6-7 What are the characteristics of the energy that we see as visible light? What structures in the eye help focus that energy?

Our eyes receive light energy and transduce (transform) it into neural messages that our brain then processes into what we consciously see. How does such a taken-for-granted yet extraordinary thing happen?

When you look at a bright red tulip, the stimuli striking your eyes are not particles of the color red but pulses of electromagnetic energy that your visual system perceives as red. What we see as visible light is but a thin slice of the whole spectrum of electromagnetic energy, ranging from imperceptibly short gamma waves to the long waves of radio transmission (FIGURE 6.12). Other organisms are sensitive to differing portions of the spectrum. Bees, for instance, cannot see what we perceive as red but can see ultraviolet light.

Two physical characteristics of light help determine our sensory experience. Light’s wavelength—the distance from one wave peak to the next (FIGURE 6.13a below)—determines its hue (the color we experience, such as the tulip’s red petals or green leaves). Intensity—the amount of energy in light waves (determined by a wave’s amplitude, or height)—influences brightness (FIGURE 6.13b). To understand how we transform physical energy into color and meaning, consider the eye.

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Light enters the eye through the cornea, which bends light to help provide focus (FIGURE 6.14). The light then passes through the pupil, a small adjustable opening. Surrounding the pupil and controlling its size is the iris, a colored muscle that dilates or constricts in response to light intensity—or even to imagining a sunny sky or a dark room (Laeng & Sulutvedt, 2014). The iris also responds to our cognitive and emotional states. When you feel disgust or you are about to answer No to a question, your pupils constrict (de Gee et al., 2014; Goldinger & Papesh, 2012). When you’re feeling amorous, your telltale dilated pupils and dark eyes subtly signal your interest. Each iris is so distinctive that an iris-scanning machine can confirm your identity.

Behind the pupil is a transparent lens that focuses incoming light rays into an image on the retina, a multilayered tissue on the eyeball’s sensitive inner surface. The lens focuses the rays by changing its curvature and thickness in a process called accommodation.

For centuries, scientists knew that when an image of a candle passes through a small opening, it casts an inverted mirror image on a dark wall behind. If the image passing through the pupil casts this sort of upside-down image on the retina, as in Figure 6.14, how can we see the world right side up? The ever-curious Leonardo da Vinci had an idea: Perhaps the eye’s watery fluids bend the light rays, reinverting the image to an upright position as it reaches the retina. Unfortunately for da Vinci, that idea was disproved in 1604, when the astronomer and optics expert Johannes Kepler showed that the retina does receive upside-down images of the world (Crombie, 1964). So how could we understand such a world? “I leave it,” said the befuddled Kepler, “to natural philosophers.”

Today’s answer: The retina doesn’t “see” a whole image. Rather, its millions of receptor cells convert particles of light energy into neural impulses and forward those to the brain. There, the impulses are reassembled into a perceived, upright-seeming image. And along the way, visual information processing percolates through progressively more abstract levels. All this happens with astonishing speed. As a baseball pitcher’s fastball approaches home plate, the light signals work their way from the batter’s retina to the visual cortex, which then informs the motor cortex, which then sends out orders to contract the muscles—all in the 4/10ths of a second that the ball is in flight.

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Retinal Processing

6-8 How do the rods and cones process information, and what is the path information travels from the eye to the brain?

Imagine that you could follow behind a single light-energy particle after it entered your eye. First, you would thread your way through the retina’s sparse outer layer of cells. Then, reaching the back of the eye, you would encounter its buried receptor cells, the rods and cones (FIGURE 6.15). There, you would see the light energy trigger chemical changes. That chemical reaction would spark neural signals, activating nearby bipolar cells. The bipolar cells in turn would activate the neighboring ganglion cells, whose axons twine together like the strands of a rope to form the optic nerve. The optic nerve is an information highway to your brain, where your thalamus stands ready to distribute the information it receives from your eyes. The optic nerve can send nearly 1 million messages at once through its nearly 1 million ganglion fibers. (The auditory nerve, which enables hearing, carries much less information through its mere 30,000 fibers.) We pay a small price for this eye-to-brain highway. Where the optic nerve leaves the eye, there are no receptor cells—creating a blind spot (FIGURE 6.16). Close one eye and you won’t see a black hole, however. Without seeking your approval, your brain fills in the hole.

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Rods and cones differ in where they’re found and in what they do (TABLE 6.1). Cones cluster in and around the fovea, the retina’s area of central focus (see Figure 6.14). Many cones have their own hotline to the brain: Each cone transmits its message to a single bipolar cell. That cell helps relay the cone’s individual message to the visual cortex, which devotes a large area to input from the fovea. These direct connections preserve the cones’ precise information, making them better able to detect fine detail.

Rods don’t have dedicated hotlines. Rods share bipolar cells which send combined messages. To experience this rod-cone difference in sensitivity to details, pick a word in this sentence and stare directly at it, focusing its image on the cones in your fovea. Notice that words a few inches off to the side appear blurred? Their image strikes the outer regions of your retina, where rods predominate. Thus, when driving or biking, you can detect a car in your peripheral vision well before you perceive its details.

Cones also enable you to perceive color. In dim light they become ineffectual, so you see no colors. Rods, which enable black-and-white vision, remain sensitive in dim light. Several rods will funnel their faint energy output onto a single bipolar cell. Thus, cones and rods each provide a special sensitivity—cones to detail and color, and rods to faint light.

When you enter a darkened theater or turn off the light at night, your eyes adapt. Your pupils dilate to allow more light to reach your retina, but it typically takes 20 minutes or more before your eyes fully adapt. You can demonstrate dark adaptation by closing or covering one eye for up to 20 minutes. Then make the light in the room not quite bright enough to read this book with your open eye. Now open the dark-adapted eye and read (easily). This period of dark adaptation matches the average natural twilight transition between the Sun’s setting and darkness. How wonderfully made we are.

To summarize: The retina’s neural layers don’t just pass along electrical impulses. They also help to encode and analyze sensory information. (The third neural layer in a frog’s eye, for example, contains the “bug detector” cells that fire only in response to moving fly-like stimuli.) In human eyes, information follows this pathway:

The same sensitivity that enables retinal cells to fire messages can lead them to misfire, as you can demonstrate. Turn your eyes to the left, close them, and then gently rub the right side of your right eyelid with your fingertip. Note the patch of light to the left, moving as your finger moves.

Why do you see light? Why at the left? This happens because your retinal cells are so responsive that even pressure triggers them. But your brain interprets their firing as light. Moreover, it interprets the light as coming from the left—the normal direction of light that activates the right side of the retina.

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6-9 How do we perceive color in the world around us?

One of vision’s most basic and intriguing mysteries is how we see the world in color. In everyday conversation, we talk as though objects possess color: “A tomato is red.” Recall the old question, “If a tree falls in the forest and no one hears it, does it make a sound?” We can ask the same of color: If no one sees the tomato, is it red?

The answer is No. First, the tomato is everything but red, because it rejects (reflects) the long wavelengths of red. Second, the tomato’s color is our mental construction. As Isaac Newton (1704) noted, “The [light] rays are not colored.” Like all aspects of vision, our perception of color resides not in the object itself but in the theater of our brains, as evidenced by our dreaming in color.

How, from the light energy striking the retina, does our brain construct our experience of color—and of such a multitude of colors? Our difference threshold for colors is so low that we can discriminate more than 1 million different color variations (Neitz et al., 2001). At least most of us can. For about 1 person in 50, vision is color deficient—and that person is usually male, because the defect is genetically sex linked.

Modern detective work on the mystery of color vision began in the nineteenth century, when Hermann von Helmholtz built on the insights of an English physicist, Thomas Young. Any color can be created by combinations of different amounts of light waves of three primary colors—red, green, and blue. Knowing this, Young and von Helmholtz formed a hypothesis: The eye must have three corresponding types of color receptors. The Young-Helmholtz trichromatic (three-color) theory thus implies that the eye’s receptors do their color magic in teams of three. Years later, researchers measured the response of various cones to different color stimuli and confirmed that the retina does have three types of color receptors, each especially sensitive to one of three colors. And those colors are, in fact, red, green, and blue. When we stimulate combinations of these cones, we see other colors. We see yellow when light stimulates both red-sensitive and green-sensitive cones.

Most people with color-deficient vision are not actually “color-blind.” They simply lack functioning red-or green-sensitive cones, or sometimes both. Their vision—perhaps unknown to them, because their lifelong vision seems normal—is monochromatic (one-color) or dichromatic (two-color) instead of trichromatic, making it impossible to distinguish the red and green in FIGURE 6.18 (Boynton, 1979). Dogs, too, lack receptors for the wavelengths of red, giving them only limited, dichromatic color vision (Neitz et al., 1989).

But why do people blind to red and green often still see yellow? And why does yellow appear to be a pure color and not a mixture of red and green, the way purple is of red and blue? Trichromatic theory left some parts of the color vision mystery unsolved, and this sparked researcher Ewald Hering’s curiosity.

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Hering, a physiologist, found a clue in afterimages. Stare at a green square for a while and then look at a white sheet of paper, and you will see red, green’s opponent color. Stare at a yellow square and its opponent color, blue, will appear on the white paper. (To experience this, try the flag demonstration in FIGURE 6.19.) Hering formed another hypothesis: There must be two additional color processes, one responsible for red-versus-green perception, and one for blue-versus-yellow perception.

Indeed, a century later, researchers also confirmed Hering’s opponent-process theory. Three sets of opponent retinal processes—red-green, yellow-blue, and white-black—enable color vision. Recall that the thalamus relays visual information from the retina to the visual cortex. In both the retina and the thalamus, some neurons are turned “on” by red but turned “off” by green. Others are turned on by green but off by red (DeValois & DeValois, 1975). Like red and green marbles sent down a narrow tube, “red” and “green” messages cannot both travel at once. Red and green are thus opponents, so we do not experience a reddish green. But red and blue travel in separate channels, so we can see a reddish-blue magenta.

So how do we explain afterimages, such as in the flag demonstration? By staring at green, we tire our green response. When we then stare at white (which contains all colors, including red), only the red part of the green-red pairing will fire normally.

The present solution to the mystery of color vision is therefore roughly this: Color processing occurs in two stages.

6-10 Where are feature detectors located, and what do they do?

Once upon a time, scientists believed that the brain was like a movie screen, on which the eye projected images. But then along came David Hubel and Torsten Wiesel (1979), who showed that our brain’s computing system deconstructs visual images and then reassembles them. Hubel and Wiesel received a Nobel Prize for their work on feature detectors, nerve cells in the brain that respond to a scene’s specific features—to particular edges, lines, angles, and movements.

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Using microelectrodes, they had discovered that some neurons fired actively when cats were shown lines at one angle, while other neurons responded to lines at a different angle. They surmised that these specialized neurons in the occipital lobe’s visual cortex—now known as feature detectors—receive information from individual ganglion cells in the retina. Feature detectors pass this specific information to other cortical areas, where teams of cells (supercell clusters) respond to more complex patterns.

One temporal lobe area by your right ear (FIGURE 6.20) enables you to perceive faces and, thanks to a specialized neural network, to recognize them from varied viewpoints (Connor, 2010). If stimulated in this area, you might spontaneously see faces. If this region were damaged, you might recognize other forms and objects, but not familiar faces.

Researchers can temporarily disrupt the brain’s face-processing areas with magnetic pulses. When this happens, people cannot recognize faces, but they can recognize houses, because the brain’s face-perception occurs separately from its object-perception (McKone et al., 2007; Pitcher et al., 2007). Thus, functional MRI (fMRI) scans have shown different brain areas activating when people viewed varied objects (Downing et al., 2001). Brain activity is so specific (FIGURE 6.21) that, with the help of brain scans, “we can tell if a person is looking at a shoe, a chair, or a face, based on the pattern of their brain activity,” noted one researcher (Haxby, 2001).

For biologically important objects and events, monkey brains (and surely ours as well) have a “vast visual encyclopedia” distributed as specialized cells (Perrett et al., 1988, 1992, 1994). These cells respond to one type of stimulus, such as a specific gaze, head angle, posture, or body movement. Other supercell clusters integrate this information and fire only when the cues collectively indicate the direction of someone’s attention and approach. This instant analysis, which aided our ancestors’ survival, also helps a soccer player anticipate where to strike the ball, and a driver anticipate a pedestrian’s next movement.

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6-11 How does the brain use parallel processing to construct visual perceptions?

Our brain achieves these and other remarkable feats by parallel processing: doing many things at once. To analyze a visual scene, the brain divides it into subdimensions—motion, form, depth, color—and works on each aspect simultaneously (Livingstone & Hubel, 1988). We then construct our perceptions by integrating the separate but parallel work of these different visual teams (FIGURE 6.22).

To recognize a face, your brain integrates information projected by your retinas to several visual cortex areas, compares it to stored information, and enables you to recognize the face: Grandmother! Scientists have debated whether this stored information is contained in a single cell or, more likely, distributed over a vast network of cells. Some supercells—grandmother cells—do appear to respond very selectively to 1 or 2 faces in 100 (Bowers, 2009; Quiroga et al., 2013). The whole facial recognition process requires tremendous brain power: 30 percent of the cortex (10 times the brain area devoted to hearing).

Destroy or disable a neural workstation for a visual subtask, and something peculiar results, as happened to “Mrs. M.” (Hoffman, 1998). Since a stroke damaged areas near the rear of both sides of her brain, she has been unable to perceive movement. People in a room seem “suddenly here or there but I have not seen them moving.” Pouring tea into a cup is a challenge because the fluid appears frozen—she cannot perceive it rising in the cup.

After stroke or surgery has damaged the brain’s visual cortex, others have experienced blindsight. Shown a series of sticks, they report seeing nothing. Yet when asked to guess whether the sticks are vertical or horizontal, their visual intuition typically offers the correct response. When told, “You got them all right,” they are astounded. There is, it seems, a second “mind”—a parallel processing system—operating unseen. These separate visual systems for perception and action illustrate once again the astonishing dual processing of our two-track mind.

***

Think about the wonders of visual processing. As you read this page, the letters are transmitted by reflected light rays onto your retina, which triggers a process that sends formless nerve impulses to several areas of your brain, which integrates the information and decodes meaning, thus completing the transfer of information across time and space from my mind to your mind (FIGURE 6.23). That all of this happens instantly, effortlessly, and continuously is indeed awesome. As Roger Sperry (1985) observed, the “insights of science give added, not lessened, reasons for awe, respect, and reverence.”

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6-12 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 6.24: 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 Ground To 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 6.25, the figure-ground relationship continually reverses—but always we organize the stimulus into a figure seen against a ground.

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Grouping Having 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 6.26.

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6-13 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 6.27). 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 Cues Try 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 6.28.) 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 Cues How 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 6.29 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.

6-14 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 Constancies Our 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 6.31 below 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 6.32). 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 Constancies Sometimes an object whose actual shape cannot change seems to change shape with the angle of our view (FIGURE 6.33). More often, thanks to shape constancy, we perceive the form of familiar objects, such as the door in FIGURE 6.34, 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 6.35 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.

***

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.

Philosophers have debated whether our perceptual abilities should be credited to our nature or our nurture. To what extent do we learn to perceive? German philosopher Immanuel Kant (1724–1804) maintained that knowledge comes from our inborn ways of organizing sensory experiences. Indeed, we come equipped to process sensory information. But British philosopher John Locke (1632–1704) argued that through our experiences we also learn to perceive the world. Indeed, we learn to link an object’s distance with its size. So, just how important is experience? How radically does it shape our perceptual interpretations?

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6-15 What does research on restored vision, sensory restriction, and perceptual adaptation reveal about the effects of experience on perception?

Restored Vision and Sensory Restriction Writing to John Locke, William Molyneux wondered whether “a man born blind, and now adult, taught by his touch to distinguish between a cube and a sphere” could, if made to see, visually distinguish the two. Locke’s answer was No, because the man would never have learned to see the difference.

Molyneux’s hypothetical case has since been put to the test with a few dozen adults who, though blind from birth, later gained sight (Gregory, 1978; von Senden, 1932). Most were born with cataracts—clouded lenses that allowed them to see only diffused light, rather as you might see a foggy image through a Ping-Pong ball sliced in half. After cataract surgery, the patients could distinguish figure from ground and could sense colors—suggesting that these aspects of perception are innate. But much as Locke supposed, they often could not visually recognize objects that were familiar by touch.

Seeking to gain more control than is provided by clinical cases, researchers have outfitted infant kittens and monkeys with goggles through which they could see only diffuse, unpatterned light (Wiesel, 1982). After infancy, when the goggles were removed, these animals exhibited perceptual limitations much like those of humans born with cataracts. They could distinguish color and brightness, but not the form of a circle from that of a square. Their eyes had not degenerated; their retinas still relayed signals to their visual cortex. But lacking stimulation, the cortical cells had not developed normal connections. Thus, the animals remained functionally blind to shape. Experience guides, sustains, and maintains the brain neural organization that enables our perceptions.

In both humans and animals, similar sensory restrictions later in life do no permanent harm. When researchers cover the eye of an adult animal for several months, its vision will be unaffected after the eye patch is removed. When surgeons remove cataracts that develop during late adulthood, most people are thrilled at the return to normal vision.

The effect of sensory restriction on infant cats, monkeys, and humans suggests that for normal sensory and perceptual development, there is a critical period—an optimal period when exposure to certain stimuli or experiences is required. Surgery on blind children in India reveals that children blind from birth can benefit from removal of cataracts. But the younger they are, the more they will benefit, and their visual acuity (sharpness) may never be normal (Sinha, 2013). Early nurture sculpts what nature has endowed. In less dramatic ways, it continues to do so throughout our lives. Our visual experience matters. For example, despite concerns about their social costs, playing action video games sharpens spatial skills such as visual attention, eye-hand coordination and speed, and tracking multiple objects (Jeon et al., 2012; Spence & Feng, 2010).

Experiments on the perceptual limitations and advantages produced by early sensory deprivation provide a partial answer to the enduring question about experience: Does the effect of early experience last a lifetime? For some aspects of perception, the answer is clearly Yes: “Use it soon or lose it.” We retain the imprint of some early sensory experiences far into the future.

Perceptual Adaptation Given a new pair of glasses, we may feel slightly disoriented, even dizzy. Within a day or two, we adjust. Our perceptual adaptation to changed visual input makes the world seem normal again. But imagine a far more dramatic new pair of glasses—one that shifts the apparent location of objects 40 degrees to the left. When you first put them on and toss a ball to a friend, it sails off to the left. Walking forward to shake hands with the person, you veer to the left.

Could you adapt to this distorted world? Baby chicks cannot. When fitted with such lenses, they continue to peck where food grains seem to be (Hess, 1956; Rossi, 1968). But we humans adapt to distorting lenses quickly. Within a few minutes your throws would again be accurate, your stride on target. Remove the lenses and you would experience an aftereffect: At first your throws would err in the opposite direction, sailing off to the right; but again, within minutes you would readapt.

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Indeed, given an even more radical pair of glasses—one that literally turns the world upside down—you could still adapt. Psychologist George Stratton (1896) experienced this. He invented, and for eight days wore, optical headgear that flipped left to right and up to down, making him the first person to experience a right-side-up retinal image while standing upright. The ground was up, the sky was down.

At first, when Stratton wanted to walk, he found himself searching for his feet, which were now “up.” Eating was nearly impossible. He became nauseated and depressed. But he persisted, and by the eighth day he could comfortably reach for an object in the right direction and walk without bumping into things. When Stratton finally removed the headgear, he readapted quickly.

In later experiments, people wearing the optical gear have even been able to ride a motorcycle, ski the Alps, and fly an airplane (Dolezal, 1982; Kohler, 1962). The world around them still seemed above their heads or on the wrong side. But by actively moving about in these topsy-turvy worlds, they adapted to the context and learned to coordinate their movements.