Seeing Invisible Colors: Part II

In a recent Knowing Neurons piece, we explored the exotic visual abilities of other animals, while lamenting the limitations of human color vision.  In this article, we stretch those limits and celebrate some peculiarities of trichromatic color vision.

Recall that a typical person has color receptors (cone cells) for red, green, and blue wavelengths of light and perceives all other colors as combinations of these primary colors resulting from differential activation of cone cells.  This is known as trichromatic color vision. Television and computer monitors cater to human physiology.  When viewing a yellow shape on a computer monitor, not a single pixel on your computer monitor is yellow.  A mixture of red and green pixels equally activating red and green color receptors creates the perception of the color yellow.  Different cone cells also “compete” to create the perception of different colors, an idea known as opponent process theory.  This is why you often see a bright blue blotch of color after staring at a yellow flashbulb.

opponent color processing theory

But color receptors aren’t just activated: they can also be fatigued by continuous activation such that they are less active following a strong sustained stimulus.  Considering opponent process theory, a curious consequence is that fatiguing one or more color receptors creates the perception of a “supernaturally” strong color yielded by the remaining color receptor(s).  For instance, a bright magenta light will equally fatigue red and blue photoreceptors, leaving green color receptors with less competition when their signals travel down the optic nerve to the visual system of the brain.  The result is a green hue which is somehow greener than green, i.e., greener than any green which can be perceived without fatiguing other color receptors.  Try it!  The engineers who designed the Epcot park at Disney World took advantage of this fact by painting pavement pink to make the grass appear greener than green.

You can create a ‘greener than green’ afterimage by fixating on the cross in the upper half of the image for 20 seconds, then shifting your gaze to the cross in the lower half.
You can create a ‘greener than green’ afterimage by fixating on the cross in the upper half of the image for 20 seconds, then shifting your gaze to the cross in the lower half.

Some women can view “invisible” colors without fatiguing any photoreceptors.  These women have a mutation in a gene encoding a green or red color receptor, shifting the sensitivity, or preferred wavelength, of cone cells expressing the gene to a different wavelength.  The original or “wild type” color receptor is encoded by the non-mutated allele, allowing for four primary colors (i.e., tetrachromatic vision) and perception of 100 times as many hues as individuals with typical vision. The locus of these genes is found on the X chromosome.  Men, who only possess one X chromosome, cannot enjoy tetrachromatic vision, even if one of these genes is mutated.  Concetta Antico, a San Diego artist, is a woman with confirmed tetrachromatic vision.  She claims that her tetrachromatic vision “takes her beyond previously known artistic color and value use.”

To take color photographs, cameras typically have three different color filters simulating trichromatic vision.  Sometimes, however, the exact wavelengths of these color filters vary from the sensitivities of human cone cells.  Your phone camera, for instance, can likely detect a faint infrared light emitted by your television remote when you change the channel.  Try it!


Generally, we can compare the colors we see in photographs with the actual objects depicted in the photograph to determine whether the colors are accurately represented.  But when NASA sends space probes to other planets, there are no humans there to judge the accuracy of color images retuned by probes.  For this reason, NASA generally includes a reference on Martian rovers to calibrate the rovers’ cameras.  Last July, when the New Horizons space probe returned the first close up images up Pluto, the Internet was abuzz with surprise that Pluto appeared reddish-beige, rather than blue, as many had naively imagined.  Although astronomers have known for years that Pluto is reddish in color, New Horizons’s color camera Ralph has color filters which do not precisely match up with human vision: blue, green, and two infrared (rather than red).  While the images returned are close to what the human eye would perceive, they are not exact.


But if colors are simply labels created by the brain, why limit our scientific instruments to what the human body can achieve?  Even if you will never experience invisible colors, take heart: the tools of modern science can shows us the world from radio waves to gamma rays and everything in between.

Electromagnetic Spectrum Knowing Neurons

Part2.1_cover_Knowing Neurons


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