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!
