
The Mystery of Tetrachromacy - Amorymeltzer
http://theneurosphere.com/2015/12/17/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/
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bendykstra
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.

~~~
such_a_casual
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?

~~~
ajuc
I don't know how it works, but I have it - I can see red and green and pink in
big blobs, but when there's lots of small red and green (or pink) details
mixed - they seem the same to me (if their darkness is similar).

Also I have problems with big uniform colour areas it there's small amount of
green mixed with pink or red and vice-versa. I was making a sunset skybox for
a game, and friends were wondering why the sky is slightly green - for me it
was pinkish-red-yellow, but I put there some green by mistake.

~~~
such_a_casual
That sounds like standard red-green colorblindness.

~~~
ajuc
Good to know, I thought standard red-green color blindness is when you don't
recognize these colors at all.

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PeterWhittaker
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
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.

------
peter303
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
> 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.

~~~
nl
_Utter poppycock._

That seems... strong, especially if referring to the "Night vision glasses no
longer needed for enhanced soldiers" bit.

 _They eventually formulate a chlorin e6 solution for human use. A few drops
are dripped into Licina’s eyes, and they had him look for people hidden among
trees as well as symbols on objects in dim light. Licina seemed to perform a
lot better than the four other people who did not get eyedrops._

[http://gizmodo.com/the-real-science-behind-the-crazy-
night-v...](http://gizmodo.com/the-real-science-behind-the-crazy-night-vision-
eyedrops-1694955347)

~~~
OopsCriticality
My frustration was more directed towards the UV-Vis-NIR part (and I read night
vision as thermal IR, but who knows), but I wouldn't say that your linked
article is something of substance. To quote the article, quoting the
experimenter/ee, "In Licina’s own words: 'Let’s be fair here. It’s kind of
crap science.'"

I wouldn't even know where to begin in criticizing their study as disseminated
on their website except to say it is completely unscientific. There are no
proper controls, for one. I think it's wholly irresponsible of the press to
report on this work in this fashion at such a premature stage. I respect the
enthusiasm of the citizen science crowd and think it's a neat idea, but stuff
like this is going to quickly earn it a very bad reputation.

------
dspeyer
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
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.)

------
qb45
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
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
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
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.

~~~
qb45
You aren't paying attention, by "inputs" I meant "inputs from retina to the
nervous system", not "inputs to the eye".

Most metamerism can be trivially explained by different combinations of light
stimulating cones in the same way. But it looks like red+blue and violet are
different and there is just no reason for them to be perceived the same,
except that apparently there is also no reason to distinguish between them.

~~~
Someone
The sensitivity of the red cones appears to go up near violet
([http://www.yorku.ca/eye/specsens.htm](http://www.yorku.ca/eye/specsens.htm))

Wikipedia claims that is because of _" a second resonancy of the red-sensitive
cone cells."_
([https://en.m.wikipedia.org/wiki/Violet_light](https://en.m.wikipedia.org/wiki/Violet_light))

~~~
qb45
Well, this finally makes sense.

None of the red cone sensitivity plots I've seen previously showed secondary
peak, but that's because they were always cut somewhere in the blue region.

------
acchow
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?

~~~
neurosphere
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.

------
peter303
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
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.

