The Forbidden Colors Your Eyes Can't See

Don't even bother making reddish green or yellowish blue colors. Your eyes simply can't see them:

Red-green and yellow-blue are the so-called "forbidden colors." Composed of pairs of hues whose light frequencies automatically cancel each other out in the human eye, they're supposed to be impossible to see simultaneously.

The limitation results from the way we perceive color in the first place. Cells in the retina called "opponent neurons" fire when stimulated by incoming red light, and this flurry of activity tells the brain we're looking at something red. Those same opponent neurons are inhibited by green light, and the absence of activity tells the brain we're seeing green. Similarly, yellow light excites another set of opponent neurons, but blue light damps them. While most colors induce a mixture of effects in both sets of neurons, which our brains can decode to identify the component parts, red light exactly cancels the effect of green light (and yellow exactly cancels blue), so we can never perceive those colors coming from the same place.

Link | Impossible Colors at Wikipedia


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(did I double post?)

"(your recepters are for red, green, and blue)"

Kind of. The receptors are for short, medium, and long wavelengths.

Their ranges overlap a bit. Wavelengths that trigger the short receptors are translated as blue. The medium green, and short blue (at least as far as I can tell, depending on how you read the various descriptions), but as the receptors respond to the same light, the relative strengths of the signal (I think) are translated into the varying intermediate colors. Long (red) and medium (green) receptors firing at the same time are interpreted as yellow, for example. We can't "see" a reddish green, because red+green (long + medium) is already perceived as yellow. We *already* have a color that describes that combination.

We perceive colors that can be produced by the full range of physically possible overlapping wavelengths within the visible light spectrum. Take Magenta: there's no single wavelength that can trigger both the long and short receptors while NOT stimulating the middle receptors. But since they can be simultaneously stimulated by *separate* wavelengths, we have the "extraspectral" color Magenta - which you will not find in a rainbow, or in an illustration of the visible light spectrum. Wrapping a spectrum diagram around in a circle so the long red end overlaps with the short blue end is how we get a diagram - the color wheel - that shows all the colors we can see: all the possible combinations of length/receptor overlap that can be triggered by actual light sources.

Take a cup of hot water and mix it with cold, you get warm. We have a sensation mapped onto that combination of degrees of heat. Somehow these experiments are stimulating some kind of response where the brain isn't perceiving warm, its like the subjects are their hands in hot and cold at the same time. It doesn't make any sense physically, but the receptors are being stimulated that way nonetheless, and perceiving something new and completely imaginary, that is not mapped directly to anything in the real world. Or, I could be full of it.
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"(your recepters are for red, green, and blue)"

Kind of. The receptors are for short, medium, and long wavelengths.

Their ranges overlap a bit. Wavelengths that trigger the short receptors are translated as blue. The medium green, and short blue (at least as far as I can tell, depending on how you read the various descriptions), but as the receptors respond to the same light, the relative strengths of the signal (I think) are translated into the varying intermediate colors. Long (red) and medium (green) receptors firing at the same time are interpreted as yellow, for example. We can't "see" a reddish green, because red+green (long + medium) is already perceived as yellow. We *already* have a color that describes that combination.

We perceive colors that can be produced by the full range of physically possible overlapping wavelengths within the visible light spectrum. Take Magenta: there's no single wavelength that can trigger both the long and short receptors while NOT stimulating the middle receptors. But since they can be simultaneously stimulated by *separate* wavelengths, we have the "extraspectral" color Magenta - which you will not find in a rainbow, or in an illustration of the visible light spectrum. Wrapping a spectrum diagram around in a circle so the long red end overlaps with the short blue end is how we get a diagram - the color wheel - that shows all the colors we can see: all the possible combinations of length/receptor overlap that can be triggered by actual light sources.

Take a cup of hot water and mix it with cold, you get warm. We have a sensation mapped onto that combination of degrees of heat. Somehow these experiments are stimulating some kind of response where the brain isn't perceiving warm, its like the subjects are their hands in hot and cold at the same time. It doesn't make any sense physically, but the receptors are being stimulated that way nonetheless, and perceiving something new and completely imaginary, that is not mapped directly to anything in the real world. Or, I could be full of it.
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"Many V4 neurons represent color, instead of raw wavelength. A double-opponent cell in V1 may fire whenever a set amount of, say, middle wavelength light impinges onto its receptive field, whereas a V4 cell responds to the middle wavelength region of the spectrum within its receptive field relative to the spectral distribution of stimuli in an extended region of the entire visual field." - Quest For Consciousness: A Neurobiological Approach, by Christoph Koch, pp 137-8

Process architecture of the visual system
http://helen.pion.ac.uk/outreach/images/how/Figure-1.png Van Essen and Felleman (1991)
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"Ack – an article explaining color perception in terms of “red light”, “green light”, and “yellow light” hitting the retina. Fail."

Total Fail...I was onboard for the yellow/blue not being able to occur together (yellow is all red & green, no blue), but red and green are fundamental components of vision (your recepters are for red, green, and blue). Red plus green is yellow!

http://imgur.com/9OsLm

In this little picture I made the square on the left is blue & yellow pixels which looks kind of grey-ish and the right is red & green pixels only, which looks yellow!
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