How does human brain keep track the order of events in sequence?
New research suggests that ‘time cells’ – neurons within the hippocampus thought to represent temporal information – might be the glue that sticks our memories together within the right sequence in order that we can easily and properly recall the right order in which things happened.
Evidence for these sorts of sequence-tracking time cells was previously found in rats, where specific neuron assemblies are thought to support the recollection of events and therefore the planning of action sequences – but less is understood about how episodic-memory’ is encoded in human brain.
To investigate, a team led by neuroscientist Leila Reddy from the Brain & Cognition Research Centre (CerCo) in France monitored electrical activity within the brains of 15 epilepsy patients, using microelectrodes implanted in hippocampus.
“Creating episodic memories requires linking together distinct events of an experience with temporal fidelity,” the researchers explain in their study.
“Given the importance of the hippocampus in sequence order learning & temporal order judgments, we tested whether human hippocampal neurons represented temporal information while participants learned the order of a sequence of things .”
The experiments were conducted during medical tests that used the electrodes to localize the source of their seizures within the brain. As a result, the research didn’t require any invasive or risky implantations that the patients wouldn’t already be undergoing for the needs of prospective epilepsy treatment.
In the experiments, the participants were presented with a sequence of images in predetermined order and were asked to memorize the sequence.
During the sessions, the electrodes recorded specific neurons within the hippocampus firing in response to the experiment, both during specific moments as images were displayed, during gaps when no images were shown, and at pauses where participants were asked to predict what image would be shown next from a sequence already displayed.
According to the researchers, the neurons involved are evidence of your time cells: “neurons whose activity is modulated by temporal context within a well-defined time window”.
The researchers say a number of these neurons were actively engaged in memorizing or recalling the sequence of images in experiments, but some were also active when no visual stimulus was present, suggesting they were encoding the flow of your time even when nothing specific was happening.
“Time cells were observed to fire’ at successive moments in these blank periods,” the researchers write in their paper.
“Temporal modulation in these gap periods couldn’t are driven by external events; rather they seem to represent an evolving temporal signal as a results of changes in patients’ experience during this point of waiting.”
According to the researchers, time cells within the human brain are “multi-dimensional”, capable of encoding information in reference to time but also responding to different sorts of sensory information or stimuli.
It’s possible, the team thinks, that the multi-dimensional behavior of those time neurons could be what records the ‘what’, ‘where’, and ‘when’ of experiences, stitching elements together to create coherent memories from a jumble of inputs.
“The phenomenon of subjective ‘mental time travel’ may be a cornerstone of episodic memory’ ,” the researchers say.
“Central to our experience of reliving the past is our ability to vividly recall specific events that occurred at a selected place and during a specific temporal order… Our results provide further evidence that human hippocampal neurons represent the flow of time’ in an experience.”
The findings are reported in The Journal of Neuroscience.
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