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Tuesday, July 31, 2012

Color as a Super Power?

Art by Jimmy Turrell 

The science of how we see color is a truly fascinating and essential part of understanding color. We are still discovering so much about the vision of not only humans, but animals and insects as well. Humans are technically trichromatic, meaning we have three cones in the retina of our eyes; red, green and blue. They allow us to see 3 color ranges which equal about 10 million different hues.

 While we've known about color blindness in the human eye for decades, there appears to be research that some humans have the ability to see more colors than the average person. I guess you can say they have super human color powers! Sorry guys, this looks like it's mainly a superpower for the ladies.

Those with super color vision are called tetrachromats and may be able to see as many as 100 million colors with an extra cone believed to be in the orange range.

This color vision superpower is related to our X chromosomes. Because men only have one of these, colorblindness is most common in men. The genes for the pigments in green and red cones lie on the X chromosome, and because women have 2 X chromosomes, this creates the chance for one type of red cone to be activated on one X chromosome and the other type of red cone on the other one. Women may also have two distinct green cones on either X chromosome. 

Many researchers think this might be connected to women who are genetically linked to men with color blindness. While true color blindness (achromatopsia) is very rare, there is more commonly a deficiency in one of the cones spectral sensitivity:

Protanomaly is a mild color vision defect in which an altered spectral sensitivity of red retinal receptors (closer to green receptor response) results in poor red–green hue discrimination. It is hereditary, sex-linked, and present in 1% of males. 

Deuteranomaly, caused by a shift in the green retinal receptors, is by far the most common type of color vision deficiency, mildly affecting red–green hue discrimination in 5% of males. It is hereditary and sex-linked.   

Tritanomaly is a rare, hereditary color vision deficiency affecting blue–yellow hue discrimination. It is not sex-linked.  

The below picture illustrates some of the visual difference in the most common types of color blindness.  
This image (when viewed in full size, 1000 pixels wide) contains 1 million pixels, each of a different color. 

 The idea that they're might be tetrachromats among us goes back all the way to 1948 when it was mentioned by Dutch scientist HL de Vries . Over the course of two decades, Newcastle University neuroscientist Gabriele Jordan and her colleagues have been researching this possibility by testing the mothers of sons with color impairment. Initial tests came up empty until they switched methods in 2007.

In the new test each woman would look into an optical device showing her three tiny discs in rapid succession. One of the discs was a nearly identical mixture of red and green, while two of the discs were a pure orange wavelength. The women aren't told which is which. 

Women with two distinct red cones would see the red-green disc differently than the orange discs. One woman, A doctor in northern England, referred to only as cDa29, was able to see the difference and is the first tetrachromat known to science. She is now conducting genetic tests on the woman's saliva to verify whether she has the genes for distinct red cones.  This puts a human more within the range of vision of most birds, reptiles and amphibians who can often see into the Ultraviolet wavelength.

While some women may technically have the fourth cone,  their red cones may be too close together in wavelength to notice a spectral difference.  Researchers differ on what percentage of the population of women this relates to but the most common estimate I've read is 2-3% of all woman are tetrachromats. How different they see from the rest of us is up for debate. Some say they may see a larger range of subtle color and undertones while other researchers muse whether the brain may need to learn to use a fourth cone. This is a superpower I could embrace!