Jan 25 2011

Tetrachromacy In Humans

Are you a tetrachromat? Probably not, but it is possible that the rare person is, with the super mutant power of enhanced color vision. OK – I would rather have Wolverine’s regeneration, but enhanced color vision would be cool.

Color vision in vertebrates is a result of the cones in the retina. Vertebrate retinas have two types of light-sensing neurons: rods see in black and white but have good light sensitivity, and so are specialized for low-light (night) vision. Cones are less sensitive than rods, but they respond to a specific range of wavelengths of light – i.e. color. By combining the color information from different cones with different wavelength sensitivities the brain is able to perceive a wide range of colors.

Different groups of vertebrates have different numbers of cones, and therefore a different range and ability to discriminate colors. Birds, for example, are tetrachromats – they have four different cones and can see farther into the ultraviolet than humans. In fact the common ancestor of tetrapod vertebrates was likely a tetrachromat. Most mammals are dichromats with only two cones. It is thought this reduction occurred during the early years of mammal evolution when our mammal ancestors were nocturnal and burrowing animals, and so needed night vision more than color vision.

Many primates, however, (including humans and our close relatives) are trichromats with three cones, and therefore have rich color vision, but not as good as birds. In fact our understanding of the genetics of cones and color vision provided yet another compelling line of evidence for evolution. Trichromatic primates do not have the same cones as their vertebrate ancestors. They did not regain one of the two cones that were previously lost. Mammals have two cones – an autosomal S-cone (a short wavelength sensitive cone), and an X-linked L/M cone (sensitive to median and long wavelength visible light and located on the X-chromosome).

Sometime after the divergence of new-world and old-world monkey, an old-world monkey ancestor underwent gene duplication of the X-linked cone gene. At first these genes would have been identical, but over time they diverged to become distinct cones with separated wavelength sensitivity. In humans these cone genes are 98% identical. The cones added sensitivity to red wavelengths and resulted in trichromacy.

The research into the evolution of color vision has also led to some interesting findings about human color vision specifically. It seems that humans have a significant degree of variability in the sensitivity of the cones. You have probably heard that some people are partially color blind, because it is standard (at least in the US) to test all school children for color blindness. But you may not have known that there is variability in the other direction as well, and that there are cases of tetrachromacy in humans.

One possible mechanism for this is that women may inherit two different versions of an X-linked gene for color vision. Women have two X-chromosomes, and in each cell one X-chromosome is inactivated essentially at random. So the retina would have a mixture (a mosaic) of cones from the two versions on the two different X-chromosomes, functionally producing four different cones in the retina.

In one study they found that most women with this condition did not demonstrate tetrachromacy on color vision tests – they still functionally were trichromats. This is likely due to the fact that the cones were not different enough. Although some hypothesize that the optic nerve or perhaps the brain combines the information from these distinct cones and treats them as one stream of color information. However, going against this hypothesis is the fact that 1 in 24 such women (according to one study) demonstrated four-dimensional (or tetrachromatic) color vision. This means that the optic nerve is capable of carrying tetrachromatic vision and the brain is capable of interpreting it.

There may be other mechanisms as well that could result in true tetrachromatic vision in humans. These cases demonstrate the plasticity of biology and the brain in particular. It also demonstrates that spontaneous mutations can result in the addition of function – in this case expanded color vision. Not only has this almost certainly happened in our evolutionary past, but it is happening today in living humans. This is not likely to result in the evolution of tetrachromacy in humans in general for two reasons. The first is that, in our modern society, there likely isn’t any selective advantage to tetrachromacy. Our primate ancestors probably benefited from trichromacy – the speculation being that it enabled them to forage for fruit and vegetables better. But unless we lived in a world dominated by fashion designers and painters, it’s hard to see how tetrachomacy would provide a significant survival advantages.

Second, humans are a large out-bred population. This does not mean that we are not evolving, but it makes it very unlikely that such a mutation will significantly spread throughout the population. It could by chance become prominent in an isolated population – the so-called founder effect. This has been demonstrated for inherited diseases, but can also occur with favorable mutations like tetrachromacy.

For now tetrachromacy remains in isolated individuals who are lucky enough to have their own mutant power.

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14 responses so far

14 Responses to “Tetrachromacy In Humans”

  1. Todd W.on 25 Jan 2011 at 8:54 am

    I wonder how often optic mutations like this might play some role in seeing “ghosts”. Could those who claim to see ghosts just be seeing added colors that the rest of the population can’t? Maybe similar for some aura claims? While I am fairly sure that most such claims stem from psychological factors and/or outright deception, perhaps there is a physical basis for some rare few.

  2. impaktdeviceson 25 Jan 2011 at 10:09 am

    Obligatory Calamities Of Nature link: http://www.calamitiesofnature.com/archive/?c=267

  3. Timmysonon 25 Jan 2011 at 10:43 am

    Is it possible that a mutation like this could lead tetrachromats to concentrate in, for example, visual arts communities?

  4. Marshallon 25 Jan 2011 at 11:15 am

    Todd–I’m going to say flat out not. Seeing additional colors will simply help you with hues–shades of colors, color discrimination, etc. What it won’t do is provide you with new outlines and edges that you hand’t previously seen (unless those edges are defined by colors that you can distinguish but others can’t–which is highly unlikely).

  5. SARAon 25 Jan 2011 at 12:22 pm

    Because vision is such an individual experience, I wonder if these folks ever knew they saw the world so differently than the rest of us until tested.

    It would be an awesome thing to have, but only if I got it AFTER having regular vision. Otherwise, I don’t suppose I would recognize the coolness.

  6. impaktdeviceson 25 Jan 2011 at 2:41 pm

    I propose a simple experiment. Shine light (presumably something with lower- and upper- frequencies outside the normal “visible” range) through a prism. Ask a number of Tetrachromats (yes I’m making up a word for them) to identify the boundaries of the resulting rainbow. Compare to Trichromat controls.

    1) Shape the results to draw tenuous connections to “auras,”
    2) Get published in a heretofore reputable science journal
    3) …
    4) Profit!

  7. bachfiendon 25 Jan 2011 at 6:44 pm

    I can’t remember which book I read it in, but I have read something similar recently about tetrachromacy in females. The (male) author suggested that this is the reason why some women take hours to dress before an outing. Their superior colour discrimination causes them to see subtle clashes of colour no one else can see!

  8. bachfiendon 25 Jan 2011 at 8:08 pm

    I know that my memory is unreliable, but I’ve just remembered which book I read about tetrachromacy. It was in Robin Dunbar’s book “How many friends does one person need? Dunbar’s number and other evolutionary quirks” chapter 2, which is very good provided one doesn’t mind reading ‘facts’ that aren’t actually true (for example, he claims DNA was recovered from a T Rex fossil).

  9. petrucioon 26 Jan 2011 at 2:51 pm

    @Marshall: Birds see many more contours in flowers than we see due to their ultraviolet vision. Non-flower objects in our day-to-day lives are unlikely to have such well defined ultraviolet contours, but I see no reason for such strong and quick denial of Todd’s interesting hypothesis, even if it is probably (but not certainly) a false one.

  10. SimonWon 26 Jan 2011 at 4:52 pm

    @petrucio Birds and insects are seeing in the ultraviolet – so colours that we can not distinguish. Since Marshall excluded those individuals (if any exist) who can distinguish colours we can’t, then his statement is likely correct. We are talking about 4 frequencies in the normal human visible spectrum of light, not 3 plus something very different.

    There are racial and gender difference in relevant eye morphology, so it is quite possible different folks see the world slightly differently in a very literal sense. However I suspect in most cases the effects on the overall effectiveness of vision are limited, as any radical difference is likely to be selected strongly for or against.

  11. Marshallon 27 Jan 2011 at 1:14 am

    @petrucio – Sure, there are a few cases where edges will appear that didn’t used to. But the number of these “new” edges will be very, very small compared to the number of edges that already exist–from the edges of objects, between different materials etc.

    I was responding to Tom’s comment with a very firm negative for an obvious reason: his entire hypothesis, that tetrochromats can see ghosts whereas us normal trichromats can’t, rests upon the fact that the ghosts are “hiding” in a very ridiculously small color space that they can distinguish but we can’t. He’s essentially saying that “ghosts” have the uncanny ability to blend into the background perfectly, yet offset the hue by a very small extent such that most people can’t perceive the hue difference, but tetrachromats can. It’s a wild and highly unlikely (understatement of the year) theory.

  12. Dianeon 27 Jan 2011 at 2:01 am

    @Marshall

    I think you are misinterpreting Todd’s hypothesis. He was suggesting that tetrachromats may be able to see edges that the rest of us don’t, and some of them may interpret those edges as ghosts or auras. He was not suggesting that ghosts and auras are real, but only detectable by tetrachromats.

  13. petrucioon 27 Jan 2011 at 4:11 pm

    @Diane
    He got that from what I read.

    But he’s saying that the added frequency able to be seen by tetrachromats is too narrow to show significant differences. Which I do agree with, but not enough to discard the whole hypothesis as silly upfront (we have been more wrong before).

    I may indeed be giving the hypothesis too much credit, and it may be sillier than I’m supposing, but I probably lack understanding on the topic to make an informed assessment.

  14. BillyJoe7on 28 Jan 2011 at 5:14 am

    I had deja vu whilst reading this arrticle. Richard Dawkins has something similar to say in his book “The Ancestors Tale”.

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