96 FIGURE Chapter 3 Sensation and Perception 3.10 Individual Differences in Cone Types These photographs show that people can differ widely from one another in the distribution of blue, green, and red cones in their retinas (Roorda & Williams, 1999). J.W., whose retina is shown in Part A, has an especially high population of red cones, whereas green cones predominate in A.N., whose retina is shown in Part B. Both have normal color vision, but J.W. will be somewhat more sensitive to long wavelengths of light, whereas A.N. will be somewhat more sensitive to light of medium wavelengths. (A) (B) The Opponent-Process Theory of Color Vision Although essentially correct, the trichromatic theory cannot explain some aspects of color vision, such as afterimages. To see an afterimage, stare at the black dot in the ﬂag in Figure 3.11 for thirty seconds, and then look at the black dot in the white space below it. What was yellow in the original image will be blue in the afterimage. What was green before will appear red, and what was black will now appear white. This type of observation led Ewald Hering to offer another theory of color vision, called the opponent-process theory. Hering suggested that color-sensitive visual elements in the eye are arranged into three kinds of pairs and that the members of each pair oppose, or inhibit, each other. Each element signals one color or the other (red or green, blue or yellow, black or white), but never both. This theory explains color afterimages. When one member of an opponent pair is no longer stimulated, the other is activated. So, in Figure 3.11, if the original image you look at is green, the afterimage will be red. Summing Up Together, the trichromatic and opponent-process theories encompass FIGURE 3.11 Afterimages Produced by the Opponent-Process Nature of Color Vision Stare at the black dot in the ﬂag for at least thirty seconds, by and then focus on the dot in the white space below it. The afterimage you will see can be explained by the opponent process theory of color vision. What colors appeared in the afterimage you saw? doing 2 learn most of what we now know about the complex process of color vision. We see color because our three types of cones have different sensitivities to different wavelengths. We sense different colors when the three cone types are stimulated in different ratios. Because there are three types of cones, any color can be produced by mixing three pure wavelengths of light. But there is more to it than that. The cones connect to ganglion cells containing pairs of opposing elements that respond to different colors and inhibit each other. This arrangement provides the basis for afterimages. Therefore, the trichromatic theory explains color vision as it relates to rods and cones, whereas the opponentprocess theory explains color vision as it relates to the ganglion cells. Both theories are needed to account for the complexity of our visual sensations of color. (“In Review: Seeing” summarizes our discussion of vision.) F e a t u r e Ty p e : I n R e v i e w Colorblindness opponent-process theory A theory of color vision stating that the visual elements sensitive to color are grouped into red-green, blue-yellow, and blackwhite pairs. Cones normally contain three kinds of chemicals, each of which responds best to a particular wavelength of light. People who have cones containing only two of these three color-sensitive chemicals are described as colorblind (Carroll et al., 2004). They are not really blind to all color, but they discriminate fewer colors than do other people, as Figure 3.12 shows. Red-green colorblindness, for example, means that reds and greens look the same brownish gray color. Colorblindness is more common in men than in women. 97 in review Seeing Online Study Center Improve Your Grade Tutorial: Opponent Processes SEEING Aspect of Sensory System Elements Key Characteristics Energy Visible light: electromagnetic radiation with a wavelength of about 400 nm to about 750 nm The intensity, wavelength, and complexity of light waves determine the brightness, hue, and saturation of visual sensations. Accessory structures of the eye Cornea, pupil, iris, lens Light rays are bent to focus on the retina. Conversion of visual stimuli to neural activity Photoreceptors (rods and cones) in the retina Rods are more sensitive to light than cones, but cones discriminate among colors. Sensations of color depend ﬁrst on the cones, which respond differently to different light wavelengths, and then on processing by ganglion cells. Pathway to the brain Optic nerve to optic chiasm to thalamus to primary visual cortex Neurons in the brain respond to particular aspects of the visual stimulus, such as shape. ? FIGURE 3.12 Are You Colorblind? At the upper left is a photo as it appears to people whose by cones have all three types of color-sensitive chemicals. The other photos simulate how colors appear to people who are missing chemicals for short wavelengths (lower left), long wavelengths (upper right), or medium wavelengths (lower right). If any of these photos look to you just like the one at the upper left, you may have a form of colorblindness. doing 2 learn 1. The ability to see in very dim light depends on photoreceptors called 2. Color afterimages are best explained by the theory of color vision. 3. Nearsightedness and farsightedness occur when images are not focused on the eye’s . .