4.2 Vision I: How the Eyes and the Brain Convert Light Waves to Neural Signals

Though controversial for other reasons, research has shown that action-shooter games improve attention and even basic visual acuity (Green & Bavelier, 2007; Li et al., 2009).

STEVE SKJOLD ALAMY

You might be proud of your 20/20 vision, even if it is corrected by glasses or contact lenses. 20/20 refers to a measurement associated with a Snellen chart, named after Hermann Snellen (1834–1908), the Dutch ophthalmologist who developed it as a means of assessing 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 if you dropped into the Birds of Prey Ophthalmologic Office, your visual pride would wither. Hawks, eagles, owls, and other raptors have much greater visual acuity than humans: in many cases, about 8 times greater, 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. Humans have sensory receptors in their eyes that respond to wavelengths of light energy. When we look at people, places, and things, patterns of light and color give us information about where one surface stops and another begins. The array of light reflected from those surfaces preserves their shapes and enables us to form a mental representation of a scene (Rodieck, 1998). Understanding vision, then, starts with understanding light.

Sensing Light

Visible light is simply the portion of the electromagnetic spectrum that we can see, and it is an extremely small slice (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 properties of light waves, each of which has a physical dimension that produces a corresponding psychological dimension (see TABLE 4.3). In other words, light doesn’t need a human to have the properties it does: Length, amplitude, and purity are properties of the light waves themselves. What humans perceive from those properties are color, brightness, and saturation.

• 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 the brightness of light.
• Purity is the number of distinct wavelengths that make up the light. Purity corresponds to what humans perceive as saturation, or the richness of colors.

Figure 4.2: Electromagnetic Spectrum The sliver of light waves visible to humans as a rainbow of colors from violet-blue to red is bounded on the short end by ultraviolet rays, which honeybees can see, and on the long end by infrared waves, upon which night-vision equipment operates. Someone wearing night-vision goggles, for example, can detect another person’s body heat in complete darkness. Light waves are minute, but the scale along the bottom of this chart offers a glimpse of their varying lengths, measured in nanometers (nm; 1 nm = 1 billionth of a meter).

Table 4.3: Properties of Light Waves

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, which bends the light wave and sends it 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.

Figure 4.3: Anatomy of the Human Eye Specialized organs of the eye evolved to detect light.

Immediately behind the iris, muscles inside the eye control the shape of the lens to bend the light again 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.

Figure 4.4: Accommodation Inside the eye, the lens changes shape to focus nearby or faraway objects on the retina. (a) People with normal vision focus the image on the retina at the back of the eye, both for near and far objects. (b) Nearsighted people see clearly what’s nearby, but distant objects are blurry because light from them is focused in front of the retina, a condition called myopia, (c) Farsighted people have the opposite problem: Distant objects are clear, but those nearby are blurry because their point of focus falls beyond the surface of the retina, a condition called hyperopia.

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.

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 lightsensitive 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).

Figure 4.5: Close-up of the Retina The surface of the retina is composed of photoreceptor cells, the rods and cones, beneath a layer of transparent neurons, the bipolar and retinal ganglion cells (RGCs), connected in sequence. The axon of a retinal ganglion cell joins with all other RGC axons to form the optic nerve. Viewed close up in this cross-sectional diagram is the area of greatest visual acuity, the fovea, where most color-sensitive cones are concentrated, allowing us to see fine detail as well as color. Rods, the predominant photoreceptors activated in low-light conditions, are distributed everywhere else on the retina.

OMIKRON/PHOTO RESEARCHERS

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 color or in shades of gray? Rods and cones differ in several other ways as well, most notably in their numbers. 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, but it can be overcome. For example, when amateur astronomers view dim stars through their telescopes at night, they know to look a little off to the side of the target so that the image will not fall on the rod-free fovea, but on some other part of the retina that contains many highly sensitive rods.

The full-color image on the left is what you’d see if your rods and cones are fully at work. The grayscale image on the right is what you’d see if only your rods are functioning.

MIKE SONNENBERG/ISTOCKPHOTO

In contrast to rods, each retina contains only about 6 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. The more fine detail encoded and represented in the visual system, the clearer the perceived image. The process is analogous to the quality of photographs taken with a 6-megapixel digital camera versus a 2-megapixel camera.

The image on the left was taken at a higher resolution than the image on the right. The difference in quality is analogous to light falling on the fovea versus the periphery of the retina.

THINKSTOCK

The retina is thick with cells. As seen in Figure 4.5, the photoreceptor cells (rods and cones) form the innermost layer. They are beneath a layer of transparent neurons, the bipolar and retinal ganglion cells. The bipolar cells collect neural signals from the rods and cones and transmit them to the outermost layer of the retina, where neurons called retinal ganglion cells (RGCs) organize the signals and send them to the brain.

The bundled RGC axons—about 1.5 million per eye—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.

Figure 4.6: Blind Spot Demonstration To find your blind spot, close your left eye and stare at the cross with your right eye. Hold the book 6 to 12 inches (15 to 30 centimeters) away from your eyes and move it slowly toward and away from you until the dot disappears. The dot is now in your blind spot and not visible. At this point the vertical lines may appear as one continuous line because the visual system fills in the area occupied by the missing dot. To test your left eye’s blind spot, turn the book upside down and repeat with your right eye closed.

Perceiving Color

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.

Figure 4.7: Seeing in Color We perceive a spectrum of color because objects selectively absorb some wavelengths of light and reflect others. Color perception corresponds to the summed activity of the three types of cones. Each type is most sensitive to a narrow range of wavelengths in the visible spectrum—short (bluish light), medium (greenish light), or long (reddish light). Rods, represented by the white curve, are most sensitive to the medium wavelengths of visible light but do not contribute to color perception.

You’ll recall that all rods are ideal for low-light vision but bad for distinguishing colors. Cones, by contrast, 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 that respond to the wavelengths corresponding to the three primary colors of light. 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.

Figure 4.8: Color Mixing The millions of shades of color that humans can perceive are products not only of a light’s wavelength, but also of the mixture of wavelengths a stimulus absorbs or reflects. Colored spotlights work by causing the surface to reflect light of a particular wavelength, which stimulates the red, blue, or green photopigments in the cones. When all visible wavelengths are present, we see white.

FRITZ GORO, Time & LIFE Pictures/Getty Images

Many people (including about 5% of all males) inherit conditions in which either the “red” or the “green” photoreceptors do not transduce light properly. Such people have difficulty distinguishing hues that to typical individuals appear as red or green. Unfortunately, in the United States, traffic signals use red and green lights to indicate whether cars should stop or go through an intersection. Why do drivers with red-green blindness not risk auto accidents every time they approach an intersection?

Age Fotostock/Superstock

The fact that three types of cones in the retina respond preferentially to different wavelengths (corresponding to blue, green, or red light) means that the pattern of responding across the three types of cones provides a unique code for each color. In fact, researchers can “read out” the wavelength of the light entering the eye by working backward from the relative firing rates of the three types of cones (Gegenfurtner & Kiper, 2003). 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.

Figure 4.9: Color Afterimage Demonstration Follow the accompanying instructions in the text, and sensory adaptation will do the rest. When the afterimage fades, you can get back to reading the chapter.

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. Just like the rest of your body, cones need an occasional break too. 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:

• Stare at the small cross between the two color patches for about 1 minute. Try to keep your eyes as still as possible.
• After a minute, look at the lower cross. You should see a vivid color aftereffect that lasts for a minute or more. Pay particular attention to the colors in the afterimage.

Were you puzzled that the red patch produces a green afterimage and the green patch produces a red afterimage? This result reveals something important about color perception. The explanation stems from the color-opponent system, where pairs of visual neurons work in opposition: red-sensitive cells against green-sensitive (as in Figure 4.9) and blue-sensitive cells against yellow-sensitive (Hurvich & Jameson, 1957). The color-opponent system explains color aftereffects. 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.

The Visual Brain

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 lateral geniculate nucleus (LGN), located in the thalamus. As you will recall from the Neuroscience and Behavior chapter, the thalamus receives inputs from all of the senses except smell. 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.

Figure 4.10: Visual Pathway from Eye through Brain Objects in the right visual field stimulate the left half of each retina, and objects in the left visual field stimulate the right half of each retina. The optic nerves, one exiting each eye, are formed by the axons of retinal ganglion cells emerging from the retina. Just before they enter the brain at the optic chiasm, about half the nerve fibers from each eye cross. The left half of each optic nerve (representing the right visual field) runs through the brain’s left hemisphere via the thalamus, and the right half of each optic nerve (representing the left visual field) travels this route through the right hemisphere. So, information from the right visual field ends up in the left hemisphere and information from the left visual field ends up in the right hemisphere.

Neural Systems for Perceiving Shape

Figure 4.11: Single-Neuron Feature Detectors Area V1 contains neurons that respond to specific orientations of edges. Here a single neuron’s responses are recorded (left) as the monkey views bars at different orientations (right). This neuron fires continuously when the bar is pointing to the right at 45°, less often when it is vertical, and not at all when it is pointing to the left at 45°.

FRITZ GORO/TIME & LIFE PICTURES/GETTY IMAGES

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 distinguish individual shapes from one another. Imagine not being able to reliably differentiate between a warm doughnut with glazed icing and a straight stalk of celery and you’ll get the idea; breakfast could become a traumatic experience if you couldn’t distinguish shapes. Perceiving shape depends on the location and orientation of an object’s edges. It is not surprising, then, that area V1 is specialized for encoding edge orientation. As you also read in the Neuroscience and Behavior chapter, neurons in the visual cortex selectively respond to bars and edges in specific orientations in space (Hubel & Weisel, 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.

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):

• The ventral (below) stream travels across the occipital lobe into the lower levels of the temporal lobes and includes brain areas that represent an object’s shape and identity, in other words, what it is, essentially a “what” pathway (Kravtiz et al., 2013; Ungerleider & Mishkin, 1982).
• 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 (Kravtiz 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).

Figure 4.12: Visual Streaming One interconnected visual system forms a pathway that courses from the occipital visual regions into the lower temporal lobe. This ventral pathway enables us to identify what we see. Another interconnected pathway travels from the occipital lobe through the upper regions of the temporal lobe into the parietal regions. This dorsal pathway allows us to locate objects, to track their movements, and to move in relation to them.

How do we know there are two pathways? The most dramatic evidence comes from studying the impairments that result from brain injuries to each of the areas.

Figure 4.13: Testing Visual Form Agnosia (left) When researchers asked D.F. to orient her hand to match the angle of the slot in the testing apparatus, she was unable to comply. (right) However, when asked to insert a card into the slot at various angles, D.F. accomplished the task virtually to perfection.

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, and not her memory for objects, was damaged. D.F.’s brain damage belongs to a category called visual form agnosia, the inability to recognize objects by sight (Goodale & Milner, 1992, 2004). Oddly, although D.F. could not recognize objects visually, she could accurately guide her actions by sight, as demonstrated in FIGURE 4.13. When D.F. was scanned with fMRI, researchers found that she showed normal activation of regions within the dorsal stream during guided movement (James et al., 2003).

Conversely, other people 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. Still, the two streams must work together during visual perception in order to integrate “what” and “where,” and researchers are starting to examine how they interact. One intriguing possibility is suggested by recent fMRI research indicating that some regions within the dorsal stream are sensitive to properties of an object’s identity, responding differently, for example, to line drawings of the same object in different sizes or viewed from different vantage points (Konen & Kastner, 2008; Sakuraba et al., 2012). This may be what allows the dorsal and ventral streams to exchange information and thus integrate the “what” and “where” (Farivar, 2009; Konen & Kastner, 2008).