How does the human brain track the order of events?
Recent research shows that the 'time cells neurons' in the hippocampus, which are thought to represent temporal info, could be the glue that holds our memories together in the right order so that we can correctly recall what happened.
These sequence-tracking time cells were previously demonstrated in rats. While specific neuron assemblies have been shown to support the recollection and planning of events, less is known about the human brain's episodic memory.
Leila Reddy, a neuroscientist from France's Brain and Cognition Research Center, (CerCo), monitored the electrical activity of 15 epilepsy patients using microelectrodes placed in the hippocampus.
Researchers explain that creating episodic memories involves linking distinct events with temporal fidelity to create them.
"Given the importance the hippocampus has in sequence order learning, and temporal order judgements, we tested whether the human hippocampal neurons represented temporal information. Participants learned the order of a sequence by studying the sequence of items."
The electrodes were used to locate the source of the seizures in the brain. The research did not require any risky or invasive implantations that patients would not already have undergone for potential epilepsy treatment.
Participants were given a sequence of images and asked to remember it.
The electrodes recorded the firing of specific neurons in the hippocampal hippocampus during the sessions. These were at specific times when images were displayed, gaps between images, and pauses during which participants were asked to guess the next image from a sequence.
Researchers claim that the neurons are evidence of time cells. They include "neurons whose activity can be modulated by temporal context within well-defined time windows".
Researchers believe that some neurons were active in recalling or memorizing the sequence of images, while others were also active when there was no visual stimulus, which suggests they were encoding time flow even though nothing was.
Researchers write that time cells were observed firing at different moments during these blank periods.
"Temporal modulation in these gap periods could not be driven by external events. Rather, they seem to reflect an evolving temporal signal due to changes in patients' experiences during this period of waiting."
Researchers found that time cells in the brain of humans are multi-dimensional, capable of both encoding and responding to different types of sensory information.
The team believes that it is possible that these multi-dimensional behaviors of time neurons could be what records the "what", "where" and "when" of experiences. They can also stitch elements together to create coherent memories out of a mess of inputs.
Researchers claim that the phenomenon of subjective "mental time travel" is an important component of episodic memories.
"The central feature of our experience of reliving our past is our ability vividly to recall specific events that took place at a particular time and place. Our results further support the hypothesis that human hippocampal neurons represent the flow of experience in time."
The Journal of Neuroscience reports the findings.