Can some women see colours the rest of us can’t?

Fine artist Concetta Antico says the moment she realised her students couldn’t see the colours she did felt like “cold water in the face”.

 

Antico says she sees “a myriad of colours” in everyday things, like shadows, where most people only report seeing one shade.

 

She’d assumed everyone saw the world as she did, until it became clear from her students that they did not.

 

“I asked someone later: ‘Why did you never say anything?’,” she recalls.

 

“They said: ‘Oh, well, you’re the teacher. We… just assumed that you were doing something artistic’.”

 

Antico later learned, through genetic testing, that she had the biological potential for tetrachromacy – a type of enhanced colour vision – which may allow an individual to see colours that others cannot.

 

 

Most people have three types of specialised cells in the eye called cones.

 

Each type is triggered by a range of different wavelengths of light, which roughly correlate to red, green and blue.

 

The cone cells then send their signals to the brain. The specific colour that we see depends on the combination of signals received by the brain from the different cone cells.

 

But some people may possess a fourth type of cone cell. In theory, this may mean their brain receives more – and more varied – information, which could boost colour sensitivity across the spectrum of visible light.

 

The genes coding for the red and green colour-sensitive cone cells are on the X chromosome. In order to be tetrachromatic, a person needs to carry and express two different variants of either gene.

 

Since women have two X chromosomes and men usually only have one, generally only biological women can be tetrachromatic. The same variant in men tends to lead to some form of colour vision deficiency.

 

Possessing a fourth cone type is called retinal tetrachromacy, and genetic testing can identify people with the gene variants to make the four types.

 

Around 12% of women are thought to have this genetic potential, according to the Tetrachromacy Project at Newcastle University in the UK, although research suggests this figure may vary between populations.

 

But having an extra cone cell doesn’t necessarily mean the person has enhanced colour vision.

 

This would be called functional tetrachromacy, according to Dr Kimberly A Jameson from the UC Urvine Brunson Center for Translational Vision Research in the United States – and is much harder to prove.

 

However, there are reasons to believe this genetic potential can lead to enhanced colour vision.

 

One study by Jameson found women with four types of cone cells split a spectrum of colours into more separate shades, suggesting they could perceive more distinct colour appearances than trichromats – people with three types of cone cell.

 

There’s evidence from the animal world, too. In some species of New World monkey, the males are dichromats, with two types of cone cells, while females are trichromats with three. Scientists have observed that trichromats eat red fruit more quickly than dichromats, suggesting they may see the colour more obviously.

 

If this was applied to tetrachromatic humans, then women with the extra type of cone cell might see colours differently, according to Dr Jenny Bosten, a visual neuroscientist at the University of Sussex in the UK.

 

But what this would look like is up for debate.

 

 

“There’s a line of thinking… if you have a further colour receptor… you may see colours that will just be indescribable in the same way that we can’t convey to a colourblind person what red or green is like if they’re red-green colourblind,” says Dr Michael Newall, a philosopher at Adelaide University, who authored a paper on what tetrachromats see.

 

“There is another possibility… it might just be that they can see finer gradations of the ordinary colours.”

 

Scientifically assessing the colours that people see is difficult.

 

“It’s impossible to measure even a normal trichromatic subjective experience of colour,” says Bosten.

 

It’s complicated by the fact there are likely many different types of tetrachromacy, according to Jameson, with significant variation possible depending on the sensitivity of the extra cone cell type.

 

But analysing things like how well people can discriminate colours, or how they rate the dissimilarity between shades, can help researchers understand differences in how people see, Bosten says.

 

 

An internet search will yield multiple sites offering tetrachromacy tests online, but experts say it is impossible to identify a tetrachromat this way.

 

Most modern screens only have three colour channels – red, green and blue, meaning that, if tetrahcromats can see novel colours, they likely won’t be able to see them as well on a screen.

 

While that makes it hard to test, noticing consistent differences between the colours you see in real life and on screens could suggest tetrachromacy, according to Newall, although it varies hugely depending on the technology you’re using.

 

Another indicator that someone is potentially a tetrachromat, Jameson says, is that they consistently notice their perception of colour appears to differ from other people’s.

 

“They have a profound awareness… since they were young, they had colour perception that was more acute, more sensitive [than others],” she says.

 

Another intriguing thing that could suggest somebody might be a tetrachromat is extreme sensitivity to harsh LED light in comparison with warmer shades.

 

“Tetrachromat women… they can’t handle light bulbs that don’t have a strong long wavelength component,” she says.

 

“They will argue, with their housemates: ‘I can’t stand this light, I can’t stand these fluorescent tubes, they make me unhappy, they make me feel sick’.”

 

People whose fathers have a mild form of colour deficiency are also more likely to be tetrachromatic.

 

But, while these can give some clues, a genetic test and lab-based study are much more reliable indicators.

 

 

Another thing researchers still don’t understand about tetrachromacy is what, if anything, turns retinal tetrachromacy into functional tetrachromacy.

 

“Do they need to have a very specific type of cone? Or is it something about how they’ve learned to use those signals?” says Bosten, suggesting it could also be related to the flexibility of other parts of the visual system.

 

“Another possibility is that they may need to use those colour signals for something relevant in their lives in order to be able to develop the ability to discriminate them.”

 

Painter Antico believes that’s what happened to her.

 

“I’ve grown my colour vision. So it’s kind of a muscle,” she says.

 

“I’ve been a painter since I was a little girl… so it’s like [I’ve spent] 100,000 hours mixing colour. I do it every day, all the time.”

 

Without a “perfect experiment”, Bosten says, “you just try to have as much information as possible from different tests, different case studies, and once you’ve put it all together, you say: ‘Well, the evidence is very largely in favour of this'”.

 

Antico does not let other peoples’ doubt dampen her spirits, and says she’s “lucky” to have this genetic mutation.

 

“It’s wild that I can see things that other people cannot see,” she says.

 

“It has made my life wonderful.”