Brain complexity: feedback systems (Introduction)

by David Turell @, Monday, April 16, 2018, 20:24 (162 days ago) @ David Turell

Studies of the brain show reverse connections that are now understood:

https://medicalxpress.com/news/2018-04-scientists-hidden-enigmatic-neural.html

"For decades, the neuroscience community has been baffled by the existence of dense connections in the brain that seem to be going "backwards." These connections, which span extensively across distant areas of the neocortex, are clearly conveying important information. But until now, the organization of the connections, and therefore their possible role, was largely unknown.

"scientists at the Champalimaud Centre for the Unknown in Lisbon report for the first time that these connections form an exquisitely organised map of the visual space and provide important insights into how they may be involved in visual perception.

***

"'Hidden in the tangle of wires we found that there is a beautiful organisation, where feedback connections target specific neurons in lower structures depending on the signals they carry."

***

"'In many separate structures of the visual system, beginning with the eye itself, neighboring neurons encode neighboring areas of the visual space. This way, the individual structures contain an almost one-to-one map of the image," Marques explains.

"This map exists in the primary visual cortex (also called V1), which is the entry point of visual information to the neocortex. This was the researchers' starting point. They asked: whether feedback connections matched the visual map encoded in V1.

"'The majority of feedback inputs formed the same spatial map as the areas they connected to in V1. In other words, the V1 and feedback maps were superimposed on each other. This observation had already been reported in other species, such as primates, so we weren't surprised. However, in the mouse, we also observed something new. The feedback connections also encoded information from further locations in the visual space.

***

"'The world is made up of objects," Petreanu explains. "The phone in your hand, the cars on the road, these are all objects that are defined by continuous lines. Therefore, it's not surprising that neurons in the visual system care a lot about these lines."

"How could feedback connections help accentuate the lines that make up objects? There are two possibilities—they can either amplify the activity in V1 where the lines are, or they can dampen activity where they are not supposed to be.

"We found that the second option is the most likely to be true," says Petreanu. "The feedback connections were abundant in V1 in areas outside the lines. We therefore hypothesise that this organisation is probably silencing neurons in the areas that lie outside the line, and thereby enhancing the contrast between objects and their surroundings."

"Next, the researchers asked whether feedback connections might participate in motion detection. To their surprise, they found not only that they do, but that they use the same strategy to do it. "This time, the visual feature was different, but the feedback connections played the same role," says Marques. "We observed that feedback connections that respond to moving objects were enriched in V1 in regions opposite to the direction of movement."

"'We believe that these results imply that this set of feedback connections learn through experience what to expect from the world and then use this knowledge to shape incoming visual information," says Petreanu. "In the world, objects are defined by continuous lines, not scattered dots, and moving objects tend to maintain their trajectory, not move around randomly. So feedback connections try to accentuate these particular features that they have learned to anticipate. Surprisingly, they do so by pointing to locations that are opposite to the expected ones."

"The results of Petreanu and Marques provide an important piece of the puzzle of how the neocortex is organized and suggest how visual perception could be generated in the brain. "

Comment: The complexity of this feedback system reeks of resign.


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