The mystery of tetrachromacy: If 12% of women have four cone types in their eyes, why do so few of them actually see more colours?

* Scientific study references are indicated in (). These can be found at the bottom of the article, and can be clicked on to access the original reports.Tetra1In these paintings, Australian artist Concetta Antico aims to capture her extraordinary visual experiences, which she describes as consisting of a mosaic of vibrant colours. In an interview with the BBC, Concetta reflected on the sight of a pebble pathway, which most people perceive as grey: The little stones jump out at me with oranges, yellows, greens, blues and pinks’ (1).

In 2012, a genetic analysis confirmed that Concetta’s enhanced colour vision can be explained by a genetic quirk that causes her eyes to produce four types of cone cells, instead of the regular three which underpin colour vision in most humans. Four cones give Concetta the potential for what researchers call tetrachromacy (from Greek ‘tetra’ – four, and ‘khrōma’ – colour), instead of normal trichromatic colour vision (from Greek ‘tria‘– three). This means that her eyes can enjoy a diversity of colours that is about 100 times greater than what is accessible to the rest of us.

While tetrachromacy is so rare that it makes headlines every time a new case emerges, it might come as a surprise that women with four cone types in their retinas are actually more common than we think. Researchers estimate that they represent as much as 12% of the female population (4). So why aren’t we surrounded by women with extraordinary colour vision? Researchers have found that only a small fraction of women who possess an extra cone type actually get to enjoy more colours. So what does it take to be a true tetrachromat? How does the human retina come to produce four cone types, and why does it only concern women? More importantly, why don’t all women fulfil their genetic potential? And how do we find the special women who do?

The fourth cone – science fiction? 

The three cone types that most of us have in our retinas allow us to see millions of colours. Each cone’s membrane is packed with molecules, called opsins, which absorb lights of some wavelengths and cause the cone to send electrical signals to the brain. The opsin molecules vary between the three cone types, so that each type is sensitive to different wavelengths from the visible spectrum (3). Together, these cone cells allow the brain to identify the wavelengths of light that our eyes encounter – colour is experienced as a way of registering this information in our consciousness.

Trichro

Individuals who happen to be born with a fourth cone type containing a new light-absorbing opsin molecule technically have the potential to distinguish a greater number of wavelengths, and thus perceive more colours. So are these extra colours like something taken from a sci-fi movie?

So far, there are no documented cases of humans with a fourth cone that captures light beyond the wavelength range of 400-700 nanometres, which is the normal visible spectrum. Thus, the existence of four cones isn’t quite the epic sci-fi scenario in which the eye becomes a hybrid between a human and some other species, like a bee or snake that can see ultraviolet light (7,8). Instead, the most common cause of a fourth cone is if an individual inherits a subtle change in DNA sequence (mutation) in one of the already existing genes for the light-absorbing opsin molecules that fill either the M- or L- cones. The human eye gains slightly superhuman abilities within the visible spectrum.

The genetic origins of four-cone retinas

An extra cone might come about if a mutation in one of the opsin genes affects the physical structure of the resulting opsin molecule in a way which influences its sensitivity to light. This change can essentially create a new cone type, because cone cells which contain the altered molecule react differently to various wavelengths compared to cones which contain the original opsin made from the non-mutated gene.

Since the M- and L- cone opsin genes are located on the X-chromosome, only women could possibly enjoy the benefits of such a mutation. A male inherits only one X-chromosome. Thus, if the single X-chromosome he receives from his mother carries a change in the M-cone opsin gene, his retina will ultimately produce three cone types: normal S-cones with opsins from a gene chromosome 7, and regular L-cones as well as abnormal M-cones containing mutated opsins from the same X-chromosome. This man would be classified as an anomalous trichromat since, like in most humans, his three cone types allow him to experience roughly the same number of colours, albeit slightly differently.

A woman, on the other hand, has the potential to produce four cone types because she inherits two X-chromosomes. So if one of them contains a mutated opsin gene, she will have one X-chromosome to provide the normal M- and L-cone opsins, and an additional chromosome to produce the mutated ‘new’ opsin. The illustration below provides some more detail.TetragenesAs mentioned, researchers estimate that women born with four cones are quite common, while the actual capacity to see more colours is exceptionally rare. So how do we objectively test whether women with four cones experience a greater range of colours? And once we identify those who indeed see more hues, how do we explain why some, but not others, can enjoy the genetic potential of tetrachromacy?

Testing for tetrachromacy with different colours designed to seem identical to the rest of us

Researchers aiming to investigate how many women actually have superior colour vision first need to fish for potential tetrachromats in the massive human population. Since women with four cones have one mutated X-chromosome, they have a 50% chance of passing that X-chromosome to their sons. This makes them much more likely than other women to have sons who are anomalous trichromats, which I described earlier. Researchers use this when seeking candidates for tetrachromacy, as they advertise for female participants whose sons have colour vision anomalies (4). The next important dilemma is to figure out how to objectively measure these women’s visual abilities. Where do we even begin searching for hues that seem identical to us but might seem distinct to tetrachromats? This challenge is by no means trivial – if we were to test for tetrachromacy by asking women if they see differences between randomly selected colour mixtures, we’d have a ridiculously long experiment.

Conveniently enough, the anomalous trichromats born to these women provide a useful starting point. While they are poorer than most people at discriminating some colours that seem obviously different to us (which is why they are often considered ‘colour-deficient’), they can in fact distinguish some colours that we perceive as identical (2). Researchers assume that if a woman with four cones sees extra colours, they must be the same ones that her sons see, given that her retina possesses the same mutated cone type (although the mother also has a fourth cone type and thus avoids the impairment her sons have with some other colours).

The surprising existence of extra colours that are visible to anomalous trichromats means that we can test for tetrachromacy by asking women if they see differences between colours that appear identical to normal trichromats, but seem different to their sons. How do we design these colours? For starters, we can use valuable findings from scientific experiments.

In 1992, researchers used bits of human DNA to produce the S-, M- and L-cone opsins inside cells and study their reactions to lights of different wavelengths (5). This experiment showed that we can easily calculate the signal that each cone type will produce when stimulated with various wavelengths. As an example, let’s take the M-cone, shown below.

Tetra4

Knowing what we do about how different cones respond to various lights, we can design mixtures of wavelengths that would produce the exact same signals across the three cone types in the normal human eye, but not in the eye of an anomalous trichromat. These mixtures would seem identical to an individual with three regular cone types, but not to one with a mutated cone.  Here’s a scenario where a normal trichromat can’t see the difference between two physically distinct colours, while an anomalous trichromat can.

Let’s start with the normal trichromat:

Tetra5The signals that the regular cones ultimately produce when stimulated with 590 nm light are exactly the same for a mixture of 540 nm + 670 nm light! When the brain receives these identical signals, it has no way of distinguishing between the two types of light, and the trichromat perceives them as identical.

Now let’s look at an anomalous trichromat who has a mutated M-cone with a light sensitivity profile that, compared to the original M-cone, falls slightly closer to the regular L-cone.

Tetra6Notice that the signals produced by these three cone types are quite different for 590 nm light and the mixture of 540 nm + 670 nm lights. This means that the anomalous trichromat’s brain can sense a distinction between these two types of light, and so the man himself can experience the difference in colour. As mentioned, this man’s mother has the same mutated M-cone in addition to three regular cones, making these types of colour mixtures ideal for testing if she can experience more colours.

This is exactly what researchers did in 2010 (4). They presented women with pairs of colour mixtures designed to appear identical to regular trichromats, but which their anomalous trichromat sons could distinguish. They were then asked to rate how similar these mixtures appeared on a scale of 1 to 10, and their answers were compared to those of normal trichromats’ mothers, who were unlikely to have four cones.

Here, the first signs appeared that four cones don’t automatically grant you superior colour vision. The mothers of regular trichromats and most mothers of anomalous trichromats behaved similarly in this experiment. The similarity ratings they gave to various pairs of colour mixtures on one occasion were not the same ones they gave when asked about the same pairs some other time. These women seemed to be giving pretty random responses, making it doubtful that any of them really saw differences between the colour mixtures. Genetic analyses confirmed that at least seven of the nine anomalous trichromats’ mothers did in fact have four distinct cone types in their retinas. And yet, their colour vision wasn’t any better than that of women with three cones. Quite the enigma.

Only one of the seven women with four cones behaved as if she actually perceived differences between the colour mixtures that were invisible to everyone apart from her sons. For any given pair of colour mixtures that she was asked to rate in terms of similarity, she gave the same number when asked on separate occasions. She clearly wasn’t just picking a random number every time, but seemed to actually see the colour differences. What makes her different from the other women with four cone types?

If having four cone types isn’t enough, what does it take to see more colours?

When it comes to genetic mutations, some are insignificant, as they produce molecules that differ only slightly, or not at all, from those made by non-mutated genes. Other mutations can have a dramatic effect on the structure of the protein that a gene goes on to produce.  With opsin genes, some mutations cause massive shifts in the light sensitivity of the resulting opsin molecule, while other mutations make a smaller difference.

The challenge for most women with four cones is that their extra cone is simply not different enough from an already existing cone type to be useful to the brain. Let’s look at two women with four cone types.

CandidatesThe light sensitivity profile of the first woman’s extra cone overlaps heavily with the profile of the normal L-cone. So, when her retina is stimulated by lights of different wavelengths, the signals that the fourth cone sends to the brain don’t really differ from what the L-cone already provides. Remember – the only way cones allow us to see colours is by sending the brain different signals for different wavelengths. If cone signals remain the same for various wavelengths, how could the brain, and so the brain’s owner, possibly see a difference?  Unfortunately, this woman’s fourth cone is so similar to the L-cone that the visual system doesn’t even notice its existence.

On the other hand, the light sensitivity profile of the second woman’s extra cone is comfortably couched between the normal M- and L-cone profiles. This cone is different enough from the rest that when the retina is stimulated by lights of various wavelengths, all four cone types produce different signals. This fourth cone becomes useful for discriminating more wavelengths, and its owner might see 100 times more colours than the rest of us. This is exactly what researchers found with the only true tetrachromat they discovered in their experiment. Analyses of the opsin genes on her X-chromosomes revealed that the light sensitivity of her fourth cone type was ideally separated from the neighbouring M- and L-cones by a comfortable 12 nanometers (4)!  In most other candidates, the fourth cone was too similar to the closest existing cone, making it incapable of enhancing colour vision.

Ultimately, experiments teach us that cones are necessary tools for seeing colour. But if one tool is no different from the next, the brain simply discards it and settles for what it has. Of the millions of women in the world whose eyes have four cone types, only a few will have won the ‘ideal’ mutation lottery that allows them to experience a seashore of colours like the tetrachromat artist Concetta Antico.

Tetra8

PS. If you are interested in learning more about how regular trichromatic vision works, have a look my previous article.

References

  1. BBC article: The women with superhuman vision. September, 2014.
  1. Bosten, J. M. et al. (2005). Multidimensional scaling reveals a color dimension unique to ‘color-deficient’ observers. Current Biology 15, R950.
  1. Hofer, H. et al. (2005). Organization of the human trichromatic cone mosaic. Journal of Neuroscience 25, 9669-9679.
  1. Jordan, G. et al. (2010). The dimensionality of color vision in carriers of anomalous trichromacy. Journal of Vision, 10.
  1. Merbs, S. L. and Nathans, J. (1992). Absorption spectra of human cone pigments. Nature 356, 433-435.
  1. Ray, P. F. et al. (1997). XIST expression from the maternal X chromosome in human male preimplantation at the blastocyst stage. Human Molecular Genetics 6, 1323-1327.
  1. Sillman, A. J. et al. (1999) The photoreceptors and visual pigments in the retina of a boid snake, the ball python (Python Regius). Journal of Experimental Biology 202, 1931-1938.
  1. Townson, S. M. et al. (1998). Honeybee blue- and ultraviolet-sensitive opsins: cloning, heterologous expression in Drosophila, and physiological characterization. Journal of Neuroscience 18, 2412-2422.

 

 

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31 thoughts on “The mystery of tetrachromacy: If 12% of women have four cone types in their eyes, why do so few of them actually see more colours?

  1. I was surprised that this article contained no mention of the function of experience and brain development in the capability to distinguish colors. Studies of some members of tribes in Africa have shown that they are capable of distinguishing between different shades of green, brown, blue, and other hues found in their natural environment with radically greater precision than those of us in western cultures are capable of. Those same tribal people are incapable of distinguishing between colors which appear extremely different to westerners, bright vibrant colors that don’t occur in nature where the tribe lives. The ability to distinguish between different colors is, at the very least, heavily influenced by brain development. If distinguishing between extremely similar shades of one color is very important for a person, their brain will develop more precision in distinguishing between such things. I believe the tribe studied was the Himba tribe.

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  2. Hi Dustin! Thanks for commenting. From what I understand, you are referring to the Sapir-Whorf hypothesis, which suggests that cultures with different linguistic colour categories see these colours differently – meaning that since the Himba people apparently have distinct words for shades of green, they might be better at perceptually discriminating these shades than individuals from cultures where green is described using a single word. I have tried to looking into this tribe, and found that some experiments with them were featured in a BBC documentary about colour vision, where researchers suggested that members of the tribe respond more quickly to differences in green shades than to differences between green and blue (their language doesn’t make that distinction). However, it appears that the research from this documentary has not resulted in a scientific publication. This is something we need to be a bit careful about, because the process of publication tends to involve the submitted work being reviewed by fellow academics with the aim of ensuring that research stands up to standards, and is thus important for verifying the quality of research. It’s quite possible that I just didn’t find the publication about colour perception in the Himba tribe, so please feel free to provide a link and prove me wrong! In case you are interested, here’s an excellent experiment arguing against the idea that our language categories truly influence our colour discrimination abilities.

    On another note though, you are probably right that our ability to distinguish between different colours is to an extent shaped by which distinctions are important to us in life. Much like an expert cheese taster (first thing that came to mind) acquires the capacity to taste differences in flavour that they couldn’t previously perceive, it’s very likely that being required to discriminate certain shades (eg. working as an artist) makes your brain more capable of capturing and noticing colour differences which appear very subtle to most other people. Having said that though, I think perceiving a colour difference is entirely unrealistic if the two surfaces you are looking at produce no difference in the cone responses in your retina. In that case, there is no viable way for the brain to obtain any information about the physical difference between two colours, and so there is essentially no potential for training / learning to change that. With respect to the capacities provided by the extra cone in four-cone females, I think the extent to which the fourth cone differs from the rest heavily determines how much additional information that cone can provide. If the cone is too similar to an already existing cone type, I see no way for the woman to ‘learn’ to distinguish colours that the rest of us see as identical. Ultimately, you are correct in that the demands of life shape brain development and how it processes colour, but only to the extent that the cones can provide the brain with the initial information about colour differences. I hope that answer was somewhat satisfactory, feel free to come back with links and arguments!

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  3. Hi Sofia –
    thanks for the informative article and the excellent illustrations. I had one thought: differences in receptor responses between the two eyes might provide another way of potentially enhancing color discrimination. I’ve not looked into the research side of this, but I’ve noticed in my own case that my two eyes generate slightly different subjective color responses: one is shifted towards the blue end of the spectrum. I don’t think my color discrimination is above-average, but who knows? I think it’s an additional degree of freedom worth looking into. And, by the way, a happy new year!

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  4. Thanks for commenting and Happy New Year to you too! I’m not sure what explanation there might be for subjective differences in colour between the two eyes… While the idea that differences between the two eyes can be harnessed to further increase colour discrimination is interesting, I’m quite hesitant that this is biologically plausible. For this interocular information to be used by the brain, I would think that there need to be direct antagonistic interactions between different photoreceptor cells in the two eyes – but as far as I know, we have no evidence of this. Rather, it seems that inhibitory interactions occur primarily, if not exclusively, between tiny neighbouring clusters of cells on the same retina. Also, I think if the brain did somehow achieve direct comparison between cells from the two eyes for colour discrimination, there would be so little certainty about where exactly that signal difference originates from (as it could be anywhere in the space between the two connected cones), that we wouldn’t really be able to perceive something as having colour. Perhaps a weird coloured mist… That’s my take on it, though there is no literature on between-eye comparisons of cone responses.

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  5. There is a popular myth out there concerning the visual capabilities of the mantis shrimp. This little creature has not 3 or 4 but 12 different photoreceptors! This has led a lot of people to assume that these creatures must see an unimaginable threefold of colours, whereas in reality, most of these photoreceptors cover the same spectrum of visible light. Well, they do also see into the low ultraviolet range, that humans cannot see, and have special ommatidia for discerning polarised light, but that’s about it. In fact, studies have shown that they’re actually worse at discerning colours than humans, not being able to see a difference between colours closer than 25 nm. In humans it is 1 to 5 nm.

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  6. bees do “see” electromagnetic waves of the ultraviolet part of the spectrum – that much is correct; snakes however don’t – they instead perceive the much longer electromagnetic waves called infra-red – on the other end, and also invisible to humans…

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  7. We had a wingback chair growing up that I always thought was a very deep and beautiful purple. But everyone told me that the chair was black, and that I needed to stop talking crazy.

    I eventually DID stop calling that deep color purple and tried to just call it black – not only with that chair but when I would see it other places and people said it was black.

    I wonder if psycho-social factors ever come into play when we are taught to describe something as subjective as color.

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  8. This is very interesting as I always “thought” I saw more colours than regular people, and both my sons are colour-blind on different levels. Now I understand why ! I also have colour related synesthesia, I wonder if it’s relevent? How/where could I find a study to take part in?

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  9. Hi Catherine! Unfortunately it’s not very straightforward to take part in studies – researchers are only on the lookout for certain types of participant (eg. potential tetrachromats) if they’re currently undertaking a specific programme of study on the topic, so there is generally no way to walk into a lab and ask to take part in a study. Having said that, if you are curious, the main researchers that come to mind when it comes to researching colour vision are The Neitz Lab in Seattle,
    USA (http://www.neitzvision.com/) and the Mollon Lab in Cambridge. (http://vision.psychol.cam.ac.uk/jdmollon/). If you’re very curious, it doesn’t hurt to get in touch with someone there and ask if they are currently on the look out for potential tetrachromat participants 🙂

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  10. There are two problems with this article:

    1. The first women sees many colors in gravels that look gray to everyone else. I don’t understand how this has something to do with tetrachromacy. Usually gravels come from the same place and are relatively similar in color. What are the odds that they are actually all different in their reflection spectra and look somehow all the same to all trichromats both normal and anomalous ? This would mean that somehow, all the color difference in the spectrum is located exactly where these women have a fourth type of cone, so that only them are sensitive to it. But some trichromats also have cones with the same sensitivity, how come they don’t see this difference ? If you think about it it seems impossible that her vision is due to tetrachromacy. It rather seems like something similar to synesthesia.

    2. Our cone cells don’t send colors to the brain, they send electric signals. For the brain to understand what color they mean, it needs to “know” where the signal comes from. Apparently this is encoded as a different neuron firing rate depending on which cone got a signal. So for someone to realize that the signal comes from the normal cones or the mutated cones, the fourth type, the signal they send must have a different rate. Is it the case ? If they are not or if they are too similar, they will be treated as the same color. As a result, the person would be exactly like a trichromat, with a sensitivity for the red cones that would be the average of the normal cones and mutated cones. And in the experiment that is shown above, they test if women are behaving like anomalous trichromats, not tetrachromats.

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  11. Also if you think you are seeing more colors than other people, you are probably just “colorblind”, an anomalous trichromat. Being “colorblind” is a wrong term because, as said it in the article, you don’t see less colors, you see them differently. You can tell the difference between colors that seem the same to other people, but the opposite has to be true too, there must be colors that look different to other people that you see as the same. This is how it works.

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  12. Hi Trith! You raise some good points. Here are my responses:

    1. When it comes to the question of whether / how this artist who markets herself as tetrachromatic can see many colours in gravels where we see grey, I think there are several plausible explanations. 1 – She could of course be lying about or exaggerating her visual capabilities. We can never really know, so to some extent we should take her word for it, given that one of the top colour vision laboratories in the world (Neitz Lab) has verified that her retina does indeed express a fourth mutated cone, and that she is to some degree tetrachromatic in behavioural tests. 2. There is another way to look at the possibility of experiencing multi-coloured pebbles. Just because gravel comes from roughly the same place does not mean that individual rocks themselves don’t have structural heterogeneities which cause them to have slightly different reflectance spectra (a bit like a diamond reflecting different coloured lights from its various cut surfaces). Heterogeneity is definitely something that’s very common in nature, so I certainly don’t see why a rock can’t feature a range of reflectance properties across its surface which we don’t normally discern, possibly because seeing these differences will not have provided us with any selective advantage and thus there has been no need for us to develop greater sensitivity in that part of the visible spectrum. When it comes to the possibility that tetrachromatic women can indeed resolve these differences in reflectances, your suggestion that ‘this would mean that all the colour difference in the spectrum is located exactly where these women have a fourth cone type’ does not necessarily follow. The fourth cone type produces greater sensitivity in a part of the visible spectrum which would allow the special observer to distinguish colours reflected off some surfaces where we don’t have the same discriminating ability. In this case, it happens to be pebbles. It could have been water, sky, plant foliage, skin colour – anything really. Presumably tetrachromacy didn’t give the artist the ability to see more colours in every part of the visible spectrum. It is likely that she merely became particularly interested in the parts of the visible spectrum where she did appear to see more than others, and thus was more likely to want to talk about those particular objects and depict them in her art. I don’t think that her claiming to see multi-coloured pebbles necessarily means that all the colour difference possible happens to be exactly where a tetrachromat’s colour vision abilities exceed ours – it just happens to be in one part of the spectrum, and that happens to be the one that the tetrachromat would like to talk about (and the one that you’re reading about now!).

    2. You are absolutely correct that cone cells don’t send colours, but electrical signals to the brain. The idea behind the possibility of tetrachromacy is that when the retina contains a mutated cone, which responds differently to particular wavelengths compared to its unmutated counterpart, then the bipolar and ganglion cells which receive inputs from a cluster of cones will receive something different depending on whether that cluster contains relatively more mutated or unmutated exemplars of the cone. Ultimately, such differences in inputs can lead to differences in the resulting electrical signals. Thus, what you’re trying to say is not being contradicted by the proposed scheme for how a fourth cone might confer extra colour perceptual abilities.

    Finally, the publication I describe in the article tests whether certain women behave like tetrachromats by testing whether they behave like anomalous trichromats and normal trichromats at the same time. It is true that anomalous trichromats appear to have a slightly more expanded colour space in some part of the visible spectrum and at the same time a slightly contracted colour space in some nearby part of the spectrum. This means you can spot an anomalous trichromat by testing whether they can discern colours along a specific ‘anomalous’ dimension of L’:L’+M signals (where L’ denotes a mutated / abnormal L cone). The anomalous observer should be able to make distinctions along the dimension which a regular observer wouldn’t see, but they will also be worse than the regular observer along the normal dimension of L:L+M (featuring an unmutated cone). The way that the study tested whether mothers of anomalous trichromats are tetrachromatic was by getting them to discern colours along the anomalous dimension as well as the normal dimension. They found that these women had an expanded colour space along the anomalous dimension, just like their sons, but did not have a contracted colour space along the regular dimension. Thus, it can be safely concluded that they have four cones to cover the visible spectrum, as they do not experience the perceptual caveats that come with being an anomalous trichromat. I hope that clarified things!

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  13. I’m an amateur, and I assume you’ve already considered epigenetics, which I understand is a mechanism for turning genes off (and is why we don’t have tails that would get surgical removal). And I had read that the genetic distribution was two percent but I assume that you’ve already considered that or didn’t need to.

    I’ll add a psychological component. Ours is, I think, the only species that trains its young to understand its visual perceptions by substantially relying on artificial visualizations such as TV and print media, and those are almost wholly designed for trichromatic perceptions (I suppose another species might paint by spreading a colorful powder around a surface but I don’t know). I knew a female who said that the sky once looked green to her. Doubtless, if she said so to, say, a trichromatic parent or schoolteacher, she would have been corrected, so to speak, to understand physical things the way other people do. In the same way that babies learn to distinguish a human voice from its echoes that are nearly as loud, a skill we still have as adults, a child seeking the power that grownups have and seeking to understand the world as grownups do will learn to see the sky as blue because the child is told that the color the child sees overhead is called “blue”. I’m not surprised that one tetrachromatic person referenced in more than one place, Concetta Antico, is an artist of original work (not even a commercial artist doing ads); original artists are often social outliers who reject many normative explanations.

    And consider business. Amazon is an example of a business that depends on the “long tail”, carrying a long list of products that have few sales each. Many quite successful businesses do so. But I haven’t seen any business with a long tail that includes products for tetrachromats. If two percent of women are tetrachromats, i.e., one percent of the population, there should be enough customers coming in and saying “this red shirt does not match my red shoes” that retailers and manufacturers would become sensitive to these perceptions, especially since there are already many shades of red being offered now for trichromats. So the tetrachromatic market must be much tinier, so much so that it’s not worth serving. Googling for “products for tetrachromats” (without quotation marks) got me nothing for sale on the first page, even though the Internet supports profitable sales despite lower prices and thinner margins. Concetta Antico probably has difficulty buying matching paints. A search for humans who are tetrachromats will require very wide efforts and I assume snowball searches (asking one subject to refer others) have already failed.

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  14. Interesting article . I was surprised only women can have this “condition” . I see a very large spectrum of colors , I can see IR leds light up on a remote control for example , and I can see iridescent colors on birds plumes , but it’s hard to know what other people can see or not …

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