Consistent population activity on the scale of minutes in the mouse hippocampus

Abstract

Neurons in the hippocampus fire in consistent sequence over the timescale of seconds during the delay period of some memory experiments. For longer timescales, firing of hippocampal neurons also changes slowly over minutes within experimental sessions. It was thought that these slow dynamics are caused by stochastic drift or a continuous change in the representation of the episode, rather than consistent sequences unfolding over minutes. This paper studies the consistency of contextual drift in three chronic calcium imaging recordings from the hippocampus CA1 region in mice. Computational measures of consistency show reliable sequences within experimental trials at the scale of seconds as one would expect from time cells or place cells during the trial, as well as across experimental trials on the scale of minutes within a recording session. Consistent sequences in the hippocampus are observed over a wide range of time scales, from seconds to minutes. Hippocampal activity could reflect a scale-invariant spatiotemporal context as suggested by theories of memory from cognitive psychology.

Internally Generated Time in the Rodent Hippocampus is Logarithmically Compressed

“…Perhaps representational drift, which can be observed for very long time scales (Ziv et al, 2013; Rubin, Geva, Sheintuch, & Ziv, 2015; Cai et al, 2016; Mau et al, 2018; Mau, Hasselmo, & Cai, 2020) is responsible for the contiguity effect in episodic memory over autobiographical time. If representational drift codes for events over log time, as we observed for hippocampal time cells here, then it should be possible to trigger very slow sequences with specific environmental events (Liu et al, 2021).…”

“…Although time cell sequences may continue much longer than the eight second delay used in this experiments (Shikano et al, 2021; Liu et al, 2022) it is difficult to imagine that time cell sequences persist for hours and days. However, it is possible that the temporal code is logarithmically compressed, perhaps by means of distinct neural mechanisms, over time scales much longer than minutes.…”

“…Although logarithmic compression of time cells provides a solution to part of the computational puzzle posed by the behavioral contiguity effect, there are still significant challenges. For instance, the contiguity effect extends over time scales much longer than have currently been observed in time cell experiments (which peak at perhaps a few minutes Shikano et al, 2021; Liu et al, 2021). The contiguity effect can be observed when stimuli are presented separated by an hour (Mack et al, 2017).…”

“…It is theoretically important to systematically evaluate whether these regions show the same quantitative distributions; this would require careful experimentation. Because the experimental challenges of estimating time are greatest for times near zero, it may be preferable to conduct this experiment over time scales greater than a few seconds; it has been argued that time cells fire sequentially over several minutes (Shikano, Ikegaya, & Sasaki, 2021; Liu et al, 2021).…”

“…Because the experimental challenges of estimating time are greatest for times near zero, it may be preferable to conduct this experiment over time scales greater than a few seconds. It has been argued that hippocampal time cells at least fire sequentially over several minutes (Shikano, Ikegaya, & Sasaki, 2021; Liu et al, 2022).…”