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Perception of color

Humans can see a nearly  infinite variety of colors within the visible portion of the electromagnetic spectrum.

On top of this humans can see a nearly infinite variety of other colors, made from combinations of visible colors of light.

However, basic physics and human physiology, combined with the way that our brain decodes signals, shows that for all practical purposes, there are many color combinations that humans can not see.

The color opponent process is a color theory : it states that the human visual system interprets information about color by processing signals from cones and rods in an antagonistic manner.

The three types of cones (L for long, M for medium and S for short) have some overlap in the wavelengths of light to which they respond, so it is more efficient for the visual system to record differences between the responses of cones, rather than each type of cone’s individual response.

This suggests that there are three opponent channels: red versus green, blue versus yellow, and black versus white (the last type is achromatic and detects light-dark variation, or luminance).

Responses to one color of an opponent channel are antagonistic to those to the other color. That is, opposite opponent colors are never perceived together – there is no “greenish red” or “yellowish blue”.

{Wikipedia, Opponent Process}

These impossible-to-see-colors are in fact known as “impossible colors” or “forbidden colors.”

However, they are reported to be seen in special circumstances. How would that even be possible?

On Seeing Reddish Green and Yellowish Blue, Hewitt D. Crane and Thomas P. Piantanid, Science, 9 September 1983:
Vol. 221 no. 4615 pp. 1078-1080
“Four color names—red, yellow, green, and blue—can be used singly or combined in pairs to describe all other colors. Orange, for example, can be described as a reddish yellow, cyan as a bluish green, and purple as a reddish blue. Some dyadic color names (such as reddish green and bluish yellow) describe colors that are not normally realizable. By stabilizing the retinal image of the boundary between a pair of red and green stripes (or a pair of yellow and blue stripes) but not their outer edges, however, the entire region can be perceived simultaneously as both red and green (or yellow and blue).”

Perception of forbidden colors in retinally stabilized equiluminant images: an indication of softwired cortical color opponency?
Vincent A. Billock, Gerald A. Gleason and Brian H. Tsou, J. Opt. Soc. Am. A/ Vol. 18, No. 10/October 2001

Illusory color mixing upon perceptual fading and filling-in does not result in ‘forbidden colors’., Hsieh, P.-J., and Tse, P.U. , 2006, Vision Research. 46, 2251-58. (PDF).(Demo of stimuli.)

Publications of Po-Jang (Brown) Hsieh, Ph.D.

“Impossible” Colors: See Hues That Can’t Exist People can be made to see reddish green and yellowish blue—colors forbidden by theories of color perception. These and other hallucinations provide a window into the phenomenon of visual opponency. By Vincent A. Billock and Brian H. Tsou, Scientific American, Feb 2010

Train Yourself to See Impossible Colors, Alasdair Wilkins, Io9.Com

Evidence for human ability to see impossible colors : Wikipedia



The Art Blog: More Than The Eye Can See, by Tucker Harris




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