All The Colours You Cannot See: Tetrachromacy
What if I told you the way you see the world, the very colours it is rendered in, might appear bleak and lifeless when compared to the way in which the person sat next to you sees them? What if they experience a broad range of the colour spectrum you simply don’t have access to? That you cannot even try to imagine? And what if this was not due to colour-blindness in yourself, but an extraordinary genetic gift in the other person? Increasing interest and research into what is dubbed ‘tetrachromacy’ in humans may indicate that this is the world we live in; that a, presumably small, selection of the population has higher colour sensitivity than everyone else, perhaps even seeing colours that do not exist to trichromats (those with ‘normal’ colour vision).
Diagram showing cross section of human eye
Image Credit: Freepik
According to our current understanding of eye function, the eyeball contains two types of light receptors: rods, which have higher light sensitivity, making them ideal for seeing in low light conditions, yet only providing images in greyscale; and cones, concentrated at the fovea of the eye, which are responsible for colour reception, providing coloured and high resolution images to the brain through the optic nerve. There are three types of these prementioned cones, each one activated by either short, medium, or long wavelengths of light. These wavelengths then correspond respectively to the perception of blue, green, and red, the combined activation of which results in the perception of the visible light spectrum. Colour perception is also known to be affected by cone fatigue, which is why, for example, if you stare at a red object for a prolonged period, if you then look at something white, you’ll see the outline of the original object in cyan as the green and blue cones are not fatigued, so function more than the red.
A tetrachromat is the term used to describe those who possess the unique ability to process colour within four photopigment classes instead of the original three, expanding the normal visible light spectrum by millions of hues. The condition is thought to be sex-linked, as the genes for green and red cones are carried on the X chromosome. In individuals with XX chromosomes, two different alleles (versions) of a photopigment gene with differing sensitivities can be inherited and expressed, resulting in four types of cones: S (small), M (medium), L (large), and an additional L’. The same does not occur in those with a XY chromosomes as the X chromosome faces no competing alleles on the Y.
The Visible Light Spectrum
Image Credit: Armasight
First theorised by de Vries, he suggested that tetrachromats were the daughters of anomalous trichromats (those for whom a cone is spectrally shifted). Supported by Gabriele Jordan and John Mollon, they add that these specific anomalous trichromats that parent tetrachromats likely perform normally in colour tests and have matching ranges close to zero on the anomaloscope (in contrast to other anomalous trichromats who express the same colour perception of dichromats, essentially exhibiting colour blindness). These daughters inherit, as a result, normal L and M (red and green) photopigment genes and an anomalous gene from a parent (thought to be the result of the L and M, similarly shaped genes, misaligning at meiosis, creating a gene with part of the normal L sequence and part of the normal M sequence, resulting in a photopigment gene with a peak sensitivity between L and M wavelengths).
Another suggested hypothesis is that tetrachromacy is caused by site 180 on the L cone, in which 62% of Caucasians have the amino acid serine, the other 38% having alanine (differing in peak sensitivity by 4-5nm, serine being sensitive to longer wavelengths). The theory suggests that women with differing inherited alleles (one X chromosome coding for serine and the other alanine) become tetrachromats. However, were this to be the explanation, 47% of Caucasian women should have tetrachromacy, and unlikely high percentage, the question of whether a difference of 4-5nm is enough to cause such a distinct change is also debateable.
Alanine.serine
Image Credit: National Library of Medicine
Empirical studies are lacking, most of all due tetrachromacy’s complex nature. How can one demonstrate they can see colour other people cannot? Tetrachromats will believe their perception is the norm, and trichromats are physically incapable of seeing what tetrachromats can. In addition, the question of colour as nature or nurture needs to be raised. Psychological studies show many nuances in social conditioning of colour, mainly caused by language (shown in Winawer’s 2007 study in which Russians could discriminate cross-boundary dark and light blues faster than English speakers due to the language having two separate words for the colours, as opposed to English only having ‘blue’). Studies must also account for arts training as artists usually become adept at colour matching, which could skew results.
As interest in the theory increases, as do attempts to pin down the cause and effects of tetrachromacy. And so, although today our knowledge on the subject is severely limited, providing research continues, hopefully in the future theories and explanations can be confirmed and we will know for certain whether individuals are just better with colour, or are genetically gifted to have access to all the colours the rest of the population cannot see.
Bibliography
Kimberly A Jameson, Alissa D Winkler, Keith Goldfarb, "Art, interpersonal comparisons of color experience, and potential tetrachromacy" in Proc. IS&T Int’l. Symp. on Electronic Imaging: Human Vision and Electronic Imaging, 2016, https://doi.org/10.2352/ISSN.2470-1173.2016.16.HVEI-145
Jameson, Kimberly A., Timothy A. Satalich, Kirbi C. Joe, Vladimir A. Bochko, Shari R. Atilano, and M. Cristina Kenney. Human Color Vision and Tetrachromacy. of Elements in Perception. Cambridge: Cambridge University Press, 2020.
Martinovic J, Paramei GV, MacInnes WJ. Russian blues reveal the limits of language influencing colour discrimination. Cognition. 2020 Aug;201:104281. doi: 10.1016/j.cognition.2020.104281. Epub 2020 Apr 7. PMID: 32276236.
Jordan, Gabriele, and John Mollon. 2019. “Tetrachromacy: The Mysterious Case of Extra-ordinary Color Vision.” Current Opinion in Behavioral Sciences 30 (September): 130–34. https://doi.org/10.1016/j.cobeha.2019.08.002.
Images
