Wobbly Universe

The title is just a fancy way of saying that some women seem to have four colour receptors in their eyes rather than the usual three. Actually some people also have a different set of three and this was known for some time before the discovery that quite a few women see extra colours than the rest of the population. Of course some men are missing a receptor and have only two and as a result are called colour blind. Compared to tetrachromats we are all colour blind.

[For Americans, some of the world spell color as colour and colorblind as colourblind and so on. This is just to help the search engines to find this page.]

"Oh, everyone knows my color vision is different," chuckles Mrs. M, a 57-year-old English social worker. "People will think things match, but I can see they don't." What you wouldn't give to see the world through her deep blue-gray eyes, if only for five minutes.

Preliminary evidence gathered at Cambridge University in 1993 suggests that this woman is a tetrachromat, perhaps the most remarkable human mutant ever identified. Most of us have color vision based on three channels; a tetrachromat has four.

The theoretical possibility of this secret sorority -- genetics dictates that tetrachromats would all be female -- has intrigued scientists since it was broached in 1948. Now two scientists, working separately, plan to search systematically for tetrachromats to determine once and for all whether they exist and whether they see more colors than the rest of us do. The scientists are building on a raft of recent findings about the biology of color vision.

This is from Looking for Madam Tetrachromat which goes on to describe the search for women who can see more colours. I guess that we men, and the rest of the female population just have to accept that some women who give interior design advice should be listened to so that the results are suitable for all people, not just trichromats and bichromats (or is that dichromats?).

According to The Science Show site, Robyn Williams states that each year the London Daily Telegraph gives a prize for science writing. The prize for 2004 junior winner was Caoimhe McKenna who’s 18 and who thinks she knows why women are so much better men at seeing colour coding and properly matching clothes. Here’s Caoimhe McKenna's winning piece.

As an Irish, teenage girl, I have often wondered if the world of colour that I perceive with my eyes is the same as that of others. Not only my fellow XX chromosome holders but also those holders of the X chromosome paired with the puny and rather ineffectual Y chromosome, commonly known as "men''. I wonder this as my father sets out to work in his mismatched tie and my brother with his odd socks, while my mother and I discuss various shades of colour to match our newly decorated living room.

Let me begin with a look at the history of our colour vision. We are descendants of nocturnal tree dwellers. Colour vision is thought to have evolved in our ancestors about 35 million years ago, giving them more opportunities to find fruit and leaves to eat. Humans have inherited a colour visual system that is dependant upon three forms of iodopsin, or colour pigments, each responding to light of a different wavelength region. Each form of iodopsin occurs in a different cone type and the relative stimulation of each type is interpreted by the brain as a particular colour.

I can see a demonstration of this when I get up close to my television; there are only three colours: red, green and blue. These coloured spots on the screen stimulate the colour pigments in the retina of my eye to different extents, so that when I stand back I can see all the colours of the rainbow. Maybe. All this is down to that amazing chemical iodopsin. Iodopsin is a pigment that like all proteins is coded for in the DNA of my genes. Interestingly the genes controlling the production of the pigments in the eye that enables discrimination of red and green is sex-linked.

The green and red genes encode photopigments that respond to different, overlapping regions in the middle-to-long wavelength spectrum and are adjacent to each other on the X chromosome. Strangely, the blue photopigment gene is on its own on another chromosome - feeling blue in solitude? This explains why the most common form of colour-blindness, red-green, is hereditary and why it affects about eight per cent of Caucasian males and less than 0.5 per cent of females.

I have two X chromosomes, one from my mother and one from my father's mother (thank you, girls). My brother has to manage with mummy's two photopigment genes on his X chromosome but I have four between my two X chromosomes. This is where it gets interesting.

One of my X chromosomes may have a slightly different green photopigment gene from the other and "X inactivation'' might happen. This well-known biological phenomenon causes some cells to rely on one X chromosome and other cells to rely on the other. I might have four different types of photopigment; blue, red, green and shifted green (or red and shifted red). All I need now for four colour vision (tetrachromacy) is a superior brain.

A recent paper by Kimberly Jameson, Susan Highnote and Linda Wasserman of the University of California, San Diego, concerning females who may have tetrachromacy shows amazing results. Up to 50 per cent of women are tetrachromatic and can use their extra pigments in "contextually rich viewing circumstances". For example, when looking at a rainbow, tetrachromat females can segment it into, on average, 10 different colours, whereas their trichromat brothers and sisters can see only seven, much as Isaac Newton's red, orange, yellow, green, blue, indigo and violet. Consequently, for those special tetrachromat women, this island that they inhabit may be seen in emerald, jade, verdant, olive, lime, bottle and 34 other shades of green. Apparently, men and women do see the world differently.

However, the tetrachromats among us should not think themselves too superior. If you want truly advanced colour vision it might be a good idea to become a bird. Pigeons, according to a paper by the late Francisco Varela of the University Hospital in Paris and colleagues, have five colour receptors and can process visual imagery up to 10 times faster than human beings. While we see a television producing smooth movement in realistic colour they will see dull flickering lights - this may be why you won't see a lot of pigeons watching Birds of a Feather.

In an article A Life More Colorful Cynthia Wood describes that "Jumping spiders are natural tetrachromats, with four kinds of receptors, and while there are no known mammalian tetrachromats, there are believed to be tetrachromats among birds, insects, reptiles, and amphibians."

The normal human retina's color receptors are tuned to green, blue, and red. Working together, the three give us our colorful view of the world. When one or more of those color receptors is missing the result is color-blindness. The genes for our red and green color receptors are located on the X-chromosome, giving women a redundant set of receptor genes. This is why men are far more prone to color-blindness than women. In order to be functionally color-blind a woman not only has to be missing a receptor gene on both X-chromosomes, it must be the same gene on each one. The chances of this happening are so slim that only 0.4% of the US female population is affected. By contrast male color-blindness is far more prevalent with 8% of the US male population affected - 95% of them with red or green receptor problems. Color-blindness makes it difficult or impossible to distinguish some colors, depending on which receptor is affected. The term color-blindness itself is somewhat of a misnomer, since color perception is altered, not eliminated. True color-blindness, wherein a person can distinguish no color at all, requires a malfunction of all three kinds of color receptors, and affects only 0.003% of the population regardless of gender.

With reference to searching for tetrachomats in humans, Cymthia Wood goes on to examine the question of whether having the two types of colours receptors in the two X chromosomes would be matched by the necessary brain development to use them ...

Dr. Gabriele Jordan of Cambridge University may have answered that one. She tested the color perception of fourteen women who each had at least one son with the right kind of color-blindness. She set up a test where the subjects had to manipulate and blend two wavelengths of colored light to produce any hue they liked. They then had to match their own results a second time. With normal color vision, several different combinations would match any given hue, with a tetrachromat the possible combinations to produce a visible match would be much reduced. Dr. Jordan reported that two of the fourteen women showed exactly the results she would have expected from a tetrachromat. At least one of the two women reports having a different sense of color from the people around her, with both better color matching and better color memory. While not completely conclusive, this initial study has so far provided our best candidates for natural human tetrachromats.

This is an interesting subject, and I note that the wikipedia article on primary colours does mention tetrachromats, but as other species

To generate optimal color ranges for species other than humans, other primary colors would have to be used. For example, for species known as tetrachromats, with four different color receptors, one would use four primary colors (since humans can only see to 400 nanometers (violet), but tetrachromats can see into the ultraviolet to about 300 nanometers, this fourth primary color might be located in the shorter-wavelength range and would probably be a pure spectral magenta rather than the magenta we see which is a mixture of red and blue).

However there is a wikipedia article on tetrachromacy also which states:

Tetrachromacy is the condition of possessing four independent channels for conveying color information, or possessing four different cones, one other than RGB. Organisms with tetrachromacy are called tetrachromats. For these organisms, the perceptual effect of any arbitrarily chosen light from its visible spectrum can be matched by a mixture of no less than four different pure spectral lights.

The normal explanation of tetrachromacy is that the organism's retina contains four types of higher-intensity light receptors (called cone cells in vertebrates as opposed to rod cells which are lower intensity light receptors) with different absorption spectra. This means the animal can see colors beyond those of a normal human being's eyesight. In practice the number of such receptor types may be greater than four, since different types may be active at different light intensities.

Tetrachromacy has not yet been confirmed in any mammals, though it is likely that it occurs in some birds, fish, amphibians, reptiles, arachnids and insects. Humans and closely related primates normally have three types of cone cells and are therefore trichromats (animals with three different cones). However, at low light intensities the rod cells may contribute to color vision, giving a small region of tetrachromacy in the color space.

It has been suggested that women who are carriers for variant cone pigments may be born as full tetrachromats, having four different simultaneously functioning kinds of cones to pick up different colors.[1] One study suggested that 2-3% of the world's women may have the kind of fourth cone that lies between the standard red and green cones, giving, theoretically, a significant increase in colour differentiation.[2] Although further studies will need to be conducted to verify tetrachromacy in humans, at least one tetrachromat has been identified - "Mrs. M," an English social worker, was discovered in a study conducted in 1993.[3] Variation in cone pigment genes is widespread in most human populations, but the most prevalent and pronounced tetrachromacy would derive from female carriers of major red-green pigment anomalies, usually classed as forms of "color blindness" (protanomaly or deuteranomaly). The biological basis for this phenomenon is X-inactivation.

To take a colour blindess test if you want. Sorry, we can't give you a tetrachromat test because your monitor only has 3 colours. :-)