Book T of C
Chap T of C
This is the 2007 version. Click here for the 2017 chapter 04 table of contents.
In 1802, Thomas Young proposed that all human vision occurred through the combination of sensitivity to red, green, and blue. This theory, modified by Hermann von Helmholtz in 1852, came to be known as the Young-Helmholtz or trichromatic (three-color) theory of color vision. The basic idea was that the eye responded to three primary colors, and combining the three primary colors of additive color mixing formed all the other colors.
The finding that there are three types of color-sensitive cone receptors in the retina supported the three-color theory. One set of receptors is sensitive to long wavelengths such as red, one to medium wavelengths such as green, and one is sensitive to short wavelengths such as blue.
What is the trichromatic theory?
So there is some truth to the three-color theory. However, other aspects of color vision cannot be accounted for by the trichromatic theory. For example, there is the phenomenon of color afterimages. If you stare at a red dot, then move your gaze to a white wall, you will see a green dot as an afterimage. If you stare at a green dot, you will see a red afterimage. The same thing happens with yellow and blue.
What is the opponent-process theory? How are color afterimages explained?
Based on the existence of color afterimages, Ewald Hering proposed the opponent process theory of color vision in 1878. He suggested that color vision occurred in three channels where "opposite" colors (called complementary colors) are in a form of competition. For example, red and green are complementary colors. When you stare at something red, your redness detectors are worn out or fatigued. Their opponents, the green receptors, gain the upper hand, and you see a green afterimage after staring at a red dot.
What are the three vision channels?
The modern form of this theory assumes there are three basic channels for vision. One channel is the red/green channel; another is the yellow/blue channel. A third channel, the black/white or brightness/darkness channel, may also provide information relevant to color vision, but that is a complex issue being debated among researchers.
What activity in color receptors does the brain interpret as "yellow"?
The yellow/blue channel may seem odd, because there are no yellow-sensitive cones in the retina. Yellow light stimulates a combination of long-wavelength (red-sensitive) and medium wavelength (green-sensitive) cones. If there is more activity in blue receptors (compared to red plus green receptors) the brain interprets this as blue. If there is more red plus green activity (as compared to blue) the brain interprets this as yellow. The result is a yellow/blue channel. Yellow and blue act as opponent processes just like red and green. If you stare at a blue image, you get a yellow afterimage; if you stare at a yellow dot, you get a blue afterimage.
What is the most common type of colorblindness? Why is it more common in men?
Color-blind people usually are missing one or more cone types: red-sensitive, green-sensitive, or blue sensitive. The result is a disorder in one or both color channels. The most common type of colorblindness is red/green colorblindness. Genetic studies show this type of color-blindness is usually caused by a defective gene on the X chromosome. If this gene is defective, women (having two X chromosomes) are "protected" by a duplicate copy of the gene on the other X chromosome. Males (having one X and one Y chromosome) do not have the extra copy, so red/green colorblindness is about 20 times more common in men than in women.
What is a monochromat, dichromat or trichromat?
A person with no color-sensitive pigments, therefore no color vision, is called a monochromat (one-color person). To such a person, the world looks like a black-and-white TV picture. Colors are shades of gray. A person with a defect in one channel-either the red/green or yellow/blue channel-is called a dichromat. Both colors in a channel are affected, so if the person cannot distinguish red that same person cannot distinguish green. A person who cannot see blue as a distinct color will also not see yellow as a distinct color. People with normal color vision use all three channels (black/ white, red/green, and yellow/blue) and are called trichromats.
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