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!!!

Unfortunately, most modern browsers refuse to stop the motion during the above title bar mouse procedure, however, digital photographs of the screen show the presence of a blank gap where the eye mistakenly sees green. The RGB sensors in the digital camera are not subject to the same exhaustion exhibited by the eye - it's only a human thing!

What you are viewing is an animated GIF image composed of a gray circle and a background picture. The position of the circle is changed at 0.09 second intervals to overlay and hide a single magenta spot, one at a time in counterclockwise order. Human visual persistence, after-image, adaptation and the eye-brain combination all work together to create what you perceive. The gray circle without the square is drag enabled so you can place the cursor on it, press and hold left button and drag it around on the screen. Try staring fixedly on one the patches and drag the circle slowly over the it without moving your eyes and your can create the effect. As you can see, the perception of green is an illusion. What is the point? What is the probability that these traits of human color perception have at some time influenced our color decisions? Think about it!

Below are some more examples of phantom colors followed by a brief discussion of why we see what we think we are seeing. Scroll on down if you are interested

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.

When looking at neutral colors as white and shades of gray, all three types of cones are responding equally and our brain tells us we are seeing a neutral color. However, when looking at highly saturated colors, the cones respond differently depending upon the wavelengths coming from the object. If we avoid moving our eyes so the same area of the retinas are being stimulated by the colored circles, the affected cones will temporarily adapt by losing some sensitivity. When the rotating neutral circle overlays the colored patch our brain gets a distorted signal due to this change in sensitivity. The adapted cones are overpowered by the freshly stimulated ones and our brain determines we are not seeing a neutral gray or white but the complement of the patch color. Green cones are not stimulated by magenta so we see green for the magenta demo. Red and green cones are stimulated by yellow so we see blue. Blue does not affect the red and green cones so the blue demo produces yellow. Cyan stimulates blue and green cones thus producing phantom red, etc. Scroll back up and you will see this all starts to make sense!