Neurons caught rapidly switching gears - study

Weizmann Institute's study suggests neurons are not programmed for a single task, but rather are highly dynamic and agile.

The Egyptian fruit bat. Prof. Nachum Ulanovsky's unique model animal. (photo credit: WEIZMANN INSTITUTE OF SCIENCE)
The Egyptian fruit bat. Prof. Nachum Ulanovsky's unique model animal.
(photo credit: WEIZMANN INSTITUTE OF SCIENCE)

If you’re brushing your teeth as you should at least twice a day, but then a pipe explodes and you have to stop the gushing water, how does your brain shift gears? Or when you’re walking your dog and suddenly an electric bike nearly runs you down? These switches pose a challenge – how do the brain’s circuits deal with such dynamic and abrupt changes in behavior?

A new study of bats just published in the prestigious journal Nature by researchers at the Weizmann Institute of Science in Rehovot suggests an answer that does not fit the classical thinking about brain function.

“Most brain research projects focus on one type of behavior at a time, so little is known about the way the brain handles dynamically changing behavioral needs,” says Prof. Nachum Ulanovsky of the institute’s brain sciences department. He and his team – graduate students Dr. Ayelet Sarel, Shaked Palgi and Dan Blum, who led the study in collaboration with postdoctoral fellow Dr. Johnatan Aljadeff and supervised together with associate staff scientist Dr. Liora Las – designed an experimental setup that mimicked real-life situations. The bats or humans rapidly switch from one behavior to another – for example, from navigation to avoiding a predator or a car crash.

How did they conduct their research?

Using miniature wireless recording devices, the researchers monitored neurons in the brains of pairs of bats that had to avoid colliding with one another while flying toward each other along a 135-meter-long tunnel at a high speed of seven meters per second. This amounted to a relative speed – the rate at which the distance between the bats closed, or the sum of both bats’ speeds – of 14 meters per second or 50 kilometers an hour.

Egyptian fruit bats were the subject of a study by Tel Aviv University researchers. (credit: YUVAL BARKAI/TEL AVIV UNIVERSITY)Egyptian fruit bats were the subject of a study by Tel Aviv University researchers. (credit: YUVAL BARKAI/TEL AVIV UNIVERSITY)

To check whether in these situations the bats switched their behavioral mode, becoming more attentive, the researchers took advantage of bats’ unique ability to sense their environment using sonar (echolocation). When spotting another animal flying rapidly toward them, the bats promptly raised their rate of echolocation clicks, signifying elevated attention levels. As their attention increased, a rapid shift occurred in the neural circuits in the bats’ hippocampus – the main brain area responsible for navigation, among other functions. The scientists discovered this shift by recording electrical signals from individual neurons in this area known as place cells.

When the bats were flying solo, their place cells encoded their location in space, but as soon as the animals spotted the other, fast-approaching bat, more than half of the neurons switched modes. The scientists could tell that the neural switch had taken place because the neurons’ firing pattern changed, indicating that they now encoded not only the bat’s own, absolute location but also a relative measure – the distance to the other bat.

The higher the animal’s attention, the more pronounced the neural switch. To the scientists’ surprise, this switch occurred extremely fast, within some one-tenth of a second. Whether the fast-approaching bat was a regular, familiar partner or a mere “acquaintance” had no effect on the neural coding, suggesting that the switch was intended to avoid a collision and had nothing to do with social behavior.

“Our study suggests that we may need to revise some basic assumptions about the brain’s circuits,” Ulanovsky continued, noting that, over the past century, the prevailing view was that each brain region performs its own function and that different behaviors are encoded in different parts of the brain. According to this classical notion, one may expect that when a switch occurs in behavior, for example, from navigation to collision avoidance, different brain regions would “light up” one after another.

The new study, however, reveals an entirely different picture: an amazingly fast switch in neural coding not just within the same brain area, but in the same neurons.

“Of course, the division of labor between different brain regions still holds. If the visual cortex, for example, is damaged, the result is visual impairment – not deafness or loss of smell,” said Ulanovsky. “But we’ve shown in our new study that the brain is much more dynamic than previously thought. Contrary to most models of the brain that assume that neurons have a stable function, we’ve found that in response to a rapid switch in behavioral needs, neurons can very rapidly switch to performing a totally new function.”

Some parts of the brain have already been shown to be less rigidly determined than thought – a phenomenon known as plasticity – but this occurs on slower time scales, reflecting longer-term biochemical changes in the brain’s synapses (the connections between neurons). In contrast, the newly revealed switch works much faster, likely reflecting a swift reorganization in neural network activity.

Future studies may look for switches between different neural coding modes in various brain areas and in a wide variety of behaviors and situations. These studies might also ask how prevalent this switching is and whether it slows down in the aging brain.

“But we’ve shown in our new study that the brain is much more dynamic than previously thought. Contrary to most models of the brain that assume that neurons have a stable function, we’ve found that in response to a rapid switch in behavioral needs, neurons can very rapidly switch to performing a totally new function.”

Prof. Nachum Ulanovsky, Weizmann Institute's brain sciences department

Yet another compelling research direction raised by the new study is learning how the brain keeps our reality from appearing fragmented. “Now that we know that neurons can change what they do within one-tenth of a second, it would be fascinating to find out how we still perceive the world as smooth and stable,” Ulanovsky concluded.