The Himba people's colour vision.

A hypothesis for why the sky is black even during the day.

The accumulation of small heritable changes within populations over time.

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The Himba people's colour vision.

#1  Postby DavidMcC » Sep 12, 2012 12:59 pm

This is the continuation of a discussion in the "Mary and the black and white room" thread
http://www.rationalskepticism.org/philosophy/mary-and-the-black-white-room-t34063-60.html#p1459574
that started a little while ago, in the Philosophy forum, but which I feel should belongs in the evolution forum, because it seems to involve NS.
A likely selection scenario to explain what I think is the Himba's lack of SW cones is that blue light focusses at a different depth in the retina from red and green, due to wavelength dispersion as the wavelength approaches the absorption edge of the lens material, which is in the near UV. Thus a bright blue sky would blur distance vision (even though there are few SW cones in the fovea centralis), by swamping the image of dark speck on the horizon if it is even slightly off the fovea.
http://hyperphysics.phy-astr.gsu.edu/hbase/vision/rodcone.html

The visual perception of intensely blue objects is less distinct than the perception of objects of red and green. This reduced acuity is attributed to two effects. First, the blue cones are outside the fovea, where the close-packed cones give the greatest resolution. All of our most distinct vision comes from focusing the light on the fovea. Second, the refractive index for blue light is enough different from red and green that when they are in focus, the blue is slightly out of focus (chromatic aberration).
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Re: The Himba people's colour vision.

#2  Postby DavidMcC » Sep 12, 2012 1:58 pm

Another point: perhaps it would be an advantage on a typically bright, sunny day in Namibia, not to have to squint, or wear sunglasses, when you look at the horizon. I'm not sure they were asked about that in the video, though.
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Re: The Himba people's colour vision.

#3  Postby DavidMcC » Sep 12, 2012 2:34 pm

It is also possible that they are tritanopians, but with both MW opsin alleles, so that they don't see blue, but see a lot of shades of green:

http://www.colblindor.com/2006/05/08/tritanopia-blue-yellow-color-blindness/

Note that violet would then appear as a dark shade of red.

Comment #1, by a certain "Digby", is especially interesting from the point of view of the Himba's powers of green vision. Perhaps he should have been given the eyesight test used on the Himba, as well as genetic testing for opsin alleles that may have been missing in the work on the Himba.
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Re: The Himba people's colour vision.

#4  Postby Varangian » Sep 12, 2012 3:47 pm

It is known that sunglasses with anti-glare lenses reduce the blue end of the spectrum, which makes them recommended for driving, sailing, etc.

Are you an ophtalmologist or optometrist, or do you just have an interest in ophtalmology?
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Re: The Himba people's colour vision.

#5  Postby DavidMcC » Sep 12, 2012 6:06 pm

Varangian wrote:It is known that sunglasses with anti-glare lenses reduce the blue end of the spectrum, which makes them recommended for driving, sailing, etc.

Are you an ophtalmologist or optometrist, or do you just have an interest in ophtalmology?

No, no, yes! 8-)
(And eye evolution.)
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Re: The Himba people's colour vision.

#6  Postby DavidMcC » Sep 12, 2012 6:30 pm

... I also did optical coating design and fabrication, professionally. (Although I never actually designed any sunglasses!)
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Re: The Himba people's colour vision.

#7  Postby DavidMcC » Sep 12, 2012 6:49 pm

On the subject of blue blocking, I was surprised to read in Nature Eye, that it is "controversial" that artificial lenses for cataract surgery should be blue-blocking, just like the real thing!
http://www.nature.com/eye/journal/vaop/ncurrent/full/eye2012178a.html
Controversy
Eye , (7 September 2012) | doi:10.1038/eye.2012.178

Blue-blocking intraocular implants should be used routinely during phacoemulsification surgery – Yes


I no longer have access to "Eye" to see the arguments put forward by the "No" people but, to me, it's a no-brainer - you could say I'm a "yes" man on that! OK, most of the people who receive artificial lenses aren't going to be young, and the damage done to the retina by blue and UV light takes years to accumulate, but some recipients will have many years ahead of them, and they don't want extra AMD just because someone thinks they should have extra blue on their retinae!
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Re: The Himba people's colour vision.

#8  Postby DavidMcC » Sep 13, 2012 12:05 pm

Back to Digby's complaint (post #3, above): I now realise that he was probably complaining about a problem with the colour printing of the rainbow as it is supposed to be seen by tritanopes, but probably printed for the benefit of standard trichromats, who see blue. It probably had too much blue in the "green" region, which Digby would therefore have seen as dark, and not representative of his rainbow.
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Re: The Himba people's colour vision.

#9  Postby DavidMcC » Sep 20, 2012 9:27 am

I found some more detailed information about the Himba's colour vocabulary:
http://en.wikipedia.org/wiki/Himba#Color_perception

Color perception
Several researchers studied the Himba color perception.[21] The Himba use four color names: Zuzu stands for dark shades of blue, red, green and purple; Vapa is white and some shades of yellow; Buru is some shades of green and blue; and Dambu is some other shades of green, red and brown. It is thought that this may increase the time it takes for the Himba to distinguish between two colours that fall under the same Herero colour category compared to people whose language separates the colours into two different colour categories. However, this time difference is only noticeable when measured in milliseconds and remains unproven.[22]

"Vapa" is interesting, because the confusion between yellow and white is exactly what you would expect of a tritanope, whose monochromatic neutral corresponds to the trichromat's yellow. Unfortunately, ref 21 also omits to consider actual eye biology.
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Re: The Himba people's colour vision.

#10  Postby DavidMcC » Sep 21, 2012 2:24 pm

http://www.nature.com/news/2010/100217/full/463857a.html

Africa yields two full human genomes

To date, none of the large human genome projects has focused on minority populations. "We've been pushing for many years for the importance of studying diverse African populations," says Sarah Tishkoff, a genetic anthropologist at the University of Pennsylvania in Philadelphia. "It's very exciting to see this has now been done."
...
Meanwhile, Tishkoff notes that the falling price and accelerating pace of sequencing mean that other African genomes are likely to be sequenced in the future. "We don't want to see any population being left out of this genomics revolution," she says.


So, eventually, we'll know more about why the Himba have an unusual colour vocabulary.
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