Snake infrared receptors: thermal or photochemical mechanism?

It appears that the two most sensitive infrared receptors known in the biological world are found in two widely different families of snakes, the pit vipers and the boas. After an infrared stimulus from a carbon dioxide laser, which has a monochromatic output at 10.6 micrometers, we find evoked potentials in boas with chronically implanted electrodes. Our data suggest that the receptors operate on a thermal principle.