Slashdot Mirror
Mutant Tetrachromat Females Found
Hydrophobe writes "Red Herring reports that
at least one living human female has
four-color (tetrachromat) vision. Apparently, genetics dictates that all such tetrachromat mutants would be female. Compared to them, the rest of us are partly colorblind - they would be able to see colors beyond the standard three-axis RGB scale."
--
"How many six year olds does it take to design software?"
dinner: it's what's for beer
Forget an extra receptor, when I was growing up I could have sworn that my mom had a whole extra eye in the back of her head.
The slashdot headline is premature in stating that a tetrachromat had actually been found.
"Nevertheless, Dr. Jordan declines to say that she has finally found a tetrachromat, partly because her testing is still a work in progress."
Sure, the dyes each represent vectors in the full infinite-dimensional spectral space, and not simply particular wavelengths -- but so long as they're linearly independent (i.e. you can't generate the spectrum of any one dye out of a weighted sum of the other dyes' spectra), they're useful for distinguishing color.
The primary additive colors (R, G, and B) are determined by the spectra of the dyes. You can't pick any set of primary colors you want -- the color wheel was discovered experimentally long before we knew the cellular biology to do direct experiments on the human eye. The primary subtractive colors (C, M, Y) are made by subtracting the corresponding (R, G, or B) from white light -- cyan light has G and B components, but no R.
When you get into detailed color vision, things (as always) get more complex. It turns out that there are no precise primary colors that everyone can agree on, because not everyone uses the same dyes in his cones! There are slight variations across the population, so that the R, G, and B primary colors correspond to different pieces of spectrum depending on who's looking.
Because of the overlap of (for example) the R and G spectra, it's not normal possible to generate a pure R signal in the human retina with any single wavelength of visible light. But we're wired to do the linear decomposition ourselves: in effect, the differential gain is really high between the R and G "raw" channels coming out of our retinas. Cool, eh? As laser pointing becomes more accurate, we ought to be able to stimulate directly our individual cones -- one day somone could perceive "superred" by directly stimulating only the red cones in his fovea. I wonder how different it would look than the more common red?
There's a really interesting overview article on color vision in the Feynman Lectures, volume I. It includes typical spectra for R, G, and B dyes. If I recall right, R and G are actually rather similar spectrally, with somewhat broad humps in the long end of the spectrum, while the B dye has a very different spectrum with a sharp peak near the short end of blue.
HER: Honey, can you find my red shirt for me?
HIM: Yeah, here it is.
HER: No, dear, that's the magenta one. I wanted the red one.
HIM: Is this it?
HER: No, that's burgundy. Forget it. Just give me my cream sweater instead.
HIM: Cream? Is that white?
HER: It's almost white but has a little yellow in it.
HIM: Here it is!
HER: You moron! That's a khaki colored sweater. I wanted the cream one! MEN!
>they just have extra reception
Er, I don't thinks so. They have a different *distribution* of receptors - four kinds (instead of three) with relatively tight color-bands, and one type which responds to the full visible light spectrum. This is why you can see B/W in very low light - still enough to trigger enough of the broad-spectrum receptors, but not enough for the tight-spectrum color recievers. This is why animals with very good night vision usually can't see color - they punt the color entirely for extra broad-spectrum receptors.
The space for those extra receptors in a tetrachromat came from somewhere, presumably other color receptors. I would *guess* that means they need more light to see in color than we do, but see finer color gradients....??
Pigeons have tetrachromat vision as well. My question though, is do they see a fourth and different color?, OR are the colors we see spread out a larger spectrum for them?? I know the frequencies are higher (or lower), Im talking about what shade it looks like in their brain, the whole how do you know when I look at grass I dont see red and call it green? According to a theory, this is similar as the difference between the vision of a dichromat (a color-blind) and a normal trichromat, like most of us. It means that a tetrachromat can have a novel pair of colors similar to our yellow-blue and red-green pairs. I would really really like to have the sensory output from her eyes fed into my brain, dont you think they could hook that up? Does that mean that there are an infinite amount of colors, because a pentachromat (some animals have five color receptors) would see even more colors.
Roses are red,
violets are blue,
trichromats can't see
the other amazing hues
________________
________________
Private Essayist