9 ms·

The Mystery of Tetrachromacy

85 points by Amorymeltzer 11y ago

PeterWhittaker 11y ago
Very inaccurate summary, since I skimmed: Up to 12% of women have an X chromosome mutation that creates a fourth cone. This cone generally overlaps the spectral region covered by the cones sensitive to red and green. The exact area of overlap determines whether or not each woman sees "more": If the fourth cone completely overlaps the region covered by an existing cone, no new information is presented to the visual system and the input is effectively discarded (she sees "just as much" red or green, not more, not less). If, however, the fourth cone has peak sensitivity in the gap between the cones covering red and green, she will she up to 100% more colour than women with normal vision.
- Someone 11y ago
  That's not quite correct. Where one cone type has a peak response, another may respond with a signal that is only one percent or less of its peak response, but the frequency regions in which hey have _any_ response basically overlap. Given the sensitivity curves, that difference is academic for blue versus red and green sensitive cones, but not quite for red versus green sensitive cones. What differs is the amount in which having a fourth type of come allows one to get rid of metameries (https://en.m.wikipedia.org/wiki/Metamerism_(color) https://en.m.wikipedia.org/wiki/Metamerism_(color), a term that the article surprisingly doesn't mention) If your fourth come type has a response curve that is very similar to one of te 'normal' three, there should still be _some_ effect, but it will be hard to devise an experiment that shows the ability to discriminate additional colors. But given the impact that not having red or green cones with their fairly similar sensitivity curves has, I suspect having a fourth curve, even if it falls between the two, will have a measurable (in the lab) effect on one's ability to discriminate colors.
bendykstra 11y ago
I learned the importance of trichromacy when a friend and I came across a patch of wild strawberries. I'm partially red-green colorblind and for every strawberry I found, my friend found ten. They were almost invisible to me. It's neat to see that the fourth cone's response curve peaks right between the red and green cones' curves in tetrachromats. I bet they are amazing at finding berries.
- Swizec 11y ago
  This probably has nothing to do with chromacy, but is related to finding berries and other odd things. My sister is gifted with finding four leaf clovers. She walks in a field and seems to just see them. Every time she picks one, it's a four leaf. Her record find is a 7 leaf clover. I always wondered how that's possible.
  - metasean 11y ago
    Is it possible she has synesthesia (https://en.wikipedia.org/wiki/Synesthesia https://en.wikipedia.org/wiki/Synesthesia)?
- such_a_casual 11y ago
  What does it mean to be partially red-green colobrlind? Isn't it the case that you either have the red-green cone or you don't?
  - nitrogen 11y ago
    It's conceivable that a mutation in the gene for the red or green light-sensing protein gets a mutation that just shifts its sensitive wavelength closer to the other. So "partial" r/g colorblindness could occur if there was still some separation between those wavelengths, but not as much as is typical.
  - deckar01 11y ago
    > Deuteranomaly ... The medium-wavelength pigment is shifted towards the red end of the spectrum resulting in a reduction in sensitivity to the green area of the spectrum. [0] The closer the spectrum perceived by the M (green) cone is to L (red) cone, the harder is is to differentiate red from green. [0] https://en.wikipedia.org/wiki/Color_blindness#cite_ref-KallColor_26-1 https://en.wikipedia.org/wiki/Color_blindness#cite_ref-KallC...
peter303 11y ago
I thought in females half of X chromosomes are deactivated as to not producing conflicting similar proteins. If the extra cone gene is on the deactivated half, then it is not produced. It is suggested that imperfect deactivation leads to female increased occurrence of autoimmune syndromes over males.
- akavi 11y ago
  The key is that which X chromosome is deactivated is not the same in all cells. It does occurs fairly early in the development process (a stage called gastrulation), and the deactivation does persist for all cells that descend from a given cell present. But critically, it's late enough that a retina could conceivably possess cells descending from two different "lineages". You can actually visibly see the "resolution" of the deactivation by looking at tortoiseshell cats. Each blotch of orange/black represents one cell present at the deactivation stage.
dspeyer 11y ago
First thought: The usual pattern is that only one X chromosome gets transcribed in any given cell.[1] This keeps the dosage of all those proteins correct. Otherwise there would need to be a separate set of dosage controls for XX and XY people. The inactivation occurs pretty early in life, and when cells replicate they keep the inactivation. This results in macroscopic regions of the body with consistent inactivation. Usually this isn't noticeable, but in cats coat pigment is on the X chromosome, and heterozygous cats often show "tortoiseshell" or "calico" coloring. The patches of contiguous color there are larger than a retina. So it seems entirely likely that all the cells in a human woman's retina would use the same X chromosome. On the other hand, that predicts half of het women would be colorblind, which isn't observed. So maybe not so simple... I have a second thought about downstream neural hookups and the mechanisms for those, but it'll have to wait for later. [1] https://en.wikipedia.org/wiki/X-inactivation https://en.wikipedia.org/wiki/X-inactivation
- xerula 11y ago
  The article explains X-inactivation, and it doesn't play the role you think here: both copies are broadly equally expressed among retinal cells. Also even if all cells in a retina used the same X-chromosome copy it doesn't predict color-blindness, since we would still have a trichromatic system. The answer to the question posed by the title is simply that all mutant fourth cone types are not created equal. Most have a spectral response curve similar enough to an existing type not to make a real difference; a few have peak responses more squarely in-between those of the usual trichromat photopigments, allowing more finely graded color perception. (The article takes its time getting round to explaining this, but it's all there.)
peter303 11y ago
Scientists are looking at direct delivery of gene therapy to retinal cells to cure serious eye diseases like macular degeneration, pigmentosa blindness, etc. Should this work well, then the next step would be lesser conditions like male color blindness. I read of some successful attempts to give dicromatic animals, genrally carnovoires, the trichromatic gene directly to the retina. These subjects can be tested by giving rewards hidden in standard color blindess test images. The next step, more controversial, would be super-vision of tetrachromacity, perhaps infra-red and ultra-violet sensitivity. Night vision glasses no longer needed for enhanced soldiers.
- OopsCriticality 11y ago
  > The next step, more controversial, would be super-vision of tetrachromacity, perhaps infra-red and ultra-violet sensitivity. Night vision glasses no longer needed for enhanced soldiers. Utter poppycock. The cornea and lens in humans are UV opaque, which is a very good thing because UV light is damaging. People without a lens (aphakia) are reported to perceive UV light, but this is otherwise an undesirable condition. As to infrared, some people can already see up to around 750nm or so (NIR) with awful quantum efficiency. Pit vipers and some fish have limited perception of IR, but I'm not aware of any animals that can see IR in the conventional sense… being warm blooded presents a real problem, for one. Plainly, there are limits to what can be achieved in hyperspectral imaging due to materials, and that's without the constraints of biology thrown in the mix.
  - scott_s 11y ago
    Your comment would be much better without the first line.
    - lsh 11y ago
      And your tepid comments are just as useless.
      - dang 11y ago
acchow 11y ago
It opens with an Australian painter who displays her extraordinary vidual experiences in her art. But if she was born this way, why would she find it extraordinary?
- mirimir 11y ago
  Maybe because people kept telling her that she must be tripping?
- neurosphere 11y ago
  Hi! I wrote the Neurosphere article you're discussing, and noticed this lively comments section. I agree with the question you're asking - if she was born this way, why did she question it? From the research I did on her life, it appears that she always assumed people could see these colours, and apparently when teaching her students in art school she would try to say things like "Try to depict all the colours you see in this leaf, like the turquoise and orange on the sides..." and her students often just found it too embarassing to tell her they didn't see any of these colours. She slowly started realising that her colour perception may be different, until one day a neurology student taking her class suggested she took a genetic test. That's it. Since then I suppose she has really harnessed the results of the test to market her vibrant art as reflecting her 'tetrachromatic reality'. Some people have commented on Twitter questioning this, and it's something I think I will write about in the near future because it's a completely fair point.
qb45 11y ago
I wonder how those poor women deal with RGB displays. It must feel weird to see a difference between red+green and actual yellow. Although, on second thought, color vision doesn't seem to work perfectly anyway. I never understood how red+blue can be perceived the same as far violet, it just doesn't make sense if you look at those frequency response curves.
- jhanschoo 11y ago
  I doubt it will be that bad. It will probably seem a little more dull, like some of the colorblind simulators you can find around the net. It's probably not just screens; pigments have the color they have because they reflect certain wavelengths, tuned to the proportion that a trichromat would consider identical. A tetrachromat is almost certain to notice that pigments of pictures on paper might be drastically different in color to the color of the objects the pictures are supposed to represent. The brain doesn't perceive color just by addition of the stimulation of the cones. Your red and green cones are still (slightly) stimulated when you see blue, and your brain tells you that blue is the color; with violet, the stimulation among the cones have a different proportion, hence violet. Incidentally, this is also why the sky appears blue; the sky is too polluted by other wavelengths to appear violet.
  - qb45 11y ago
    Actual violet (like UV LED or mercury lamp) should stimulate almost exclusively S cones. But it turns out that if you take blue, which stimulates M and L more than violet, and then add even more L stimulation, you end up with something that "feels" more violet than blue (at least to me). This is what bothers me. It looks like two distinct inputs producing the same output, even though there exists an intermediate input which produces different output.
    - Someone 11y ago
      Two distinct inputs producing the same output is metamery. https://en.m.wikipedia.org/wiki/Metamerism_(color) https://en.m.wikipedia.org/wiki/Metamerism_(color). Without it, color printing and color televisions would be way harder to create.

85 points by Amorymeltzer 11y ago

Very inaccurate summary, since I skimmed: Up to 12% of women have an X chromosome mutation that creates a fourth cone. This cone generally overlaps the spectral region covered by the cones sensitive to red and green. The exact area of overlap determines whether or not each woman sees "more": If the fourth cone completely overlaps the region covered by an existing cone, no new information is presented to the visual system and the input is effectively discarded (she sees "just as much" red or green, not more, not less). If, however, the fourth cone has peak sensitivity in the gap between the cones covering red and green, she will she up to 100% more colour than women with normal vision.

Someone 11y ago
That's not quite correct. Where one cone type has a peak response, another may respond with a signal that is only one percent or less of its peak response, but the frequency regions in which hey have _any_ response basically overlap. Given the sensitivity curves, that difference is academic for blue versus red and green sensitive cones, but not quite for red versus green sensitive cones. What differs is the amount in which having a fourth type of come allows one to get rid of metameries (https://en.m.wikipedia.org/wiki/Metamerism_(color) https://en.m.wikipedia.org/wiki/Metamerism_(color), a term that the article surprisingly doesn't mention) If your fourth come type has a response curve that is very similar to one of te 'normal' three, there should still be _some_ effect, but it will be hard to devise an experiment that shows the ability to discriminate additional colors. But given the impact that not having red or green cones with their fairly similar sensitivity curves has, I suspect having a fourth curve, even if it falls between the two, will have a measurable (in the lab) effect on one's ability to discriminate colors.

bendykstra 11y ago

I learned the importance of trichromacy when a friend and I came across a patch of wild strawberries. I'm partially red-green colorblind and for every strawberry I found, my friend found ten. They were almost invisible to me. It's neat to see that the fourth cone's response curve peaks right between the red and green cones' curves in tetrachromats. I bet they are amazing at finding berries.

Swizec 11y ago
This probably has nothing to do with chromacy, but is related to finding berries and other odd things. My sister is gifted with finding four leaf clovers. She walks in a field and seems to just see them. Every time she picks one, it's a four leaf. Her record find is a 7 leaf clover. I always wondered how that's possible.
- metasean 11y ago
  Is it possible she has synesthesia (https://en.wikipedia.org/wiki/Synesthesia https://en.wikipedia.org/wiki/Synesthesia)?
such_a_casual 11y ago
What does it mean to be partially red-green colobrlind? Isn't it the case that you either have the red-green cone or you don't?
- nitrogen 11y ago
  It's conceivable that a mutation in the gene for the red or green light-sensing protein gets a mutation that just shifts its sensitive wavelength closer to the other. So "partial" r/g colorblindness could occur if there was still some separation between those wavelengths, but not as much as is typical.
- deckar01 11y ago
  > Deuteranomaly ... The medium-wavelength pigment is shifted towards the red end of the spectrum resulting in a reduction in sensitivity to the green area of the spectrum. [0] The closer the spectrum perceived by the M (green) cone is to L (red) cone, the harder is is to differentiate red from green. [0] https://en.wikipedia.org/wiki/Color_blindness#cite_ref-KallColor_26-1 https://en.wikipedia.org/wiki/Color_blindness#cite_ref-KallC...

peter303 11y ago

I thought in females half of X chromosomes are deactivated as to not producing conflicting similar proteins. If the extra cone gene is on the deactivated half, then it is not produced. It is suggested that imperfect deactivation leads to female increased occurrence of autoimmune syndromes over males.

akavi 11y ago
The key is that which X chromosome is deactivated is not the same in all cells. It does occurs fairly early in the development process (a stage called gastrulation), and the deactivation does persist for all cells that descend from a given cell present. But critically, it's late enough that a retina could conceivably possess cells descending from two different "lineages". You can actually visibly see the "resolution" of the deactivation by looking at tortoiseshell cats. Each blotch of orange/black represents one cell present at the deactivation stage.

dspeyer 11y ago

First thought: The usual pattern is that only one X chromosome gets transcribed in any given cell.[1] This keeps the dosage of all those proteins correct. Otherwise there would need to be a separate set of dosage controls for XX and XY people. The inactivation occurs pretty early in life, and when cells replicate they keep the inactivation. This results in macroscopic regions of the body with consistent inactivation. Usually this isn't noticeable, but in cats coat pigment is on the X chromosome, and heterozygous cats often show "tortoiseshell" or "calico" coloring. The patches of contiguous color there are larger than a retina. So it seems entirely likely that all the cells in a human woman's retina would use the same X chromosome. On the other hand, that predicts half of het women would be colorblind, which isn't observed. So maybe not so simple... I have a second thought about downstream neural hookups and the mechanisms for those, but it'll have to wait for later. [1] https://en.wikipedia.org/wiki/X-inactivation https://en.wikipedia.org/wiki/X-inactivation

xerula 11y ago
The article explains X-inactivation, and it doesn't play the role you think here: both copies are broadly equally expressed among retinal cells. Also even if all cells in a retina used the same X-chromosome copy it doesn't predict color-blindness, since we would still have a trichromatic system. The answer to the question posed by the title is simply that all mutant fourth cone types are not created equal. Most have a spectral response curve similar enough to an existing type not to make a real difference; a few have peak responses more squarely in-between those of the usual trichromat photopigments, allowing more finely graded color perception. (The article takes its time getting round to explaining this, but it's all there.)

peter303 11y ago

Scientists are looking at direct delivery of gene therapy to retinal cells to cure serious eye diseases like macular degeneration, pigmentosa blindness, etc. Should this work well, then the next step would be lesser conditions like male color blindness. I read of some successful attempts to give dicromatic animals, genrally carnovoires, the trichromatic gene directly to the retina. These subjects can be tested by giving rewards hidden in standard color blindess test images. The next step, more controversial, would be super-vision of tetrachromacity, perhaps infra-red and ultra-violet sensitivity. Night vision glasses no longer needed for enhanced soldiers.

OopsCriticality 11y ago
> The next step, more controversial, would be super-vision of tetrachromacity, perhaps infra-red and ultra-violet sensitivity. Night vision glasses no longer needed for enhanced soldiers. Utter poppycock. The cornea and lens in humans are UV opaque, which is a very good thing because UV light is damaging. People without a lens (aphakia) are reported to perceive UV light, but this is otherwise an undesirable condition. As to infrared, some people can already see up to around 750nm or so (NIR) with awful quantum efficiency. Pit vipers and some fish have limited perception of IR, but I'm not aware of any animals that can see IR in the conventional sense… being warm blooded presents a real problem, for one. Plainly, there are limits to what can be achieved in hyperspectral imaging due to materials, and that's without the constraints of biology thrown in the mix.
- scott_s 11y ago
  Your comment would be much better without the first line.
  - lsh 11y ago
    And your tepid comments are just as useless.
    - dang 11y ago

acchow 11y ago

It opens with an Australian painter who displays her extraordinary vidual experiences in her art. But if she was born this way, why would she find it extraordinary?

mirimir 11y ago
Maybe because people kept telling her that she must be tripping?
neurosphere 11y ago
Hi! I wrote the Neurosphere article you're discussing, and noticed this lively comments section. I agree with the question you're asking - if she was born this way, why did she question it? From the research I did on her life, it appears that she always assumed people could see these colours, and apparently when teaching her students in art school she would try to say things like "Try to depict all the colours you see in this leaf, like the turquoise and orange on the sides..." and her students often just found it too embarassing to tell her they didn't see any of these colours. She slowly started realising that her colour perception may be different, until one day a neurology student taking her class suggested she took a genetic test. That's it. Since then I suppose she has really harnessed the results of the test to market her vibrant art as reflecting her 'tetrachromatic reality'. Some people have commented on Twitter questioning this, and it's something I think I will write about in the near future because it's a completely fair point.

qb45 11y ago

I wonder how those poor women deal with RGB displays. It must feel weird to see a difference between red+green and actual yellow. Although, on second thought, color vision doesn't seem to work perfectly anyway. I never understood how red+blue can be perceived the same as far violet, it just doesn't make sense if you look at those frequency response curves.

jhanschoo 11y ago
I doubt it will be that bad. It will probably seem a little more dull, like some of the colorblind simulators you can find around the net. It's probably not just screens; pigments have the color they have because they reflect certain wavelengths, tuned to the proportion that a trichromat would consider identical. A tetrachromat is almost certain to notice that pigments of pictures on paper might be drastically different in color to the color of the objects the pictures are supposed to represent. The brain doesn't perceive color just by addition of the stimulation of the cones. Your red and green cones are still (slightly) stimulated when you see blue, and your brain tells you that blue is the color; with violet, the stimulation among the cones have a different proportion, hence violet. Incidentally, this is also why the sky appears blue; the sky is too polluted by other wavelengths to appear violet.
- qb45 11y ago
  Actual violet (like UV LED or mercury lamp) should stimulate almost exclusively S cones. But it turns out that if you take blue, which stimulates M and L more than violet, and then add even more L stimulation, you end up with something that "feels" more violet than blue (at least to me). This is what bothers me. It looks like two distinct inputs producing the same output, even though there exists an intermediate input which produces different output.
  - Someone 11y ago
    Two distinct inputs producing the same output is metamery. https://en.m.wikipedia.org/wiki/Metamerism_(color) https://en.m.wikipedia.org/wiki/Metamerism_(color). Without it, color printing and color televisions would be way harder to create.