“I saw it with my own eyes!” “Seeing is believing.” “I’m from Missouri, the Show Me State.” When we need reliable information, in vision, we trust.

The visual system delivers four different types of information, namely, information about (1) distance and size; (2) motion; (3) shape; and (4) brightness and color.

DISTANCE AND SIZE. Stop reading for a moment and look around your environment. (But don’t forget to return to the reading!) No matter where you are, you’ll immediately notice that some objects are closer than others. Your phone is nearby. A car seen through a window is farther away. The window is closer than the car but farther than the phone. The perception of distance—called depth perception, because you are judging how deep into three-dimensional space an object lies in relation to you—is so easy that, until you take a psychology course, you might not even think about it. But how do you do it?

Before we answer this question, note how distance relates to perceptual information about another feature of the world: objects’ size. Judgments about size are not determined solely by the amount of space an object takes up in your visual field, which is the range of the potentially observable world that can be perceived at any point in time. If you make a circle with your thumb and forefinger and look through it at the moon, your fingers take up more space in your visual field than the moon. However, you perceive the moon as bigger because you also perceive it as very distant.

To judge distance and size, your visual system uses cues to depth, which are sources of information that enable organisms to judge the distance between themselves and the objects they perceive. There are two types of depth cues:

To experience the power of monocular cues, look around your environment again—but this time, cover one of your eyes. Do you still perceive depth? Sure you do! Numerous monocular cues provide distance information. Two examples are shown in Figure 5.1.

At first glance, the two people appear to be of different sizes: A big one behind a little one. But how different are they? If you measure them, you’ll find that their physical size on the page is identical. Yet you perceive that the monster on top is farther away and bigger. This perception is caused by your visual system’s use of two monocular cues:

A third monocular distance cue is occlusion, a visual phenomenon in which one object partly blocks another from view. The blocking indicates that the first object is in front of the other—in other words, that it is closer to you. Occlusion alone is sufficient to indicate distance. Figure 5.2 eliminates converging verticals and texture yet, due to occlusion, one runner still appears closer than the other.

Another monocular distance cue is shading, which is the presence within a visual image of a relatively dark area that appears to have been created by blocking a source of light. Objects can appear closer to or farther from you based solely on how they are shaded. Look at Figure 5.3. Some of the circles seem to rise up above the square, and thus to be closer to you than is the square itself. Other circles seem to sink into the square, away from you. But none are actually closer or farther away—they’re all just sitting there on the page. The figure’s shading creates the perception of distance.

Another monocular cue to depth is an object’s clarity, which is the degree of distinctness, as opposed to fuzziness, of a visual image. Clearer objects appear to be closer. In general, they are; you usually can see objects right next to you more clearly than those that are farther away. However, sometimes clarity varies while actual distance remains constant. If so, the variations in clarity can create an illusory perception of change in distance that is startling. If you live in an area with fog, mist, or smog, a mountain range can appear far in the distance. But if the atmosphere clears up, the mountain suddenly appears to have jumped forward closer to you.

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Finally, the perception of objects’ distance and size is affected by environmental context, that is, the overall set of visual cues in the environment in which the object is placed. The role of context is illustrated by the odd-looking group of people in Figure 5.4.

It turns out that the oddity is not the people but the room; it is an Ames room, an apparatus for studying the perception of size. Its key feature is that it is not cubic. As you can see in Figure 5.5, the wall slants back, away from the viewer. In addition, the floor and ceiling are not parallel; the ceiling height is higher on the left than the right. The room is designed so that it’s impossible to detect these deviations from a room’s typical cubic shape. Your visual system (1) assumes that a room (the context in which the people are viewed) is cubic and (2) interprets people’s height in light of this assumption (O’Reilly, Jbabdi, & Behrens, 2012). The only interpretation that fits the assumption is that the people on the right are big.

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With all these monocular cues to depth, you might wonder why we evolved to have two eyes. Binocularity has its advantages. With two eyes, organisms have a “spare”; vision is maintained even if one eye is injured. Two eyes also provide a wider visual field; organisms can take in more information with two eyes than one. In addition, binocular vision provides more accurate depth perception (Bingham & Pagano, 1998), thanks to two binocular cues to depth: stereopsis and convergence.

Stereopsis is the perception of three-dimensional space produced by the fact that the images reaching your two eyes are not identical (Figure 5.6). The images vary because your eyes are a couple of inches apart. You can see the effect this has by holding your hand in front of your face, with your thumb right near your nose and palm facing to the left, and closing one eye and then the other. Your hand appears to “jump” because your eyes are receiving different images. When you look at distant objects, the difference between the images is tiny. But when you look nearby, it is large. The degree of difference between the two images thus is a binocular cue that provides information about distance.

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Convergence is a depth cue that involves the muscles that move your eyes. When you look at close objects (e.g., a few inches from your face), your eye muscles must exert effort to focus your eyes on it. If the object gets ever closer, more muscular effort is required to make your eyes converge. Feedback from these muscles to your brain provides an extra source of information about the distance of objects (Wexler & van Boxtel, 2005).

You’ve learned about a wide range of cues to depth. Table 5.1 summarizes them for you.

CULTURAL OPPORTUNITIES

PERCEIVING MOTION. In addition to size and distance, your visual system delivers information about motion. This information is of two types: relating to (1) the movement of objects in the environment (with respect to you, who are not moving), and (2) your own movement through an environment (which is stationary). Let’s consider them in order.

People are very good at perceiving the movement of objects. Even slight movements are detected immediately. If you were to hold this book in front of you at arms’ length, the amount of movement required for you to notice that motion occurred is no larger than the tiny space between two letters on this page (Sekuler, Watamaniuk, & Blake, 2002). People also are good at detecting where objects are going and how fast. In perception research, participants rapidly detect even quite small variations in the direction and speed of stimuli, such as dots moving on a computer screen (Sekuler et al., 2002).

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The visual system is so inclined to detect motion that we sometimes perceive it even when it’s not occurring. In the phi phenomenon, observers view two (or more) objects that are stationary but flash in an alternating sequence. What they perceive is not stationary flashing objects, but motion; the display appears to viewers to consist of one object that moves from one location to another.

Motion also conveys information about the type of object that is moving—especially if it is a living being. Swedish psychologist Gunnar Johansson conducted research (reviewed in Cutting, 2012) in which light sources were attached to the limbs of people who were filmed while they walked around in a dark room. The resulting film depicted merely a collection of moving dots: white points of light on a black background (Figure 5.9). Yet viewers immediately recognized that they were looking at human beings. Movement, in the absence of any other visual cues (faces, clothing, etc.), instantly indicates that the moving object is a person. From the motion of dots alone, research participants often can identify people’s sex and even their emotional state (the actors being filmed were asked to move as if they were experiencing various emotions; Atkinson et al., 2004; Gold et al., 2008).

Now imagine a different case: The objects in your view are stationary, but you are moving—for example, you are jogging through a wooded park. What is your perceptual experience like? There are two facts to note:

SHAPE. In addition to distance and motion, your visual system also provides information about shape, that is, the outline or contour of objects. In fact, the visual system is so inclined to detect shape that you end up seeing shapes that aren’t really present. Look at Figure 5.11, which depicts a Kanizsa triangle, devised by Italian psychologist Gaetano Kanizsa (1976). You perceive a bright white triangle in the center, on top of another triangle and three black circles. But in the open area in the center, no lines exist; in other words, there is no bright white triangle—a shape formed by three continuous lines—on the page. In fact, if you focus intently on one of those open, line-free areas, your perception of a white triangle will fade away. But if you shift your attention back to the image as a whole, the bright white triangle returns.

Note also that the image does not explicitly show a triangle or circles underneath the bright white triangle. The drawing merely depicts three V-shaped lines and three “Pac-Man” images. Yet the tendency to experience familiar geometric shapes causes you to perceive triangles and circles.

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Your visual system’s interpretation of shapes depends on the context in which they occur. The middle box in Figure 5.12 displays a vertical line next to a curved “m” shape on its side. What do you see? If you read up-and-down, the context is numbers and the shape in the middle is perceived as a 13. If you read left-to-right, the context is letters and the shape suddenly is a “B.” The idea that context guides the perception of any object is often called a Gestalt principle of perception, named for a nearly century-old school of thought, Gestalt psychology, which emphasized the perception of whole, organized structures (Wagemans et al., 2012). Figure 5.13 illustrates a number of Gestalt principles.

Sometimes your visual system does not know precisely “what to do with” shapes and their contours. Some figures, in other words, are open to two interpretations, and the visual system “flips” back and forth between them as it struggles to make sense of the environment. The edges of the vase in Figure 5.14 can also be perceived as the outline of a human face in profile. Your varying perceptions of the vase illustrate the principle of figure-ground perception, which is the visual system’s tendency to divide a scene into (1) objects, or “figures,” that are the focus of attention, and (2) the background context in which these figures occur. For example, when you look at Figure 5.14 and see profiles, they move to the foreground, and the white area of the vase temporarily appears to be behind them, in the background.

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BRIGHTNESS AND COLOR. In addition to information about distance, motion, and shape, your visual system provides information about brightness and color. Human vision is extremely sensitive to these visual qualities. Cells in your eyes respond to the smallest possible amount of light, that is, one photon (the basic elementary unit of light; Rieke & Baylor, 1998).

Your visual system detects not only light’s presence, but also variation in its brightness (e.g., if room lights brighten or dim). How much change is required for you to notice? This is a question about just noticeable differences. A just noticeable difference (JND) is the minimal variation in a physical stimulus (light, sound, etc.) that a person can detect. JNDs have been explored since the nineteenth century by researchers in psychophysics, a branch of psychology that studies relations between physical stimuli and psychological reactions.

What, then, is the answer? How much change in light is required for you to notice? It turns out that there is no one fixed answer. Instead, it depends. With a bit of thinking, you can figure out for yourself what the answer depends on. Compare these two circumstances:

In both cases, the amount of change is the same: the light of one candle. But in #1 you notice the change and in #2 you don’t. In other words, in #1 the light of one candle exceeds the JND and in #2 that same light is less than the JND. The lesson is that the JND is not constant; its value depends on how much light is already in the environment. If a lot of light is present, then a bigger change in illumination is needed for you to notice the change in brightness.

Your perception of an object’s brightness depends on not only light coming from that object, but also light coming from nearby objects. The visual system responds to the contrast between brightness in different areas of the visual field. Figure 5.15 illustrates this. It appears to display a dark gray square on the left and a light gray square on the right. But those two squares are identical! The perception of their difference is caused by their contexts: The left square is much darker than its surrounding context and thus is perceived as darker than the square on the right.

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Now let’s consider the perception of color. We need to begin with the physics of light. Visible light is one little sliver of the overall electromagnetic spectrum, which is the full range of physical energy that is made up of electricity and magnetism. Electromagnetic energy exists in the form of physical waves. The waves vary in wavelength (Figure 5.16). Many long (e.g., radio waves) and short (e.g., x-rays) wavelengths cannot be detected by the visual system, which is sensitive to one sliver of the spectrum—the region of visible light.

Within that region, variations in wavelength correspond to variations in color. The shortest detectable wavelengths appear violet, and the longest are experienced as red. Note that the color is not present in the light itself; light energy is not literally “colored.” Color is a psychological experience that occurs when the visual system is activated by light energy.

Wavelength does not entirely determine the psychological experience of color. We know this because wavelengths and color may fail to correspond in two ways: (1) Sometimes different wavelengths of light are experienced as the same color; and (2) sometimes the same wavelength of light produces, in different circumstances, different color experiences. Both phenomena are shown in Figure 5.17.

In panels (a) and (b), the apples look roughly the same: a few red apples nestled between some yellow and green fruits. The light reaching your eye from panels (a) and (b) differs considerably, though, due to a bluish filter placed over the image in panel (b). You thus have the same color experience, red, despite varying wavelengths of light. The tendency for a given object to be perceived as having the same color despite changes in illumination is called color constancy.

In panels (b) and (c), the apples look different. The apples in (c) are strange-looking: of a blue-purple variety. They appear quite unlike the normal red apples in (b). Yet the apple images in panels (b) and (c) are identical. You thus experience different colors of apple when looking at (b) and (c), despite the fact that the wavelengths of light reaching your eyes from the apples are the same.

The lesson here is that your visual system does not respond to individual points of color but to the visual environment as a whole (Shevell & Kingdom, 2008). This makes sense evolutionarily. The ability to perceive colors—like other abilities—evolved because it contributed to organisms’ survival and reproduction. This biological success requires organisms to be attuned to their overall environment. Figure 5.18 shows how color perception aids an organism’s survival. If you’re a bee, you need to find flowers. Without color vision, it’s difficult to find them; they blend into the background of a plant’s leaves (see the left panel of the figure). But with color vision, they “pop” right out from the environmental background (the right panel). It is not surprising, then, that bees have been able to perceive color for hundreds of millions of years (Chittka, 1996).

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One last key fact about color perception is that colors have opposites. You can imagine a reddish brown but not a reddish green. Red and green are opposites. It’s easy to imagine a yellowish green but not a yellowish blue. Blue and yellow are opposites. Opponent processes in perception create visual experiences featuring two opposing color pairs: red-green and yellow-blue (Hurvich & Jameson, 1957). In special circumstances, you can experience these opposites at play. Look at Figure 5.19 and follow its instructions. You’ll find that the colors (green, yellow) will jump rapidly to their opposites (red, blue) in the afterimage, the image you see after looking at the depicted image. Why? Although more than one biological mechanism is involved, one key process involves biochemicals in cells within the eye that respond to color (Ritschel & Eisemann, 2012). When you stare at one color for a long time, biochemicals which react to that color become temporarily depleted. When you then look at a blank wall, the chemical that responds to an opposite color is abundant, and it dominates your perceptual experience.

Let’s now look more deeply at the biology of perception.

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The first piece of biological machinery that enables you to see is, of course, the eye.

THE EYE. Four biological mechanisms near the front of the eye (Figure 5.20a) begin the work of visual perception: the cornea, the pupil, the iris, and the lens. The cornea is the transparent material at the eye’s very front. Its curvature begins the process of focusing light, to produce a sharp image of the outside world. The pupil is the opening through which light passes (Figure 5.20b). The size of the opening is controlled by the iris, the colorful structure that surrounds the pupil. The iris dilates the pupil (i.e., widens the opening) in dim light and constricts it in bright light. Finally, the lens is a second transparent biological mechanism that focuses incoming light. Unlike the cornea, the lens’s focusing mechanism is adjustable. Muscles in the eye adjust the flexible lens to sharpen images of objects at different distances. With age, the lens loses some of its flexibility, causing people to need supplementary lenses—that is, eyeglasses—to see both near and far objects clearly.

The inner volume of the eye is filled with a transparent fluid. Light passes through this fluid and reaches the retina, the rear wall of the eye, which contains a system of nerve cells that respond to light and send signals toward the brain.

As we noted in this chapter’s opening story about upside-down glasses, the image that reaches the retina is upside down; due to the curvature of the lens, the image “flips.” So why don’t you see the world upside down when you look at the image on the retina? Or why didn’t the man who wore upside-down glasses see the world upside down when he looked at the image on his retina? It’s because people don’t literally “look at the image on the retina.” People look at the world outside themselves. The retina is merely one of a series of biological tools that enable you to perceive information about the world that arrives in the form of light. If something alters the usual way that light strikes the retina—as when you put on upside-down glasses; or stand on your head; or try reading the material in Figure 5.21—you still can, with a little practice, accurately pick up information about the world.

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Once light reaches the retina, the body’s nervous system swings into action. The nerve cells that transduce light energy are called photoreceptors, as noted earlier in this chapter. The retina contains two types of photoreceptors, whose names reflect their shapes: cones and rods (Figure 5.22). An average human retina has about 5 million cones and 100 million rods (Winkler, 2013).

Cones are photoreceptors that provide two critical visual qualities: detail and color. Detail is possible thanks to a dense concentration of cones near the center of the retina, in a region known as the fovea (Curcio et al., 1990; Figure 5.23). You rely on the fovea, and its concentration of cones, when focusing attention to get a sharp, detailed look at an object. When you focus on any one object, you’ll notice that others, in the periphery of your visual field, appear blurrier and less detailed. Light from these objects falls outside of the fovea, where there are fewer cones.

Cone cells also enable us to see in color. Different types of cone cells are sensitive to different wavelengths of light—that is, to wavelengths we perceive as differently colored. The combination of these sensitivities gives us color perception. Amazingly, although we can identify a huge number of different colors, there is not a huge number of different cone types. There are just three: cones that are maximally sensitive to wavelengths of light corresponding to (1) blue, (2) green, and (3) red (Figure 5.24). They combine to provide the full spectrum of color experiences (Sharpe et al., 1999). (A TV similarly combines red, green, and blue beams of light to produce a spectrum of colors.)

Due to genetic variations, some people lack normal functioning in certain cone types. The result is color blindness, an insensitivity to one or more of the colors red, green, or blue (Sharpe et al., 1999). If all of your color-sensitivity cones are working properly, you’ll see the number 74 in Figure 5.25. If they aren’t, you may see nothing but dots.

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Rods are photoreceptors that enable you to see in low illumination. They are the receptors that, as noted earlier, can respond to as little as one photon of light; a cone cell, by comparison, requires about 100 times as many photons to become active. Thanks to the sensitivity of rods, you can see outdoors after the sun sets, or indoors when only a small light is on.

Although rods allow people to see in low illumination, they do not enable us to see color. Unlike cones, rods are not color sensitive. They do not respond at all to long wavelengths of light in the red range. You experience this at night; when the sun sets, the environment looks not only darker, but also less colorful, and red objects appear black. You do not, however, entirely lose your color vision at night. Research shows that, even if only rods are active, the visual system as a whole provides some experience of color, in part by making “educated guesses” about the color of familiar objects based on past experience of them in bright light (Pokorny et al., 2006).

The retina picks up most of its visual information during visual fixations, periods when a person’s gaze is held in one location. Fixations occur between saccades, which are rapid movements of the eyes from one position to another. A challenge for researchers is to track the rapid movements and fixations of a perceiver (see Research Toolkit).

RESEARCH TOOLKIT

Once its cones and rods are stimulated, the eye’s job is to send visual information to the brain. Key to this information flow are ganglion cells, which receive information from the retina and transmit it into the brain. The long fibers of the ganglion cells, along which these signals travel, form the optic nerve (see Figure 5.20a). There are far fewer ganglion cells than there are rods and cones; each ganglion cell, then, receives and integrates information from multiple photoreceptors.

At the spot along the retina at which the optic nerve exits the eye, there are no photoreceptors. Light striking that spot thus creates no visual experience. It’s the blind spot—the location in the visual field at which nothing is seen. This might seem odd to you because, when you look around, you seem to see everything; you don’t usually experience a blind spot. Your brain normally “fills in” the blind spot, providing you with a continuous visual experience. But you can experience it—see Figure 5.29.

FROM EYE TO BRAIN. The route along which signals travel from the eye into the brain contains three major steps. The first two involve the optic chiasm and the lateral geniculate nucleus.

The third step of processing, which we’ll consider in detail, occurs in the visual cortex.

THE VISUAL CORTEX. Signals from the lateral geniculate nucleus reach the visual cortex, a region in the rear of the brain devoted to processing visual information (Belliveau et al., 1991). The biological processing required for the perception of visual features discussed earlier (size, shape, motion, etc.) occurs in the visual cortex.

The visual cortex contains layers of cells that progressively integrate signals coming from multiple photoreceptors in the eyes. The first layer of processing maps retinal activation into visual cortex activation (Tootell et al., 1982). In this mapping, adjacent areas of the retina are represented in adjacent areas of the cortex (Figure 5.31).

These mappings from retina to visual cortex provide insight into visual illusions. Earlier in this chapter, you saw a size illusion: Sometimes two identical objects appear to differ in size (see Figure 5.1). It turns out that, during this visual illusion, cortical activation corresponds to their perceived size, not their actual size (Figure 5.32). Objects of the same actual size that appear larger/smaller have larger/smaller activation patterns in the cortex (Huk, 2008).

This result provides a link between levels of analysis (Figure 5.33). People experience illusions. The visual system’s processing of cues such as texture and converging vertical lines explains the illusions in terms of the workings of the mind. Research on the visual cortex shows what happens at the level of the brain.

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More than a half-century ago, neuroscientists David Hubel and Torsten Wiesel conducted Nobel Prize–winning work on exactly how the visual cortex does its job (Hubel, 1963). They discovered individual cells in the cortex that are “feature detectors.” The cells respond to specific visual features such as size, shape, and motion. For example, a cortical cell may respond maximally to lines that appear at a specific angle (Figure 5.34).

More recent research on the visual cortex highlights the ways in which the cortex combines the processing of different types of information. Visual properties such as the shape, brightness, texture, and color of objects influence one another, due to complex information processing in the cortex that integrates information about these perceptual features (Shapley & Hawken, 2011). This integration produces more accurate perception of objects. If you view, for example, a blue car that’s partly in the shade, partly in the sun, and partly dented, the physical light that reaches your retina varies from one part of the car to another. Yet you do not perceive a multicolored car. You perceive, accurately, a car that is of a continuous shade of blue, as a result of your cortex’s ability to integrate multiple visual features.