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I understand that cone cells vary in the color they sense, is this because of wavelength, frequency, something else, or a combination of the previous? I also understand that tetrachromats can see an extended spectrum of color because they have four as opposed to three (for trichromats) color channels, hence, I believe, an extended variation of cone cells.

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The range of color perception in humans is primarily limited by the sensitivity of cone cells (specifically, the opsin proteins in cone cells) to various frequencies of light. This sensitivity drops off at 700 nm on one side and 380 nm on the other, which is where infrared and ultraviolet begin, respectively.

Even with hypothetical infrared or ultraviolet cones, we wouldn't necessarily be able to perceive color much further beyond our current range anyway. The various parts of the eye (cornea, lens, and the liquid inside) block both ultraviolet (starting around 400 nm) and infrared (starting maybe at 1400 nm) beyond certain wavelenghts.

http://en.wikipedia.org/wiki/Color_vision

Tetrachromaticity, at least in humans, does NOT confer an extended spectrum of color perception. Tetrachromaticity results from a mutation on one of the light sensing proteins, changing its peak sensitivity by a few nanometers. So a tetrachromat can't see beyond the normal color spectrum, but they can probably distinguish shades of green and blue more precisely than trichromats. This has been an area of active scientific research recently; I think most scientists didn't think they existed. Now studies suggest they're 2-50% of the population. Humans with 5+ cone types are also hypothesized, and with the ubiquity of genome sequencing I imagine we'll be learning a lot more about that soon.

Other species have more diverse color perception, with ultraviolet being especially common in insects and birds. The mantis shrimp has an astounding 16 different kinds of photo receptors, plus a few specialized eyes for detecting polarized light. For a loving account of mantis shrimp, I recommend this comic. http://theoatmeal.com/comics/mantis_shrimp

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Wikipedia says "the rods and cones located at the back of the human eye cannot detect the short ultraviolet wavelengths" but you say otherwise, could you possibly give evidence for your claim? Edit: Also mutation could also change the trough, right? –  5ives Apr 28 '13 at 10:45
    
I also understand that ultraviolet radiation is absorbed by varios tissues of the eye (which is what causes photokeratitis, is this the same for infrared? –  5ives Apr 28 '13 at 10:54
    
Sorry, I meant that as a hypotical, i.e. if we DID have extra chromophores, they still might not work. I did see one reference that infrared doesn't actually start getting absorbed until 1400 nm by the eye though, so we might be able to extend a fair distance in that direction, but not infinitely. –  Alexander D. Scouras Apr 28 '13 at 10:59
    
I indeed read somewhere (and have never been able to find the source) that it would only take a single base pair mutation for us to see much further into ultraviolet. But that it wouldn't matter because our lens starts absorbing it very close to our existing limit. There is apparently no true infrared vision, possibly because the photons are too low energy. Animals that "see" infrared do it through other types of organs. So I wouldn't bet on a mutation on that end. –  Alexander D. Scouras Apr 28 '13 at 11:04
    
You mean that if we had extra chromophores, humans still might not be able to see them or that the chromophores just wouldn't work? –  5ives Apr 28 '13 at 11:16
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