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A tiny group of people can see ‘invisible’ colours that no-one else can perceive (bbc.com)
194 points by austinz on Sept 13, 2014 | hide | past | web | favorite | 86 comments

Antico's painting of an eucalyptus doesn't "give us a hint of the extra shades she is seeing". It's just another instance of the [El Greco fallacy](http://www.huffingtonpost.com/wray-herbert/a-new-look-at-per...): art historians used to think that El Greco's elongated figures were a result of his having a severe form of astigmatism. But, of course, if that had been the reason, he would have also seen an elongated canvass when painting, with the net result of figures painted in standard proportions.

Here as well, if the tree painting was somewhow a reflection of Antico's tetrachromatism, she would have chosen colors that matched those that she sees on the eucalyptus. Such colors would be indistinguishable to us trichromats from the colors the tree does have, with the net result of a normally colored tree painting.

At best, Antico is using something of a metaphor to convey her tetrachromatism. But no real insight on her vision.

I'm a printmaker (among other things), mostly making monotypes in recent years. The technique allows subtle color transitions which I deeply enjoy, and I notice particular pleasure of the visual effects of colors that range from green, yellow-green through orange and red.

I've discovered that many art colleagues "don't get it", i.e., who seem not to discern the same subtleties that I appreciate. I'm beginning to think this may be due to differences in color discrimination. IOW whatever the basis for the difference I'm sure it would be measurable within limits of color vision test resolution.

I don't know about the illustration in the article. Reproduction (especially reduced resolution) may very well coarsen the color so that the distinctions are run together, so no difference to see. You might want to try the on-line Farnswell-Munsell 100 test alluded to above which might be more informative.

As far as I can see, you are helping my point (perhaps that is what you intended): suppose that you saw those subtle transitions in, say, the wood of a kitchen table, and decided to make a printing that portrayed them. Suppose, as well, that your art colleagues have less color discrimination than you, and didn't see those transitions in wood color. They would also fails to see them in your printed portrait of them.

So, you would have two entities, wood and print, which would match each other in color both for you and your colleagues. It's just that the colors thus matched would be beautiful and subtle for you, boring for them.

Paintings don't exactly match reality. They can emphasize the details you're deliberately copying. It's very easy for a painting of a subtle effect to be less subtle. The colleagues could even use the painting to help them understand the original.

I think it's different: You can see the color green in multiple ways: Monochromatic light of 520nm wavelength looks green to you, but so does dicromatic light composed of a mixture of yellow (580nm) and blue (440nm). The two situations activate your 3 cone cells in the same way.

When painting, you might 'see' the 520nm green in your scene, but choose paint that is a mixture of yellow and blue to recreate it since the net color looks the same to you. However, the paint will look different from the scene to someone missing (eg) red cones.: Your red cones contributed to the blue+yellow looking green to you, since the yellow paritally activates the red cones.

Your point is valid, and while I agree there is not strong enough evidence for a causal connection, I think the idea is interesting.

My mother has painted Impressionist-style oil paintings my whole life, and taught painting classes. In her classes, she always tells people to use colors that they don't see ("You need the pink on the water", etc). Once you've painted enough, you learn to realize what colors are in the scene but not quite obvious. You definitely exaggerate the colors ("hint of pink" becomes "pink streaks") while painting in this style, but it also forces you to think harder about what colors your eye actually sees.

So it seems to me that impressionist-style coloring can definitely come from "faking it" (being taught it, and trying hard), but I would definitely believe it could come from tetrachromism, where those small differences stick out to you a lot. It would be fascinating to know if any early impressionists had evidence of being tetrachromatic.

I agree with this. Often the colors are there, that edge is warm and that edge is cool, but sometimes a shape is just not very interesting, the color isn't interacting and your job as the artist is to make everything work. Through that process you learn to see color.

After reading your comment I thought of the following:

1) Considering that a tetrachromatic colour X is seen by regular folks as a trichromatic colour Y.


2) Are all artificial representations (paint) of the tetrachromatic colour X also seen by regular folks as the trichromatic colour Y?

I guess your hypothesis is true only if the answer to the above question is "yes".

I don't agree with the logic here.

Antico may see some color in the bark that I don't see---say, green.

So she adds actual green paint into her painting, which, of course, I do see.

No, she adds some grey paint that looks green to here, but you of course, like the bark, can only see it as grey.

But it turns out she doesn't have that exact pigment needed to have the same uncommon spectrum, so she uses an almost perfect match that looks green to both.

Sure, that might happen.

It's almost guaranteed to happen. Do you really think she has paints that can match any material in all parts of the visible spectrum? That's what you're asking for with this hypothetical "green to her, grey to us" paint. If she sees green, she's going to put down green paint. Furthermore, if to her these colors are the interesting part of the scene, it's natural that she'll exagerate them a bit. She's an artist for pity's sake, not a camera.

> she's going to put down green paint.

But what appears green to her may not appear green to a trichromat.

In paint, where there's only one pigment, green is green, whoever looks at it.

Paint pigments are not pure single-wavelength colors.

They're not pure wavelengths but there's not many pigments. You're not going to get a cross-spectrum match.

what about the "night vision" painting. if it's pitch black and someone has the ability to see a vase, and then remembers the vase and paints the vase green in another room brightly lit...

well, they wouldn't paint a pitch black canvas and claim they can see a vase in that painting.

if el greco had some kind of condition that made him see elongated figures at thirty feet away, would a small normally proportioned figure drawing two feet away really look elongated also?

The night vision painting recovers quite a bit of the visual experiences: how shapes are arranged in space, for example. What is recovered in the multicolor eucalyptus from the tetrachromat's visual experience?

El Greco usually paints figures close by. Portraits, mostly. Your question does not apply to his most famous paintings, I don't think.

He could paint it with night vision paint that then could look like pitch black or have some other unintended side effect for a normal viewer.

It's hard to compress what's known about the subject of color perception differences among people with nominally normal color vision.

Hypothetical tetrachromacy has been demonstrated, that is, M and L cones with minor differences in peak spectral response existing in a retinal mosaic. The question arises whether this difference is capable of being propagated through the bipolar connecting neuronal paths (which form the "blue-yellow" and "red-green" channel paths conveying color info to the brain visual cortex).

Several studies found no evidence for a "fourth channel", and some perceptual studies did not show confirmation of tetrachromacy. However a couple of studies do hint at rare female subjects who do seem to have good evidence for this trait.

It is postulated that because of the differences in the exact genetic encodings on the 2 X chromosomes in females, that up to 50% of women have a retinal mosaic of 4 populations of cones, S M M' L, S M L L', and so on. The theory is that this allowed greater distinction of yellow to red color distinction which may have helped survival by avoiding toxic plants based on subtle color distinctions.

In art, there are great colorists and those who are virtually incapable of subtle use of color. If anyone interested in finding out about their own abilities, check out: http://www.xrite.com/online-color-test-challenge

The on-line test is only a rough estimate, but it's fun and challenging. I suspect there's a spectrum, aka normal distribution of color discrimination ability. I should't say how well I did, though the results of trying it several times were remarkably consistent despite different computer systems that I used.

BTW I encountered one very intriguing article reporting on a large sample of retinas obtained post-mortem. As expected, ~50% of females showed retinal mosaic of 2 M or L cones where only one each was expected. Most fascinating was the fact that of male retinas 8% showed a similar tetrachromat pattern. How that could arise is a mystery considering males have only one X chromosome. (There are pigment gene variations with greater number of CNV and TR which could be activated? But it's still a mystery.)

> Hypothetical tetrachromacy has been demonstrated, that is, M and L cones with minor differences in peak spectral response existing in a retinal mosaic. The question arises whether this difference is capable of being propagated through the bipolar connecting neuronal paths (which form the "blue-yellow" and "red-green" channel paths conveying color info to the brain visual cortex).

Adding genes for extra cone types into normally dichromatic mammals (mice or something? I don’t remember off hand and I don’t have the citation at my fingertips) has seemed in studies to result in trichromatic vision, so it’s not too big a leap to suspect that tetrachromacy of the type described in the article might happen for some humans.

As you say there hasn’t been any study which showed this conclusively (that I’ve seen anyhow). I’d love to see more thorough research on these subjects who supposedly possess such vision.

> In art, there are great colorists and those who are virtually incapable of subtle use of color. If anyone interested in finding out about their own abilities, check out: http://www.xrite.com/online-color-test-challenge

Note, this online test is a bit easier than the paper version of the Farnsworth–Munsell hue test, since the colors in the screen version end up with some lightness differences that make it a bit easier to keep some sections of the chart in order. Either way, the test IMO mostly measures (a) whether someone has “normal” trichromatic vision, and (b) how patient and willing to fiddle with fine details they are. I don’t personally think Farnsworth–Munsell test scores are super meaningful, though a very poor score does indicate some color vision deficiency. [FWIW, if I take the time to do the test slowly, either on screen or on paper, I consistently score ~0, occasionally mixing up one pair or another.]

But anyway, I think the ability to be a good colorist in art has a whole lot to do with practice. Spending a lot of time mixing paint or color correcting photographs is likely to heighten awareness of tiny distinctions. For instance, I know that after a couple years of photography courses, several of my friends got much better at noticing color casts in photographs. Not that they couldn’t physically see them before, but after experience they more often spontaneously noticed the casts and started to have a feel for just how much adjustment in which direction would be necessary to counteract them.

\tangent A computer-based color test might not be able to detect tetrachromacy, because monitors do not display a continuum of frequencies of light. They are intrinsically tied to trichromacy, by combining only three frequencies of light in different proportions. RGB.

It's mathematically possible that a continuous spectrum of light also wouldn't pick it up, if its only effect was that different combinations of frequency that appear identical to an ordinary person could be distinguished. (Not that this would confer any evolutionary advantage.)

I got 39. I found the test interesting in terms of sorting algorithms. Typically when we think of sorting stuff, the comparison function is deterministic and cheap.

In this case, it gets increasingly hard and even erratic to tell adjacent colors apart; essentially the comparison function is increasingly expensive and non-deterministic.

I wonder if that has any consequences for the "optimal" strategy. In my case -- and I suspect most people did it the same way --, I used a kind of two-way insertion sort: for each field, I first decided if it should move left or right, then moved it as far as it "made sense". So I ended up with a list that got sorted from the outside towards the inside. In the end I had a few passes of "bubble sort", checking adjacent pairs for correctness. This was when it got expensive and erratic. :)

It would be a fun experiment to have an algorithm sort the list (based on whatever sorting algorithm) and simply use the human as a comparison robot: display two hues at the same time and let the human decide which of the two is "smaller". I'm sure you could easily measure and even predict the increase in "expense" (i.e. response time) and the increase in errors and ambiguity for close hues.

You could also do stuff like force the user to decide within a very short time, to determine if there's actually an increase in accuracy when staring at a given pair for 10 seconds vs a 500ms "hunch" decision (or heck, 10 500ms hunches, still a two-fold improvement).

> I think some of this has to do with practice. Spending a lot of time mixing paint or color correcting photographs is likely to heighten awareness of tiny distinctions.

I wouldn't doubt that practice could help, but the test (I believe) was designed to minimize practice effect. I've mixed a lot of color, but one can mix color to a shade only as far as one could determine it's right.

It used to be, color matching for printed material was done in large shops by workers visually comparing swatches to customer desired shades. This required training, but not every trainee could master it despite extra effort and time. Seemed it required native ability, talent as it were, to be able to be good at it.

Anyway, the online test is solely for one's own interest and information. I don't see there's anything substantial at stake.

I scored 0 on the test first time (perfect color vision), no practice, and I have pretty much no experience with any kind of art. (27 male for ref)

EDIT: Had another 2 tries, got 4 and 12. Turns out practice does more harm than good - after 3 tries it's a bit disorienting trying to read off the screen. Gonna have to take a break from machine.

A test like this needs to isolate the subject to only one spectrum at a time. I gave up after the first two because of too much noise.

I'm not sure how to describe it, I got 25... I could see the variances, in the tiles that looked "off" but wasn't always sure which direction to split to... I finally got frustrated and hit score. :-(

This test went around at work a while ago. At the time, I worked with a number of artists.

Many people got very different results depending on the monitor they were using.

This would probably be a very large factor in the score. I scored a 7 on a calibrated Dell UltraSharp (IPS) monitor at work. If I were to do this same test on my cheap uncalibrated Acer at home, I don't believe my score would be nearly as good. Even the difference between a calibrated and an uncalibrated cheaper LCD should be noticeable to most people.

> I think some of this has to do with practice. Spending a lot of time mixing paint or color correcting photographs is likely to heighten awareness of tiny distinctions.

I've never done either, score 12, age +- 50, male.

Interesting test, harder than I thought it would be.

I too have never done either, and also scored 12.

> But anyway, I think the ability to be a good colorist in art has a whole lot to do with practice.

Indeed. I scored 12 on that test, despite being colorblind with moderate-strong protanopia.

I just did the test, and whoohoo I got perfect colour vision: http://i.imgur.com/WFoktvP.png (yay!)

But does that really mean I got all of them correct?! I was squinting and squinting (and cursing insertion sort), and I was pretty tired out near the end. I thought I might've been able to squeeze out some last tiny errors if I would have gone over all those bars again, flipping a few boxes that were not just quite right.

Also if I look at the rainbow bar in the screenshot, I'm assuming that's the full range of colours I sorted, I can clearly see there's a few errors and discontinuities in there still.

I wonder if it maybe just rounds down really small errors to zero because they're "close enough"? That's too bad, because I'm tempted to go back and try to do better :) But if it won't increase my score (decrease my error rate), I only have my own judgement of that final full rainbow hue bar to go by :)

(also, remember to disable Flux/Redshift. it probably doesn't help accuracy :) )

I suspect there's a spectrum [of color perception among people with nominally normal color vision].

Indeed there is. The most common is a variant carried by around 2/3 of the human population (depending on ethnicity and gender), which results in a slightly different red response curve, as shown in color matching tests [1]; and might also be involved in some forms of actual color blindness. Other, less common variations involve mosaicism of both red and green pigments as well [see ibid, Fig. 8].

The tetrachromacy thing is a perennial media favorite these days; but IMO these subtler but actually widespread differences are much more interesting, at least from a philosophy of mind point of view.

[1] Deeb 2005 - The molecular basis of variation in human color vision. Link below:


Thanks for the reference, I hadn't seen it previously. In males, the variation appeared to be more toward decreased color perception, except for one variation associated with increased red sensitivity.

It does raise the interesting but perhaps unanswerable question of the similarity or difference of the effect of the molecular variation vs. tetrachromacy on color perception.

Would greater red sensitivity of L cones in effect produce equivalent visual experience as having a fourth type of cone?

Since the visual system computes color on the basis of differential response in M and L cones at time t, having a greater separation of M and L spectra would be sufficient information to compute a larger range of intermediate colors vs. the normal L-M difference. The presence of another node L' between L and M could just be redundant input and not necessarily adding information to the color computation.

A question that arises here is if S M L L' females have better color perception is it because of having L and L' or would it reflect L' being more distant from M than the typical single L of female trichromats.

That may be significant in light of lack of clear findings of direct benefit to female color perception in those having L L' retinal mosaics.

EDIT: Actually, to my way of thinking, the issue is more precisely color discrimination, which is fairly directly measurable vs color perception, a less tractable concept.

"considering males have only one X chromosome": Around 1 in 500 males have two X chromosomes (XXY), so it should be possible for males to end up as tetrachromats too (although this wouldn't explain 8%). See https://en.wikipedia.org/wiki/Klinefelter_syndrome

I'm from Australia but hadn't heard of the Rainbow Eucalypt before (the tree that the featured tetrachromat painted). That's not so unlikely as there are a lot of Eucalyptus species.

Still, I was curious and so looked it up and was very interested to find that it is a Eucalypt from outside Australia and, more than that, the only one whose native region partly lies within the northern hemisphere. Another name for it is the Mindanao gum, as it is native to the tropical rainforests of Mindanao in the Philippines (north of the equator).


From my photography experience I noticed a lot more abnormalities than just tetrachromacy, which is not uncommon amongst artists. I had a model that was "seeing" colors in the sounds (synaesthesia) - she was often saying you talk to me in blue or that person has such a nice white voice.

I've been tested for color vision and I am 100% accurate meaning I can discern subtle shades from each other precisely with no effort. I also have a very light form of synaesthesia manifesting itself by seeing a flash of light in the night right before an unexpected sudden loud noise (yet with observable latency) - like seeing a lightning just before hearing a thunder, even if the noise is not accompanied by any light.

I was also always wondering if what I perceive as "blue" is the same as other people perceive as "blue"? What if the neural response in my brain wires the color sensation in the same way as other people perceive "green"? That would help to understand individual preferences for colors. Also, we know there are special cells in retina doing direction detection, edge detection etc. - what if this had a profound impact on how we individually perceive world around us? Somebody can have a strong edge or directional detection present in their view all the time, logically assuming it's normal for the others as well that are lacking that ability. We are just too diverse, and people are rather quiet in order not to risk being considered abnormal and marginalized, a common problem for artists in general.

I once had a friend, over some beers, describe his synaesthesia. He thought it was just him being crazy, until I handed him my phone with the Wikipedia page on it.

He said that people who were being dishonest sounded purple- the man was a walking lie detector, and never knew how he was doing it.

Do you also see a pattern in the white flash (zebra stripes, lines, checkers)? I always assumed that was a normal side-effect of being startled while resting; changing from your brain-vision to eye-vision, though I'm not sure of the patterns significance.

I can't recall any pattern - all I remember is a dynamic amorphous shape with different light intensities in different parts, and this just quickly flashes in front of my "eyes", right before loud sound hits.

>seeing a flash of light in the night right before an unexpected sudden loud noise

Does this improve your reaction time to sudden noises? You should totally test it :)

It's only a few milliseconds before the noise, I would have to develop Bruce Lee-grade reflexes to do anything about it ;-)

I see this flash at nighttime as well and always thought that it was my sleepy brain jolting awake.

I sometimes see a flash at nighttime (not accompanied by any sound, though).

My first hunch was that my computer or phone may have been bugged to take a flash photo. But that's hard for me to believe. Maybe it's just my brain doing something weird (or something normal).

Maybe you are seeing Cherenkov radiation? Cosmic particles passing through the eye.

"Cherenkov radiation can be generated in the eye by charged particles hitting the vitreous humour, giving the impression of flashes"


You'd have to be in the path of a cosmic ray shower ,near some dangerously critical nuclear decay, or near the beam-line of a particle accelerator.

Better check under the bed, then.

Mantis Shrimps have 12 photoreceptors, they can also see in all polarisations. They don't do paintings so it's hard to know how they experience this. http://en.wikipedia.org/wiki/Mantis_shrimp#Eyes

It's not that they see a few colors that no one perceives, it's that for them color is a 4 dimensional space instead of a 3 dimensional one. So it's a whole new world of different colors.

Even though our eyes detect color in 3 dimensional space, that doesn't seem to be how we perceive it. E.g. I can't tell you how much red, green, or blue is in a color, just that it's close to the cluster of colors I recognize as "purple" or "blue" or "orange" or whatever.

See this chart for example: http://imgs.xkcd.com/blag/satfaces_map_1024.png

Interestingly there is some evidence that culture and language might affect our perception of color.

You are incorrect about that, and Randall’s drawing is misleading you (it shows the gamut boundary of an RGB cube, rather than the full three-dimensional color space he asked about in his survey).

For people with “normal” trichromatic color vision, the color we perceive when looking at a particular spot does indeed fall into a three dimensional “color space”. You’re right that without careful attention/training it’s hard to state colors in terms of coordinates in terms of cone cell responses directly or color opponent signals (blue–yellow, red–green, white–black). It’s easy to train someone to give reasonably accurate color coordinates in terms of lightness, hue, and chroma, however.

If you want to understand human color vision in detail, I recommend this online resource: http://www.handprint.com/LS/CVS/color.html

> Interestingly there is some evidence that culture and language might affect our perception of color.

The evidence for effects of language on color perception is very weak, or it indicates that the effects are themselves very weak.

The big paper in this field is this: http://www.languagescience.umd.edu/~ellenlau/courses/ling440...

They find that Russian speakers, who have separate words for two shades of blue, can discriminate between those shades of blue something like 50 ms faster than they can discriminate between two shades of blue that fall into the same color word category.

First thing to note, that's a very small difference.

Second thing: the effect goes away under verbal interference (like when you have the subjects repeat a word over and over again while doing the color discrimination task). So I guess there's an effect of language on color perception, but not while you're talking? It definitely doesn't seem to me that the colors I am perceiving change when I am talking vs. when I am not.

Third, there's a huge overall reaction time difference between the Russian and English speakers in the paper, which calls into question how well the experiment was really run.

So it's not clear that there really are effects of culture/language on color perception; and if they do exist they are very small. (At least that's the conclusion I would draw from this paper, which is widely cited in that field. I'd be interested to hear of different results if they're out there.)

We seem to perceive it in something closer to hsl space (3d) than rgb space (also 3d). You can tell that it's "purple" but you can also tell whether it's saturated or not, and if it's bright or dark purple. Also check out Lab space: http://en.wikipedia.org/wiki/Lab_color_space

There was a good study published recently documenting the ability for patients with eye surgery who could see into the UV spectrum.

The cornea blocks some UV light so if you have eye correction surgery your cornea is now modified/thinner and you can see slightly into the UV spectrum.

Probably has some downsides too.. Cataracts are probably more probable seeing as people tend to get them more frequently at higher altitude.

The Tibetan people apparently have a high rate of cataracts since there is higher UV light at the higher altitude.

It wasn't the cornea, but an artificial lens that was responsible. [0] UV passed through the artificial lens, but not through our natural one. It was replaced because of cataracts.

[0] http://science.slashdot.org/story/12/02/14/165202/followup-u...

The thing is, the woman doesn't seem to perceive new colors that other people do not; the colors others don't see are being aliased to familiar colors like "pink", and "red". (When she says "do you see that pink", is it a pink that we could see elsewhere? Or some color that we have no concept of, but which she relates to pinkness.)

I'd be curious if she still sees the same thing in a digital image of the scene, or does the RGB system destroy it.

Also of interest is that birds have little oil droplets, often colored, as part of their visual set up. There are five different known types of color-filtering oil droplets (a sixth that is transparent) but it doesn't appear every bird has all of them.

I'm no bird sight expert but it's suggested here [0] that a particular bird may have 8 effective color receptors (5 cones, 3 cone-droplet pairs) for a huge number of colors humans can't see. Diagrams of the receptors themselves can be found at the wiki [1].

[0] - http://watchingtheworldwakeup.blogspot.com/2008/11/mountain-... [1] - http://en.wikipedia.org/wiki/Bird_vision#Light_perception

It's learnable and takes a lot of practice mixing colors and you need the right teacher. I picked up this ability in art school and it absolutely makes a sunset more beautiful. But to keep seeing this way you have to practice regularly. Use it or lose it. If I'm not painting regularly, it's gone.

Like if you play chess for hours and hours every day, eventually you start to experience everything in terms of chess moves. It's the same with color. Look at just about any flat surface long enough and you'll notice it has a hue, saturation and value. Just go into Photoshop and you'll see that boring white wall has variations in hue, saturation and value, it's no medical mystery in my opinion.

This part is a tiny more green, that part is a tiny more blue. I'm not giving you an art lesson here but you get the idea. Artists know they can make things look farther away by painting them blue/gray, decrease saturation with distance, etc. The painter learns to exaggerate because she just has a 2D surface to work with. Anyway, through the process of painting our perception of color improves, even when we're not painting.

K-12 the goal is just paint the sky blue and the grass green, color inside the lines and get your grade. But if you want to be a serious painter at some point, you may want to look at things fresh, more intently. When people see your paintings, don't expect them to care how long it took to mix the color. Only a few artists will notice and they're not the ones buying your work.

So even for artists, seeing color is not terribly useful, because mixing color is very time-consuming. It's more a hindrance than anything because I can finish an ink wash in 1/100th the time and sell that for the same price as an oil painting of the same subject.

If you have all the time in the world and don't need money, I say go for it, buy some expensive oil paint and send me an email, I'll tell you what to buy. Stretch the canvas yourself and go all out.

> Jordan’s “acid test” involved coloured discs showing different mixtures of pigment, such as a green made of yellow and blue. The mixtures were too subtle for most people to notice: almost all people would see the same shade of olive green, but each combination should give out a subtly different spectrum of light that would be perceptible to someone with a fourth cone.

From what I understand, a modern monitor is capable of displaying the entire range of colors perceptible to the human eye. Am I mistaken about that?

So would it not be possible to create a web-based version of these "coloured discs", and then we can test ourselves.

> From what I understand, a modern monitor is capable of displaying the entire range of colors perceptible to the human eye. Am I mistaken about that?

It kinda depends on what you think as a color. Computer monitors rely heavily on that (normal) human vision can not distinguish from eg. red-green and "true"[1] yellow, even though they are completely different from physical perspective. So a typical computer monitor does not bother to create "true" yellow and instead cheats by outputting red-green when asked for yellow.

If we had an hypothetical tetrachromat with additional cones sensitive for yellow light, her color vision would span the same range as normal (trichromat) color vision, ie cover the same piece of EM spectrum. But normal computer monitors would not be able to reproduce for her what she sees in real world.

Then there is the somewhat related discussion about violet and red-blue which is quite curious when you begin think about it.

[1] I'm using "true" color to refer light with specific wavelength

From a purely physical point of view, a computer monitor can only produce a spectrum of light (photon histogram) that has spikes on the wavelengths for red, green and blue. These spectra reproduce the vast majority of colors that are perceptible to humans with normal vision. However, the "colored discs" in the article exploit the fact that two colors that are perceived as identical, can in fact be produced by (at least) two different spectra of light. Tetracromats can differentiate between some of these spectra, whereas people with normal vision can not.

(A simple example: The color cyan can be produced by additively combining the spectra of blue and green, but can also be produced by creating photons with only the wavelength of cyan, 490-520nm according to Wikipedia. A person with normal color vision would not be able to differentiate between these two light sources, whereas a measurement device or perhaps a tetrachromat could. A computer monitor can only produce the first of these).

So the answer to your question is no, unfortunately.

Look at this image (http://upload.wikimedia.org/wikipedia/commons/thumb/6/60/CIE...) from Wikipedia. It represents all the colors (chromaticities) that humans can see. A device that reproduces images using primary colors can reproduce all the colors within the vertices of the primaries used. For example, an RGB monitor can reproduce all colors within the triangle formed by the particular R, G, and B subpixel primaries as plotted on that diagram.

What you'll notice though is that because the diagram has a curved edge (this is the spectrum of visible light actually), no polygon can encompass all colors.

I've always thought it would be cool if someone made a CRT using two prisms (splitting light from a blackbody), such that a narrowband of light from one prism is combined with a narrowband of light from the other prism in different intensities. This would be able to reproduce all colors a human can see, assuming the ability to tune to a specific portion of the spectrum is fast enough to scan over all pixels that constitute the image.

> no polygon can encompass all colors.

But I suppose that a polygon could encompass all colors if it was big enough to include also wavelengths of non-visible colors.

Ooh, a combined monitor/tanning lamp/heater! Great idea! /s

> Am I mistaken about that? Yes. See: https://en.wikipedia.org/wiki/Gamut

Google 'tetrachromacy test', first hit http://www.blogadilla.com/2008/06/08/are-you-a-tetrachromat/

that claim is quite dubious. I don't think it's possible to create metamers for people with normal 3-color vision on an RGB monitor. To do that you need an extra degree of freedom.


Hmm... I'm not sure about that. It's an interesting question. It's equivalent to the question: can we create two images on a computer monitor that appear the same to a person with normal color vision yet appear different to a colorblind person?

I think it would depend on the particular mapping from 3 to 2 dimensions... it might be possible.

> can we create two images on a computer monitor that appear the same to a person with normal color vision yet appear different to a colorblind person?

It surely depends on the type of colorblindness, there are many [1].

[1] http://en.wikipedia.org/wiki/Color_blindness

If you're monochromate or dichromate, I doubt that this is possible. Monochromates or dichromates are simply missing one or two color components out of three.

If you have anomalous trichromacy, you can by creating a monitor with pixels of different colors.

In any case, I don't see how it is possible to identify a tetrachromate with a trichromate (RGB) monitor. I tried to increase the contrast and change the hue of the color test to understand what was different between the apparently similar colors but I wasn't able to find any difference.

Using JPEG for a color test doesn't seem well thought out. Using GIMP (Colors, Map, Rotate Colors), one can make a letter appear in the center circle. I guess if the other circles had any letters in them, the compression ruined it.

Without using a monitor, a potential test would be to look at flowers like Rudbeckias(in real life of course) that have evolved to have patterns that tetrachromat insects can see. If you can see these patterns, you may be a tetrachromat.

> The crux of Jordan’s argument lay in the fact that the gene for our red and green cone types lies on the X chromosome. Since women have two X chromosomes, they could potentially carry two different versions of the gene, each encoding for a cone that is sensitive to slightly different parts of the spectrum. In addition to the other two, unaffected cones, they would therefore have four in total – making them a “tetrachromat”.

Why couln't this happen for either of the other cones?

The male equivalent is "Anomalous trichromacy" http://en.wikipedia.org/wiki/Color_blindness#Anomalous_trich.... You have one of the color receptors tuned to a slightly different frequency, so you see the word in a different way. (Which way is the correct way?) This condition is easy to spot with the standard Ishihara color test, so it's easy to measure the % of the population in these cases.

--- Green

The most common case of male anomalous trichromacy is the "wrong/unusual" green receptor case, so the most common case of woman tetracromat is one that has 2 copies of the usual blue receptor, 2 copies of the usual red receptor and 1 copy of the usual green receptor 1 copy of the unusual green receptor. This is the case discussed in the article.

To reduce the text size I'll denote this kind of tetracromat with BBGgRR, where the capital letter denote the usual color receptor and the noncapital letter denote the unusual color receptor. (This is unrelated to recessive and dominant genes that are usually denoted by capital and noncapital letters.)

--- Red

The second most common case of male anomalous trichromacy is the "wrong/unusual" red receptor case, so the seccond most common case of woman tetracromat is BBGGRr (1 copy of the usual red receptor an 1 copy of the unusual red receptor).

Obviously, you can have both mixed copies, so a woman can have BBGgRr and be a pentacromat. At least have the genes to "see" in a 5-dimensional color space. If she can use this ability is less clear.

--- Blue

The least common case of male anomalous trichromacy is the "wrong/unusual" blue receptor case, so the least common case of woman tetracromat is BbGGRR.

It can be mixed with the other cases, so a woman can be pentacromat or hexacromat.

The interesting part is that the blue gene is not in the X chromosome, so both male and females have two copies. So a male can be tetracromat BbG_R_ . But this chromosome is not inactivated as the 50% of the X chromosomes in females. So I'm not sure if it's possible to have a different kind of vision in this case.

--- It's more complicated

Actually you can have more than 1 copy of the color genes in each chromosome, and there are more than two variations of each gene, so it's more complicated.

The gene for the “short” wavelength cone† is located on chromosome 7, whereas the genes for the others are located on the X chromosome. Since females have two X chromosomes, it is possible for them to have 2 variant copies of the long or medium cone genes. (Since males only have one X chromosome, color vision deficiencies caused by a missing or abnormal copy of one of the medium/long cone pigment genes are much more common than for females.)

It would certainly be possible for a genetic mutation to result in an extra mutated copy of the short cone pigment gene, but I don’t think I’ve heard of variants of this gene with slightly different light sensitivity than the usual type (they might exist though?). By contrast “non-standard” version of the medium cone pigment gene is relatively common.

See: http://ghr.nlm.nih.gov/gene/OPN1SW http://ghr.nlm.nih.gov/gene/OPN1MW http://ghr.nlm.nih.gov/gene/OPN1MW2 http://ghr.nlm.nih.gov/gene/OPN1LW

† We should properly refer to the cones as long, medium, and short rather than red, green, and blue, since color is computed from differences in cone responses.

The radiolab episode about colors talks about tetrachromats and some other very interesting stuff, worth a listen: http://www.radiolab.org/story/211119-colors/

For an excellent fantasy take on this, see Brent Weeks Lightbringer books.

The frequency sensitivity curve of rods is different from reach of the cone types, too. If some rods contribute to daylight vision, then everyone is tetrachromatic to some extent.

Invisible colors? Boring. How about imaginary colors?


How about David Lindsay's Arcturians (Voyage to Arcturus) who were able to see two extra colors namely jale and ulfire. https://en.wikipedia.org/wiki/A_Voyage_to_Arcturus

... and a lot more fun to be had over here: http://en.wikipedia.org/wiki/List_of_fictional_colors

Octarine or GTFO ;)

I can actually see infrared on the dark quite easily. But I think it should be due to some falloff of the light spectrum emitted by the LED.

Am I the only one who thinks her painting of the tree looks the same as the photo color wise?

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