Adaptation and Visual Encoding - things are not always as they might seem!Focus your eyes on the center X and stare fixedly without moving your eyes and note what happens to the blinking color patches. If done properly, the magenta spots will fade somewhat while being mixed with flashes of green, magenta's complementary color. The longer you stare at the center, the more vivid this phantom color becomes. Try blinking your eyes rapidly or moving your head from side to side and you can interrupt the illusion allowing you to see only the magenta spots again. Believe it, there is no green in this picture! Try putting your mouse pointer on the top title bar above as indicated and hold down the left mouse button. On some browsers the motion will cease while the button is depressed causing the green to vanish - out of mind, out of sight! Repeat several times. Luckily you might see both possible cases, a blank gray area or the complete circle of 12 magenta patches - never the mysterious green spot!!!
The trichromatic theory of human color vision was born in 1802 when Thomas Young proposed the existence of three different kinds of color receptors in the eye. The encoded activity ratio of these three receptors, each with different spectral sensitivity, combined to produce our ability to perceive different colors. Additionally, in 1878 Ewald Hering also proposed a different color coding system. Still three channels, one carrying lightness as black, white, or shades of gray and two color channels, one responding to red or green, the other to blue or yellow. The latter was known as the opponent theory because it involved three opposed channels. The earlier, involving three single components became the component theory. These theories arose from researchers and scientists trying to explain various visual phenomena, one of which was the complementary after-image demonstrated in the previous exhibits. Much evidence existed to support both theories and each had many champions through the years causing much debate and controversy.
As it turns out, we know today that both theories had some validity! The human eye does contain three types of color receptors with different spectral sensitivity curves. They are called cones and are referenced as L, M, and S ala the wavelengths of their sensitivity peaks - Long, Medium and Short or Red, Green and Blue for this discussion. These cones are coupled to the optic nerve and subsequently to the brain via two 'color information' channels just as Hering predicted, one carrying blue/yellow coding and the other red/green. Also, as predicted, the brightness is a separate pathway.