Scientists say ‘time cells’ in the human brain code for the passage of time

(ORDO NEWS) — How the human brain keeps track of the order of events in a sequence Research shows that “time cells” neurons in the hippocampus thought to represent temporal information may be the glue that glues our memories together in the correct sequence so we can remember the correct order of events.

Evidence for such transient sequence-tracking cells has previously been found in rats, where certain groups of neurons are thought to support event recall and sequence planning, but for a long time, less was known about how episodic memory is encoded in the human brain. .

For the study, a team led by neuroscientist Leila Reddy of the Center for the Study of the Brain and Cognition (CerCo) in France monitored electrical activity in the brains of 15 epileptic patients using microelectrodes implanted in the hippocampus.

“In order to create episodic memories, it is necessary to link together the individual events of an experience with temporal accuracy,” the researchers explain in their study published last year.

“Given the importance of the hippocampus in sequencing learning and temporal order judgment, we tested whether human hippocampal neurons represent temporal information while participants learn sequencing of items.”

The experiments were conducted during medical tests that used electrodes to localize the source of their seizures in the brain.

As a result, the study did not require any invasive or risky implants that patients would not already undergo for the promising treatment of epilepsy.

During the experiments, participants were presented with a sequence of images in a predetermined order and asked to remember this sequence.

During the sessions, the electrodes recorded how certain neurons in the hippocampus fired in response to the experiment, both at specific times when images were shown, in between when no images were shown, and in pauses when participants were asked to predict which image would be shown next. from the sequence already shown.

According to the researchers, the neurons involved in this process are time cells: “neurons whose activity is modulated by temporal context within a well-defined time window.”

Some of these neurons were actively involved in remembering or recalling sequences of images in the experiments, but some were active in the absence of a visual stimulus, suggesting they code for the passage of time even when nothing in particular is happening, the researchers said.

“The time cells were observed to fire at successive moments of these blank periods,” the researchers explain in their paper.

“The temporal modulation in these intervening periods could not be caused by external events; rather, they represent an evolving temporal signal resulting from changes in patients’ experiences during waiting.”

Temporal cells in the human brain are “multi-dimensional,” able to encode information about time but also respond to different kinds of sensory information or stimuli, the researchers say.

It’s possible, the team thinks, that the multidimensional behavior of these time neurons could be what records the “what”, “where”, and “when” of experience, stitching the elements together to create cohesive memories from the jumble of inputs.

The phenomenon of subjective “mental time travel” is the cornerstone of episodic memory,” the researchers say.

“Central to our experience of the past is the ability to vividly recall specific events that occurred in a specific place and in a specific temporal order… Our results provide additional evidence that human hippocampal neurons represent the passage of time in experience.”

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