You might be proud of your 20/20 vision, even if it is corrected by glasses or contact lenses. The expression 20/20 refers to a measurement of visual acuity, the ability to see fine detail; it is the smallest line of letters that a typical person can read from a distance of 20 feet. But hawks, eagles, owls, and other raptors have up to eight times greater visual acuity than humans, or the equivalent of 20/2 vision (meaning that what the normal human can just see from 2 feet away can be seen by these birds at a distance of 20 feet away). Your sophisticated visual system has evolved to transduce visual energy in the world into neural signals in the brain. Understanding vision, then, starts with understanding light.

Visible light is simply the portion of the electromagnetic spectrum that we can see, and it is an extremely small slice of the pie (see FIGURE 4.2). You can think about light as waves of energy. Like ocean waves, light waves vary in height and in the distance between their peaks, or wavelengths. There are three physical properties of light waves, each of which produces a corresponding psychological dimension (see TABLE 4.3). The length of a light wave determines its hue, or what humans perceive as color. The intensity or amplitude of a light wave—how high the peaks are—determines what we perceive as brightness. Purity is the number of distinct wavelengths that make up the light, and it is purity that determines what we perceive as saturation, or the richness of colors.

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The Human Eye

Eyes have evolved as specialized organs to detect light. FIGURE 4.3 shows the human eye in cross-section. Light that reaches the eyes passes first through a clear, smooth outer tissue called the cornea and then continues through the pupil, a hole in the colored part of the eye. This colored part is the iris, which is a translucent, doughnut-shaped muscle that controls the size of the pupil and hence the amount of light that can enter the eye.

Immediately behind the iris, muscles inside the eye control the shape of the lens to bend the light and focus it onto the retina, light-sensitive tissue lining the back of the eyeball. The muscles change the shape of the lens to focus objects at different distances, making the lens flatter for objects that are far away or rounder for nearby objects. This is called accommodation, the process by which the eye maintains a clear image on the retina. FIGURE 4.4a shows how accommodation works.

If your eyeballs are a little too long or a little too short, the lens will not focus images properly on the retina. If the eyeball is too long, images are focused in front of the retina, leading to nearsightedness (myopia), which is shown in FIGURE 4.4b. If the eyeball is too short, images are focused behind the retina, and the result is farsightedness (hyperopia), as shown in FIGURE 4.4c. Eyeglasses, contact lenses, and surgical procedures can correct either condition. For example, eyeglasses and contacts both provide an additional lens to help focus light more appropriately, and procedures such as LASIK physically reshape the eye’s existing lens.

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From the Eye to the Brain

How does a wavelength of light become a meaningful image? The retina is the interface between the world of light outside the body and the world of vision inside the central nervous system. Two types of photoreceptor cells in the retina contain light-sensitive pigments that transduce light into neural impulses. Cones detect color, operate under normal daylight conditions, and allow us to focus on fine detail. Rods become active under low-light conditions for night vision (see FIGURE 4.5).

Rods are much more sensitive photoreceptors than cones, but this sensitivity comes at a cost. Because all rods contain the same photopigment, they provide no information about color and sense only shades of gray. Think about this the next time you wake up in the middle of the night and make your way to the bathroom for a drink of water. Using only the moonlight from the window to light your way, do you see the room in shades of gray? About 120 million rods are distributed more or less evenly around each retina except in the very center, the fovea, an area of the retina where vision is the clearest and there are no rods at all. The absence of rods in the fovea decreases the sharpness of vision in reduced light.

In contrast to rods, each retina contains only about six million cones, which are densely packed in the fovea and much more sparsely distributed over the rest of the retina, as you can see in FIGURE 4.5. This distribution of cones directly affects visual acuity and explains why objects off to the side, in your peripheral vision, aren’t so clear. The light reflecting from those peripheral objects is less likely to land in the fovea, making the resulting image less clear.

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As seen in FIGURE 4.5, the photoreceptor cells (rods and cones) form the innermost layer of the retina. Above them, the bipolar cells collect neural signals from the rods and cones and transmit them to the retinal ganglion cells (RGCs), which organize the signals and send them to the brain. The bundled RGC axons form the optic nerve, which leaves the eye through a hole in the retina. Because it contains neither rods nor cones and therefore has no mechanism to sense light, this hole in the retina creates a blind spot, a location in the visual field that produces no sensation on the retina. Try the demonstration in FIGURE 4.6 to find the blind spot in each of your own eyes.

Sir Isaac Newton pointed out around 1670 that color is not something “in” light. In fact, color is nothing but our perception of wavelengths from the spectrum of visible light (see FIGURE 4.2). We perceive the shortest visible wavelengths as deep purple. As wavelengths increase, the color perceived changes gradually and continuously to blue, then green, yellow, orange, and, with the longest visible wavelengths, red. This rainbow of hues and accompanying wavelengths is called the visible spectrum, illustrated in FIGURE 4.7.

Cones come in three types; each type is especially sensitive to either red (long-wavelength), green (medium-wavelength), or blue (short-wavelength) light. Red, green, and blue are the primary colors of light; color perception results from different combinations of the three basic elements in the retina. For example, lighting designers add primary colors of light together, such as shining red and green spotlights on a surface to create a yellow light, as shown in FIGURE 4.8. Notice that in the center of the figure, where the red, green, and blue lights overlap, the surface looks white. This demonstrates that a white surface really is reflecting all visible wavelengths of light.

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A genetic disorder in which one of the cone types is missing—and, in some very rare cases, two or all three—causes a color deficiency. This trait is sex-linked, affecting men much more often than women. Color deficiency is often referred to as color blindness, but in fact, people missing only one type of cone can still distinguish many colors, just not as many as someone who has the full complement of three cone types. You can create a kind of temporary color deficiency by exploiting the idea of sensory adaptation. Staring too long at one color fatigues the cones that respond to that color, producing a form of sensory adaptation that results in a color afterimage. To demonstrate this effect for yourself, follow these instructions for FIGURE 4.9:

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Were you puzzled that the red patch produces a green afterimage and the green patch produces a red afterimage? When you view a color, let’s say, green, the cones that respond most strongly to green become fatigued over time. Now, when you stare at a white or gray patch, which reflects all the colors equally, the green-sensitive cones respond only weakly compared with the still-fresh red-sensitive cones, which fire strongly. The result? You perceive the patch as tinted red.

Streams of action potentials containing information encoded by the retina (neural impulses) travel to the brain along the optic nerve. Half of the axons in the optic nerve that leave each eye come from retinal ganglion cells (RGCs) that code information in the right visual field, whereas the other half code information in the left visual field. These two nerve bundles link to the left and right hemispheres of the brain, respectively (see FIGURE 4.10). The optic nerve travels from each eye to the thalamus. From there, the visual signal travels to the back of the brain, to a location called area V1, the part of the occipital lobe that contains the primary visual cortex. Here the information is systematically mapped into a representation of the visual scene.

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Neural Systems for Perceiving Shape

One of the most important functions of vision involves perceiving the shapes of objects; our day-to-day lives would be a mess if we couldn’t reliably differentiate between a warm doughnut with glazed icing and a straight stalk of celery. Perceiving shape depends on the location and orientation of an object’s edges. As you read in the Neuroscience and Behavior chapter, neurons in the visual cortex selectively respond to bars and edges in specific orientations in space (Hubel & Wiesel, 1962, 1998). In effect, area V1 contains populations of neurons, each “tuned” to respond to edges oriented at each position in the visual field. This means that some neurons fire when an object in a vertical orientation is perceived, other neurons fire when an object in a horizontal orientation is perceived, still other neurons fire when objects in a diagonal orientation of 45° are perceived, and so on (see FIGURE 4.11). The outcome of the coordinated response of all these feature detectors contributes to a sophisticated visual system that can detect where a doughnut ends and celery begins.

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Pathways for What, Where, and How

Two functionally distinct pathways, or visual streams, project from the occipital cortex to visual areas in other parts of the brain (see FIGURE 4.12). One pathway, the ventral (below) stream, travels across the occipital lobes to the temporal lobe and includes brain areas that represent an object’s shape and identity—in other words, what it is, essentially a “what” pathway (Kravitz et al., 2013; Ungerleider & Mishkin, 1982). The other pathway, the dorsal (above) stream, travels up from the occipital lobe to the parietal lobes (including some of the middle and upper levels of the temporal lobes), connecting with brain areas that identify the location and motion of an object—in other words, where it is (Kravitz et al., 2011). Because the dorsal stream allows us to perceive spatial relations, researchers originally dubbed it the “where” pathway (Ungerleider & Mishkin, 1982). Neuroscientists later argued that because the dorsal stream is crucial for guiding movements, such as aiming, reaching, or tracking with the eyes, the “where” pathway should more appropriately be called the “how” pathway (Milner & Goodale, 1995).

Some of the most dramatic evidence for the existence of these two pathways comes from studying patients with brain injuries. For example, a woman known as D.F. suffered permanent damage to a large region of the lateral occipital cortex, an area in the ventral stream (Goodale et al., 1991). Her ability to recognize objects by sight was greatly impaired, although her ability to recognize objects by touch was normal. This suggests that her visual representation of objects, not her memory for objects, was damaged. D.F.’s condition is called visual form agnosia, the inability to recognize objects by sight (Goodale & Milner, 1992, 2004). Conversely, other patients with brain damage to the parietal lobe, a section of the dorsal stream, have difficulty using vision to guide their reaching and grasping movements (Perenin & Vighetto, 1988). However, their ventral streams are intact, meaning they recognize what objects are. We can conclude from these two patterns of impairment that the ventral and dorsal visual streams are functionally distinct; it is possible to damage one while leaving the other intact.

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