By now everybody will have heard the news, Technology Review: Color-Blind Monkeys Get Full Color Vision (original paper Katherine Mancuso, William W. Hauswirth, Qiuhong Li, Thomas B. Connor, James A. Kuchenbecker, Matthew C. Mauck, Jay Neitz & Maureen Neitz, Gene therapy for red–green colour blindness in adult primates, Nature advance online publication 16 September 2009 doi:10.1038/nature08401). The nice thing about this result is that it is gene therapy in adults rather than done in the embryonic state (like the knock-in mice of Jacobs et al). Here the new pigment was expressed in cones already doing vision and connected into the colour-processing pathways, but the new kind of signals caused plastic change in the processing so that they could distinguish between the new colours. I wonder how they experienced the difference?
In discussing this with transhumanist friends we immediately looked at the potential of not just becoming tetrachromats (which would increase chromatic discrimination) but extending the visual range by expressing some opsins sensitive to longer or shorter wavelengths. There are clear limits to this: due to the lens and vitreous humor being UV-opaque below ~300 nm we can only see near UV, and real "heat vision" for seeing other mammals in the dark will be hard to achieve given that the interior of our eyes is shining with the same IR wavelengths. Besides, the water will likely absorb IR beyond ~1500 nm (see this diagram of eye transmittance).
This discussion led me to look up one of those weird old research strands: replacing vitamin A in food with retinoids intended to change spectral sensitivity. Thanks to Edward Keyes page I found these two papers:
Yoshikami, Pearlman, and Crescitelli, Visual pigments of the vitamin A-deficient rat following vitamin A2 administration, Vision Research 9:633-646 1969. This demonstrates that that rats with a diet deficient in vitamin A but supplemented with A2 (3-dehydroretinol) did get different retinal pigments, with sensitivity shifted redward by 20 nm.
Ernest B. Millard and William S. McCann, Effect of vitamin A2 on the red and blue threshold of fully dark adapted vision, Journal of Applied Physiology 1:807-810 1949 This paper did this experiment on humans back in the 40's, noticing a slight increase in sensitivity.
However, as Riis points out, going from A1 to A2 in humans would just bring down our sensitivity peak from 565 nm to 625 nm. Not an enormous improvement! But we would be able to see a bit more of the very near infra-red.
It would be really interesting to be able to see so far down in IR (> 5000 nm) that one could tell the chemical differences between substances by doing visual IR spectroscopy. "Oh, that looks like it has a lot of trans-disubstituted alkenes!" But we need very different kinds of eyes to do it. It is not enough to tweak them slightly.