Hippocampal ensemble dynamics timestamp events in long-term memory.

Alon Rubin,Yaniv Ziv,Nitzan Geva,Liron Sheintuch

doi:10.7554/elife.12247

Abstract

The capacity to remember temporal relationships between different events is essential to episodic memory, but little is currently known about its underlying mechanisms. We performed time-lapse imaging of thousands of neurons over weeks in the hippocampal CA1 of mice as they repeatedly visited two distinct environments. Longitudinal analysis exposed ongoing environment-independent evolution of episodic representations, despite stable place field locations and constant remapping between the two environments. These dynamics time-stamped experienced events via neuronal ensembles that had cellular composition and activity patterns unique to specific points in time. Temporally close episodes shared a common timestamp regardless of the spatial context in which they occurred. Temporally remote episodes had distinct timestamps, even if they occurred within the same spatial context. Our results suggest that days-scale hippocampal ensemble dynamics could support the formation of a mental timeline in which experienced events could be mnemonically associated or dissociated based on their temporal distance.

Highlights

The capacity to remember temporal relationships between different events is essential to episodic memory, but little is currently known about its underlying mechanisms
Such environment-nonspecific dynamics could support a linkage in long-term memory between dissimilar events that occur at temporal proximity. If this is the case, we would expect the hippocampal representations of events that occur in different spatial environments but in temporal proximity to share common time-varying components. To test these alternative hypotheses we investigated hippocampal neuronal representations of different spatial contexts over multiple days and weeks
We quantified the correlations between the ensemble cell activity patterns in different sessions, in either the same or different environments for pairs of sessions separated by different intervals (Figure 2A,B). Both the degree of overlap between populations of cells active on different days, and correlations in their activity patterns gradually declined as a function of elapsed time (Figure 2B, green data)

Summary

Introduction

The capacity to remember temporal relationships between different events is essential to episodic memory, but little is currently known about its underlying mechanisms. Such gradual changes in the ensembles of place cells active during similar events on different days have been recently reported, but the extent that these dynamics carry temporal information remains unclear (Mankin et al, 2012; Ziv et al, 2013).

Results

Conclusion