Our eyes have in-built night vision mode, say scientists who found that to see under starlight and moonlight, the retina changes both the software and hardware of its light-sensing cells.
Retinal circuits that were thought to be unchanging and programmed for specific tasks are adaptable to different light conditions.
However, scientists have now identified how the retina reprograms itself for low light.
"To see under starlight, biology has had to reach the limit of seeing an elementary particle from the universe, a single photon," said Greg Field, an assistant professor at Duke University in the US.
The findings, published in the journal Neuron, show that the reprogramming happens in retinal cells that are sensitive to motion.
Even in the best lighting, identifying the presence and direction of a moving object is key to survival for most animals.
However, detecting motion with a single point of reference does not work very well. So, the retinas of vertebrates have four kinds of motion-sensitive cells, each specifically responsive to a motion that is up, down, right or left.
In humans, these directional neurons account for about 4 percent of the cells that send signals from the retina to the brain.
In a study with mouse retinas conducted under a microscope equipped with night vision eye pieces in a very dark room, researchers found that the retinal cells sensitive to upward movement change their behaviour in low light.
The 'up' neurons will fire upon detecting any kind of movement, not just upward.
When there is much less light available, a weak signal of motion from the 'up' neurons, coupled with a weak signal from any of the other directional cells can help the brain sense movement.
The loss of motion perception is a common complaint in human patients with severe vision loss. Field said this finding about the adaptability of retinal neurons may help the design of implantable retinal prosthetics in the future.
