Color-blindness

Following up on Grim's color test:

Obviously both my husband and I have a good deal of color acuity, but his is on a higher plane than mine.  It's something like tone pitch acuity:   my sense of relative sound pitch is good enough to let me improvise and sing harmony, but my grasp of absolute pitch is fuzzy and intermittent.  People with real absolute pitch simply know what the tone is without hesitation, in or out of context, whether they've heard it recently or not.   My husband's color acuity is like perfect pitch: he knows the color when he sees it and continues to know it when he's not looking at it any more, whereas I have to have the two colors together in order to judge--though if I can see them together, I know the difference with confidence.

Color-blindness is a deficiency in one of the three kinds of cones in the retina, each of which specializes in a particular freqency range.  "Blindness" is perhaps not a very accurate term, because the impairment of cone function can occur all along the spectrum from barely detectable to complete; it's nothing like a simple on/off switch.  The problem with the cones is usually a sex-linked congenital condition, meaning it results from an area on the X-chromosome and therefore manifests more often in men (8% to women's 0.5%) because they haven't got a second X-chromosome to mask it.   Women would have to have the gene on both X-chromosomes in order to manifest the deficit.   Of course, they remain carriers, so a common pattern of inheritance is from grandfather to grandson.

One never knows about the evo-pop explanations for these things, but Wiki has these interesting observations to offer:

Some studies conclude that color blind people are better at penetrating certain color camouflages.  Such findings may give an evolutionary reason for the high prevalence of red–green color blindness.  And there is also a study suggesting that people with some types of color blindness can distinguish colors that people with normal color vision are not able to distinguish.

When I was a child, my sisters and I were fascinated with the little book of Ishihara color-blindness tests that were on my father's shelf.  They're available on the Internet now, of course:

Wiki says that one of the earliest color-blindness tests was created in response to the Lagerlunda train crash of 1875 in Sweden.  Because Physiologist Alarik Frithiof Holmgren suspected that the train engineer's color-blindness caused him to misinterpret a warning signal, he used skeins of dyed wool fiber to test the ability to distinguish colors.  This site discusses even earlier works dating back to the 17th century.

Here's an interesting online test that's like the one Grim posted, but with more specific diagnostic results.  I experimented with this one.   To me, there's only one way to arrange the colors, no matter how many times I do it, and apparently it agrees with the site's notion of the ideal arrangement.   I tried randomly introducing errors to see if I could understand the diagnostic tools, but I guess the errors have to fall into a standard pattern to make it work.

I was taught as a child that red-green color-blindness was just one of the possibilities, which happens to be the most common because of the likely failure of one of the three kinds of rod.  There should be two other theoretical possibilities in addition to [blue/yellow-red] a/k/a [green/red], which are: [yellow/red-blue] a/k/a [orange/blue], and [red/blue-yellow] a/k/a [purple/yellow].   This site assures me that the categories of potential dysfunction are considerably different, and that "red-green color-blindness" is something of a misnomer, though it is in fact the most common type. Taking "red-green colorblindness" as a generic term for protanopia (red-blindness), protanomaly (red- weakness), deuteranopia (green-blindness), and deuteranomaly (green-weakness), it accounts for about 99% of all color-blindness.  This downloadable book is full of interesting facts, such as the following:

-- Approximately every 500st handshake is between two colorblind people. 

-- It is almost sure (probability: 94%) that at least one member of a football team is colorblind. 

-- If you pick out 100 persons, the chance is very low (about 1.5%) that none of them is colorblind.

 If that link doesn't work, the free download can be triggered from here.

There's a possibility of gene-therapy treatment in the works.  It's said to work brilliantly in squirrel monkeys, but is still undergoing safety testing in animals before it can be tried in humans.  What would it be like suddenly to be able to detect a new range of colors?  It's reasonably well established, I take it, that some organisms are tetrachromatic; i.e., distinguish four peaks of color frequency to our three.  Some people think a certain percentage of humans are tetrachromatic.  In addition, although the normal cornea filters out UV light, people lacking a cornea reportedly can perceive the lower ranges of UV light as a separate bluish-white color.  They can see in something like a more full octave, with the frequency of the high-purple color nearly twice that of the lower-purple end of the rainbow.  Do the colors look as similar to them as high- and low-C sound to us?