LO 4 Explain how electromagnetic energy is transduced into a sensation of vision.

When you look at a stop sign, would you believe you are sensing light waves bouncing off the stop sign and into your eyes? The eyes do not sense faces, objects, or scenery. They detect light. Remember, if you don’t have light, you don’t have sight. But what exactly is light? Light is an electromagnetic energy wave, composed of fluctuating electric and magnetic fields zooming from place to place at a very fast rate. And when we say “fast,” we mean from Atlanta to Los Angeles in a tenth of a second. Electromagnetic energy waves are everywhere all the time, zipping past your head, and bouncing off your nose. As you can see in Figure 3.1, light that is visible to humans falls along a spectrum, or range, of electromagnetic energy.

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WAVELENGTH The various types of electromagnetic energy can be distinguished by their wavelength, which is the distance from one wave hump to the next (like the distance between the crests of waves rolling in the ocean; Figure 3.1). Gamma waves have short wavelengths and are located on the far left of the spectrum. At the opposite extreme (far right of the spectrum) are the long radio waves. The light humans can see falls in the middle of the spectrum, measuring between 400 and 700 nanometers (nm) or billionths of a meter (Brown & Wald, 1964). Wavelength also plays an important role in determining the colors humans and animals can detect.

Although dogs can only see the world in blues, yellows, and grays (Coren, 2008, October 20), primates, including humans, can detect a wider spectrum of colors, including reds and oranges. This ability to see reds and oranges may be an adaptation to spot ripe fruits against the green backdrop of tree leaves (Rowe, 2002). Other creatures can see “colors” that we can’t. Snakes can detect infrared waves radiating off the bodies of their prey, and birds size up potential mates using the ultraviolet waves reflected by feathers (Bennett, Cuthill, Partridge, & Maier, 1996; Gracheva et al., 2010).

FEATURES OF COLOR The colors you see result from light reflecting off of objects and reaching your eyes. Every color can be described according to three factors: hue, brightness, and saturation. The first factor, hue, is what we commonly refer to as “color” (blue jeans have a blue hue). Hue is determined by the wavelength reflecting off of an object: Violet has the shortest wavelength in the visible spectrum (400 nm), and red has the longest (700 nm). The brightness of a color represents a continuum from bright to dim, or the intensity of the color. Brightness depends on wave height, or amplitude, the distance from midpoint to peak (or from midpoint to trough; Figure 3.1). Just remember, the taller the height, the brighter the light. Saturation, or color purity, is determined by uniformity of wavelength. Saturated colors are made up of same-size wavelengths. Objects we see as pure violet, for instance, are reflecting only 400-nm light waves. We can “pollute” the violet light by mixing it with other wavelengths and the result will be a less saturated, pale lavender. Most colors in the environment are not pure. The red pigments in Kerry Washington’s red lipstick probably reflect a mixture of waves in the 600–700 nm range, rather than pure 700-nm red. Combinations of these three basic features—hue, brightness, and saturation—can produce an infinite number of colors; our eyes are just not sensitive enough to tell them all apart. Amazingly, though, the average person sees some 2.3 million colors (Linhares, Pinto, & Nascimento, 2008).

PERCEPTION OF COLORS And now, the 2.3 million-dollar question: Most of us agree that stop signs are red and taxicabs are yellow, but can we be sure that one person’s perception of color is identical to another’s? Is it possible that your “red” may be someone else’s “yellow”? Without getting inside the mind of another person and perceiving the world as he does, this question is very difficult to answer. The colors we perceive do not actually exist in the world around us; they are the product of what is in our environment, including properties of the object and the wavelength of the light it reflects, and the brain’s interpretation of that light. When these factors change, so too do the colors we see.

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Your three perceptions of this same yellow object will be very different—not because the object itself has changed, but rather because the light shining upon it (and thus, the light it reflects) has changed. Color perception results from the interaction between light and brain activity. The eyes take light energy and transform it into neural code for the brain to interpret. The exquisite specificity of this code allows us to distinguish between electric blue and cobalt, lime green and chartreuse. All this is made possible by the eye’s remarkable biology.

The human eye is nothing short of an engineering marvel (Infographic 3.2). When you look in the mirror, you see only a fraction of each ping-pong-sized eyeball; the rest is hidden inside your head. Let’s explore this biological wonder, starting from the outside and working our way toward the brain.

THE CORNEA The surface of the eye looks wet and glassy. This clear outer layer over the colored portion of the eye is called the cornea, and it has two important jobs: (1) shielding the eye from damage by dust, bacteria, or even a poke, and (2) focusing incoming light waves. About 65–75% of the focusing ability of the eye comes from the cornea, which is why imperfections in its shape can lead to blurred vision (National Eye Institute, 2012). Corneal flaws are primarily to blame for so many people (about a third of the American population) needing glasses or contact lenses. LASIK eye surgery, a popular alternative to corrective lenses, uses a laser to reshape the cornea so that it can focus light properly.

THE IRIS AND THE PUPIL Directly behind the cornea is the donut-shaped iris. When you say someone has velvety brown eyes, you are really talking about the color of her irises. The black hole in the center of the iris is called the pupil. In dim lighting, the muscles of the iris relax, widening the pupil to allow more light inside the eye. In bright sunlight, the iris muscles squeeze, constricting the pupil to limit the amount of light. Studies show that men are more enticed by women with enlarged pupils (Hess, 1975; Tombs & Silverman, 2004). When looking at someone she finds attractive, a woman’s pupils dilate—especially during the most fertile time of her monthly cycle (Laeng & Falkenberg, 2007).

THE LENS AND ACCOMMODATION Behind the pupil is the lens, a tough, transparent structure that is similar in size and shape to an “M&M’s candy” (Mayo Clinic, 2014). Like the cornea, the lens specializes in focusing incoming light, but it can also change shape in order to adjust to images near and far, a process called accommodation. If you take your eyes off this page and look across the room, faraway objects immediately come into focus because your lens changes shape. As we age, the lens begins to stiffen, impairing our ability to focus on up-close images like the words you are reading right now. You can tell if a person has this condition, called presbyopia, when she strains to read a book or restaurant menu at arm’s length. Most people develop some degree of presbyopia between 40 and 65 years of age (Mayo Clinic, 2014).

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After passing through the cornea, pupil, and lens, light waves travel through the eyeball’s jellylike filling and land on the retina, a carpet of neurons covering the back wall of the eye. This area was the source of the triplets’ visual impairments. Emma, Zoe, and Sophie all suffered from retinopathy of prematurity (ROP), a condition in which blood vessels in the retina grow incorrectly (Chawla et al., 2012). The last 3 months of pregnancy are critical for fetal eye development, when webs of blood vessels rapidly branch from the center of the retina outward, delivering crucial oxygen and nutrients to the developing tissue. After the triplets’ very early birth, these vessels began to branch abnormally, eventually pulling the retina from the back of the eye.

The retina is responsible for the transduction of light energy into neural activity; that is, sensing light and relaying a message to the brain. Without the retina, vision is impossible.

LO 5 Describe the function of rods and cones.

PHOTORECEPTORS AND OTHER NEURONS The retina is home to millions of specialized neurons called photoreceptors, which absorb light energy and turn it into chemical and electrical signals for the brain to process. Two types of photoreceptors are located along the back of the retina, rods and cones, which get their names from their characteristic shapes. Rods are extremely sensitive, firing in response to even a single photon, the smallest possible packet of light (Rieke & Baylor, 1998). If rods were all we had, the world would look something like an old black-and-white movie. Cones enable us to enjoy a visual experience more akin to HDTV (except for those of us with color deficiencies, which we will discuss later). In addition to providing color vision, cones allow us to see fine details, such as the small print on the back of a shampoo bottle.

The rods and cones are just the first step in a complex neural signaling cascade that ultimately leads to a visual experience in the brain (see Infographic 3.2 on page 97). Near the rods and cones are bipolar cells, another specialized type of neuron, located approximately in the middle of the retina. When a rod or cone is stimulated by light energy, it conveys its signal to nearby bipolar cells. These in turn convey their signal to ganglion cells, yet another type of neuron, located toward the front of the retina. Axons of the ganglion cells bundle together in the optic nerve, which is like an electrical cable (one extending from each eye) hooking the retina to the brain. The optic nerve exits the retina at the optic disc, causing a blind spot, since this area lacks rods and cones. You can find your blind spot by following the instructions in the Try This.

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THE FOVEA The retina in each eye is home to some 120 million rods and 6 million cones (Amesbury & Schallhorn, 2003). Rods are found everywhere in the retina, except in the optic disc (mentioned earlier) and a tiny central spot called the fovea. Cones are packed most densely in the fovea, but are also sprinkled through the rest of the retina. When you need to study something in precise detail (like the tiny serial number on the back of a computer), hold it under a bright light and stare at it straight-on. The cones in the fovea excel at sensing detail and operate best in ample light. If, however, you want to get a look at something in dim light, focus your gaze slightly to its side, stimulating the super-light-sensitive rods outside the fovea.

The eye has an amazing ability to adjust to drastic fluctuations in light levels. This process starts with the pupil, which rapidly shrinks and expands in response to light changes, and then continues with the rods and cones, which need more time to adjust to changes in lighting. When you walk into a dark movie theater after being outside in the bright sun, you can barely see an inch in front of your face. After a few minutes, your eyes start to adjust to the dark in a process called dark adaptation, which takes about 8 minutes for cones and 30 minutes for rods (Hecht & Mandelbaum, 1938; Klaver, Wolfs, Vingerling, Hoffman, & de Jong, 1998). Cones respond more quickly, so they are more useful in the first few minutes of dark adaptation. Then the rods kick into action, allowing you to make out silhouettes of objects and people. Why are these photoreceptors so sluggish in their responses to darkness? To restore their sensitivity to light, rods and cones must undergo a chemical change associated with protein molecules, and this takes time (Caruso, 2007, August 13; Ludel, 1978). Also keep in mind that, for most of human evolution, dark adaptation meant adjusting to the gradual setting of the sun (Caruso, 2007, August 13).

When you leave the dark theater and return to the blinding light of day, the eyes also adjust. With light adaptation, the pupil constricts to reduce the amount of light flooding the retina, and the rods and cones become less sensitive to light. Light adaptation occurs relatively quickly, lasting at most 10 minutes (Ludel, 1978).

Let’s stop for a moment and examine how information flows through the visual pathway (Infographic 3.2 on page 97). Suppose you are looking at Oprah Winfrey’s face. Remember, you’re actually seeing the light that her face reflects. Normally, light rays bouncing off Oprah would continue moving along a straight-line path, but they encounter the bulging curvature of the cornea covering your pupil, which bends them. (The light is further bent by the lens, but to a lesser extent.) Rays entering the top of the cornea bend downward and those striking the base bend upward. The result is an inverted, or flip-flopped, projection on your retina. It’s like your eye is a movie theater, the retina is its screen, and the feature film being played is Your World, Turned Upside Down (Kornheiser, 1976; Stratton, 1896). The neurons of the retina respond to the stimulus; signals are sent from the photoreceptor cells to the bipolar cells, which then signal the ganglion cells that bundle into the optic nerves.

The optic nerves (one from each eye) intersect at a place in the brain called the optic chiasm. From there, information coming from each eye gets split, with about half traveling to the same-side thalamus and half going to the opposite-side thalamus. Interneurons then shuttle the data to the visual cortex, located in the occipital lobes in the back of your head. Neurons in the visual cortex called feature detectors specialize in detecting specific features of your visual experience, such as angles, lines, and movements. How these features are pieced together into a unified visual experience (I see Oprah!) is complex. Hubel and Wiesel (1979) proposed that visual processing begins in the visual cortex, where teams of cells respond to specifically oriented lines (as opposed to just pixel-like spots of light), and then continues in other parts of the cortex, where information from both eyes is integrated. Scientists have now identified at least 30 different areas in the brains of humans and other primates that play a role in visual processing (Ramachandran & Rogers-Ramachandran, 2009).

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Although we have discussed various features of color, we have not yet explained how waves of electromagnetic energy result in our perception of colors. How does the brain know red from maroon, green from turquoise, yellow from amber? Two main theories explain human color vision—the trichromatic theory and the opponent-process theory—and you need to understand both because they address different aspects of the phenomenon.

LO 6 Compare and contrast the theories of color vision.

THE TRICHROMATIC THEORY Proposed in the 1800s by an English physician–scientist, Thomas Young (1773–1829), and expanded upon decades later by Hermann von Helmholtz (1821–1894), a Prussian physicist, the trichromatic theory (trī-krō-ˈma-tik) suggests that we have three types of cones. Red cones are excited by electromagnetic energy with wavelengths in the red range (about 620–700 nm); green cones fire in response to electromagnetic energy with wavelengths in the green realm (500–575 nm); and blue cones are activated by electromagnetic energy with wavelengths corresponding to blues (about 450–490 nm; Mollon, 1982).

So how is it that we can detect millions of colors when our cones are only sensitive to red, green, and blue? The primary colors of light are also red, green, and blue, and when mixed together in equal proportions, they appear as white light (see the photograph to the left). According to the trichromatic theory, the brain identifies a precise hue by calculating patterns of excitement among the three cone populations. When you look at a yellow banana, for example, both the red and green cones fire, but not the blue ones. The brain interprets this pattern of red and green activation as “yellow.” And because white light is actually a mixture of all the wavelengths in the visible spectrum, it excites all three cone types, creating a sensation of “white” in the mind’s eye. Thus, it is the relative activity of the three types of cones that the brain uses to make its color calculations.

COLOR DEFICIENCY AND COLOR BLINDNESS Loss or damage to one or more of the cone types leads to color deficiency, more commonly known as “color blindness.” These terms are often used interchangeably, but true color blindness is extremely rare. Sometimes color blindness is accompanied by extreme sensitivity to light and poor vision for detail, both resulting from deficient or missing cones (Tränkner et al., 2004). The condition associated with red–green color defects occurs in about 8% of men and less than 1% of women with European ancestors (Deeb, 2005). Most people with color deficiencies have trouble distinguishing between red and green because they are missing or deficient in the red or green receptors, but they can see other colors very well. If you can’t see the number shown in the Ishihara color plate to the right, then you may have a red–green deficiency (Wong, 2011).

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Ample research backs up the trichromatic theory, but there are some color-related phenomena it cannot explain. A prime example is the afterimage effect. An afterimage is an image that appears to linger in your visual field after its stimulus, or source, is gone.

OPPONENT-PROCESS THEORY German physiologist Ewald Hering (1834 –1918) realized that the trichromatic theory could not explain the visual phenomenon of the afterimage effect and thus developed the opponent-process theory of color vision. Hering proposed that in addition to the color-sensitive cones, a special group of neurons responds to opponent colors—pairs of colors such as red–green and blue–yellow that cannot be perceived simultaneously (there is no such thing as reddish-green or bluish-yellow light). One neuron in an opponent pair fires when you look at red, but is inactive when you see green. The other neuron gets excited by green but turned off by red. If you spend enough time staring at the red and blue picture in the Try This above, the neurons excited by these colors become exhausted and stop responding. When you shift your gaze to the white surface, the opponent neurons fire in response to the green and yellow wavelengths. Meanwhile, the red and blue neurons are too fatigued to fire. You end up seeing a green and yellow afterimage. Research has provided strong support for the opponent-process theory, identifying particular types of neurons in a region of the thalamus (DeValois & DeValois, 1975; Jameson & Hurvich, 1989).

As it turns out, we need both the trichromatic and opponent-process theories to clarify different aspects of color vision. Color perception occurs both in the light-sensing cones in the retina and in the opponent cells serving the brain. The ability to perceive color is not based on processing at a single point along the visual pathway, or even one area of the brain (Gegenfurtner, 2003; Solomon & Lennie, 2007).

Vision is a highly complex—and fragile—sensory system. The same could be said of hearing, the subject of our next section. Let’s find out how the auditory system takes sound waves from the environment, and turns them into perceptions of clapping thunder, buzzing mosquitos, and joyful giggles.

Question 1

Question 2

1. opponent-processing

2. photoreceptors

3. fovea

4. feature detectors

Question 3

Question 4

1. presbyopia.

2. optic disc.

3. cones.

4. rods.