How the brain feels time

Original author: Rita Elmkvist Nilsen
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Researchers from the Norwegian Institute of Systems Neuroscience. Kavli (Kavli Institute for Systems Neuroscience) discovered a network of brain cells that determine our sense of time in our experiences (experiences) and memories.

“This network provides time stamps for events and follows the experience of past experiences,” said Professor Edward Moser, Nobel Prize winner and Director of the Kavli Institute, which is located at the Norwegian University of Science and Technology NTNU). The area of ​​the brain that senses time is located in close proximity to the area responsible for the perception of space.

Time expression

Clocks are devices created by mankind to measure time. By agreeing to an unwritten social contract, we coordinate our own activities with hourly time. However, our brain does not perceive the duration of time periods in standard units - minutes and hours, as on your wristwatch. Temporary signatures in our experiences and memories have completely different temporal characteristics.

In the course of evolution, living organisms, including humans, have developed several biological clocks that help us keep track of time. Differences between different chronometers in the brain are not only in the time scales that they measure, but also in the phenomena that the neural clocks are tuned to. Some chronometers are controlled by external processes: for example, circadian rhythms are configured for sunrise and sunset. This watch helps the body to adapt to the daily rhythm.

Other chronometers are set by internal phenomena, such as hippocampal cells, which create a chain signal, like a domino effect, which rather accurately measures lengths of time up to 10 seconds. Today we know most of the brain mechanisms used to measure time on a small scale (for example, in seconds). Conversely, little is known about the timeline that the brain uses when recording our experiences and memories that can last from a few seconds to minutes and hours.

Neural clock for the experienced time

The neural clocks that track time in the course of their experiences are exactly what was discovered, according to Albert Tsao and his colleagues at the Kavli Institute for System Neuroscience at NTNU.

By recording the activity of a population of brain cells, researchers identified a steady signal that encodes time deep inside the brain.

“Our research reveals how the brain gives meaning to time as an event is experienced,” says Cao, “This network does not explicitly encode time. What we measure is, rather, a subjective feeling of time, which is generated by the flow of events experienced. ”

Neural clocks manage the organization of our experience in an orderly sequence of events. This activity generates a clock in the brain for subjective time. Thus, the experience and sequence of events in it are the substance from which the subjective time is generated and measured by the brain.

Temporary space and memory in the brain

“Today we have a fairly good understanding of how our brain perceives space, whereas our knowledge of time is not so complete,” says Professor Moser.

“The space in the brain is relatively easy to explore. It consists of specialized cell types that are dedicated to specific functions. Together they form the basis of the system, ”he added.

In 2005, May-Britt and Edward Moser discovered neural lattices that plot our environment on different scales on a map consisting of hexagonal blocks. In 2014, Moser shared the Nobel Prize in Physiology and Medicine for the discovery of cells that form a positioning system in the brain, with his colleagues and mentor John O'Keefe of University College in London.

In 2007, inspired by the discovery of Moser's neural lattices encoding space, Albert Cao (at that time PhD candidate at the Kavli Institute) set himself the task of unraveling the phenomena occurring in the mysterious lateral entorhinal cortex (LEC). This area of ​​the brain is located near the medial entorhinal cortex (medial entorhinal cortex, MEC), where neuron grids were discovered by its leaders — Moser and others.

“I was hoping to discover similar key acting cells that would reveal the functional feature of this neural network,” says Cao. This task resulted in a long project.

“It seemed that there was no template in the activity of these cells. The signal changed all the time, ”says Professor Moser.

And it was only in the last couple of years that researchers began to suggest that the signal actually changes over time. Suddenly, the previously recorded data found meaning.

This illustration shows the episodic time from 4-hour skiing from a steep mountain, including events that influenced the perception of time by the skier. The idea is that the experienced time depends on the events and can be perceived faster or slower than the time on the clock.

The newly discovered record of surviving time is in the lateral entorhinal cortex (LEC), marked in green. About LEC is MEC, the area of ​​the brain responsible for spatial perception (not marked in the illustration). Next to the MEC is the hippocampus - a structure in which information from networks of time and space are combined to form episodic memories.

The rights to the image from belong to the Kolbjørn Skarpnes & Rita Elmkvist Nilsen / NTNU Communication Division & Kavli Institute for Systems Neuroscience.

“Time is an unbalanced process. It is always unique and changeable, ”says Professor Moser,“ If this network actually encodes time, the signal must change over time in order for the experience to be recorded as a unique memory. ”

Technological advances

For Moser and the company, it was enough to decipher the signal from only one neural lattice in order to discover how space is encoded in the medial entorhinal cortex. Decoding the time in the lateral entorhinal cortex was much more difficult. Only by looking at activity in hundreds of cells, Cao and his colleagues were able to see that the signal encodes time.

“The activity in these neural networks is so distributed that the mechanism itself probably consists in the structure of connections within these networks. The fact that it can take the form of various unique patterns implies a high degree of plasticity, ”says Professor Moser,“ I suppose distributed networks and combinations of activity structures deserve special attention in the future. In this work, we found an area so closely related to the time of events or experiences that it can generate a whole new field of research. ”

Time form

The structure of time for a long time is a topic of debate among philosophers and physicists. What does the newly discovered brain mechanism for episodic time tell us about how we perceive time? Is our perception of time linear, like the flow of a river, or cyclical, like a wheel or a spiral? Research data from Kavli says that both are true and that the signal on the time-encoding network can take many different forms, depending on the experience.

In 2016, PhD candidate Jørgen Sugar joined the Kavli project to conduct a new set of experiments that would test the hypothesis that the LEC network encodes episodic time. In one experiment, the rat was presented with a wide range of experiments and options for action. She was free to run, explore, and chase pieces of chocolate while attending a series of open environments.

“The uniqueness of the time signal during this experiment suggests that the rat had a very good recording of the time and time sequence of events during the two-hour experiment,” says Sugar, “We managed to use the signal from the time-coding network to accurately track when something happened or another event during the experiment. "

In the second experiment, the task was more structured, with a narrower spectrum of experiments and options for action. The rat was trained to chase pieces of chocolate by turning it to the left or right in an 8-shaped maze.

“In this activity, we saw that the time-coding signal changed from unique sequences in time to a repetitive and partially overlapping pattern,” says Cao, “On the other hand, the time signal became more accurate and predictable during repetitive tasks. The data suggests that the rat had a refined sense of time during each round, but a poor sense of time from circle to circle and from beginning to end of the experiment. ”

Professor Moser argues that the study shows that when you change the activities in which you are involved, when you change the contents of your experience, you actually change the time signals in the LEC, and thus, how you perceive time.

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