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To see under the starlight and moonlight, the retina of the eye changes both the software and the material of its light-sensitive cells to create a kind of night vision. According to Duke scientists, the retinal circuits that were considered immutable and programmed for specific tasks are adaptable to different light conditions, which have identified the reprogramming of the retina in low light.
"To see under the light of the stars, biology had to reach the limit of seeing an elementary particle of the universe, a single photon," said Greg Field, an assistant professor of neurobiology and biomedical engineering at the university. Duke University. "It is remarkable at night how few photons exist."
The results that appear early online in neuron show that reprogramming occurs in movement-sensitive retinal cells.
Even in the best lighting conditions, identifying the presence and direction of a moving object is essential to the survival of most animals. But detecting a movement with a single point of reference does not work very well. Thus, vertebrate retinas have four types of motion-sensitive cells, each specifically responding to a movement that is up, down, right or left.
When an object moves precisely in one of these directions, this population of neurons fires strongly, Field said. However, if the motion is halfway between the top and the left, both cell populations will fire, but not so strongly. The brain interprets this type of signal as a movement going both up and to the left.
"For complex tasks, the brain uses large populations of neurons because a single neuron can only perform one," Field said.
In humans, these directional neurons account for about 4% of the cells that send signals from the retina to the brain. In rodents, it's more like 20 to 30%, Field adds, because motion detection is of vital importance to an animal that other animals really love to eat.
In a study with mouse retinas conducted under a microscope equipped with night vision eye pieces in a very dark room, Xiaoyang Yao, a student at Field's lab, found that retinal cells sensitive to upward movement change their behavior. The "up" neurons will trigger when detecting any type of movement, not just upwards.
A small sample of mouse retina was placed on an array of electrodes capable of measuring the individual triggering of hundreds of neurons at a time "and we then show the films," Field said. "Xiaoyang's insight was to go and see what these cells do day and night, she noticed a difference and wondered why."
When there is much less light available, a weak motion signal from the "up" neurons, coupled with a weak signal from one of the other directional cells, can help brain movement as it interprets two directional signals as a motion that is something in between.
Perception of loss of motion is a common complaint in human patients with severe vision loss.
Field said that this discovery regarding the adaptability of retinal neurons could help design implantable retinal prostheses in the future.
"Many animals choose to feed at night, probably because it's harder to see for predators," Field said. "But of course, nature is an arms race, owls and cats have developed highly specialized eyes to see at night, the prey has changed what they have to survive."
For reasons that are not yet clear, only the "rising" cells become generalists of the movement in low light conditions.
Field suspects that the most important direction for a prey animal is to spot a predator that rises to approach its prey, but it does not have these data yet.
What is important at the moment, is that the eye and the brain modify their calculation of motion in a dim light.
"We have learned that large populations of retinal neurons can adapt their function to compensate for different conditions," Field said.
The retina is made up of many circuits running in parallel, said Jeffrey Diamond, senior researcher at the National Institute of Neurological Disorders and Stroke, who is also studying visual processing in the retina.
"We learn that these tours do different things at different times of the day," Diamond added.
Now Field has found that this adaptation to low light, driven by changes in circuits and chemical signals between cells, raises the question of how many other adaptations will be found, he added.
"There are 50 types of amacrine cells, the retinal medicine cabinets, and most of them probably release several neurotransmitters that can influence the retinal circuit," concluded Diamond. "We only know about 20% of these cells."
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