Is human tetrachromacy real?
Can some people really see one hundred times more colours thanks to an extra type of cone cell in their eyes?
Tetrachromacy, a characteristic that allows the eye to perceive an increased range of colours, is common among birds and fish. But there’s also been plenty of speculation about human tetrachromats, and every so often, an online test comes along promising to tell you whether you’re one of them. Do people with exceptional colour vision really exist, and how can we identify them?
The science of colour vision
Most of us are trichromats, meaning we have three types of cone cell in our retinas. Each type of cone cell is sensitive to different wavelengths of light. Short-wave cones are sensitive to short wavelengths, i.e. purple and blue. Middle-wave cones are sensitive to yellow and green, and long-wave cones are sensitive to red and orange.
Each type of cone cell can distinguish 100 different variations in colour. Since the brain combines information from different types of cone cell, a human trichromat can see about 1 million colours.
If you lack functionality in one type of cone cell, you’re a dichromat. Theoretically, this reduces the number of colours you can see 100-fold. Most commonly, this will give you trouble distinguishing between shades of red, yellow and green, although variations exist depending on which type of cone cell is non-functional.
Now let’s go the other way and consider someone with four types of cone cell. Theoretically, this would allow them to see 100 times more colours — for humans, that’s an astonishing 100 million distinct hues. This is what we mean by tetrachromacy. To a tetrachromat, the average person would seem colour-blind.
Where the extra cone cell comes from
So are some humans really tetrachromats? The short answer is yes — some people have four types of cone cell.
In 1948, Dutch scientist HL de Vries observed that although the daughters of colour-blind men were not colour-blind themselves, they perceived colour slightly differently in tests. He theorised that these women actually had enhanced colour vision. Genes related to colour vision are located on the X chromosome. The colour-blind fathers had two normal types of cone cell and one mutant type. Thanks to their two X chromosomes, the daughters inherited this mutant cone cell type and the three normal cone cells, giving them four types of cone cell in total. Based on the proportion of colour-blind males, around 12% of females could be tetrachromats.
Researchers have even suggested that tetrachromacy could explain the persistence of colour-blindness in males. Although colour-blindness is a clear disadvantage, if female carriers gain enhanced colour perception, it makes sense for the trait to be maintained in the population.
More recent research points to variation in the long-wave cone cell gene as a likely cause of human tetrachromacy. The human long-wave and middle-wave cone cell genes are positioned close to one another, allowing them to misalign and recombine during meiosis. This process can produce variants of one of these genes — usually the long-wave gene. The hybrid variant encodes for a photopigment with an absorption spectrum between those of typical long- and middle-wave genes.
Someone with XX chromosomes could have two versions of the long-wave gene, giving them two types of long-wave cone cell sensitive to slightly different wavelengths of light. Theoretically, this would give the person better colour vision between the red and green part of the spectrum.
Unfortunately, even if you do happen to be a genetic tetrachromat, your colour vision isn’t necessarily better than a trichromat’s. Having an extra type of cone cell in your retinas doesn’t mean that it picks up significantly different wavelengths to your other cone cells, or that your brain knows what to do with the extra information. Our brains aren’t adapted to perceive colour in four dimensions.
The one true tetrachromat
After two decades of searching, Gabriele Jordon at the University of Newcastle has only been able to confirm functional tetrachromacy in one person. 24 carriers of colour-blindness took a colour vision test. They were shown coloured disks with subtly different mixtures of pigment, only distinguishable by someone with a fourth type of cone. Only one woman, referred to as cDa29, was able to respond swiftly and accurately.
Genetic analyses suggested cDa29 had normal genes for the long-wave and medium-wave photopigment as well as a gene that appeared to be a hybrid between the two. As a result, she had three well-separated cone photopigments towards the long-wave end of the light spectrum. Behaviourally and genetically, she resembled a true tetrachromat.
All this being said, it’s not clear whether these well-separated photopigments are enough to explain cDa29’s abnormally sharp colour vision on their own.
In summary, human tetrachromats seem to be real. But we don’t know a whole lot about them, and we don’t know a whole lot of them.
Could you be a tetrachromat?
Maybe you suspect you’re a tetrachromat. It’s admittedly unlikely, but not impossible. How do you go about testing your suspicions?
Although there are plenty of online tests for tetrachromacy out there, they’re not accurate. Your computer screen doesn’t provide enough colour information for a tetrachromat to see anything special. There’s really no way to know for sure if you’re a tetrachromat outside of participation in a proper research study.
Note: Colour blindness is rather common, affecting 8% of males and 0.5% of females, and many people don’t realise they have the condition until adulthood. If this article has got you thinking about your own colour vision, you may be better off checking for dichromacy than tetrachromacy.
