made up of a single wavelength and are therefore not seen as a pure hue. Rather, they are made up of light waves of more than one wavelength. The more wavelengths in a light, the less saturated (or pure) its hue is.
The Eye: How Does It Work?
The eye is an almost perfect sphere composed of two fl uid-fi lled chambers. Light passes through the clear cornea into the fi rst chamber. At the back of this cham- ber, the colored iris opens and closes to regulate how much light passes through the pupil into the lens . The lens is held in place by ligaments attached to the ciliary muscle . This muscle focuses images by controlling the thickness of the lens, so that a clear image falls onto the light-sensitive retina at the back of the second chamber (see fi gure 5.2 ). When the ciliary muscle is uncontracted, the tension of the ligaments stretches the lens relatively fl at. When the ciliary muscle contracts, it lessens the ten- sion of the ligaments and the lens thickens. The lens must be thickened to focus on close objects; that is why reading for long periods—which involves prolonged con- traction of the ciliary muscle—makes your eyes feel tired. When the shape of the eye is not spherical, the lens cannot focus light on the retina, resulting in near sightedness or far sightedness.
The real business of transducing light waves is carried out in the retina by two types of receptor cells named the rods and the cones because of their shapes (see fi gure 5.3 ). The cones are far less numerous than the rods—about 6 million cones compared with 125 million rods in each eye (Pugh, 1988; Solomon & Lennie, 2007).
Cones are concentrated in the center of the retina, with the greatest concentration at a cen- tral spot called the fovea . In good light, visual acuity (the clarity and sharpness of vision) is best for images that are focused directly on the fovea, partly because of the high concentration of cones (Rossi & Roorda, 2010).
The rods are located throughout the retina, except in the center (the fovea). Their role in vision differs from that of the cones in four main ways. First, because of their location, they are largely responsible for peripheral vision—vision at the top, bottom, and sides of the visual fi eld—whereas the cones play little role in this aspect of vision. Second, the rods are hundreds of times more sensitive to light than the cones. This means that they play a more important role in vision in dim light than do the cones. Third, the rods produce sensa- tions that are perceived with less visual acuity than do cones. This is largely because neurons leading from several rods often converge, so that their impulses are sent to the brain on a single nerve fi ber (shown in fi gure 5.3 ). In contrast, cones more commonly send their messages to the brain along separate nerve fi bers, giving the brain more precise information about the location of the stimulation on the retina.
The fourth difference between the rods and the cones concerns color vision. Both types of receptors respond to variations in light and dark (in terms of the number of receptors that fi re and the frequency with which they fi re), but only the cones can code
cornea (kor ´ ne¯ -ah) Protective coating on the surface of the eye through which light passes.
iris (ı¯ ´ ris) Colored part of the eye behind the cornea that regulates the amount of light that enters.
pupil (pyoo ´ pil) Opening of the iris.
lens Transparent portion of the eye that adjusts to focus light on the retina.
ciliary muscle (sil ´ e¯ -ar´´e) Muscle in the eye that controls the shape of the lens.
Rod Cone
Retina
Cone cell Rod cell
Nerve fibers Surface of retina
Light waves
Impulses to optic nerve
Figure 5.3
Diagram of the microscopic structure of a section of the retina showing the rods and cones and their principal neural interconnections. The blowup shows individual rod and cone cells.
lah35163_ch05_120-161.indd 126
lah35163_ch05_120-161.indd 126 3/16/11 6:37 PM3/16/11 6:37 PM
information about color. Because the rods do not detect color, and because the cones can respond only in bright light, we can see only indistinct forms of black and gray in an almost dark room.
Would you be surprised to learn that you are partially blind in each eye? The spot near the center of the retina where the optic nerve is attached contains no rods or cones. Because there is no visual reception at this point, it is known as the blind spot . We are not normally aware of this blind spot because we “fi ll in” the missing informa- tion during the process of seeing by using information coming in from the other parts of the retina. However, look at fi gure 5.4 for a demonstration of its existence.
Neural messages from the rods and cones are processed in a preliminary way in the neurons of the retina and are then sent to the visual areas of the left and right occipital lobe of the cerebral cortex for interpretation. Recall from chapter 3 that the neural messages from the eyes is transmitted to the visual areas in a complicated fashion. As shown in fi gure 5.5 (p. 128), stimuli that are on your right fall on the left side of each eye. Information from the right visual fi eld of both eyes is sent to the visual area in the occipital lobe of the left visual hemisphere after the optic nerves cross over at the optic chiasm in the brain. Information from stimuli on your left falls on the right side of each eye and is sent to the visual area in the right cerebral hemisphere. It’s a bit confusing when you read about it for the fi rst time, but the brain manages to keep it all straight.
Dark and Light Adaptation
When you walk into a dark movie theater from the daylight, you are “blind” at fi rst;
your eyes can pick up very little visual information. Within about 5 minutes, how- ever, your vision in the darkened room has improved considerably, and very slowly it improves over the next 25 minutes until you can see fairly well. When you exit the theater from the matinee performance, you have the opposite experience. At fi rst the intense light “blinds” you. You squint and block out the light, but in a little while you can see normally again. What is going on? How can you be sighted one moment and blind the next just because the intensity of light has suddenly changed?
The phenomena are called dark adaptation and light adaptation. Here is what happens in the retina during dark adaptation . In a lighted room, the rods and cones are being used frequently, so they are not very sensitive. When we enter darkness, the rods and cones are not sensitive enough to be stimulated by the low-intensity light.
This gives the receptors a “rest,” so they begin to gain sensitivity by making a fresh
retina (ret ´ i-nah) Area at the back of the eye on which images are formed and that contains the rods and cones.
rods The 125 million cells located outside the center of the retina that transduce light waves into neural impulses, thereby coding information about light and dark.
cones The 6 million receptor cells located mostly in the center of the retina that transduce light waves into neural impulses, thereby coding information about light, dark, and color.
fovea (foˉ ´ ve¯ -ah) Central spot of the retina, which contains the greatest concentration of cones.
visual acuity (vizh ´ u-al ah-ku ´ i-te¯ ) Clarity and sharpness of vision.
optic nerve Nerve that carries neural messages about vision to the brain.
blind spot Spot where the optic nerve attaches to the retina; it contains no rods or cones.
optic chiasm Area in the brain where half of the optic nerve fi bers from each eye cross to the opposite side of the brain.
dark adaptation Increased sensitivity of the eye in semidarkness following a reduction in overall illumination.
Blind Spot
Figure 5.4 You can demonstrate to yourself the existence of the blind spot in the following way below: Hold your book at about arm’s length with the word Spot in front of your eyes. Close your right eye and stare at the word Spot. Move the book in slowly until the word Blind disappears. At this point, its image is falling on the spot in the retina where the optic nerve is attached and there are no receptors. We are not normally aware of the existence of this blind spot because we “fi ll in” our perceptions to compensate for the missing information. In this case, we see the dotted line as continuous after the word Blind disappears.
In dim light, the color of this red apple hasn’t changed, but its color appears to fade. Cones in the eye pick up color, but they do not work well in dim light.
supply of the chemicals used in light recep- tion, which have been “bleached out” by the intense light (Wang & others, 2009).
At fi rst, both the rods and the cones are recovering their sensitivity, so improvement is fairly rapid. But the cones become fully sensitive (remember, they are not very sen- sitive in weak light) within about 5 minutes, so the rate of improvement slows after that.
The rods continue to improve in sensitivity slowly, reaching a level of sensitivity to light that is an amazing 100,000 times greater than in bright illumination after about 30 minutes in the dark.
In light adaptation, eyes that have been in the dark for a while become very sensi- tive to light, partly because they have built up a full supply of chemicals used in light reception. When we are suddenly exposed to intense light, the rods and cones are highly responsive and, in essence, “overload” the visual circuits. It’s not until the intense light has had a chance to reduce the sensitivity of the receptors—partly by bleaching out some of the receptor chemicals—that we can see comfortably again. Fortunately, this process takes place in about a minute.
Color Vision
Energy of any wavelength within the spec- trum of visible light evokes a sensation of color when it stimulates the human visual system. But light energy is just that—energy;
it has no color of its own. Color is the experi- ence that results from the processing of light energy by the eye and nervous system.
It’s obviously useful to be able to discriminate among different wavelengths of light: blueberries are ready to be eaten; green berries are not. A blueberry refl ects the wavelength of light that we perceive as blue and absorbs the rest, but how does the human visual system produce the sensation of blue? It has taken psychologists and other scientists more than 150 years to reach the current understanding of the complex mechanisms of color vision.
In the early 1800s, Thomas Young and Hermann von Helmholz observed that any color can be created by shining different combinations of the wavelengths of light for red, blue, and green on a single spot. For example, as illustrated in fi gure 5.6 , the com- bination of red and green light produces yellow. Based on this observation, Young and Helmholz guessed that there are three kinds of cones in the retina that respond mostly to light in either the red, green, or blue range of wavelengths. Their theory is referred to as the trichromatic theory of color vision. According to this theory, all sensations of colors result from different levels of stimulation of the red, green, and blue recep- tors in the retina.
Over the years, many types of studies have confi rmed that there are indeed three kinds of cones (Solomon & Lennie, 2007; Mancuso & others, 2009). As shown in
light adaptation Regaining sensitivity of the eye to bright light following an increase in overall illumination.
Left eye Left visual
field
Right visual field
Right eye
Optic nerve
Visual cortex Optic chiasm Optic tract
Optic nerve
Visual cortex Optic tract
Figure 5.5
Images of objects in the right visual fi eld are focused on the left side of each retina, and images of objects in the left visual fi eld are focused on the right side of each retina. This information is conveyed along the optic nerves to the optic chiasm and the thalamus. The thalamus then relays the information to the visual cortex of the occipital lobes. Note that images of objects in the right visual fi eld are processed by the left occipital lobe and images of objects in the left visual fi eld are processed by the right occipital lobe.
lah35163_ch05_120-161.indd 128
lah35163_ch05_120-161.indd 128 3/16/11 6:37 PM3/16/11 6:37 PM
fi gure 5.7 , each kind of cone contains pigments that mostly absorb light of the wave- lengths that correspond to red, green, and blue. Does that mean that the trichromatic theory of Young and Helmholz was correct? Yes and no. Color vision is fascinatingly complex.
Soon after Young and Helmholz stated the trichromatic theory, other scientists pointed out that it could not explain the intriguing phenomenon of color afterimages:
If you stare at a patch of color for a while and then shift your eyes to a white surface, you will see a ghostly afterimage of the patch in the color that is complementary to that of the original patch. 1 For example, stare intensely for about 30 seconds at the white dot in the center of the word red that is printed in the color red in fi gure 5.8 . Then
1 Note that complementary colors are different to artists. In art, a complementary color of a primary color (red, blue, and yellow) is the one you get when you mix the other two primary colors. Hence, orange is the complement of blue because mixing yellow and red yields orange. Don’t be confused by these two different meanings of the term complementary color.
trichromatic theory (trı¯ ´´kro¯ -mat´ik) Theory of color vision contending that the eye has three different kinds of cones, each of which responds to light of one range of wavelength.
Figure 5.6
The trichromatic theory of color vision is based on the observation that all colors can be produced by various combinations of red, blue, and green light (and that all three lights together create white).
Wavelength of light (nanometers)
Red Yellow
Green Violet
700 600
500 400
Amount of responsiveness of receptors
Figure 5.7
Our ability to see color is partly based on the fact that three kinds of cones contain pigments that respond mostly to light in the wavelengths for blue, green, and red. Note, however, that each type of cone also responds to other nearby wavelengths of light.
This means, for example, that light in the yellow range stimulates both the red and the green cone receptors, but not as strongly as light in the red or green ranges, respectively.
Figure 5.8
Stimulus used in the demonstration of afterimages. Stare at the white dot in the center of the word red for 30 seconds. Then look at the white space above the word. What do you see?
stare at the blank white space above it. You will see an afterimage of the word red, but it will be green. The same thing occurs for all four of the complementary colors.
The opponent-process theory was developed to explain phenomena that cannot be explained just by the existence of three kinds of cones. The opponent-process the- ory states that there are also two kinds of color-processing mechanisms, which receive messages from the three kinds of cones (see fi gure 5.9 ). These two color-processing mechanisms respond in opposite ways that correspond to the two pairs of complemen- tary colors. For example, suppose you look at a lemon (and light in the yellow range of wavelength refl ects from the lemon onto your retinas). Light of this wavelength stimu- lates both the red and green receptors, but not the blue receptors (see fi gure 5.7 ). The rate of fi ring of the yellow-blue (Y-B) processing mechanism is increased by signals from the red and green receptors but is slowed down by inhibitory signals from the blue receptors. Therefore, light in the yellow wavelengths leads the Y-B mechanism to send a high-frequency message along the visual system to the brain. This signal is the primary information used by the brain to produce the sensation of yellow on the peel of the lemon. However, the rate of fi ring of the red-green (R-G) processing mecha- nism is increased by signals from the red receptors but is slowed by signals from the green receptors. When light in the green wavelengths stimulates the green receptors, the receptors send a strong inhibitory message to the red-green (R-G) opponent mech- anism, which causes it to send low-frequency neural signals to the brain. In similar ways, combinations of signals from the opponent-processing mechanisms supply the brain with the information necessary for all color sensations.
Note that opponent-process theory explains the phenomenon of color afterimages, because the R-G and Y-B mechanisms cannot signal both of their opponent colors at the same time. Staring at the red stimulus reduces the sensitivity of the red receptors (through sensory adaptation), leading the R-G system to send a signal to the brain that is interpreted as the sensation of green. Perhaps the strongest evidence for the opponent-process theory, however, is that we now know that neurons throughout the visual system—from the retina to the cerebral cortex—respond to light in an opponent- process fashion (Engle, 1999; Solomon & Lennie, 2007). For example, if a specifi c neuron in the retina is excited by green light, then it is inhibited from fi ring by red light.
Thus, the trichromatic theory accurately describes events very well at the fi rst level of neurons in the visual system—the cones within the retina—but the opponent- process theory best describes the activities of neurons in the rest of the visual system.
In other words, they are both correct when combined (Gegenfurtner, 2003).
Color Blindness. Few people are completely unable to distinguish among all col- ors, but partial color blindness affects about 8% of males and 1% of females. Most people with partial color blindness have diffi culty distinguishing only between two
opponent-process theory Theory of color vision contending that the visual system has two kinds of color processors, which respond to light in either the red-green or yellow-blue ranges of wavelength.
Fast Slow Slow Fast
Three kinds of cones
Opponent-process mechanisms
Neurons to brain
Blue Green Red
Green Red Yellow Blue
– + –
+ +
Figure 5.9
The modern theory of color vision combines the three kinds of cone color receptors of trichromatic theory with the two processing mechanisms of opponent-process theory. When light in the red wavelengths stimulates the red receptors, the receptors send a strong excitatory message to the red-green (R-G) opponent mechanism, which stimulates rapid fi ring of neural signals to the brain. Light in the yellow wavelengths stimulates both the red and the green receptors somewhat. They both send weak excitatory messages to the yellow- blue (Y-B) opponent mechanism, but together they are strong enough to stimulate it to send fast signals to the brain. When light is in the blue wavelengths, it stimulates the blue receptors, which send strong inhibitory messages to the Y-B opponent mechanism, leading it to fi re slowly. Light in the green wavelengths leads to slow fi ring of the R-G opponent mechanism in a similar fashion.
lah35163_ch05_120-161.indd 130
lah35163_ch05_120-161.indd 130 3/16/11 6:37 PM3/16/11 6:37 PM