Human ‘Time Neurons’ Encode Specific Moments in Time

Summary: Specific hippocampal neurons may be involved in time-related memory encoding. Findings reveal the human brain contains time-tracking neurons.

Source: SfN

Neurons in the hippocampus fire during specific moments in time, according to research recently published in Journal of Neuroscience. The cells may contribute to memory by encoding information about the time and order of events.

Episodic memories involve remembering the “what, where, and when” of past experiences. The “where” may be encoded by place cells in the hippocampus, which fire in response to specific locations.

Rodents have hippocampal neurons that fire in response to specific moments in time — the “when” — but until recently it was not known if the human brain contained them too.

Reddy et al. recorded the electrical activity of neurons in the hippocampus of epilepsy patients undergoing diagnostic invasive monitoring for surgery. During the recording, the participants viewed and memorized a sequence of 5 to 7 images.

At random intervals, the participants were quizzed on the next image in the sequence before it resumed. Time-sensitive neurons fired during specific moments in time between quizzes, irrespective of the image.

This shows the firing activity on a graph
Hippocampal neurons fire at successive moments of a temporal interval. This shows the firing activity of the population of time cells (N=128). Each row shows the firing activity for an individual time cell, averaged across trials. The x-axis corresponds to time of the median trial length. The neurons are sorted by the latency of the maximum firing rate. Credit: Reddy et al., JNeurosci 2021

The neurons still tracked time even during 10-second gaps with no images while the participants waited. The researchers could decode different moments in time based on the activity of the entire group of neurons.

These results demonstrate the human brain contains time-tracking neurons.

About this neuroscience research news

Source: SfN
Contact: Calli McMurray – SfN
Image: The image is credited to Reddy et al., JNeurosci 2021

Original Research: Closed access.
Human hippocampal neurons track moments in a sequence of events” by Leila Reddy, Benedikt Zoefel, Jessy K. Possel, Judith C. Peters, Doris Dijksterhuis, Marlene Poncet, Elisabeth C.W. van Straaten, Johannes C. Baayen, Sander Idema and Matthew W. Self. Journal of Neuroscience


Human hippocampal neurons track moments in a sequence of events

An indispensable feature of episodic memory is our ability to temporally piece together different elements of an experience into a coherent memory. Hippocampal “time cells” – neurons that represent temporal information – may play a critical role in this process. While these cells have been repeatedly found in rodents, it is still unclear to what extent similar temporal selectivity exists in the human hippocampus.

Here we show that temporal context modulates the firing activity of human hippocampal neurons during structured temporal experiences. We recorded neuronal activity in the human brain while patients of either sex learned predictable sequences of pictures. We report that human time cells fire at successive moments in this task.

Furthermore, time cells also signaled inherently changing temporal contexts during empty 10-second gap periods between trials, while participants waited for the task to resume. Finally, population activity allowed for decoding temporal epoch identity, both during sequence learning and during the gap periods.

These findings suggest that human hippocampal neurons could play an essential role in temporally organizing distinct moments of an experience in episodic memory.

Significance Statement:

Episodic memory refers to our ability to remember the “what, where, and when” of a past experience. Representing time is an important component of this form of memory.

Here, we show that neurons in the human hippocampus represent temporal information. This temporal signature was observed both when participants were actively engaged in a memory task, as well as during 10s-long gaps when they were asked to wait before performing the task. Furthermore, the activity of the population of hippocampal cells allowed for decoding one temporal epoch from another.

These results suggest a robust representation of time in the human hippocampus.

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