Searching for the women that can see 99 million more colours than us

Humans have evolved to see around a million different colours, which is why getting exactly the right shade of paint mixed at B&Q is such a horrendous ordeal. That makes us pretty unusual among mammals, with dogs and monkeys seeing around 10,000 shades of colour.

The reason for this discrepancy comes down to the number of cone cells. A normal human eye, has three types of cone cells, and each one can distinguish around 100 shades. Combine the various permutations of the 300 shades, and you’ve got a palette of around 1,000,000 colours to play with.

The reason other mammals see far fewer shades is that they only have two flavours of cone cells, reducing the spectrum of colour visible. A defective cone cell is also the reason that some people are colour-blind – it’s defective, rather than non-existent. Keep that in mind, it’ll become important later.

It’s astonishing to think that the world around us is actually far more vibrant that we can ever truly appreciate, with just the three cone cells, but it is. We’re not top of the food chain when it comes to cone cells, or even close. While we see more colours than dogs, butterflies have five cone cells and the mantis shrimp has an incredible 16. To other mantis shrimps, the colourful beast probably looks even more fabulous than it does to you or I:

But I digress. The point is that this world of extra colour is off-limits to humans. Or is it? We’ve slowly getting some evidence of something that has been considered theoretically possible since 1948: a living, breathing tetrachromat.

A tetrachromat is someone with four types of cone cell, which in theory means that they would be able to seen 99 million more colours than us. People with an extra type of cone cell are rare, although not unheard of – but until recently there has been no evidence to show that people with the extra type of cell see the world any differently.

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But to back up a little bit, why do some of us develop an extra cone cell? Remember when I said colour-blindness was important? It’s about the genetics they pass down to their offspring. Colour-blind men have two normal functioning cone cells and a mutant cone cell that is less sensitive to green or red light. Dutch scientist HL de Vries found that the daughters of these men inherited four cones: three functioning cones and a mutant one. Around 40 years later, John Mollon from the University of Cambridge began to survey genetically likely women to see if any of them might have the holy grail of four working cones. He came up empty – the women surveyed could see the same humdrum million-colour palette that you and I can see.

In 2007, a neuroscientist named Dr Gabriele Jordan at Newcastle University picked up where de Vries left off, only with a different way of testing. She examined 25 women identified as having the extra cone, and put them in a dark room where three coloured circles of light were flashed in front of the women. Those with regular vision would see these as the same – as the majority of participants did – but one of the women (codenamed cDa29) managed to spot the difference in each test. “I was jumping up and down,” Jordan told Discover Magazine.

But as colour-blind people can’t ever fully understand the colours that we see, it’s just as impossible for us to comprehend what the world looks like to patient cDa29 – if it even looks that different at all. Jay Neitz, a vision researcher from the University of Washington, told Discover that it could be that cDa29’s extra visual powers are underutilised in a world that’s designed for us trichromats. “It could be that our whole world is tuned to the world of the trichromat,” he said.

It’s extremely hard to comprehend the full range of colours that different species can see, but the best demonstration I’ve ever heard comes from the podcast Radiolab, where they enlisted a choir to simulate how a dog, human, butterfly and mantis shrimp would see the same rainbow.

It’s definitely a brilliant way of getting your head around exactly what the majority of us can only ever imagine. You can find the amazing section here – listen from 9:50.

Images: Charlene McBride and Kerri used under Creative Commons