Mice use a specific neural pathway to detect shadows, and it can detect just about the faintest shadows possible, according to new research from Aalto University and the University of Helsinki. The human eye has the same neural circuitry, which researchers believe could be used to investigate visual diseases with unprecedented resolution.
To test shadow detection, the researchers put mice in a maze with almost no light. The exit was marked by a black spot, barely distinguishable from the surrounding darkness. By tracking how the mice moved through the maze and measuring the activity of neurons at the back of the eye — the retina — the team showed that a group of retinal cells known as OFF ganglion cells could control the extremely small drop in light levels. detected.
‘Our goal is to go from molecules all the way to behaviour’ says Professor Petri Ala-Laurila, who has a joint appointment at Aalto and the University of Helsinki. This study builds on previous work by his group showing that ON ganglion cells are used to detect a very faint spot of light in the dark. ‘The opposite task is to detect the faintest shadows, where only a few photons are missing. We hypothesized that that’s what the most sensitive UIT cells do in starlight, because they increase their firing rate in response to shadows,” says Ala-Laurila.
The team also calculated the fundamental limit of shadow detection based on the physical properties of the light receptors and neural pathways. After taking into account unavoidable losses — for example, not every photon hitting a receptor is absorbed — they found that the mice’s behavior and retinal activity came very close to a perfect response.
Our tightly contained modeling emphasizes that both visually guided behavior and the most sensitive UIT ganglion cells are near-perfect shadow detectors.”
dr. Johan Westö, one of the joint first authors of the study.
The study had to be done in almost complete darkness to detect these differences. Nataliia Martyniuk, the study’s other lead author, says that “the superb sensitivity of the visual system to the faintest shadows places high technical demands on the experiments we conducted at these extremely low light levels.” At higher light levels, many more retinal circuits are activated, which would have made the analysis prohibitively expensive.
Insanely faint shadows can be detected! Even a few missing photons from a few thousand rod receptors is enough for the animals to detect a shadow. That probably has to do with the huge evolutionary need to detect shadows, as mice and other animals evolved to avoid predators in really low light levels.†
Professor Petri Ala-Laurila
These findings show how the process of making sense of incoming light — turning it into a mental image — is distributed among different cell types that perform different computational tasks in the retina. In the eye, input from thousands of receptors flows to the ON and OFF ganglion cells, which act as modules to specifically detect light and shadows, respectively.
‘That puts a lot of computation on the retina, and doing this detection at an early stage simplifies the task for downstream processes in the brain’ says Ala-Laurila. ‘We can demonstrate this principle in the eye at very low light levels. I suspect it is also true at higher light levels and in other senses and other areas of neurobiology.’
The retinal circuitry responsible for transmitting information to ON and OFF ganglion cells is nearly identical in humans. Ala-Laurila explains that the ability to test the function of retinal cell types specifically in starlight may also have clinical implications. Many visual diseases are specific to certain cell types in the retina. By performing tests at very low light levels, such diseases could perhaps be detected earlier and more accurately, because at those light levels every photon counts. “During my life I want to see revolutionary techniques to detect visual diseases using very low light levels as a tool,” he says.
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