19-6 How did the Gestalt psychologists understand perceptual organization, and how do figure-ground and grouping principles contribute to our perceptions?

It’s one thing to understand how we see colors and shapes. But how do we organize and interpret those sights so that they become meaningful perceptions—a rose in bloom, a familiar face, a sunset?

Early in the twentieth century, a group of German psychologists noticed that when given a cluster of sensations, people tend to organize them into a gestalt, a German word meaning a “form” or a “whole.” As we look straight ahead, we cannot separate the perceived scene into our left and right fields of view. It is, at every moment, one whole, seamless scene. Our conscious perception is an integrated whole.

Consider FIGURE 19.13: The individual elements of this figure, called a Necker cube, are really nothing but eight blue circles, each containing three converging white lines. When we view these elements all together, however, we see a cube that sometimes reverses direction. This phenomenon nicely illustrates a favorite saying of Gestalt psychologists: In perception, the whole may exceed the sum of its parts, rather as water differs from its hydrogen and oxygen parts.

Over the years, the Gestalt psychologists demonstrated many principles we use to organize our sensations into perceptions (Wagemans et al., 2012a,b). Underlying all of them is a fundamental truth: Our brain does more than register information about the world. Perception is not just opening a shutter and letting a picture print itself on the brain. We filter incoming information and construct perceptions. Mind matters.

Imagine designing a video-computer system that, like your eye-brain system, can recognize faces at a glance. What abilities would it need?

Figure and GroundTo start with, the video-computer system would need to separate faces from their backgrounds. Likewise, in our eye-brain system, our first perceptual task is to perceive any object (the figure) as distinct from its surroundings (the ground). Among the voices you hear at a party, the one you attend to becomes the figure; all others are part of the ground. As you read, the words are the figure; the white space is the ground. Sometimes the same stimulus can trigger more than one perception. In FIGURE 19.14, the figure-ground relationship continually reverses—but always we organize the stimulus into a figure seen against a ground.

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GroupingHaving discriminated figure from ground, we (and our video-computer system) must also organize the figure into a meaningful form. Some basic features of a scene—such as color, movement, and light-dark contrast—we process instantly and automatically (Treisman, 1987). Our minds bring order and form to stimuli by following certain rules for grouping, also identified by the Gestalt psychologists. These rules, which we apply even as infants and even in our touch perceptions, illustrate how the perceived whole differs from the sum of its parts (Gallace & Spence, 2011; Quinn et al., 2002; Rock & Palmer, 1990). Three examples:

Such principles usually help us construct reality. Sometimes, however, they lead us astray, as when we look at the doghouse in FIGURE 19.15.

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19-7 How do we use binocular and monocular cues to perceive the world in three dimensions, and how do we perceive motion?

From the two-dimensional images falling on our retinas, we somehow organize three-dimensional perceptions. Depth perception enables us to estimate an object’s distance from us. At a glance, we can estimate the distance of an oncoming car or the height of a house. Depth perception is partly innate, as Eleanor Gibson and Richard Walk (1960) discovered using a model of a cliff with a drop-off area (which was covered by sturdy glass). Gibson’s inspiration for these visual cliff experiments occurred while she was picnicking on the rim of the Grand Canyon. She wondered: Would a toddler peering over the rim perceive the dangerous drop-off and draw back?

Back in their Cornell University laboratory, Gibson and Walk placed 6-to 14-month-old infants on the edge of a safe canyon and had the infants’ mothers coax them to crawl out onto the glass (FIGURE 19.16). Most infants refused to do so, indicating that they could perceive depth.

Had they learned to perceive depth? Learning seems to be part of the answer because crawling, no matter when it begins, seems to increase infants’ wariness of heights (Campos et al., 1992). As infants become mobile, their experience leads them to fear heights (Adolph et al., 2014).

How do we do it? How do we transform two differing two-dimensional retinal images into a single three-dimensional perception?

Binocular CuesTry this: With both eyes open, hold two pens or pencils in front of you and touch their tips together. Now do so with one eye closed. With one eye, the task becomes noticeably more difficult, demonstrating the importance of binocular cues in judging the distance of nearby objects. Two eyes are better than one.

Because your eyes are about 2½ inches apart, your retinas receive slightly different images of the world. By comparing these two images, your brain can judge how close an object is to you. The greater the retinal disparity, or difference between the two images, the closer the object. Try it. Hold your two index fingers, with the tips about half an inch apart, directly in front of your nose, and your retinas will receive quite different views. If you close one eye and then the other, you can see the difference. (Bring your fingers close and you can create a finger sausage, as in FIGURE 19.17.) At a greater distance—say, when you hold your fingers at arm’s length—the disparity is smaller.

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We could easily build this feature into our video-computer system. Moviemakers can simulate or exaggerate retinal disparity by filming a scene with two cameras placed a few inches apart. Viewers then wear glasses that allow the left eye to see only the image from the left camera, and the right eye to see only the image from the right camera. The resulting effect, as 3-D movie fans know, mimics or exaggerates normal retinal disparity. Similarly, twin cameras in airplanes can take photos of terrain for integration into 3-D maps.

Monocular CuesHow do we judge whether a person is 10 or 100 meters away? Retinal disparity won’t help us here, because there won’t be much difference between the images cast on our right and left retinas. At such distances, we depend on monocular cues (depth cues available to each eye separately). See FIGURE 19.18 for some examples.

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Imagine that you could perceive the world as having color, form, and depth but that you could not see motion. Not only would you be unable to bike or drive, you would have trouble writing, eating, and walking.

Normally your brain computes motion based partly on its assumption that shrinking objects are retreating (not getting smaller) and enlarging objects are approaching. But you are imperfect at motion perception. In young children, this ability to correctly perceive approaching (and enlarging) vehicles is not yet fully developed, which puts them at risk for pedestrian accidents (Wann et al., 2011). But it’s not just children who have occasional difficulties with motion perception. Our adult brains are sometimes tricked into believing what they are not seeing. When large and small objects move at the same speed, the large objects appear to move more slowly. Thus, trains seem to move slower than cars, and jumbo jets seem to land more slowly than little jets.

Our brain also perceives a rapid series of slightly varying images as continuous movement (a phenomenon called stroboscopic movement). As film animators know well, a superfast slide show of 24 still pictures a second will create an illusion of movement. We construct that motion in our heads, just as we construct movement in blinking marquees and holiday lights. We perceive two adjacent stationary lights blinking on and off in quick succession as one single light moving back and forth. Lighted signs exploit this phi phenomenon with a succession of lights that creates the impression of, say, a moving arrow.

19-8 How do perceptual constancies help us construct meaningful perceptions?

So far, we have noted that our video-computer system must perceive objects as we do—as having a distinct form, location, and perhaps motion. Its next task is to recognize objects without being deceived by changes in their color, brightness, shape, or size—a top-down process called perceptual constancy. Regardless of the viewing angle, distance, and illumination, we can identify people and things in less time than it takes to draw a breath, a feat that challenges even advanced computers and has intrigued researchers for decades. This would be a monumental challenge for a video-computer system.

Color and Brightness ConstanciesOur experience of color depends on an object’s context. This would be clear if you viewed an isolated tomato through a paper tube over the course of a day. The tomato’s color would seem to change as the light—and thus the wavelengths reflected from its surface—changed. But if you viewed that tomato as one item in a salad bowl, its color would remain roughly constant as the lighting shifts. This perception of consistent color is known as color constancy.

Though we take color constancy for granted, this ability is truly remarkable. A blue poker chip under indoor lighting reflects wavelengths that match those reflected by a sunlit gold chip (Jameson, 1985). Yet bring a bluebird indoors and it won’t look like a goldfinch. The color is not in the bird’s feathers. You and I see color thanks to our brain’s computations of the light reflected by an object relative to the objects surrounding it.

FIGURE 19.20 dramatically illustrates the ability of a blue object to appear very different in three different contexts. Yet we have no trouble seeing these disks as blue. Nor does knowing the truth—that these disks are identically colored—diminish our perception that they are quite different. Because we construct our perceptions, we can simultaneously accept alternative objective and subjective realities.

Brightness constancy (also called lightness constancy) similarly depends on context. We perceive an object as having a constant brightness even while its illumination varies. This perception of constancy depends on relative luminance—the amount of light an object reflects relative to its surroundings (FIGURE 19.21). White paper reflects 90 percent of the light falling on it; black paper, only 10 percent. Although a black paper viewed in sunlight may reflect 100 times more light than does a white paper viewed indoors, it will still look black (McBurney & Collings, 1984). But if you view sunlit black paper through a narrow tube so nothing else is visible, it may look gray, because in bright sunshine it reflects a fair amount of light. View it without the tube and it is again black, because it reflects much less light than the objects around it.

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This principle—that we perceive objects not in isolation but in their environmental context—matters to artists, interior decorators, and clothing designers. Our perception of the color and brightness of a wall or of a streak of paint on a canvas is determined not just by the paint in the can but by the surrounding colors. The take-home lesson: Comparisons govern our perceptions.

Shape and Size ConstanciesSometimes an object whose actual shape cannot change seems to change shape with the angle of our view (FIGURE 19.22). More often, thanks to shape constancy, we perceive the form of familiar objects, such as the door in FIGURE 19.23, as constant even while our retinas receive changing images of them. Our brain manages this feat thanks to visual cortex neurons that rapidly learn to associate different views of an object (Li & DiCarlo, 2008).

Thanks to size constancy, we perceive objects as having a constant size, even while our distance from them varies. We assume a car is large enough to carry people, even when we see its tiny image from two blocks away. This assumption also illustrates the close connection between perceived distance and perceived size. Perceiving an object’s distance gives us cues to its size. Likewise, knowing its general size—that the object is a car—provides us with cues to its distance.

Even in size-distance judgments, however, we consider an object’s context. This interplay between perceived size and perceived distance helps explain several well-known illusions, including the Moon illusion: The Moon looks up to 50 percent larger when near the horizon than when high in the sky. Can you imagine why?

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For at least 22 centuries, scholars have wondered (Hershenson, 1989). One reason is that monocular cues to objects’ distances make the horizon Moon seem farther away. If it’s farther away, our brain assumes, it must be larger than the Moon high in the night sky (Kaufman & Kaufman, 2000). Take away the distance cue, by looking at the horizon Moon through a paper tube, and the object will immediately shrink.

Size-distance relationships also explain why in FIGURE 19.24 the two same-age girls seem so different in size. As the diagram reveals, the girls are actually about the same size, but the room is distorted. Viewed with one eye through a peephole, the Ames room’s trapezoidal walls produce the same images you would see in a normal rectangular room viewed with both eyes. Presented with the camera’s one-eyed view, your brain makes the reasonable assumption that the room is normal and each girl is therefore the same distance from you. Given the different sizes of the girls’ images on your retinas, your brain ends up calculating that the girls must be very different in size.

Perceptual illusions reinforce a fundamental lesson: Perception is not merely a projection of the world onto our brain. Rather, our sensations are disassembled into information bits that our brain then reassembles into its own functional model of the external world. During this reassembly process, our assumptions—such as the usual relationship between distance and size—can lead us astray. Our brain constructs our perceptions.

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Form perception, depth perception, motion perception, and perceptual constancies illuminate how we organize our visual experiences. Perceptual organization applies to our other senses, too. Listening to an unfamiliar language, we have trouble hearing where one word stops and the next one begins. Listening to our own language, we automatically hear distinct words. This, too, reflects perceptual organization. But it is more, for we even organize a string of letters—THEDOGATEMEAT—into words that make an intelligible phrase, more likely “The dog ate meat” than “The do gate me at” (McBurney & Collings, 1984). This process involves not only the organization we’ve been discussing, but also interpretation—discerning meaning in what we perceive.