Seeing in colour: The Tetrachromacy myths

[Kat Clements | Contributing Writer]

There’s a lot of myths and misinformation about seeing in colour. From online tests claiming to diagnose you with colour-blindness or tetrachromacy, to “fun fact” lists circulating lies or half-truths about people or animals who see colour differently, the internet – as always – is a mixed blessing. So we’ve rounded up a few of the biggest or coolest facts about colour vision for you to check out.

Let’s start with the basics…

How do we see colour?

Humans have two types of light-sensitive cells in their eyes; rods and cones. Rods are responsible for vision in low light conditions, and they only see in greyscale. If you go outside at night, you’ll notice that everything appears greyish, and lacks the colour you could see in daylight. That’s because your eyes are using rod cells to make the most of what little light there is, but they cannot detect colour.

In daylight, however, your eyes will use cone cells. Cone cells do detect colours, but they only function in higher light intensities.

The average human has three types of cone cells. These types are sensitive to different regions of the spectrum of visible light, and their capabilities often overlap. However, each type has a region that only it can detect, and the cones are thus referred to as red, green and blue cone cells.

There are thousands of both rods and cones in the eye; they are located in the retina, which is the region at the back of the eye where light will land. When a cell detects the type of light it is sensitive to, the photons will trigger the release of pigment molecules which change the electrochemical balance of the cell, which instigates an action potential – an electric current which sends a message to the brain.

Cones and Rods in the eye | AstroSurf
Cones and Rods in the eye | AstroSurf

Now for the weird stuff…

Tetrachromacy

It sounds like something straight from the pages of Uncanny X-Men. There are – at least anecdotally – people who can see more colours than the rest of us. These people have two copies of the X chromosome, and they have different alleles on each one. The two alleles together mean that they have an extra cone – an extra version of the cells that allow us to perceive colour. So they can detect a wider range of light, and thus more distinct colours, than the rest of us – most humans have 3 cones (trichromatic) rather than 4 (tetrachromatic).

This happens because there are two slightly different versions of one of the cones, and they detect slightly different ranges of spectra – frequencies of light. Most people will have either one or the other, meaning that two people might see slightly different colours in the same scene. Tetrachromats are people who have both variants, meaning that they have 4 different cones and can see both of the ranges of light detected by the variant cones.

Almost all potential tetrachromats are women (or afab) because the gene variants for the cones are carried on the X chromosome (usually, women are XX, men are XY). In very rare cases, someone can be born with XXY chromosomes (usually this manifests as amab, although there may be exceptions), in which case they would also have the potential to be tetrachromats.

There are a lot of “tests” online which promise to tell you if you are tetrachromatic, which sadly lack scientific backing and provide a lot of conflicting statistics. For example, the site Hellou claims that 25% of the population are tetrachromatic.

How many colours can you see?
How many colours can you see?

It’s hard to say exactly how many people are actually tetrachromatic, since not everyone who has four cones – “putative retinal tetrachromats” – will actually be able to see a wider spectrum of colour (true tetrachromat). According to Jameson (2009), between 15% and 50% of women are potential tetrachromats, but only a few individuals have ever come forward claiming to have a wider perception of colour.

One of them is artist Concetta Antico. She says that she can see thousands of colours where most people can’t – in a BBC article she explains that “The grocery store is a nightmare. It’s like a trash pile of colour coming in at every angle. People find that extraordinary that white is my favourite colour, but it makes sense because it is so peaceful and restful for my eyes. There is still a lot of colour in it, but it’s not hurting me.”

Concetta Antico and her work
Concetta Antico and her work

In the next part of “Seeing in colour” we’ll be investigating colour blindness, so check back soon!

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Seeing in colour: The Tetrachromacy myths