‘Time cells’ organize our memories chronologically

“It’s like a movie playing inside my head.” Usually, this is what we say when we think about an experience or where we leave the phone – remembering what we did as if rewinding our memories. A group of scientists at the University of Texas found that not everyone can do this without wrapping the film: they don’t have what they called “time cells.”

The so-called episodic memories are those that we keep as if printed on a film film – and as such, they have a marker that indicates their chronological order, inserted by the time cells in the memories as they are being formed and being stored.

“By making time cells create this indexing over time, you can put everything together in a way that makes sense,” says neurologist and neurosurgeon Bradley Lega, lead author of the study now published in the journal Proceedings of the National Academy of Sciences.

The place of memory

The existence of these cells responsible for ordering the events that we keep in memory has been known for decades, in experiments with rodents – not in humans. The experiment that resulted in the publication of the study involved 27 patients with severe epilepsy.

As part of the pre-surgical preparation, they received electrodes in the brain – more specifically, in the hippocampus and in another area involved in spatial orientation, memory and perception of time.

The hippocampus, considered the seat of memory.Source: Shutterstock / Reproduction

With the electrodes implanted, patients were asked to try to memorize sequences of 12 or 15 words that appeared on a screen for about 30 seconds. Then, after a short break, they were asked to remember the words they had seen. While doing so, the researchers measured the electrical activity of individual brain cells.

Time windows

The result: they found a small group of cells that increased their electrical activity at specific times during the presentation of each word sequence.

“The time cells we found are marking discrete segments of time within that window of approximately 30 seconds, and these chronological markings seem to help people remember when they saw each word and in what order they appeared,” said Vega.

The discovery of these cells helps both to clarify the mechanism that our brain uses to order our memories (and, finally, to find the lost cell inside the house) as well as why certain lesions that reach the hippocampus affect not the memory itself, but how it is saved.

Out of order

According to the article’s reviewer, New York University neuroscientist György Buzsáki, “the results help to explain why people who have hippocampus damage may have strange memory problems.”

An experiment carried out six years ago by a group of researchers at the University of California at San Diego compared the memories of a group of people at the end of a tour of the institution – a tour that included staged events.

Damage to the hippocampus prevents us from remembering facts in order.Damage to the hippocampus prevents us from remembering facts in order.Source: Library of Congress / Reproduction

”The sequence of events has completely disappeared in people with injuries to the hippocampus; the new study suggests that their brains don’t have time cells, ”explained Buzsáki.

They are also responsible for our temporal perception. “They are not like watches; their rhythm is not constant, but depends on factors such as mood. When you have to wait for the covid-19 pandemic to end, time passes very slowly. But when you’re having fun, it flies, “said Buzsáki.

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