Expert: Dana Krempels, Ph.D. - 11/18/2007

Question
I've always assumed that human vision was about  average by comparison to other animals, but I recently read somewhere (re a page on Cerenkov radiation) that, in fact, human vision "peaks at" the colour green, and is relatively poor at perceiving the violet range of the spectrum. Is the human eye also poor at perceiving red colours as well, by comparison to other animals?

Lastly, do any species of animals exist which can perceive all parts of the visual (human) spectrum plus the infra-red and ultra-violet range? If so, please give me some examples - (I presume that no species can actually see x-rays, microwaves and other more extreme ranges, though a tiny number might(perhaps?)  have specialised organs which sense some such rays).
Thanks,
Geoff

Answer
Dear Geoff,

Rhodopsin, the pigment found in our rod photoreceptors, has a maximum absorbance at about 500nm (blue-green).  This pigment/these photoreceptors are responsible for night vision, and they do not confer sensitivity to colors.  I wonder if what you read was referring to night vision, since only our *rod* pigments peak in the "green" region of the spectrum.  It does mean that the light we see best at night will be around 500nm, and taper off at either end from there.  But it doesn't say anything about human ability to see color or use *cone* photoreceptors for daytime vision.

The human eye has excellent trichromatic color vision, and we see very well into the red (about 700nm) and the violet (about 400nm) regions of the spectrum.  The ability to discriminate colors is imparted by three different types of cone photoreceptors, each with a different pigment peaking in the red, green, and blue regions of the spectrum, respectively.  Humans have better red vision than almost all other mammals, and as good as that of most other animals that can discriminate colors.

Infrared vision was actually my specialty when I was doing my dissertation.  I examined whether birds of prey could see into the near infrared region of the spectrum, and although my data suggested that they could see longer wavelengths than the human eye, it did not indicate that they perceived the longer (IR) wavelengths as a different color.  

Snakes such as pit vipers and pythonids, which have heat-sensing pits or scales (depending on the group), can "see" long infrared wavelengths because the heat coming from their prey is processed by the visual cortex.  We can't really imagine what they "see", but they certainly can perceive the heat from their prey in a somewhat visual fashion.

A surprising number of species can see ultraviolet, and even discriminate wavelengths between about 300-380nm as a separate color.  Many insects (bees, butterflies), and possibly most birds and some other reptiles can perceive UV as a separate color.  Some flowers have nectar guides visible only in the ultraviolet region of the spectrum.  You can see an example here:

www.bio.miami.edu/dana/pix/nectarguide_uv.jpg

As far as I know, no living animals can see very short (x-rays) or very long (radio) wavelengths.

Hope this helps.

Dana