We can see in the infrared!

The major part of my professional life was spent doing research on how the rod cells in our retinas change light into a nerve signal. (I just got a request from ResearchGate, a site on which scientists list their work, suggesting that I upload another of my old vision articles, in this case one that appeared in Nature - in 1965 - 50 years ago! - titled "Reaction of the Rhodopsin Chromophore with Sodium Borohydride".) Even though for the past 20 years or so I have focused on the topics covered by MindBlog I occasionally see a vision article that takes me back to 'the old days'. A colleague from those days (Krzysztof Palczewski) and collaborators have recently done a nice piece of work demonstrating that we can actually expand our vision beyond the normal "visible" range of 400 (blue) to 720 (red) nanometer (nm) wavelengths into the higher frequency (lower energy) infrared regions emitted by infrared lasers. It turns out that the Rhodopsin Chromophore of my article above, retinal, which normally has its shape changed (isomerized) by absorbing one photon of visible light, can be activated by a two-photon chromophore isomerization, especially at wavelengths above 900 nm. From their significance and abstract statements:

This study resolves a long-standing question about the ability of humans to perceive near infrared radiation (IR) and identifies a mechanism driving human IR vision. A few previous reports and our expanded psychophysical studies here reveal that humans can detect IR at wavelengths longer than 1,000 nm and perceive it as visible light, a finding that has not received a satisfactory physical explanation. We show that IR light activates photoreceptors through a nonlinear optical process.

Vision relies on photoactivation of visual pigments in rod and cone photoreceptor cells of the retina. The human eye structure and the absorption spectra of pigments limit our visual perception of light. Our visual perception is most responsive to stimulating light in the 400- to 720-nm (visible) range. First, we demonstrate by psychophysical experiments that humans can perceive infrared laser emission as visible light. Moreover, we show that mammalian photoreceptors can be directly activated by near infrared light with a sensitivity that paradoxically increases at wavelengths above 900 nm, and display quadratic dependence on laser power, indicating a nonlinear optical process. Biochemical experiments with rhodopsin, cone visual pigments, and a chromophore model compound 11-cis-retinyl-propylamine Schiff base demonstrate the direct isomerization of visual chromophore by a two-photon chromophore isomerization. Indeed, quantum mechanics modeling indicates the feasibility of this mechanism. Together, these findings clearly show that human visual perception of near infrared light occurs by two-photon isomerization of visual pigments.