It's pretty amazing how well we can see even when there are very few photons scattering from objects, such as under starlight or moonlight conditions. A new study offers a possible explanation, suggesting that mammals and other vertebrates rewire both the 'software' and 'hardware' of light-sensing cells to offer a kind of natural night vision.
Adapting to the dark
Until not too long ago, retinal circuits were thought to be rigid and solely meant for specific tasks. However, the findings suggest that, in mice at least, some cells in the retina are reprogrammable.
In order to assess the presence and direction of a moving object -- a critical ability for both prey and predator -- vertebrates have evolved four kinds of motion-sensitive cells, each responsive to a specific type of motion: up, down, right, and left.
When an object is moving in one of those directions, the corresponding population of neurons will fire strongly. If the motion is halfway between up and left, both populations of neurons will fire, but not as strongly as they would have in a clear direction of motion.
“For complex tasks, the brain uses large populations of neurons, because there’s only so much a single neuron can accomplish,” said Greg Field, an assistant professor of neurobiology and biomedical engineering at Duke University.
Previously, studies have shown that, in humans, these directional neurons account for about 4% of the cells that send signals from the retina to the brain. In rodents, this ratio is between 20% and 30%, because they can mean the difference between fleeing danger or being eaten.
“A lot of animals choose to forage at night, presumably because it’s harder for predators to see,” Field said. “But of course, nature is an arms race. Owls and cats have developed highly specialized eyes to see at night. The prey have altered what they have to survive.”
Researchers in Field's lab studied mouse retinas under a microscope equipped with night vision eyepieces in a very dark room. Remarkably, they found that the "up" neurons started to fire upon detecting any kind of movement, not just upward.
The extra firepower from the up neurons, coupled with signals from any other directional cells, helps the brain sense movement when not much light is available.
It's not clear yet why only the "up" cells become motion generalists in low lighting conditions. One possible explanation is that up is the most important direction for a prey animal to spot a predator that may loom upward as it approaches.
Whatever may be the case, this is the first study that shows that the eye and brain alter their computation of motion to facilitate vision during the night.
“We’ve learned that large populations of retinal neurons can adapt their function to compensate for different conditions,” Field said.
It's likely that other types of circuitry may be adaptable. There are 50 kinds of amacrine cells -- interneurons in the retina -- but we only know something about only 20% of those cells.
In more practical terms, the findings could help researchers design implantable retinal prosthetics. Poor motion perception is one of the most important symptoms of severe vision loss.
The findings appeared in the journal Neuron.