Researchers have discovered a network of brain cells that expresses our sense of time within experiences and memories.

"This network provides timestamps for events and keeps track of the order of events within an experience," says the senior author. This area of the brain where time is experienced is located right next to the area that codes for space.

Clocks are devices created by humans to measure time. By social contract, we agree to coordinate our own activities according to clock time. Nevertheless, your brain does not perceive the duration in time with the standardized units of minutes and hours on your wristwatch. The signature of time in our experiences and memories belongs to a different kind of temporality altogether.

Over the course of evolution, living organisms including humans have developed multiple biological clocks to help us keep track of time. What separates the brain's various timekeepers is not only the scale of time that is measured, but also the phenomena the neural clocks are tuned to. Some timekeepers are set by external processes, like the circadian clock that is tuned to the rise and fall of daylight. This clock helps organisms adapt to the rhythms of a day.

Other timekeepers are set by phenomena of more intrinsic origins, like the hippocampal time cells that form a domino-like chain signal that tracks time spans up to 10 seconds precisely. Today we know a great deal about the brain's mechanisms for measuring small timescales like seconds. Little is known, however, about the timescale the brain uses to record our experiences and memories, which can last anywhere from seconds to minutes to hours.

A neural clock that keeps track of time during experiences is precisely what the researchers believe they have discovered.

By recording from a population of brain cells the researchers identified a strong time-coding signal deep inside the brain.

The neural clock operates by organizing the flow of our experiences into an orderly sequence of events. This activity gives rise to the brain's clock for subjective time. Experience, and the succession of events within experience, are thus the substance of which subjective time is generated and measured by the brain.

Decoding time in the lateral entorhinal cortex proved to be a more complex task. It was only when looking at activity from hundreds of cells that the researchers were able to see that the signal encoded time.

The structure of time has long been a disputed topic by philosophers and physicists alike. What can the newly discovered brain's mechanism for episodic time tell us about how we perceive time? Is our perception of time linear resembling a flowing river, or cyclical like a wheel or a helix? Data from the Kavli study suggest both are correct, and that the signal in the time-coding network can take on many forms depending on the experience.

In one experiment a rat was introduced to a wide range of experiences and options for action. It was free to run around, investigate and chase bits of chocolate while visiting a series of open space environments.

"The uniqueness of the time signal during this experiment suggests that the rat had a very good record of time and temporal sequence of events throughout the two hours the experiment lasted," the author says. "We were able to use the signal from the time-coding network to track exactly when in the experiment various events had occurred."

In the second experiment, the task was more structured with a narrower range of experiences and options for action. The rat was trained to chase after bits of chocolate while turning left or right in a figure-8 maze.

"With this activity, we saw the time-coding signal change character from unique sequences in time to a repetitive and partly overlapping pattern," the author says. "On the other hand, the time signal became more precise and predictable during the repetitive task. The data suggest that the rat had a refined understanding of temporality during each lap, but a poor understanding of time from lap to lap and from the start to end throughout the experiment."

The senior author says the study shows that by changing the activities you engage in, the content of your experience, you can actually change the course of the time-signal in LEC and thus the way you perceive time.

https://geminiresearchnews.com/2018/08/how-your-brain-experiences-time/
 
https://www.nature.com/articles/s41586-018-0459-6