The Trichromatic Theory The trichromatic theory (also known as the Young-Helmholtz theory) applies to the first level of processing, which occurs in the retina of the eye. The retina contains three basic types of cones. One type responds maximally to blue, another to green, and a third to red. The thousands of colors we see result from the combined activity of these three types of cones.

Total color blindness is usually due to a genetic variation that causes cones of the retina to be absent or malfunction. The visual world then consists of black, white, and shades of gray. Many species of animals are totally color-blind, but the condition is extremely rare in human beings. Most “color-blind” people are actually color deficient, due to an absence of, or damage to, one or more types of cones. Usually, the person is unable to distinguish red and green; the world is painted in shades of blue, yellow, brown, and gray. In rarer instances, a person may be blind to blue and yellow and may see only reds, greens, and grays. Color deficiency is found in about 8 percent of white men, 5 percent of Asian men, and 3 percent of black men and Native American men (Sekuler & Blake, 1994). Because of the way the condition is inherited, it is rare in women.

The Opponent-Process Theory The opponent-process theory applies to the second stage of color processing, which occurs in ganglion cells in the retina and in neurons in the thalamus and visual cortex of the brain. These cells, known as opponent-process cells, either respond to short wavelengths but are inhibited from firing by long wavelengths, or vice versa (DeValois & DeValois, 1975). Some opponent-process cells respond in opposite fashion to red and green, or to blue and yellow; that is, they fire in response to one and turn off in response to the other. (A third system responds in opposite fashion to white and black and thus yields information about brightness.) The net result is a color code that is passed along to the higher visual centers. Because this code treats red and green, and also blue and yellow, as antagonistic, we can describe a color as bluish green or yellowish green but not as reddish green or yellowish blue.

Opponent-process cells that are inhibited by a particular color produce a burst of firing when the color is removed, just as they would if the opposing color were present. Similarly, cells that fire in response to a color stop firing when the color is removed, just as they would if the opposing color were present. These facts explain why we are susceptible to negative afterimages when we stare at a particular hue—why we see, for instance, red after staring at green. (To see this effect for yourself, see Figure6.8.) A sort of neural rebound effect occurs: The cells that switch on or off to signal the presence of “green” send the opposite signal (“red”) when the green is removed and vice versa.

Figure 6.8

A Change of Heart

Form Perception To make sense of the world, we must know where one thing ends and another begins. In vision, we must separate the teacher from the lectern; in hearing, we must separate the piano solo from the orchestral accompaniment; in taste, we must separate the marshmallow from the hot chocolate. This process of dividing up the world occurs so rapidly and effortlessly that we take it completely for granted, until we must make out objects in a heavy fog or words in the rapid-fire conversation of someone speaking a language we don't know.

Gestalt psychologists, who belonged to a movement that began in Germany and was influential in the 1920s and 1930s, were among the first to study how people organize the world visually into meaningful units and patterns. In German, Gestalt means “form” or “configuration.” Gestalt psychologists' motto was “The whole is more than the sum of its parts.” They observed that when we perceive something, properties emerge from the configuration as a whole that are not found in any particular component. A modern example of the Gestalt effect occurs when you watch a movie. The motion you see is nowhere in the film, which consists of separate frames projected at (usually) 24 frames per second.

Gestalt psychologists also noted that people organize the visual field into figure and ground. The figure stands out from the rest of the environment (see Figure6.9). Some things stand out as figure by virtue of their intensity or size; it is hard to ignore the bright light of a flashlight at night or a tidal wave approaching your piece of beach. Unique objects also stand out, such as a banana in a bowl of oranges, and so do moving objects in an otherwise still environment, such as a shooting star. Indeed, it is hard to ignore a sudden change of any kind in the environment because our brains are geared to respond to change and contrast. However, selective attention—the ability to concentrate on some stimuli and to filter out others—gives us some control over what we perceive as figure and ground, and sometimes it blinds us to things we would otherwise interpret as figure, as we saw earlier.

Figure 6.9

Figure and Ground

Other Gestalt principles describe strategies used by the visual system to group sensory building blocks into perceptual units (Köhler, 1929; Wertheimer, 1923/1958). Gestalt psychologists believed that these strategies were present from birth or emerged early in infancy as a result of maturation. Modern research, however, suggests that at least some of them depend on experience (Quinn & Bhatt, 2012). Here are a few well-known Gestalt principles:

Gestalt Principles

Unfortunately, consumer products are sometimes designed with little thought for Gestalt principles, which is why it can be a major challenge to, say, operate the correct dials on a new stovetop (Norman, 2004, 1988). Good design requires, among other things, that crucial distinctions be visually obvious. Watch the video Perceptual Magic in Art 1 to learn more about how cues in the environment influence our perceptions.

Depth and Distance Perception Ordinarily we need to know not only what something is but also where it is. Touch gives us this information directly, but vision does not, so we must infer an object's location by estimating its distance or depth.

To perform this remarkable feat, we rely in part on binocular cues, cues that require the use of two eyes. One such cue is convergence, the turning of the eyes inward, which occurs when they focus on a nearby object. The closer the object, the greater the convergence, as you know if you have ever tried to cross your eyes by looking at your own nose. As the angle of convergence changes, the corresponding muscular changes provide information to the brain about distance.

The two eyes also receive slightly different retinal images of the same object. You can prove this by holding a finger about 12 inches in front of your face and looking at it with only one eye at a time. Its position will appear to shift when you change eyes. Now hold up two fingers, one closer to your nose than the other. Notice that the amount of space between the two fingers appears to change when you switch eyes. The slight difference in lateral (sideways) separation between two objects as seen by the left eye and the right eye is called retinal disparity. Because retinal disparity decreases as the distance between two objects increases, the brain can use it to infer depth and calculate distance.

Binocular cues help us estimate distances up to about 50 feet. For objects farther away, we use only monocular cues, cues that do not depend on using both eyes. One such cue is interposition: When an object is interposed between the viewer and a second object, partly blocking the view of the second object, the first object is perceived as being closer. Another monocular cue is linear perspective: When two lines known to be parallel appear to be coming together or converging (say, railroad tracks), they imply the existence of depth. These and other monocular cues are illustrated here.

Monocular Cues to Depth

Visual Constancies: When Seeing Is Believing Your perceptual world would be a confusing place without still another important perceptual skill. Lighting conditions, viewing angles, and the distances of stationary objects are all continually changing as we move about, yet we rarely confuse these changes with changes in the objects themselves. This ability to perceive objects as stable or unchanging even though the sensory patterns they produce are constantly shifting is called perceptual constancy. The best-studied constancies are visual and include the following:

1. Size constancy. We see an object as having a constant size even when its retinal image becomes smaller or larger. A friend approaching on the street does not seem to be growing; a car pulling away from the curb does not seem to be shrinking. Size constancy depends in part on familiarity with objects; you know that people and cars do not change size from moment to moment. It also depends on the apparent distance of an object. An object that is close produces a larger retinal image than the same object farther away, and the brain takes this into account. When you move your hand toward your face, your brain registers the fact that the hand is getting closer, and you correctly perceive its unchanging size despite the growing size of its retinal image. There is, then, an intimate relationship between perceived size and perceived distance.

2. Shape constancy. We continue to perceive an object as having a constant shape even though the shape of the retinal image produced by the object changes when our point of view changes. If you hold a Frisbee directly in front of your face, its image on the retina will be round. When you set the Frisbee on a table, its image becomes elliptical, yet you continue to see it as round.

3. Location constancy. We perceive stationary objects as remaining in the same place even though the retinal image moves about as we move our eyes, head, and body. As you drive along the highway, telephone poles and trees fly by on your retina. But you know that these objects do not move on their own, and you also know that your body is moving, so you perceive the poles and trees as staying put.

4. Brightness constancy. We see objects as having a relatively constant brightness even though the amount of light they reflect changes as the overall level of illumination changes. Snow remains white even on a cloudy day, and a black car remains black even on a sunny day. We are not fooled because the brain registers the total illumination in the scene and automatically adjusts for it.

5. Color constancy. We see an object as maintaining its hue despite the fact that the wavelength of light reaching our eyes from the object may change as the illumination changes. Outdoor light is “bluer” than indoor light, and objects outdoors therefore reflect more “blue” light than those indoors. Conversely, indoor light from incandescent lamps is rich in long wavelengths and is therefore “yellower.” Yet an apple looks red whether you look at it in your kitchen or outside on the patio.

Part of the explanation involves sensory adaptation, which we discussed earlier. Outdoors, we quickly adapt to short-wavelength (bluish) light, and indoors, we adapt to long-wavelength light. As a result, our visual responses are similar in the two situations. Also, when computing the color of a particular object, the brain takes into account all the wavelengths in the visual field immediately around the object. If an apple is bathed in bluish light, so, usually, is everything else around it. The increase in blue light reflected by the apple is canceled in the visual cortex by the increase in blue light reflected by the apple's surroundings, and so the apple continues to look red. Color constancy is further aided by our knowledge of the world. We know that apples are usually red and bananas are usually yellow, and the brain uses that knowledge to recalibrate the colors in those objects when the lighting changes (Mitterer & de Ruiter, 2008).

Visual Illusions: When Seeing Is Misleading Perceptual constancies allow us to make sense of the world. Occasionally, however, we can be fooled, and the result is a perceptual illusion. For psychologists, illusions are valuable because they are systematic errors that provide us with hints about the perceptual strategies of the mind.

Although illusions can occur in any sensory modality, visual illusions have been the best studied. Visual illusions sometimes occur when the strategies that normally lead to accurate perception are overextended to situations where they do not apply. Compare the lengths of the two vertical lines in Figure6.10a. You will probably perceive the line on the right as slightly longer than the one on the left, yet they are exactly the same. (Go ahead, measure them; everyone does.) This is the Müller-Lyer illusion, named after the German sociologist who first described it in 1889.

Figure 6.10

The Müller-Lyer Illusion

One explanation for the Müller-Lyer illusion is that the branches on the lines serve as perspective cues that normally suggest depth (Bulatov et al., 2015; Gregory, 1963). The line on the left is like the near edge of a building; the one on the right is like the far corner of a room (see Figure6.10b). Although the two lines produce retinal images of the same size, the one with the outward-facing branches suggests greater distance. We are fooled into perceiving it as longer because we automatically apply a rule about the relationship between size and distance that is normally useful: When two objects produce the same-sized retinal image and one is farther away, the farther one is larger. The problem, in this case, is that the two lines do not actually differ in length, so the rule is inappropriate.

Just as there are size, shape, location, brightness, and color constancies, so there are size, shape, location, brightness, and color in constancies, resulting in illusions. The perceived color of an object depends on the wavelengths reflected by its immediate surroundings, a fact well known to artists and interior designers. Thus, you never see a good, strong red unless other objects in the surroundings reflect the blue and green part of the spectrum. When two objects that are the same color have different surroundings, you may mistakenly perceive them as different (see Figure6.11).

Figure 6.11

Color in Context

The Muller-Lyer Illusion

Some illusions are simply a matter of physics. Thus, a chopstick in a half-filled glass of water looks bent because water and air refract light differently. Other illusions occur due to misleading messages from the sense organs, as in sensory adaptation. Still others occur because the brain misinterprets sensory information, as in the Müller-Lyer illusion and the illusions in Figure6.12.

Figure 6.12

Fooling the Eye

Perhaps the ultimate perceptual illusion occurred when Swedish researchers tricked people into feeling that they were swapping bodies with another person or even a mannequin (Petkova & Ehrsson, 2008). The participants wore virtual-reality goggles connected to a camera on the other person's (or mannequin's) head. This allowed them to see the world from the other body's point of view as an experimenter simultaneously stroked both bodies with a rod. Most people soon had the weird sensation that the other body was actually their own; they even cringed when the other body was poked or threatened. The researchers speculate that the body-swapping illusion could some day be helpful in marital counseling, allowing each partner to literally see things from the other's point of view, or in therapy with people who have distorted body images (Ahn, Le, & Bailenson, 2013).

In everyday life, most illusions are harmless and entertaining. Occasionally, however, an illusion interferes with the performance of some task or skill, or may even cause an accident. For example, because large objects often appear to move more slowly than small ones, some drivers underestimate the speed of onrushing trains at railroad crossings. They think they can beat the train, with tragic results.