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Find out more about colour blindness and take a test here

Why is it crucial to understand colour vision?
On one level it鈥檚 an intellectual question, if I can find out everything that happens in our heads when we see something and say 鈥渢hat鈥檚 red鈥�, it would explain a lot of things. Also, we use colour as a language now. It鈥檚 everywhere: everything is colour-coded, and colours tell us where to go and what to do all the time.

Does this mean that colour blindness is more of a disability than it used to be?
It can be. One place where it鈥檚 super-obvious is education, where so much of the material uses colour: 鈥渟elect the red circle; follow the yellow line鈥�, and so on. My wife鈥檚 brother is red-green colour-blind and when he was a child, the teachers thought he wasn鈥檛 trying because sometimes he did things wrong 鈥� colouring in a triangle with red when it was supposed to be green, for example. They thought he was belligerent because he sometimes didn鈥檛 follow the instructions.

I get emails from a lot of colour-blind people describing their problems 鈥� it can be very difficult. One guy told me about the trouble he has with key cards for hotel doors, for example. He knows the light is supposed to change from red to green when the card works, so he has to keep staring at it to try to figure out when it changes.

What causes colour blindness?
Humans have three types of colour receptor, which correspond to the three colours used on computer screens or in printing: red, green and blue. The genes for red and green receptors are next to each other on the X chromosome, and the most common defect is when one of those genes has been deleted 鈥� about 2 per cent of men are missing one of those genes. Females have two X chromosomes, so they only have a problem if there is a defect on both of them. That鈥檚 why colour blindness is more rare in women; the rate is only 1 in 230.

You developed your own test to study colour vision. Can you tell me about it?
We wanted to uncover all the different kinds of mutation that could cause red-green colour blindness, so we set out to screen thousands of people, and also test them genetically. To assess their colour vision, we developed a test that will tell you in 2 minutes if you鈥檙e colour-blind.

How does your colour blindness test work?
It is . In each image there a shape that is invisible for a specific type of colour blindness, and different patterns of responses are diagnostic for normal colour vision versus every different kind of colour blindness. We chose the paper test because it stays calibrated. The problem with tests on computers is that different screens are not calibrated the same for colour. The paper test is also portable. You can hand it out to people.

Can you treat colour blindness?
We have been able to cure colour blindness in a monkey model. We selected squirrel monkeys because the females are trichromats 鈥� with three-colour vision like humans 鈥� but the males are all colour-blind, or dichromats. Our males only had blue and green receptors.

We gave them gene therapy, using a virus we learned about from colleagues developing a therapy for a muscle defect. Their biggest problem was that the virus they were using to deliver the therapy only infected a random subset of the muscle cells, but this was just the characteristic we wanted for ours! Our idea was that the virus would infect a subset of the green cones in the retina and convert them into red cones, so you get a random mosaic of red and green sensitivity in the male monkeys.

Were you confident it would work?
I鈥檓 always optimistic, but cautious. I thought that if we can change the retina so that it is like normal, that鈥檚 the first thing. Then, if it doesn鈥檛 convey a new colour to the brain, that鈥檚 the next thing we鈥檒l work on.

But after this treatment, the squirrel monkeys gained full colour vision?
Yes. We knew if this worked people would be really sceptical, so we ran tests on the animals for two years through the process. We wake the monkeys up every day before breakfast and have them do a test, which is based on the leading colour-vision test for humans. Their colour vision is absolutely stable; we measure the thresholds and they never change.

Could this type of gene therapy cure colour blindness in humans?
We did this simply as a cognitive neuroscience experiment. But after tens of thousands of emails from people telling me about how they would love to have normal colour vision, I now have this as a life goal.

鈥淎fter thousands of emails, curing colour blindness in humans is now a life goal鈥�

I鈥檓 pretty sure we would get the same result, but with humans we would have to do it in a way that is guaranteed 100 per cent safe. Finding a way to do it with no risks is one of the problems. Some people are so enthusiastic, they have offered to come into the lab at night and be injected without telling anyone. I have to tell them: 鈥淭hat鈥檚 not going work for us!鈥�

As you point out, most of us have three colour receptors. Yet aren鈥檛 there people who have four, called tetrachromats?
As a species, humans are trichromatic. But there is a polymorphism in the red pigment, so not everyone has the same. It鈥檚 possible for a female to have two different red receptors 鈥� say, orange-red and yellow-red. We did tests on retinas from an eye bank and found that some had the gene expression for four types of receptor. This was present in about 1 in 200 women. When we carry out genetic tests on people who say they have experiences seeing hues that other people can鈥檛 see, it turns out that they have genes for the extra pigments.

How does a fourth colour receptor affect vision?
Presumably it would be multiplicative 鈥� they would be able to see a hundred times more colours than normal.

Is there a test to learn if you鈥檙e a tetrachromat?
This is the hard part. Gabriele Jordan and her colleagues at Newcastle University in the UK developed an optical system which mixes red and green light to get an exact yellow. To a trichromat, it looks the same as monochromatic yellow light, but a tetrachromat sees different colours. Yet what is amazing is that when Jordan鈥檚 team examined , only one tested convincingly as a tetrachromat.

That may be because there is a huge learning aspect to our experience of colour. Monitors and TV screens only show the colours a trichromat would see, and most commercial paints are based on similar colour mixing. Surrounded by human-made objects in our daily lives, there is nothing to stimulate that extra colour sense.

What can we learn from tetrachromats?
My plan is to bring people who have the genetic potential into the lab and see if we can train them to be tetrachromats. What is most interesting for us is the light this can shed on how the brain works. We assume that neuroplasticity goes away at a certain age, but that may be the exception rather than the rule. We鈥檙e finding that people can learn whole new sensory capacities later in life. This could open up a whole world of doing things we couldn鈥檛 do before 鈥� and not just in vision.

Additionally, I would certainly like to try gene therapy to give monkeys the fourth photopigment, to see if they become tetrachromats.

Could there one day be a tetrachromat treatment for humans?
It could certainly be popular. Look at the way people threw away their old computer monitors when there were new ones with more colours. Suppose you could just have a shot and get the fourth photopigment, so you could see a hundred times more colours 鈥� who wouldn鈥檛 go for that?

This article appeared in print under the headline 鈥淒imensions of colour鈥�