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Explainer: what is colour blindness?

Here are six test tubes filled with coloured dyes. How many different colours do you see? Most people say six, but some people would say only two or three. There are even some (very rare) people who see…

This is how a person without colour blindness would see these coloured test tubes. Bigstockphoto / Craig Colvin

Here are six test tubes filled with coloured dyes. How many different colours do you see? Most people say six, but some people would say only two or three. There are even some (very rare) people who see no colour at all.

How can it be that one person says two things have the same colour, yet somebody else says they are completely different?

Colour isn’t really there

Scientists know that Isaac Newton did more than just sit around watching apples fall into his garden. He linked gravity on Earth to the movements of planets, and his experiments with glass prisms showed that white light is a mixture of different wavelengths (he called them refrangible rays).

The eye has red, blue and green cones Flickr / Cam Incoll

One of his many brilliant insights was that unlike size or weight, colour is not a property of the objects that fill our world. Colour depends precisely on which wavelengths of light are bounced from objects before they reach the eye.

Colour is a sensation, a property of the mind. No matter how bright and vivid and real they seem, colours are inside your head, not outside.

Why do people see colours?

Light is picked up in the eye by three types of cells called cone cells. They are called cones because in the microscope they look like tiny ice-cream cones.

They are nicknamed red, green, and blue because they pick up different wavelengths and there are millions of each type cone cells. Just as a painter can mix from three tubs of paint to produce a wide and vivid palette, your brain uses these three cone types to create thousands of colour sensations.

Cone cells in the eye. The blue cones are labeled with a dye to show their shape Bigstockphoto / ugrunert

What causes colour blindness?

The cone cells are just like other cells in the body - they are controlled by genes. The genes controlling cones are prone to faulty replication during early development, and affected cones either fail to develop, or start to pick up abnormal wavelengths. The result is like taking away or diluting one of our painter’s tubs: the colour sensations are reduced or changed.

We can filter an image to show how a red-green colour blind person might see it; colour does not disappear but the range of colours is reduced.

A red-green colour blind view of the test tubes Bigstockphoto / Craig Colvin

How is colour blindness inherited?

Every cell in every woman’s body contains two gene packages called X chromosomes, but men have only one. The genes controlling red and green cones are located on X chromosomes.

If a woman has a faulty or missing gene on one X chromosome, the gene on the second X chromosome works as a backup and the cones develop normally.

But if the faulty X chromosome is transmitted from mother to son, there’s no backup, and the son will have reduced or altered colour sensations, called red-green colour blindness. Other forms of colour blindness are much more rare and are usually more severe.

This picture shows the different types of colour blindness Flickr / entirelysubjective

Can colour blindness be cured?

Most scientific studies suggest that the wiring of the eye and brain is identical in people with normal and abnormal colour vision. The only difference is at the first stage of vision, where the cones can be faulty.

The obvious solution is to fix the faulty cones, and this is what a team led by scientists Maureen and Jay Neitz at the University of Washington in Seattle have attempted.

Monkey see, monkey do.

Maureen and Jay Neitz study a species of monkey in which all the males are red-green colour blind. In gene therapy trials, their team injected colour-blind male monkeys with the gene controlling the missing cone type.

This monkey’s colour vision was improved by gene therapy. Jay and Maureen Neitz

We don’t know what monkeys see, but we can see what they do. Two monkeys tested so far learnt how to tell red from green patterns after the injections.

These are promising results but the eye is a very delicate organ and the injections are still dangerous to sight. Colour blindness may be curable but there is still some way to go.


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27 Comments sorted by

  1. mixmaxmin

    logged in via Twitter

    Great article and exciting prospects. When my kids were old enough to understand that I couldn't see all the colours as they saw them, my youngest at the time - about 4 - proceeded to take out his colouring pencils and draw a line with each colour asking me to repeat after him what the colour was. He then tested me by jumbling the pencils and randomly picking one out from the bunch. It was difficult to explain that it was not about learning the colours. I wanted to show what I experienced so I went…

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  2. John Lamp

    Senior Lecturer, School of Information Systems at Deakin University

    My only problem with this article was that, as I looked at the pictures and thought "so that's how a colour blind person perceives colours" the rather confronting thought came "but how would I know whether I was actually a colour blind person getting a colour blind person's view of what a colour blind person sees?" Back to square 1!

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    1. Peter Macinnis

      logged in via Facebook

      In reply to John Lamp

      As somebody who is usually regarded as "very colour-blind" in lay terms, I looked at these with some curiosity, because in the past, I have seen Kodak 35 mm slides which purported to show the Ishihara (confetti) tests accurately, and they didn't. The examples shown here are far better. I give this account full marks, and my slighting comments below about the idiots who pontificate on this condition do NOT apply to Professor Martin.

      Looking at the four "windmills", I now suspect that my belief…

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    2. Dianna Arthur
      Dianna Arthur is a Friend of The Conversation.

      Environmentalist

      In reply to Peter Macinnis

      " I suggest that the normal response of "normals" on hearing that one is colour blind (which is to point at something and ask what colour it is) means that colour blind people are trained to be more logical."

      Guilty as charged.

      Whatever natural impediments we humans may be born with, there are always alternatives to compensate. Hence the amplified hearing and touch of blind people, the non-hearing to vibration and so on.

      The photos used in the article helped explain the colour-blind world to me very succinctly.

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    3. Rodger Kensen

      Systems Analyst

      In reply to Peter Macinnis

      Regarding the "normal response of "normals" on hearing that one is colour blind (which is to point at something and ask what colour it is)".

      I tolerated this seemingly mandatory line of questioning for the first twenty odd years of telling people I'm colourblind, before I came up with the rather blunt and somewhat offensive response for days where I lacked patience "If I had of just told you I was a pedophile, would you now be asking me to look after your children?".

      While staggeringly effective at stopping the line of questioning, it should be used with caution.

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    4. Dianna Arthur
      Dianna Arthur is a Friend of The Conversation.

      Environmentalist

      In reply to Rodger Kensen

      You couldn't find a better analogy than that of a pedophile? Hearing impaired? A stutter? 2 left feet? Male-pattern balding?

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    5. Rodger Kensen

      Systems Analyst

      In reply to Dianna Arthur

      I appreciate that it's a very blunt instrument only for use on the most boorish, but the attraction is that it's a very stark contrast of a similar concept and almost guaranteed that nobody will ever say yes.

      Alas none of your examples fit particularly well as a comparison, although if you can come up with a similarly effective yet less offensive method I will be more than happy to change my ways.

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    6. Dianna Arthur
      Dianna Arthur is a Friend of The Conversation.

      Environmentalist

      In reply to Rodger Kensen

      If pedophilia is more analogous to colour-blindness in your world-view, far be it from me to suggest less confronting methods.

      I have learned my lesson and shall refrain from future suggestion or correspondence.

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    7. Rodger Kensen

      Systems Analyst

      In reply to Dianna Arthur

      Not sure where you get that I am saying colorblindness is analogous to pedophilia.

      The analogy is that asking someone who is colorblind what colour an object is akin to asking a pedophile to look after your kids.

      It is something borne out of the frustration of 20 years of the same question every time I tell someone I'm colourblind.

      I'm sorry I shared.

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    8. Peter Macinnis

      logged in via Facebook

      In reply to Rodger Kensen

      Given that I started this, let me offer a gentler analogy which I read in 'Scientific American' many years ago. It was in a comment to the effect that people do not, on hearing that somebody is blind, think it amusing to send them into a room with obstacles in it. There are ALWAYS giggling idiots who find it amusing to ask the colour blind to name colours so the audience can snigger and guffaw. One may think this sort of behaviour died out when they stopped letting the allegedly sane into Bedlam…

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    9. Bruce Tabor

      Research Scientist at CSIRO

      In reply to Peter Macinnis

      Thanks Peter, fascinating.
      On your comment, "...look at the four quadrants: if they all look different, you are normal. If any two of them look the same, your display may need adjusting, but more probably, some of your cones are deficient."

      The two on the right (green & red-blindness) are very similar. To my eyes the difference between them is one of colour saturation, not hue. In particular the maroon region is almost black rather than olive green.

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    10. Dianna Arthur
      Dianna Arthur is a Friend of The Conversation.

      Environmentalist

      In reply to Peter Macinnis

      Thank you Peter.

      We all have our burdens - while I do not know what it is like to be colour-blind (for which I an very grateful having some artistic skills), I DO know what discrimination from the ignorance of others is like.

      Migraine - "just a bad headache"
      Bi-polar - "pull yourself together"
      Chronic Fatigue - "malingerer and lazy"

      I have heard it all.

      I also freely admit that Rodger Kensen's attempts to explain his experience of colour-blindness went completely over my head. However…

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    11. Graham Dawson

      logged in via Facebook

      In reply to Peter Macinnis

      I'm diagnosed red-green colourblind which buggered up my childhood ambition to be a pilot. I'm hopeless with the standard test and not great at matching clothes (so I'm told by the women in my life), however I see all six colours in the test clearly and the 4 windmills are all different to me...
      I'm feeling like I've been conned!

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    12. Paul R Martin

      Professor of Experimental Ophthalmology

      In reply to Graham Dawson

      Dear Graham,

      Thank you for your comments. The examples shown here are only likely to pick up severe red-green deficiencies ("dichromacy"); yours may be milder or you may, as many other males with CVD (colour vision deficiency), have found other visual "clues" in the images. Strongly coloured edges, for example, can give clues by the tiny interference fringes, called chromatic aberrations, that they produce in the eye. These subtle signals are ignored by most viewers but CVD observers can become much better than "normal" observers at picking up and using them. It's a good example of the wonderful learning power of the human brain.

      The diagnostic CVD tests are specifically designed to prevent such clues, and are administered under controlled lighting conditions.

      Feel free to contact me directly if you have further questions, best wishes, Paul.

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    13. Graham Dawson

      logged in via Facebook

      In reply to Paul R Martin

      Hi Paul,

      Thanks very much for your answer. I think I'll take some comfort telling people that though my cone cells are a bit ordinary, my brain is really good!

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    14. Emma Anderson

      Artist and Science Junkie

      In reply to Dianna Arthur

      I have known two very artistically talented people who had two different forms of colour blindness. In both cases, it was the visual art, and in both cases, until some kind of blindness-specific test (incidental, or medical) event occurred, no one would have known.

      If only I had the appreciation for aesthetics they do, but as it is said, in the eye of the beholder.

      Also Peter in a previous comment you mentioned that if any two of the 'windmill' images above are the same to the viewer, perhaps we should adjust our monitor or maybe we are also colour blind. I am not sure that is technically correct.

      The images on the right hand side, whilst specifying two kinds of colour blindness, actually refer to the same cone type in the opponent-process model. Seeing more red means seeing less green, and vice versa.

      Whilst I am not technically colour blind, it is these two images that are almost identical to me, with the small detail that the hue in the bottom right image is darker.

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    15. Paul R Martin

      Professor of Experimental Ophthalmology

      In reply to Peter Macinnis

      Dear Peter, thank you very much for your insightful comments.

      You can see more about experiences of colour vision deficient observers at: http://www.colourmed.com.

      Best wishes and thanks again for taking time to explain your perspective, yours, Paul

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  3. Chris Plant

    Engineer

    Only red-green colour blindness is linked to the X chromosome. Blue-yellow blindness (the next most common) is linked to a totally different chromosome (whose number escapes me at the moment). Intriguing that sight is linked to so many different parts of our DNA.

    The article fails to mention that we are all born with different numbers of cones and rods, so experience colour and light intensity very differently i.e. the more cones, the fewer rods so those with good colour vision have relatively poorer perception in low light or at night, and vice versa.

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    1. Paul R Martin

      Professor of Experimental Ophthalmology

      In reply to Chris Plant

      Hi Chris,

      Thanks for your comments,

      You are correct to write that blue-yellow colour blindness ("tritanopia") is not X-linked, it is linked to chromosome 7. Because each cell has two of these chromosomes the second can act as a backup if one is faulty. The chance of getting two faulty genes is very low and tritanopia is correspondingly rare.

      To your second point the numbers of rods and cones in the human eye are quite consistent between individuals. To my knowledge there is no link between quality of colour vision and quality of night-time vision of the type you mention. There are, on the other hand, diseases which specifically affect colour vision or night-time vision. Feel free to contact me directly if you have further questions, best wishes, Paul.

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  4. Freestyle Cyclists

    logged in via Twitter

    I'm not sure that a "cure" for colour blindness is wanted or needed.

    If colour blindness were harmful wouldn't it have been eliminated from the genome, since it is always expressed?

    Is it correct that in red/green colour blindness the red cones are sensitive to a different wavelength, closer to the green? Therefore giving a heightened sensitivity in the greens along with the reduced perception of reds?

    I'm colour blind, and better than most people at picking up camoflagued birds and animals in the bush, but hopeless at finding red berries.

    I don't think of myself as handicapped, just differently abled.

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    1. Paul R Martin

      Professor of Experimental Ophthalmology

      In reply to Freestyle Cyclists

      Hi Freestyle Cyclists,

      Thank you for your comments, and for your good questions. The evolutionary biology of red-green colour vision deficiency (CVD) is still a matter of scientific debate; for example the prevalence of CVD shows variation among races (high for Caucasian, low for Inuit); some species of monkeys have almost no CVD but in other species the males all show CVD. The fact that some males with CVD can "see through" colour camouflage (as you can) may have something to do with these interesting differences, but there is no single clear answer.

      Regarding wavelength sensitivity you are correct that in mild red-green CVD both red and green cones are expressed but one or other (or, rarely, both) type has different sensitivity.

      best wishes, Paul

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    2. Emma Anderson

      Artist and Science Junkie

      In reply to Paul R Martin

      Coming from a different background, I understand that the term ethnicity might be more applicable to what you're describing, nonetheless, I find that interesting.

      I was going to ask if this variation had something to do with latitude and it's relationship to light intensity, ie actual colour variations in the environment. But then I realised that there is a second thing, that is the Inuit still live in a mostly cold climate (tundra) whereas most (but not all) of Europe has experienced it's largest population growths since the end of the last major ice age. So I would anticipate that the results would be the reverse, you'd need less red/green discrimination in the snow than in, well, a temperate zone. Perhaps this is a more recent mutation, but one distant enough in time to have occurred before the ancestors of the Inuit and Indo-European peoples branched off from each other?

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  5. Tracey Tritsch

    Lead Associate

    A very interesting article, thanks Paul. Does this mean that most colour blind people are men?

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    1. Paul R Martin

      Professor of Experimental Ophthalmology

      In reply to Tracey Tritsch

      Hi Tracey, yes you are correct, because (most commonly) the X chromosome carries the faulty gene the fault is only evident in males and can be passed from mother to son. Best wishes, Paul

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  6. Andrew McGee

    Lecturer, Faculty of Law at Queensland University of Technology

    Dear Paul

    Thanks for an interesting article. There's just one thing I'd take issue with, but it's certainly not a claim that begins (or likely) will end with you. It's the claim that objects are not really coloured or that colours are sensations. There's been a lot of work on this in philosophy (some of it awful). The good work was pioneered by J L Austin in Sense and Sensibilia, and is required reading for anyone interested in these issues. A book building partly on Austin's work (as well as…

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    1. Paul R Martin

      Professor of Experimental Ophthalmology

      In reply to Andrew McGee

      Dear Andrew,

      Many thanks for your comments and analysis.

      I agree that I am, to paraphrase Newton, writing "not philosophically and properly, but grossly, and according to such conceptions as vulgar people would be apt to frame."

      I an treating describing colour as a visual sensation: a physiologist distinguishes several sensory modalities (somatic - touch; auditory - hearing, and so on). The focus here is on sensory discrimination. I imagine that if a person reports that two objects have the same colour then I could, in principle, find that the two objects produce indistinguishable patterns of nerve activity somewhere in the brain.

      Having written that, your fine point is well taken!

      best wishes, yours, Paul

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  7. VIVEKANANDHAN THIRUNAVUKARASU

    ELECTRICAL ENGINEER

    this article is very interesting one. I thoroughly understand what you say! your article is in simple english. i like your way of expressing.

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