived as a rectangle. (b)With great effort you may be able to see this design as a collection of flat shapes.However, if you maintain shape constancy, the distorted squares strongly suggest the surface of a sphere. (From
Spherescapes-1 by Scott Walter and Kevin McMahon, 1983.)
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than nearby objects. Brightness constancy refers to the fact that the brightness of objects appears to stay the same as lighting conditions change. However, this holds true only if the blouse and surrounding objects are all illuminated by the same amount of light. You could make an area on your friend’s gray skirt look whiter than the shaded blouse by shining a bright spotlight on the skirt.
To summarize, the energy patterns reaching our senses are constantly changing, even when they come from the same object.
Size, shape, and brightness constancy rescue us from a confusing world in which objects would seem to shrink and grow, change shape as if made of rubber, and light up or fade like neon lamps. Gaining these constancies was only one of the hurdles Mr. S. B. faced in learning to see. In the next section, we will consider some others.
Perceptual Organization— Getting It All Together
We have seen that Mr. S. B. had to learn to understand his visual sensations. He was soon able to tell time from a large clock and to read block letters he had known only from touch. At a zoo, he recognized an elephant from descriptions he had heard.
However, handwriting meant nothing to him for more than a year after he regained sight, and many objects were meaningless until he touched them. Thus, while Mr. S. B. had visual sensations,
his ability to perceive remained limited.
How are sensations organized into meaningful perceptions?
The simplest organization involves grouping some sensations into an object, or figure, that stands out on a plainer background.
A basic capacity for figure-ground organization is probably inborn, as evidenced by its being the first perceptual ability to appear after cataract patients regain sight. In the brain, cells that process visual information respond more actively to figures than to backgrounds (Lamme, 1995). In normal figure-ground perception, only one figure is seen. In reversible figures, however, figure and ground can be switched. In œFigure 6.2 it is equally possible to see either a wineglass figure on a dark background or two face profiles on a light background. As you shift from one pattern to the other, you should get a clear sense of what figure-ground organization means.
Gestalt Principles
How do we separate a figure from its background? The Gestalt psychologists (see Chapter 1) studied this question in detail.
Even if you were seeing for the first time, they concluded, several factors would bring some order to your perceptions (œFig. 6.3).
1. Nearness. All other things being equal, stimuli that are near each other tend to be grouped together (Kubovy & Holcombe, 1998). Thus, if three people stand near each other and a fourth person stands 10 feet away, the adjacent three will be seen as a group and the distant person as an outsider (see œFig. 6.3a).
2. Similarity. “Birds of a feather flock together,” and stimuli that are similar in size, shape, color, or form tend to be grouped together (see œFig. 6.3b). Picture two bands marching side by side. If their uniforms are different colors, the bands will be seen as two separate groups, not as one large group.
3. Continuation, or continuity. Perceptions tend toward simplicity and continuity. In œFigure 6.3c it is easier to visualize a wavy line on a squared-off line than it is to see a complex row of shapes.
Perception The mental process of organizing sensations into meaningful patterns.
Size constancy The perceived size of an object remains constant, despite changes in its retinal image.
Native perception A perceptual experience based on innate processes.
Empirical perception A perception strongly influenced by prior experience.
Shape constancy The perceived shape of an object is unaffected by changes in its retinal image.
Brightness constancy The apparent (or relative) brightness of objects remains the same as long as they are illuminated by the same amount of light.
Figure-ground organization Part of a stimulus appears to stand out as an object (figure) against a less prominent background (ground).
Reversible figure A stimulus pattern in which figure-ground organization can be reversed.
œ Figure 6.2 A reversible figure-ground design.Do you see two faces in profile or a wineglass?
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4. Closure. Closure refers to the tendency to complete a figure so that it has a consistent overall form. Each of the drawings in
œFigure 6.3d has one or more gaps, yet each is perceived as a recognizable figure. The “shapes” that appear in the two right drawings in Figure 6.3d are illusory figures (implied shapes that are not actually bounded by an edge or an outline). Even young children see these shapes, despite knowing that they are “not really there.” Illusory figures reveal that our tendency to form shapes—even with minimal cues—is powerful.
5. Contiguity. A principle that can’t be shown in Fig. 6.3 is contiguity, or nearness in time and space. Contiguity is often responsible for the perception that one thing has caused another (Michotte, 1963). A psychologist friend of mine demonstrates this principle in class by knocking on his head with one hand while knocking on a wooden table (out of sight) with the other. The knocking sound is perfectly timed with the movements of his visible hand. This leads to the irresistible perception that his head is made of wood.
6. Common region. As you can see in œFigure 6.3e, stimuli that are found within a common region or area tend to be seen as a group (Palmer, 1992). On the basis of similarity and nearness, the stars in Figure 6.3e should be one group and the dots another. However, the colored backgrounds de- fine regions that create three groups of objects (four stars,
(a) Principle of nearness
Notice how differently a group of six objects can be perceptually organized, depending on their spacing.
(b) Principle of similarity
In these examples, organization depends on similarity of color.
Similarity and nearness can be combined to produce a new organization.
(c) Principle of continuity (d) Principle of closure (e) Principle of common region
This?
plus or This?
œ Figure 6.3 How we organize perceptions.
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two stars plus two dots, and four dots). Perhaps the principle of common region explains why we tend to mentally group together people from a particular country, state, province, or geographic region.
To learn about how the principles that guide perceptual organization can be applied to practical problems, see “Designing for Human Use.” Clearly, the Gestalt principles shape our day-to-day perceptions, but so does learning and past experience. Take a moment and look for the camouflaged animal pictured in œFigure 6.5.
Designing for Human Use
Machines are of little value unless humans can operate them. A pocket calculator that is difficult to handle might just as well be a paperweight. An automobile design that blocks large areas of the driver’s vision could be deadly. To adapt machines for human use, the engineering psychologist (human factors engineer) must make them
compatible with our sensory and motor capacities (Petroski, 2000). For example, displays must be easy to perceive,
controls must be easy to use, and the tendency to make errors must be minimized (œFig. 6.4). (A display is any dial, screen, light, or other device used to provide information about a machine’s activity to a human user. A control is any knob, handle, button, lever, or other device used to alter the activity of a machine.)
Many of the machines we rely on each day were designed, in part, by human factors engineers. Some familiar examples include push-button telephones, “user-friendly” computers, home appliances, cameras, PDAs, airplane controls, and traffic signals.
Psychologist Donald Norman (1994) refers to successful human factors engineering as natural design. Effective design makes use of perceptual signals that people understand naturally, without needing to learn them. An example is the row of vertical buttons in elevators. The buttons mimic the layout of the floors. This is simple, natural, and clear.
Effective design also provides feedback (information about the effect of making a response). The audible click designed into many computer keyboards is a good example. As Norman points out, the cause of many accidents is not just “human error.” The real culprit is poor design.
U S I N G P S YC H O LO G Y
(a)
Left roll Right roll Left roll Right roll
Standard Indicators Improved Indicators
A C B D A C B D A B C A B C D
(b) (c)
D
(d)
œ Figure 6.4 Human factors engineering. (a) Early roll indicators in airplanes were perceptually confusing and difficult to read (top). Improved displays are clear even to nonpilots.
Which would you prefer if you were flying an airplane in heavy fog? (b) Even on a stove,the placement of controls is important.During simulated emergencies,people made no errors in reaching for the controls on the top stove.In contrast, they erred 38 percent of the time with the bottom arrangement (Chapanis & Lindenbaum, 1959).(c) Sometimes the shape of a control is used to indicate its function to discourage errors.For example, the left control might be used to engage and disengage the gears of an industrial machine, whereas the right control might operate the landing flaps on an airplane. (d) This design depicts a street intersection viewed from above. Psychologists have found that painting white lines across the road makes drivers feel like they are traveling faster.This effect is even stronger if the lines get progressively closer together. Placing lines near dangerous intersections or sections of highway has dramatically lowered accident rates.
Illusory figure An implied shape that is not actually bounded by an edge or an outline.
Engineering psychology (human factors engineering) A specialty concerned with making machines and work environments compatible with human perceptual and physical capacities.
Display Any dial, screen, light, or other device used to provide information about a machine’s activity.
Control Any knob, handle, button, lever, or other device used to alter the activity of a machine.
Natural design Human factors engineering that makes use of naturally understood perceptual signals.
Feedback Information on the effects of a response; feedback is returned to the person performing the response.
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(Camouflage patterns break up figure-ground organization). If you had never seen similar animals before, could you have located this one? Mr. S. B. would have been at a total loss to find meaning in such a picture.
In a way, we are all detectives, seeking patterns in what we see. In this sense, a meaningful pattern represents a perceptual hypothesis, or initial guess about how to organize sensations.
Have you ever seen a “friend” in the distance, only to have the person turn into a stranger as you drew closer? Preexisting ideas and expectations actively guide our interpretation of sensations (Mack & Rock, 1998).
The active nature of perception is perhaps most apparent for
ambiguous stimuli (patterns allowing more than one interpretation).
If you look at a cloud, you may discover dozens of ways to organize its contours into fanciful shapes and scenes. Even clearly defined stimuli may permit more than one interpretation.
Look at Necker’s cube in œFigure 6.6 if you doubt that perception is an active process. Visualize the top cube as a wire box. If you stare at the cube, its organization will change. Sometimes it will seem to project upward, like the lower left cube; other times it will project downward. The difference lies in how your brain interprets the same information. In short, we actively
construct meaningful perceptions, we do not passively record the events and stimuli around us (Hoffman, 1999).
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