How do our eyes and brain allow us to perceive colors? Many scientists have asked this question and two theories have been written to explain the process: The Trichromatic Theory of Vision and the Opponent Process Theory of Vision. Both explain how color is perceived and both are accepted theories. The difference is what part of vision you are describing. Understanding the differences will help you understand why there are two theories and both are correct.
Trichromatic Theory of Vision
Doctor Thomas Young proposed this theory in 1801. The proposal said that the eye could perceive light by three color-sensitive receptors arrayed along the inner wall of the eye (the retina.) Later research by other scientists discovered that these receptors were part of the cells known as cones. The cones are sensitive to three different wavelengths of light: Red, Green and Blue.
As light enters the eye and impacts the retina, the cones that are sensitive to a specific color will send a signal to the brain. When all three signals are received the brain perceives white. If a signal from only one type of cone is received then the corresponding color will be seen. Other colors are combinations of the three primary colors.
This theory describes how color can be perceived by the functions of cells in the eye, but does not explain how the brain translates these signals.
Opponent Process Theory of Vision
Ewald Hering proposed this theory in 1892. His proposal said that red, yellow, green and blue were the primary colors that the human brain could perceive. He also explained that they were perceived to oppose each other respectively. This means that the brain can see red or green; or yellow or blue, but not both at the same time.
This theory also explains the after effects of color. After effects are when one signal is sent strongly against another signal (such as red over green) and then taken away. The result is that the opposite color becomes more perceptible. To see how this works, look at the white dot inside the bright red image via the picture as below for 30 seconds then look at the white area to the right and blink. The opposing color (green) should be more perceptible.
This theory describes how our brains interpret data to perceive colors, but does not explain how those color signals are sent to the brain.
Both the theories deal with how color vision is achieved, however, they are not the same. What makes these two theories different is where the process is taking place. In Trichromatic theory of vision, the process takes place on the cellular level inside the eye. In Opponent Process theory of vision, the process takes place in the brain. Both are correct as they describe how the process works on two completely different levels.
The ways the theories have been tested are different as well. The Trichromatic Theory was tested by Helmholtz in a color-matching experiment. Subjects would attempt to match a color by altering the amounts of the three wavelengths of light. It was discovered that the subjects found it impossible to match the colors if they only used two wavelengths but could match any color if they used three.
The Opponent Process Theory was tested through the use of Afterimages. If a subject was to stare at a red square for roughly a minute and then looks at a white surface a green after image would be seen. Another test used was devised by Leo Hurvitch and Dorothea Jameson called Hue Cancellation. If one was to mix green and red light together yellow would be produced as opposed to a reddish-green. If one was also to mix yellow and blue light together white light would be produced instead of a bluish-yellow.
Another difference is the cause of each of the theories. The Trichromatic’s causes are the receptors that receive the light. The three different types of Cone Receptors depend on the wavelength of the light. There are Short-wavelength Cone Receptors, Mid-wavelength Cone Receptors and Long-Wavelength Cone Receptors. These cones are found behind the retina and have a different levels of absorption because of the amounts of opsin amino acids in the cones. This has been discovered by researchers.
To make it even simpler, Trichromaticism is how the eye perceives color and Opponent Process is how the brain perceives it. And now you can astound your friends with your knowledge of vision without confusing them with too much technical jargon.