KEY THEME

The receptor cells for vision respond to the physical energy of light waves and are located in the retina of the eye.

KEY QUESTIONS

A lone caterpillar on the screen door, the pile of dirty laundry in the corner of the closet, a spectacular autumn sunset, the intricate play of color, light, and texture in a painting by Monet. The sense organ for vision is the eye, which contains receptor cells that are sensitive to the physical energy of light. But before we can talk about how the eye functions, we need to briefly discuss some characteristics of light as the visual stimulus.

THE NATURE OF LIGHT

Light is just one of many different kinds of electromagnetic energy that travel in the form of waves. Other forms of electromagnetic energy include X-rays, the microwaves you use to pop popcorn, and the infrared signals or radio waves transmitted by your TV’s remote control. The various types of electromagnetic energy differ in wavelength, which is the distance from one wave peak to another.

Humans are capable of seeing only a minuscule portion of the electromagnetic energy range. In Figure 3.2, notice that the visible portion of the electromagnetic energy spectrum can be further divided into different wavelengths. As we’ll discuss in more detail later, the different wavelengths of visible light correspond to our psychological perception of different colors.

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THE HUMAN VISUAL SYSTEM

Imagine that you’re watching your author Susan’s cat Milla sunning herself in a nearby window. Simply seeing a brown tabby cat with emerald green eyes involves a complex chain of events. To help understand the visual process, trace the path of light waves through the eye in Figure 3.3 as we describe each step.

First, light waves reflected from the cat enter your eye, passing through the cornea, pupil, and lens. The cornea, a clear membrane that covers the front of the eye, helps gather and direct incoming light. The sclera, or white portion of the eye, is a tough, fibrous tissue that covers the eyeball except for the cornea. The pupil is the black opening in the eye’s center. The pupil is surrounded by the iris, the colored structure that we refer to when we say that someone has brown eyes. The iris is actually a ring of muscular tissue that contracts or expands to precisely control the size of the pupil and thus the amount of light entering the eye. In dim light, the iris widens the pupil to let light in; in bright light, the iris narrows the pupil.

Behind the pupil is the lens, another transparent structure. In a process called accommodation, the lens thins or thickens to bend or focus the incoming light so that the light falls on the retina. If the eyeball is abnormally shaped, the lens may not properly focus the incoming light on the retina, resulting in a visual disorder. In nearsightedness, or myopia, distant objects appear blurry because the light reflected off the objects focuses in front of the retina. In farsightedness, or hyperopia, objects near the eyes appear blurry because light reflected off the objects is focused behind the retina. During middle age, another form of farsightedness often occurs, called presbyopia. Presbyopia is caused when the lens becomes brittle and inflexible. In astigmatism, an abnormally curved eyeball results in blurry vision for lines in a particular direction. Corrective glasses remedy these conditions by intercepting and bending the light so that the image falls properly on the retina. Surgical techniques like LASIK correct visual disorders by reshaping the cornea so that light rays focus more directly on the retina.

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THE RETINA

RODS AND CONES

The retina is a thin, light-sensitive membrane that lies at the back of the eye, covering most of its inner surface (see Figure 3.3). Contained in the retina are the rods and cones. Because these sensory receptor cells respond to light, they are often called photoreceptors. When exposed to light, the rods and cones undergo a chemical reaction that results in a neural signal.

Rods and cones differ in many ways. First, as their names imply, rods and cones are shaped differently. Rods are long and thin, with blunt ends. Cones are shorter and fatter, with one end that tapers to a point. The eye contains far more rods than cones. It is estimated that each eye contains about 7 million cones and about 125 million rods!

Rods and cones are also specialized for different visual functions. Although both are light receptors, rods are much more sensitive to light than are cones. Once the rods are fully adapted to the dark, they are about a thousand times better than cones at detecting weak visual stimuli (Masland, 2001). We therefore rely primarily on rods for our vision in dim light and at night.

Rods and cones also react differently to changes in the amount of light. Rods adapt relatively slowly, reaching maximum sensitivity to light in about 30 minutes. In contrast, cones adapt quickly to bright light, reaching maximum sensitivity in about 5 minutes. That’s why it takes several minutes for your eyes to adapt to the dim light of a darkened room but only a few moments to adapt to the brightness when you switch on the lights.

You may have noticed that it is difficult or impossible to distinguish colors in very dim light. This difficulty occurs because only the cones are sensitive to the different wavelengths that produce the sensation of color, and cones require much more light than rods do to function effectively. Cones are also specialized for seeing fine details and for vision in bright light.

Most of the cones are concentrated in the fovea, which is a region in the very center of the retina. Cones are scattered throughout the rest of the retina, but they become progressively less common toward the periphery of the retina. There are no rods in the fovea. Images that do not fall on the fovea tend to be perceived as blurry or indistinct. For example, focus your eyes on the word “For” at the beginning of this sentence. In contrast to the sharpness of the letters in “For,” the words to the left and right are somewhat blurry. The image of the outlying words is striking the peripheral areas of the retina, where rods are more prevalent and there are very few cones.

THE BLIND SPOT

One part of the retina lacks rods and cones altogether. This area, called the optic disk, is the point at which the fibers that make up the optic nerve leave the back of the eye and project to the brain. Because there are no photoreceptors in the optic disk, we have a tiny hole, or blind spot, in our field of vision. To experience the blind spot, try the demonstration in Figure 3.4.

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Why don’t we notice this hole in our visual field? The most compelling explanation is that the brain actually fills in the missing background information (Ramachandran, 1992a, 1992b; Weil & Rees, 2010). In effect, signals from neighboring neurons fill in the blind spot with the color and texture of the surrounding visual information (Supér & Romeo, 2011).

KEY THEME

Signals from the rods and cones undergo preliminary processing in the retina before they are transmitted to the brain.

KEY QUESTIONS

Visual information is processed primarily in the brain. However, before visual information is sent to the brain, it undergoes some preliminary processing in the retina by specialized neurons called ganglion cells. This preliminary processing of visual data in the cells of the retina is possible because the retina develops from a bit of brain tissue that “migrates” to the eye during fetal development (Hubel, 1995).

When the numbers of rods and cones are combined, there are over 130 million receptor cells in each retina. However, there are only about 1 million ganglion cells. How do just 1 million ganglion cells transmit messages from 130 million visual receptor cells?

VISUAL PROCESSING IN THE RETINA

Information from the sensory receptors, the rods and cones, is first collected by specialized neurons, called bipolar cells. Look back at the lower portion of Figure 3.3. The bipolar cells then funnel the collection of raw data to the ganglion cells. Each ganglion cell receives information from the photoreceptors that are located in its receptive field in a particular area of the retina. In this early stage of visual processing, each ganglion cell combines, analyzes, and encodes the information from the photoreceptors in its receptive field before transmitting the information to the brain (Ringach, 2009).

Signals from rods and signals from cones are processed differently in the ganglion. For the most part, a single ganglion cell receives information from only one or two cones but might well receive information from a hundred or more rods. The messages from these many different rods are combined in the retina before they are sent to the brain. Thus, the brain receives less specific visual information from the rods and messages of much greater visual detail from the cones.

As an analogy to how rod information is processed, imagine listening to a hundred people trying to talk at once over the same telephone line. You would hear the sound of many people talking, but individual voices would be blurred. Now imagine listening to the voice of a single individual being transmitted across the same telephone line. Every syllable and sound would be clear and distinct. In much the same way, cones use the ganglion cells to provide the brain with more specific visual information than is received from rods.

Because of this difference in how information is processed, cones are especially important in visual acuity—the ability to see fine details. Visual acuity is strongest when images are focused on the fovea because of the high concentration of cones there.

FROM EYE TO BRAIN

How is information transmitted from the ganglion cells of the retina to the brain? The 1 million axons of the ganglion cells are bundled together to form the optic nerve, a thick nerve that exits from the back of the eye at the optic disk and extends to the brain (see Figure 3.5).

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From the optic chiasm, most of the optic nerve axons project to the brain structure called the thalamus. (For more on the specific brain structures involved in vision, see Chapter 2). This primary pathway seems to be responsible for processing information about form, color, brightness, and depth. A smaller number of axons follow a detour to areas in the midbrain before they make their way to the thalamus. This secondary pathway seems to be involved in processing information about the location of an object.

Neuroscientists now know that there are several distinct neural pathways in the visual system, each responsible for handling a different aspect of vision (Paik & Ringach, 2011; Purves, 2009). Although specialized, the separate pathways are highly interconnected. From the thalamus, the signals are sent to the visual cortex, where they are decoded and interpreted.

Most of the receiving neurons in the visual cortex of the brain are highly specialized. Each responds to a particular type of visual stimulation, such as angles, edges, lines, and other forms, and even to the movement and distance of objects (Hubel & Wiesel, 2005; Livingstone & Hubel, 1988). These neurons are sometimes called feature detectors because they detect, or respond to, particular features or aspects of more complex visual stimuli. Reassembling the features into a recognizable image involves additional levels of processing in the visual cortex and other regions of the brain, including the frontal lobes.

Understanding exactly how neural responses of individual feature detection cells become integrated into the visual perceptions of faces and objects is a major goal of contemporary neuroscience (Celesia, 2010; Mahon & Caramazza, 2011). Experience also plays an important role in the development of perception, especially visual perception (Huber & others, 2015). In the Focus on Neuroscience, we explore how Mike May’s perceptual abilities were affected by his lack of visual experience.

We see images of an apple, a banana, and an orange because these objects reflect light waves. But why do we perceive that the apple is red and the banana yellow? What makes an orange orange?

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THE EXPERIENCE OF COLOR

WHAT MAKES AN ORANGE ORANGE?

Color is not a property of an object, but a sensation perceived in the brain (Werner & others, 2007). To explain how we perceive color, we must return to the original visual stimulus—light.

Our experience of color involves three properties of the light wave. First, what we usually refer to as color is a property more accurately termed hue. Hue varies with the wavelength of light. Look again at Figure 3.2 on page 90. Different wavelengths correspond to our subjective experience of different colors. Wavelengths of about 400 nanometers are perceived as violet. Wavelengths of about 700 nanometers are perceived as red. In between are orange, yellow, green, blue, and indigo.

Second, the saturation, or purity, of the color corresponds to the purity of the light wave. Pure red, for example, produced by a single wavelength, is more saturated than pink, which is produced by a combination of wavelengths (red plus white light). In everyday language, saturation refers to the richness of a color. A highly saturated color is vivid and rich; a less saturated color is faded and washed out.

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The third property of color is brightness, or perceived intensity. Brightness corresponds to the amplitude of the light wave: The higher the amplitude, the greater the degree of brightness.

These three properties of color—hue, saturation, and brightness—are responsible for the amazing range of colors we experience. A person with normal color vision can discriminate from 120 to 150 color differences based on differences in hue, or wavelength, alone. When saturation and brightness are also factored in, we can potentially perceive millions of different colors (Elliot & Maier, 2014).

Many people mistakenly believe that white light contains no color. White light actually contains all wavelengths—and thus all colors—of the visible part of the electromagnetic spectrum. A glass prism placed in sunlight creates a rainbow because it separates sunlight into all the colors of the visible light spectrum.

So we’re back to the question: What makes an orange orange? Intuitively, it seems obvious that the color of any object is an inseparable property of the object—unless we spill paint or spaghetti sauce on it. In reality, color is a sensation perceived in the brain (Werner & others, 2007).

Our perception of color is primarily determined by the wavelength of light that an object reflects. If your T-shirt is red, it’s red because the cloth is reflecting only the wavelength of light that corresponds to the red portion of the spectrum. The T-shirt is absorbing the wavelengths that correspond to all other colors. An object appears white because it reflects all the wavelengths of visible light and absorbs none. An object appears black when it absorbs all the wavelengths of visible light and reflects none. Of course, in everyday life, our perceptions of color are also strongly affected by the amount or type of light falling on an object or the textures and colors that surround it (Purves, 2009; Shevell & Kingdom, 2008).

HOW WE SEE COLOR

Color vision has interested scientists for hundreds of years. The first scientific theory of color vision, proposed by Hermann von Helmholtz (1821–1894) in the mid-1800s, was called the trichromatic theory. A rival theory, the opponent-process theory, was proposed in the late 1800s. Each theory was capable of explaining some aspects of color vision, but neither theory could explain all aspects of color vision. Technological advances in the last few decades have allowed researchers to gather direct physiological evidence to test both theories. The resulting evidence indicates that both theories of color vision are accurate. Each theory describes color vision at a different stage of visual processing (Hubel, 1995).

The Trichromatic Theory As you’ll recall, only the cones are involved in color vision. According to the trichromatic theory of color vision, there are three varieties of cones. Each type of cone is especially sensitive to certain wavelengths—red light (long wavelengths), green light (medium wavelengths), or blue light (short wavelengths). For the sake of simplicity, we will refer to red-sensitive, green-sensitive, and blue-sensitive cones, but keep in mind that there is some overlap in the wavelengths to which a cone is sensitive (Purves, 2009). A given cone will be very sensitive to one of the three colors and only slightly responsive to the other two.

When a color other than red, green, or blue strikes the retina, it stimulates a combination of cones. For example, if yellow light strikes the retina, both the red-sensitive and green-sensitive cones are stimulated; purple light evokes strong reactions from red-sensitive and blue-sensitive cones. The trichromatic theory of color vision received compelling research support in 1964, when George Wald showed that different cones were indeed activated by red, blue, and green light.

The trichromatic theory provides a good explanation for the most common form of color blindness: red–green color blindness. People with red–green color blindness cannot discriminate between red and green. That’s because they have normal blue-sensitive cones, but their other cones are either red-sensitive or green-sensitive (Carroll & others, 2009). Thus, red and green look the same to them. Because red–green color blindness is so common, stoplights are designed so that the location of the light as well as its color provides information to drivers. In vertical stoplights the red light is always on top, and in horizontal stoplights the red light is always on the far left.

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The Opponent-Process Theory The trichromatic theory cannot account for all aspects of color vision. One important phenomenon that the theory does not explain is the afterimage. An afterimage is a visual experience that occurs after the original source of stimulation is no longer present. To experience an afterimage firsthand, follow the instructions in Figure 3.6. What do you see?

Afterimages can be explained by the opponent-process theory of color vision, which proposes a different mechanism of color detection from the one set forth in the trichromatic theory. According to the opponent-process theory of color vision, there are four basic colors, which are divided into two pairs of color-sensitive neurons: red–green and blue–yellow. The members of each pair oppose each other. If red is stimulated, green is inhibited; if green is stimulated, red is inhibited. Green and red cannot both be stimulated simultaneously. The same is true for the blue–yellow pair. In addition, black and white act as an opposing pair. Color, then, is sensed and encoded in terms of its proportion of red or green and blue or yellow.

For example, red light evokes a response of RED-YES–GREEN-NO in the red–green opponent pair. Yellow light evokes a response of BLUE-NO–YELLOW-YES. Colors other than red, green, blue, and yellow activate one member of each of these pairs to differing degrees. Purple stimulates the red of the red–green pair plus the blue of the blue–yellow pair. Orange activates red in the red–green pair and yellow in the blue–yellow pair.

Afterimages can be explained when the opponent-process theory is combined with the general principle of sensory adaptation (Jameson & Hurvich, 1989). If you stare continuously at one color, sensory adaptation eventually occurs and your visual receptors become less sensitive to that color. What happens when you subsequently stare at a white surface?

If you remember that white light is made up of the wavelengths for all colors, you may be able to predict the result. The receptors for the original color have adapted to the constant stimulation and are temporarily “off duty.” Thus, they do not respond to that color. Instead, only the receptors for the opposing color will be activated, and you perceive the wavelength of only the opposing color. For example, if you stare at a patch of green, your green receptors eventually become “tired.” The wavelengths for both green and red light are reflected by the white surface, but since the green receptors are “off,” only the red receptors are activated. Staring at the green, black, and yellow flag in Figure 3.6 should have produced an afterimage of opposing colors: a red, white, and blue American flag.

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An Integrated Explanation of Color Vision At the beginning of this section, we said that current research has shown that both the trichromatic theory and the opponent-process theory of color vision are accurate. How can both theories be right? It turns out that each theory correctly describes color vision at a different level of visual processing.

As described by the trichromatic theory, the cones of the retina do indeed respond to and encode color in terms of red, green, and blue. But recall that signals from the cones and rods are partially processed in the ganglion cells before being transmitted along the optic nerve to the brain. Researchers now believe that an additional level of color processing takes place in the ganglion cells (Demb & Brainard, 2010).

As described by the opponent-process theory, the ganglion cells respond to and encode color in terms of opposing pairs (DeValois & DeValois, 1975; Solomon & Lennie, 2007). In the brain, the thalamus and visual cortex also encode color in terms of opponent pairs. Consequently, both theories contribute to our understanding of the process of color vision. Each theory simply describes color vision at a different stage of visual processing (Hubel, 1995; Werner & others, 2007).