‘Time cells’ in the human brain encode the passage of time, scientists say: ScienceAlert

How does the human brain follow the sequence of events?

Research suggests that ‘time cells’ – neurons in the hippocampus thought to represent time information – could be the glue that binds our memories together in the right sequence so that we can correctly recall the correct order in which things happened.

There is evidence for these types of time stations to track the streak previously found in ratswhere specific circuits of neurons are thought to support remembering events and planning action sequences – but for a long time less was known about how episodic memory is encoded in the human brain.

To investigate this, a team of researchers led by neuroscientist Leila Reddy from the Center for Brain and Cognition Research (CerCo) in France monitored electrical activity in the brains of 15 epilepsy patients, using microelectrodes implanted in the hippocampus.

“The creation of episodic memories requires linking different events of experience with temporal fidelity,” the researchers they explained in their studypublished last year.

“Given the importance of the hippocampus in sequence order learning and temporal order judgments, we tested whether human hippocampal neurons represented temporal information as participants learned the order of a sequence 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 implantations that the patients would not have already undergone for the purpose of prospective epilepsy treatment.

In the experiments, participants were shown a sequence of images in a predetermined order and asked to memorize the sequence.

During the sessions, the electrodes recorded specific neurons in the hippocampus that fired in response to the experiment, both during specific moments when the images were presented, during gaps when no images were presented, and during pauses when the participant was asked to predict which image would be displayed next from the sequence already displayed.

According to the researchersthe neurons involved are evidence of temporal cells: “neurons whose activity is modulated by temporal context within a well-defined time window”.

The researchers said that some of these neurons were actively involved in memorizing or recalling a sequence of images in the experiments, but some were also active when there was no visual stimulus, suggesting that they were encoding the passage of time even when nothing in particular was happening.

“Time cells were observed to fire at successive moments during these blank periods,” the researchers explained in their work.

“The temporal modulation during these gap periods could not have been driven by external events; rather, they appear to represent an evolving temporal signal as a result of changes in patient experience during this waiting time.”

According to the researchers, the temporal cells in the human brain are “multidimensional,” able to encode information in relation to time but also respond to different types of sensory information or stimuli.

It’s possible, the team thinks, that the multidimensional behavior of these temporal neurons could be what captures the ‘what’, ‘where’ and ‘when’ of experiences, putting the elements together to form coherent memories from the pile of input.

“The phenomenon of subjective ‘mental time travel’ is a cornerstone of episodic memory,” the researchers said.

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

The findings were published in Journal of Neuroscience.

An earlier version of this story was first published in July 2021.