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We sometimes think of colors as objective properties of objects, much like shape or volume. But research has found that we experience colors differently, depending on gender, national origin, ethnicity, geographical location, and what language we speak. In other words, there is nothing objective about colors.
It would be rather surprising if there were no variation in how we experience colors. The number of cones (photoreceptors) in the human retina is not constant. Sometimes cones are present in large numbers, and sometimes they are barely present. And this difference has been observed in so-called normal individuals who react in the same way to color stimuli.
The fact that the number of cones in our eyes varies considerably suggests that the brain must be able to automatically adjust the input from the retina. So, individual variations in color perception may not purely be a matter of the nature and number of the cones (or photoreceptors) in the retina. It can also be a result of the fact that people with different numbers of cones calibrate the input from the retina in different ways.
One approach to test for variation in color vision is to test for variations in color judgments and color discrimination abilities. Such tests have demonstrated vast variation across perceivers exposed to the same color stimulus. Malkoc and colleagues, for example, found that what some people pick as their best example of red is what others pick as their best example of orange. The researchers only tested for individual differences, not for differences in gender, national origin, ethnicity, geographical location, or native language spoken. But other research points to variations of this kind.
Recent studies indicate significant variance in a gene located on the X chromosome which codes for a protein that detects light in the long-wavelength (red/orange) regions of the color spectrum. As women have two copies of the X chromosome, it is possible for them to have two different versions of this gene, and hence it is possible for them to have a more fine-grained ability to discriminate light in the long-wavelength regions of the color spectrum. Women are thus potentially in a position to perceive a broader spectrum of colors in the long-wavelength regions than men.
Kimberly Jameson and her colleagues have taken the hypothesis that there are sex differences in color vision one step further. They speculate that up to 40 percent of women have tetrachromatic color vision. The line of argument runs as follows. Most humans have three cone types, which absorb maximally in different regions of the spectrum. So, most humans are trichromats. However, 8 percent of males (and an insignificant number of females) have only two cone types. They are dichromats (color-blind). Dichromacy results when a genetically mutant red or green photopigment gene on the X chromosome fails to express retinal photopigment.
Women who carry a deviant photopigment gene on an X chromosome typically are not color-blind, because they have two X chromosomes, but if they have a male offspring, then he is highly likely to have some degree of red or green color blindness.
The mothers and daughters of dichromats and the mothers and daughters of males with deviant red/green photopigment genes may have a typical X chromosome and an X chromosome that carries one of the deviant red or green photopigment genes. If the normal red and green photopigments and a highly altered variant are all expressed, together with the blue photopigment (from chromosome 7), then the woman could have tetrachromatic color vision.
Of course, for tetrachromacy to be present, the variant red/green photopigment must constitute a cone type that differs from the ordinary red/green cone type, and the brain must be able to process the color signal coming from the extra photopigment.
Jameson argues that evidence for the possibility of female human tetrachromacy can be found in the animal kingdom. Female spider monkeys are normally dichromats, but those possessing an extra photopigment gene variant are trichromats. The extra cone type allows some female monkeys to experience shades of color, which other female spider monkeys can’t experience.
Experiments that test for tetrachromacy in women with dichromatic offspring have also been conducted. Though still preliminary, the results indicate that women who are genetically capable of expressing more than three cone types tend to perform better on color discrimination tests. So, it could well be that some women can see more colors than the rest of us.
The variation in color categories across languages is another indicator of the variation in color vision. Many languages are so-called “grue languages.” They do not lexically discriminate blue from green but have only one basic color term that names stimuli with dominant wavelengths in the middle- and short-wavelength (blue/green) regions of the color spectrum. These include Vietnamese, Kuku-Yalanji (an aboriginal language), Tswana (a South-African language), and Zulu (a South-African language). Other languages do distinguish between blue and green but also have “mixed” color terms that name stimuli with dominant wavelengths in the middle- and short-wavelength regions of the spectrum. These include Chinese, Korean, and Japanese.
Some languages are so-called “dark languages”; they do not lexically discriminate blue from gray or black (e.g., Tswana). And some languages only have two words, one for dark and one for light (for example, Dani, a New Guinean language, and Lani, the Indonesian language). There are also languages that have more color terms than English. Russian, for example, has a term for light blue (“goluboy”) and a different term (“siniy”) for medium and dark blue.
What's more: The lexical category boundaries between the colors shift as we move across linguistic communities. For example, in Chinese, green and light blue fall in the same category as do dark blue and black.
To what extent linguistic variability reflects variation in color perception is a matter of debate. But an increasing number of studies seem to suggest that this might well be the case. I will look at the connection between color language and color perception in a future post.
References
Brogaard, B. (2009). “Color in the Theory of Colors? Or: Are Philosophers' Colors All White?”, The Center Must Not Hold: White Women on The Whiteness of Philosophy, George Yancy, ed. New York: Lexington Books, 131-152.
Brogaard, B. (2019). Seeing and Saying. Oxford University Press.
Jameson, K. A. (2007). “Tetrachromatic Color Vision”, The Oxford Companion to Consciousness, in P. Wilken, T. Bayne, and A. Cleeremans, ed. Oxford University Press: Oxford.
Malkoc, G., Kay, P., Webster, M. A. (2005). “Variations in Normal Color Vision. IV. Binary Hues and Hue Scaling,” Journal of the Optical Society of America, A 22, 2154-2168.
