According to the article, our photoreceptors respond to four different wavelengths, but our lenses block the lower wavelengths (the shaded portions in the picture). Surgical replacement of biological lenses with more transparent ones might allow us to see into the UV A range. - Sanjeev Singh

FriendFeed is slowly discovering all the links that I've been collecting for my future blog post, “We are all color blind.” - Amit Patel

This appeals to be in a bizarre fashion. - Jerry Welch

"Note that the above optical density is for a human lens of about 5 mm thickness. The optical density is proportional to the thickness of the lens. As will become apparent below, smaller animals have better ultraviolet visibility than humans because of their thinner lens. Larger animals have even less sensitivity in the ultraviolet and even blue regions for the same reason." - bob

And the fourth color look like pea soup? ;-) - Jim Norris

Yeah but I'm guessing the lens blocks UV A for a reason, which has something to do with keeping your retinas from being fried. Just a guess though ;-) - Karim

How would we process the data, though? (For that matter, how do X-chromosome heterozygous tetrachomats process the data?) Our retina and visual pipeline is pretty set up for trichromacy. - ⓞnor

nor, that's interesting, where could I learn more about "our retina and visual pipeline is pretty set up for trichromacy"? - Jason Wehmhoener

j1m probably has some more technical references, but I very highly recommend reading http://www.handprint.com/HP... if you are at all into geeking out about the fundamentals of color. From http://www.handprint.com/HP...: "Evolution could arrive at a more complex visual system, but it would require modifying a visual cortex specialized to receive and interpret the three cone outputs; adding a fourth cone would mean reengineering the brain as well." - ⓞnor

So has anyone had these new lenses installed? I thought that was a relatively common procedure. Maybe they use uv blocking replacements? - Paul Buchheit

I've heard of experiments where people (probably Army "volunteers") had their vision extended into near-UV, but with the predicted retina-burning results. - Gabe

Cataract surgery and the use of replacement intraocular lenses has been around for a while. I am not sure, but I am under the impression that originally people were encouraged to wear sunglasses or UV-blocking lenses to block UV, though lately the replacement lenses seem to block UV (see http://archopht.ama-assn.org/cgi...). This is to prevent retinal damage. - Karim

There is a fascinating article here: http://www.guardian.co.uk/science... that confirms my suspicions that UV looks like the color white under a black light: whiter than white, tinged with blue or violet. If you have ever seen someone's clothes or teeth glowing under a blacklight, you might have an idea what being able to see into the near UV is like. - Karim

I am not of the opinion that a fourth cone would require re-engineering the brain, so much as it might involve co-opting the existing channels. UV might be perceived as a change in brightness (luminance) rather than a new color (chroma). - Karim

paul, egnor: the studies that this article refers to involved people who had their lense in one eye replaced. One of the investigators is himself akaphic and can see UV: http://starklab.slu.edu/humanUV... Karim's link is good too. - Sanjeev Singh

@Karim, the problem is that the perception of brightness will continue to be needed for (a drum roll) actual brightness. - j1m

:-D good point, j1m. i guess i am thinking of UV looking "unnaturally" bright, glowing, the way the color white does under a blacklight. so the perception of UV would be of things being radioactive ;-) just a guess, mind you... - Karim

I think you might actually need to have one normal eye to see uv light: you'd need to compare the differences between the eyes and if your UV eye sees a whitish blue that the normal eye doesn't, then it's UV. - Sanjeev Singh

Well, think about how you see violet -- you don't need a violet-sensitive and a non-violet-sensitive eye, just a violet-sensitive cone, whose signal can be compared to a few non-violet-sensitive cones a few microns away from it. - j1m

From the sound of it, there is no UV cone, it's just that the regular cones are uv sensitive (though the blue more so), which is why it looks like a bluish white. - Paul Buchheit

The reason you can't "see" UV directly might be due to the processing circuitry, not the cones themselves. - Sanjeev Singh