- Some animals sense light with organs outside their eyes, but humans do not. Still, some blind people can adjust their body to the cycle of night and day.
- In recent years such nonvisual responses to light have been linked to specialized neurons in the retina that can detect light autonomously but also relay signals from rods and cones, other light-sensing cells.
- These neurons respond to blue light and may be remnants of evolutionarily ancient organs from our invertebrate past.
- The discovery may lead to new approaches to treating seasonal affective disorder, certain sleep disorders and other debilitating conditions.
In the 1920s Harvard University graduate student Clyde E. Keeler discovered two surprising facts about mice he had bred in his rented attic room. One, all the progeny were completely blind. Two, despite the animals’ blindness, their pupils still constricted in response to ambient light, albeit at a slower rate than did the pupils of sighted mice.
Many years later researchers extended Keeler’s observation, showing that mice genetically engineered to lack rods and cones (the light receptors involved in vision) nonetheless reacted to changes in light by adjusting their circadian clock—the internal timer that synchronizes hormone activity, body temperature and sleep. The animals performed the usual daytime activities when in daylight and nighttime activities when in the dark. They could do so even though their retinas lacked the photoreceptor cells that vertebrate eyes use to form images, although surgically removing their eyes abolished this ability. This phenomenon may be common to many mammals, including humans: recent experiments have shown that certain blind people can also adjust their circadian clocks and constrict their pupils in response to light.