By Emma Meyers
We live in world of color. As children, we quickly learn that the sky is blue and the grass is green. We drive our cars, directed by colored lights. They can affect our moods, excite or relax us, and allow us to express ourselves through art, clothing, and décor. Colors are so central to our culture that they’ve come to take on meanings beyond their aesthetics: pure white, green with envy, red states and blue states.
The tuning curve for the three types of cone cells. At each wavelength along the bottom axis, the cones are activated with different intensities. Each combination of activation allows you to see a color (via Wikimedia)
We perceive colors with specially adapted photoreceptor cells in our retinas called cones. There are three types of cone cells (from which the name for our ability to see color – trichromatic vision –is derived), and each is tuned to detect light in a specific range of wavelengths: long, with sensitivity peaking at red light; medium, tuned to green light; and short, which respond best to blue light. The one million (yes, million!) colors we are capable of seeing each correspond to different combinations of cone activation in response to light. Often, men (and, occasionally, women) who are colorblind have a genetic abnormality that results in the lack of one of the three types of cones. With only two cones, the full range of light wavelengths are not detected, and fewer combinations of activation lead to fewer perceived colors.
If fewer cones mean fewer colors, what if we possessed a fourth cone? Would an extra photoreceptor give its lucky possessor superhuman, super-color vision? It turns out this might actually be the case. Scientists lead by Gabrielle Jordan at Newcastle University in England long hypothesized the theoretical likelihood of a four-coned human, or tetrachromat, walking among us.
In addition to the red, green, and blue cones we all possess, a tetrachromat would also have a cone that has its peak sensitivity tuned to orange light. The extra cone would increase the tetrachromat’s color perception capacity to about 100 million hues – colors that most of us cannot even conceive of would be common shades in her world. Tetrachromacy would only be possible in women, the Newcastle group explained in an article
in Discover Magazine published in the summer of 2012. Genes for the red and green cones are both located on the X chromosome, so while men who inherit an X chromosome with a mutated cone gene are colorblind, women, who have two X chromosomes, may be able to use the same mutant gene along with her three working cones to see an array of colors we can’t begin to understand. And it’s not as rare a condition as you might think – given the prevalency of colorblindness in men, Jordan estimates that as many as 12% of women might be tetrachromats.
People with normal trichromatic vision can see the slight differences and gradations between colors in this wheel. A tetrachomat, though, may see even more colors and nuanced gradients (via Wikimedia)
Despite this high estimate, though, finding a true tetrachromat to study has proven difficult. Though Jordan and her colleagues identified women with four cones, none showed any signs of tetrachromacy in subtle color discrimination tasks. While they finally did find one veritable tetrachromat, she was the only one out of 25 four-coned women tested to see any unique colors.
Though additional tetrachromats have been identified
, none are able to explain how their world might look different from ours. Adding to the obvious lack of vocabulary is the simple fact that we have designed our world of trichromatic color vision, so any tetrachromatic benefit, aesthetic or otherwise, might not be manifest in our man-made, trichromatic daily lives. University of Washington vision researcher Jay Neitz suggests that this might also be why the other 24 women with four cones Jordan tested were not true tetrachromats: there’s just not enough color variation available for tetrachormats to tap into their abilities. Until we gain more understanding though, we mere trichromats must be content with the world as we see it, even while knowing there is a kaleidoscopic array of colors within colors that we can’t imagine hidden from us in plain sight.
(I couldn’t find an online tetrachromacy test, but to test your own color perception abilities, try this game!)