The Mystery of Tetrachromacy

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.

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?

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), 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.

> 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.

I don't think we can get good infra-red vision.

Creatures with infra-red vision are usually cold blooded. We shine too brightly in infrared from within to be able to see something outside.

But tetrachromacy should rightfully belong to us. It was lost by our pre-mammal ancestors, while reptiles still have base tetrachromacy, and some fish perceive even more colors.

Most animals only perceive two colors and grow two pigments. Makes animal coloring (and animal world) kind of boring for us. Birds use bright colors because they can distinguish them.

I just visited my retina guy and he told me about the latest developments. These days they're modifying the genome of stem cells and printing small clusters of retinal cells onto a substrate for implantation.

There have been some successes curing serious blindness issues but they will be hesitant for a good long while to "cure" things like colorblindness. If things go wrong a blind person is still blind, but a colorblind person has quite a bit to lose.

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.)

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.

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.

Maybe because people kept telling her that she must be tripping?

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.