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When neurons change gears: how the brain copes with sudden changes

New research done in bats reveals far-reaching dynamic capabilities of neural networks

Two fruit bats in Australia. Photo: depositphotos.com
Two fruit bats in Australia. Photo: depositphotos.com

Life is surprising. A routine walk to the grocery store may turn into one of avoiding a head-on collision with an electric scooter or, in contrast, crossing the road to say hello to a friend who happens to be on the way. How does the brain know how to deal with such dynamic and rapid behavior changes? It turns out that the answers to this question go beyond the classical concept of the brain and its operation - as shown by a new study in bats by Weizmann Institute of Science scientists published today in the scientific journal Nature.

"Various constraints lead to the fact that brain studies often focus on one type of behavior at a time. For this reason, we know very little about the brain's dealings with the dynamism of the real world," says Prof. Nahum Ulanovsky from the department of neuroscience at the institute, who headed the research team. His group designed an experiment designed to reflect the dynamic complexity of reality: they let pairs of bats fly rapidly towards each other; At a certain moment, in order to avoid a collision, the bats had to change behavior quickly and deviate from their course. The experiment was made possible thanks to a unique set-up:Bat tunnel” 135 meters long that was established on the institute's campus in Rehovot - and tiny wireless devices that monitored the brain activity in bats, at the level of the single neuron, while they were flying at a speed of 7 meters per second. The experiment was conducted under the leadership of Dr. Ayelet Sharel, Shaked Pelagi and Dan Blum, in collaboration with the post-doctoral researcher Dr. Yonatan Elhadef, and under the guidance of Prof. Ulanovsky and staff scientist Dr. Liora Les.

As we know, bats are excellent navigators and they orientate themselves in space with the help of biological sonar - they send out sound waves and listen to the echo that returns to them. When the bats first noticed a bat rushing towards them, they increased the production frequency of the sonar signals - a fact that indicates a change in behavior and increased attention to the environment. Simultaneously with the amplification of the sonar signals, a rapid change also occurred in the neural circuits in the hippocampus of the bats. The scientists documented these changes by monitoring the action of the place cells in this brain area responsible, among other things, for navigation and orientation in space - in bats as well as in humans.

When the bats flew alone in the tunnel, without fear of collision, their location cells encoded their location in space as expected, but as soon as they perceived that another bat was approaching them quickly, more than half of the cells in the hippocampus - about 55% - switched gears: their activity pattern changed sharply, and the researchers They showed that the cells no longer encode only the position of the bat itself, but also its distance relative to the opposite bat. The higher the attention of the bat, as reflected by the rhythm of the sonar signals, the greater was the change in the firing pattern of the nerve cells. To the surprise of the scientists, the change - which acted as a kind of "neural switch" - happened very quickly: within about 100 milliseconds. "The degree of familiarity between the bats did not affect the neural coding, that is, the change in coding was intended to prevent a collision and was not related to social behavior," notes Prof. Ulanovsky.

For the past 100 years, it has been widely believed that each brain region performs a dedicated activity, and that different behaviors are coded in distinct brain regions. According to this classical concept, it could be expected that during a change in behavior, for example, in the transition from routine navigation to collision avoidance, different areas of the brain would "light up" one after the other. But the new study reveals a completely different picture: an incredibly rapid change in the neural coding not only in the same area of ​​the brain, but in the very same nerve cells.

"The research shows that there is room to re-examine some of our basic hypotheses about the functioning of the brain," says Prof. Ulanovsky. "Of course, the idea of ​​dividing roles between different brain areas is still valid: damage to the visual cortex will lead to vision damage and not hearing loss or tinnitus. And yet, most models of the brain assume that each nerve cell performs a stable and constant function, while we have now shown that when the needs change, the nerve cells quickly change their function."

It is now known that the function of certain parts of the brain is less rigid than previously thought, but this feature, known as plasticity, occurs over longer periods of time and reflects biochemical changes in the connections between nerve cells. In contrast, the neural switching discovered in the new study is immediate, within a fraction of a second, and it probably reflects a rapid reorganization in the activity of neural networks.

Future studies will be able to check whether the findings discovered in the study are unique to the hippocampus or whether they also characterize other brain areas and other types of situations and behaviors. The findings also raise a fascinating philosophical question: how do we experience the world in a unified and continuous way and not in a fragmented or broken way. Says Prof. Ulanovsky: "After we showed that nerve cells can change their role within a tenth of a second, it will be interesting to find out how the brain allows us a continuous and stable perception of reality."

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