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According to new evidence, a small fraction of women show tetrachromacy: an enhanced color vision that enables them to discern more colors than most of us can see.
In school you probably have been taught that humans have three types of cells in the human retina, the so called “cones”, which allow us to see in full color and make us trichromats (tri=3 chromatic = ‘colored’). These cones contain light sensitive pigments which detect light of different wavelengths. Light consists of a spectrum of wavelengths, spanning the whole array of colors: from short waves (bluish colors, including ultra violet) to long wavelengths (red and infrared). Each of these color cone cells has specific wavelengths to which they are most sensitive, and these lie in the short-wave (blue), middle-wave (green), and long-wave (red) regions of the visible light spectrum. By having sensitivity to these three different wavelengths and colors, cones enable us to discriminate a large range of colors.
In the most prevalent form of colorblindness, also called anomalous trichromacy, there are only two fully functioning sets of cone cells. Often, the blue set is fine, but subtle changes in the DNA-sequence of the green or red cone pigments can lead to a shift of the sensitivity of the pigments towards shorter or longer wavelengths, indicated by the yellow curve in the figure below.
The yellow curve shows the light sensitivity of an altered cone. It could originate from either a green cone of which the sensitivity has shifted toward the red portion of the spectrum, or from a red cone with a shift toward the green. Image taken from Jordan et al. (2010) Journal of Vision.
In this case, the altered cone cell signals to the brain when being hit by light of a different wavelength, and the color gets misinterpreted by the brain. Unexpectedly, it’s also this form of colorblindness that has been argued to give rise to tetrachromacy and subsequently richer color perception.
How can anomalous trichromacy lead to tetrachromacy?
The genes coding for the light sensitive pigments of green and red cones are located on the X chromosome. Most women have two X chromosomes, of which one gets randomly inactivated in each cell during development, leaving just one functional. Thereby, they don’t interfere with each other. However, since every woman has not one, but two X chromosomes, it would be possible that they carry the normal red and green pigment genes on one of the X chromosomes and an anomalous gene on the other. Due to the random inactivation in each cell, there will be a mixture of cones (from the two different X chromosomes) expressed in the retina. This essentially means that four different types of cones would be produced and the woman would be tetrachromatic (tetra = 4, chromatic = ‘colored’). Researchers have argued that about 12% of women should be tetrachromats.
So why are we not surrounded by women with outstanding color vision?
Interestingly, the existence of tetrachromatic women has been suggested as early as 1948, by a Dutch researcher Hessel de Vries, who studied the sensitivities of the color receptors to light of different wavelengths. Since then the fourth cone has been confirmed in many women, however, only one case of enhanced color discrimination, i.e., behavioral tetrachromacy, has been reported in scientific literature so far. So it seems as if apart from having a fourth cone, something additional is required to enable superior color vision. Unfortunately, scientists still have to unravel what other criteria have to be fulfilled to make someone behaviorally tetrachromatic. Although many women would agree, a lot of them are really good at spotting the right colors…
Written by Eva Klimars, edited by Annelies van Nuland
De Vries, H.L. (1948). The fundamental response curves of normal and abnormal dichromatic and trichromatic eyes. Physica, 14(6), 367–380
Jordan, G., Deeb, S. S., Bosten, J. M., & Mollon, J. D. (2010). The dimensionality of color vision in carriers of anomalous trichromacy. Journal of Vision, 10(8), 12. doi:10.1167/10.8.12