Sunday , October 17 2021

A surprise discovery reveals a second visual system


The visual system is probably the most well understood part of the brain. Over the past 75 years, neurologists have devised a detailed description of how bright light coming into their eyes allows us to recognize our grandmother's face, to follow a hawk in flight or read a prayer. But a new study by researchers at the University of California (UC) in San Francisco in the United States questions a fundamental aspect of the science of vision, which shows that even the best studied parts of the brain are still They can contain many surprises.

According to the standard visual processing model, all visual information on the retina must first pass through the primary visual cortex (V1) behind the brain, which extracts simple features such as lines and edges, before a number "higher order visual areas" that extract more and more complex elements such as shapes, shadows, movements, and so on.

The new study, published in Friday's digital edition of science, "Science", shows for the first time that one of these visual higher visual areas, which is involved in the perception of moving objects, does not depend at all on information. of V1. Instead, this region, known as post-natal cortex (POR), appears to obtain visual data directly from an old evolutionary sensory processing center at the base of the brain called colliculus superior.

"It's like discovering a second primary visual cortex," says lead author Massimo Scanziani, professor of physiology at UCSF and an investigator at the Howard Hughes Medical Institute. This undermines the entire visual system concept in the mammalian cortex as a perfect hierarchy with V1 as a guardian and raises a variety of questions including how these two parallel visual systems have evolved and how they co-operate to produce a visual experience unified. he adds.

Ancestral superior colic (called non-mammalian optic tectum) is the main center of sensory processing for creatures with little or no crust, such as fish, amphibians, lizards and birds. It is especially attributed to the movement and related to refiective behaviors: for example, the ability of a frog to catch a tumultuous air with the tongue or ability of a fish to move away from a thriving predator.

However, the upper colic did not disappear with the development of the cortex in mammals. In primates, including humans, it has been linked to quick and unconscious forms of visual processing, such as jumping with fear when you see a stick that looks like a snake or you automatically catch a ball you throw at your face.

You can also play a role in directing visual attention (for example, when we expect a traffic light to turn green or examine a crowded scene to look for someone with a red and white hat in particular). But the new article is the first time anyone shows that this ancient evolutionary system has a dedicated space in the cortex.


Research began when the lead author, Riccardo Beltramo, a postdoctoral researcher in Scanzian's lab, recorded neural responses to the visual stimuli in motion in the POR mouse, which is known to play a role in the perception and memory of the space movement.

He used a technique called optogenetics to temporarily stop the V1 activity with light, hoping to confirm his expectations that POR responses depend on the flow of information through the standard visual hierarchy. But to his surprise, he discovered that ROP neurons continued to respond to stimuli in motion, even without V1 entry.

"It was absolutely remarkable," says Beltramo, "we stopped the main visual area of ​​the cortex and the visual responses from the ROP were not affected, this being the first extraordinary surprise moment that told us that we are in something completely unexpected."

If ROP replies to moving objects did not come out of V1, Beltramo asked how they got there? There must be another way to connect ROP with visual information in the retina, he argued. To identify this parallel visual pathway, Beltramo used customized virus-injected injections that label neurons that are connected to each other. This allowed him to demonstrate that POR neurons obtain two sources of anatomical information: one in V1 and one in the upper colic, each direction to different areas of the thalamus, the central relay station of the brain.

To confirm that the upper colic led POR responses to motion, Beltramo used optogenetics to systematically silence V1 or higher colic while recording from POR. He showed that, unlike V1 inactivation, stopping the upper colic caused the POR activity to disappear completely. In fact, the upper collider appears to be critical to ROP's ability to track objects on the move, which researchers have found that ROP inputs could not do on their own.

The study raises fundamental questions about brain development and its functioning. More research is needed to test whether visual responses in areas similar to ROP of the primate brain as well as us depend also on the contribution of higher colic, as well as on how the V1-activated activity and the collicle interact to influence the behavior of the animals , say the authors.

"We hypothesized that ROP, which was previously considered a superior visual area, could be a kind of primary visual cortex, similar to an airplane, reptile or early simple cortex, and that it can be dedicated to detecting that something is moving into the environment, be it small, a nearby booty, or a long distant predator, "says Beltramo.

"From this perspective, perhaps V1 would add to this information a more precise discrimination of the nature of the moving object, such as its exact location, or whether it is a tasty cockroach or a potentially deadly scorpion," he adds.

Based on previous studies, the recently discovered Coliculus-POR system could be related to responses to fear, spatial attention and navigation, or even facial recognition: all the specialties in the temporal cortex region they are in. BY.

The findings may also have implications for an intriguing phenomenon called "blind vision," in which people who become blind because of V1 injuries can still identify the positions of objects and navigate obstacles, even if they can not perceive them. Based on primate studies, it is believed that orbital vision depends on the upper colic, but the new study suggests that it could also involve cortical areas similar to ROP.

"This is one of these findings that generates many questions instead of responding to anyone, because, like many discoveries, they have questions that no one knew how to do them," Scanziani concludes.

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