Scientists have long known that the eyes of mammals contain neurons that respond to light and set the body’s internal, or circadian, clock. It was long assumed that this light-sensing function was performed by known visual cells called rods and cones. But in 1999, report the journal Science, researchers found that “mutant mice lacking all rods and cones [who are thus functionally blind] still have light-responsive clocks.” This led researchers to the conclusion that “some other cells in the eye had to be sensing light”
          Now these elusive light sensors have been found. Although intermingled with the image-forming rods and cones, the sensors form “a separate visual circuit, running in parallel with this image-forming visual system,” explains Science. The newly discovered circuit’s functions include governing pupil size and melatonin release, synchronizing the body’s internal clock with the cycle of light and dark and other tasks. It may even play a role in the modulation of mood.
         Significantly, the light sensors do not respond to brief flashes of light, lest they confuse the body clock, but only to longer changes in luminance levels. One scientist described the discovery as “spectacular,” adding that “it is the biggest break yet in the question of what is the photoreceptor in mammals.”