The Visual System: Learn It 3—Color Vision

Color Vision

Normal-sighted individuals have three different types of cones that mediate color vision. Each of these cone types is maximally sensitive to a slightly different wavelength of light.

trichromatic theory of color vision

According to the Young-Helmholtz trichromatic theory of color vision, shown in Figure 1, all colors in the spectrum can be produced by combining red, green, and blue. The three types of cones are each receptive to one of the colors.

A graph is shown with “sensitivity” plotted on the y-axis and “Wavelength” in nanometers plotted along the x-axis with measurements of 400, 500, 600, and 700. Three lines in different colors move from the base to the peak of the y axis, and back to the base. The blue line begins at 400 nm and hits its peak of sensitivity around 455 nanometers, before the sensitivity drops off at roughly the same rate at which it increased, returning to the lowest sensitivity around 530 nm . The green line begins at 400 nm and reaches its peak of sensitivity around 535 nanometers. Its sensitivity then decreases at roughly the same rate at which it increased, returning to the lowest sensitivity around 650 nm. The red line follows the same pattern as the first two, beginning at 400 nm, increasing and decreasing at the same rate, and it hits its height of sensitivity around 580 nanometers. Below this graph is a horizontal bar showing the colors of the visible spectrum.
Figure 1. This figure illustrates the different sensitivities for the three cone types found in a normal-sighted individual. (credit: modification of work by Vanessa Ezekowitz)

Colorblindness: A Story

William, a single father, was preparing for a public event when his 7-year-old daughter told him his clothes didn’t match. Concerned, they sought a second opinion from a nearby convenience store. The store clerk examined William’s attire—a bright green pair of pants, a reddish-orange shirt, and a brown tie—and confirmed, “Your clothes definitely don’t match.”

Prompted by these comments, William consulted friends and coworkers, who diplomatically described his style as “unique.” Realizing something might be off, he visited an eye doctor and discovered he was colorblind, unable to distinguish between certain shades of greens, browns, and reds.

The figure includes three large circles that are made up of smaller circles of varying shades and sizes. Inside each large circle is a number that is made visible only by its different color. The first circle has an orange number 12 in a background of green. The second color has a green number 74 in a background of orange. The third circle has a red and brown number 42 in a background of black and gray.
Figure 2. The Ishihara test evaluates color perception by assessing whether individuals can discern numbers that appear in a circle of dots of varying colors and sizes.

 

Aside from occasional fashion missteps, William’s colorblindness hasn’t significantly affected his daily life. This case highlights how sensory limitations can go unnoticed until a specific situation brings them to light.

Some forms of color deficiency are rare. Seeing in grayscale (only shades of black and white) is extremely rare, and people who do so only have rods, which means they have very low visual acuity and cannot see very well. The most common X-linked inherited abnormality is red-green color blindness (Birch, 2012). Approximately 8% of males with European Caucasian decent, 5% of Asian males, 4% of African males, and less than 2% of indigenous American males, Australian males, and Polynesian males have red-green color deficiency (Birch, 2012). Comparatively, only about 0.4% of females of European Caucasian descent have red-green color deficiency (Birch, 2012).

The trichromatic theory of color vision is not the only theory—another major theory of color vision is known as the opponent-process theory.

opponent-process theory

According to opponent-process theory, color is coded in opponent pairs: black-white, yellow-blue, and green-red. The basic idea is that some cells of the visual system are excited by one of the opponent colors and inhibited by the other. So, a cell that was excited by wavelengths associated with green would be inhibited by wavelengths associated with red, and vice versa.

One of the implications of opponent processing is that we do not experience greenish-reds or yellowish-blues as colors. Another implication is that this leads to the experience of negative afterimages. An afterimage describes the continuation of a visual sensation after removal of the stimulus. For example, when you stare briefly at the sun and then look away from it, you may still perceive a spot of light although the stimulus (the sun) has been removed. When color is involved in the stimulus, the color pairings identified in the opponent-process theory lead to a negative afterimage. You can test this concept using the flag in Figure 3.

An illustration shows a green flag with a thick, black-bordered yellow lines meeting slightly to the left of the center. A small white dot sits within the yellow space in the exact center of the flag.
Figure 3. Stare at the white dot for 30–60 seconds and then move your eyes to a blank piece of white paper. What do you see? This is known as a negative afterimage, and it provides empirical support for the opponent-process theory of color vision.

But these two theories—the trichromatic theory of color vision and the opponent-process theory—are not mutually exclusive. Research has shown that they just apply to different levels of the nervous system. For visual processing on the retina, trichromatic theory applies: the cones are responsive to three different wavelengths that represent red, blue, and green. But once the signal moves past the retina on its way to the brain, the cells respond in a way consistent with opponent-process theory (Land, 1959; Kaiser, 1997).