Hypothetical form of color vision that uses four channels. Human vision is trichromatic; that is, we have receptors for red, green, and blue light. All of the colors that you see are combinations of these three. Tetrachromats, if they exist, would have perceptors for a fourth color of light.

The notion of tetrachromacy emerged from the study of genetics in the late 1940s. A number of scientists who study the biology of human vision are currently conducting experiments to determine whether tetrachromacy can actually exist, and if so, how it could be detected.

If tetrachromacy is a reality, it is hypothesized that there would have to be a fourth neural channel to transmit information from the fourth color detector to the brain. This fourth channel is not naturally occuring in humans, so it would represent an adaptation, and prove the flexibility of the nervous system in adapting to genetic quirks.

According to current theories, owing to a quirk of genetics (specifically the way that the the color detector genes are transmitted) only women are possible candidates for tetrachromacy. Tetrachromatic females would likely have male children with some form of color blindness.

Tetrachromatism is the ability to see four basic colors in place of the standard three - red, green and blue. While the existance of Tetrachromatists has not been confirmed yet, Dr. Neitz and Dr. Jordan are both conducting studies and are trying to find people with this disorder. Due to the genetics of vision, these people must be female and chances are that their male children will be color-blind.

      Why is this so? The genes for green and red photopigments are found on the X chromosome, that for blue is located far off on another chromosome. Since men only have only one X chromosome, no error-checking occurs when the new X chromosome is created in the egg. Since the red and green pigments are located so proximally they somtimes cross over; some of the eggs are disadvantaged from the start. The X chromosome might lack either a red or green photopigment gene or have dual red/green photopigment genes - regardless, the male offspring is bound to be color-blind. However, if the lucky sperm cell enriches the egg with another X chromosome and a phenomenon called X inactivation occurs, there is the possibility of a Tetrachromatist. Through X inactivation, all four pigment-genes would be activated and the woman would have receptors for Red, Green, Off-green (or Off-red) and blue color vision.
      Jeremy Nathans, a pioneer in color-vision research at Johns Hopkins University School of Medicine postulates that the brain would be able to interpret the fourth signal without trouble and assign it a new 'color.' The true determining aspect for seeing the fourth color is the degree of seperation between the peak in sensitivity for the new pigment versus red or green - if the peaks are too close, the person might only see a different (although alien) shade of green or red. Possible advantages of this would include increased awareness of skin tones (see if a child is bleak or flushed, even to a small degree;) the universal knowledge of a fourth color could aid in categorization and general awareness - try to imagine life without one of the three primaries.

So there is a color we're not seeing. Has anyone else read up on this, could it be perceived through biochemical manipulation? Since all the actual interpretation and assigning of a color takes place in the mind, this should be possible... too bad they can't tell us what the new color looks like, this is slowly driving me insane =)

I've always wondered how something like this could be tested.

The problem is, no tetrachromes are known to exist, so the experiment would naturally have to be carried out by trichromes on possible tetrachromes. The problem with this is that the trichromes wouldn't be able to see the different colors (or variations in colors, depending on how the experiment is carried out) that a tetrachrome could. Further, all current optical systems, while some are more sensitive to the human eye, are designed as trichromatic, and we wouldn't really know where to begin when designing a tetrachromatic machine (there's never been a need, and most humans couldn't even see the fourth color anyway). Therefore, the scientists (who would have to be trichromes, at least as far as anyone knows) would have no reliable way of measuring their results. So how would the scientists tell that they've found one?

This is a fascinating hypothesis and concept. But what puzzles me are a few things, such as:

  • Wouldn't a failure of X inactivation be more likely to result in tetrachromatism? (Two divergent sets of genes simultaneously expressed?)

  • Why would a four-color visual perception system be interpreted in the end by the brain at all differently? Wouldn't the brain simply compensate for the variations in post-processing?

  • As for testing the hypothesis, or trying to confirm or refute any perceptual advantages/disadvantages these women might have, the first thing I would think of would be to compare a sample of mothers of known color-blind sons with a random sample of women. Test as many aspects of visual perception and color perception as possible. Do they have a higher (or lower) incidence of design skills and talents than women in the normal sample?

    Or, go the other way and compare a selective sample of women who work in visual arts (interior designers, graphic designers, fashion designers, quilters, craftswomen, etc.) to a random female sample. Then conduct a possibly longitudinal study to detect any meaningful variation in the frequency of color blind sons between the two samples?

This node was originally written when slashdot posted a story on it, but lacked some of the details those above are demanding.

Tetrachromatism is the condition where genetic expression in a female leads to there being four different color receptors instead of the standard three, which are red, green, and blue. Two researchers, Dr. Neitz at the Medical College of Wisconsin and Dr. Jordan currently at the University of Newcastle, are attempting to find women with this condition. Due to the unique characteristics of this genetic expression, women with this gene likely have color-blind male children. This is because the genes for green and red photoreceptors are found on the X chromosome. Men only have one X chromosome, which means that they only have one set of genes for those photoreceptors. Occasionally, X chromosomes swap genetic material with each other while undergoing meiosis - leading to slight alterations in these photoreceptors. This crossing-over can lead to the possibility of an X chromosome that, instead of a green and a red gene, instead express two different greens or two different reds. When combined with a "normal" X chromosome that has a red and a green receptor specified, this can lead to the ability to express four different receptors instead of three.

How do they test for this? (Paraphrased from the Red Herring article) They believe they have, in fact, found at least one tetrachromat. How did they do this? An experiment was setup in which the subjects attempted to determine whether a pair of colored lights matched. Using a joystick, they blended two wavelengths of light to produce a match. In order to eliminate the blue photoreceptor, the hues were picked to lay outside its spectrum. (The blue photoreceptor does not appear on the X chromosome). The subjects would then try to reproduce it. Because a trichromat would have only two receptors to use, they would find a large number of colors that apparently "matched". However, the tetrachromats would produce a single, precise match every time. Two subjects were capable of doing this.

Why X inactivation is important (not from the article. I am not a geneticist, but I'm married to one, and we did discuss this subject...): To express tetrachromacy, what one has is an X chromosome with two slightly different greens or two slightly different reds and an X chromosome with a green and a red. Now, what X inactivation means is that some cells will rely on one of the chromosomes and some will rely on the other -- in other words, divergent expression.

Why would the brain be able to tell? First off, the brain notices color in the first place. There are animals without the ability to distinguish color -- why should we have it? There are even different versions of color blindness. IMhO, I think it would be fairly obvious to the brain when adjacent receptors function differently. Some reading indicates, interestingly, that the brain actually does assign more space to pattern recognition than wavelength.

For more information, check out the original slashdot article at http://slashdot.org/science/00/11/28/1536204.shtml or the Red Herring article it points to at http://www.redherring.com/mag/issue86/mag-mutant-86.html, or learn about color vision genetics at http://www.arbor.edu/~michaelb/chroab2.htm. There is a lot of information out there, actually; a search on Google for color vision genetics will lead to many links and examination of color vision in many species.

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