LAUSR

When we think of how several events unfold during an experience, our sense of the duration, intervals, and order of these events is subjective and distinct from a precise representation of metric time. Such temporal information, so-called episodic time, is fundamental for encoding and retrieving episodicmemories, the record of what happened, where it happened, andwhen. Recently, several studies monitored the activity of individual neurons (Tsao et al., 2018; Bright et al., 2020) or hemodynamics (Bellmund et al., 2019; Montchal et al., 2019) to provide evidence that the neural representation of episodic time exists in the lateral entorhinal cortex (LEC). The LEC is one of the first and most severely affected regions in Alzheimer’s disease patients (Khan et al., 2014; Wisse et al., 2014; Nosheny et al., 2019), and the targeted disruption of the LEC impairs various rodent models of episodic memory (Morrissey and Takehara-Nishiuchi, 2014). Within the network of inter-connected regions supporting episodic memory, the LEC is unique in that it maintains reciprocal connections with the hippocampus on the one hand and with the neocortical regions on the other (Bota et al., 2015; Nilssen et al., 2019). This anatomical feature led to a view that the LEC is an essential relay structure between the hippocampus and neocortex (Squire, 1992; Eichenbaum, 2000, 2017). Specifically, during memory encoding, the LEC routes multiple pieces of highly processed sensory information from the neocortex to the hippocampus, thereby helping the hippocampus combine them into a single representation of an experience. During memory retrieval, the LEC transfers the stored information from the hippocampus to the neocortex to reinstate the neocortical activity patterns during the original experience. The discovery of neural representations of episodic time in the LEC provides a prime opportunity to refine the role of the LEC in the network processes supporting episodic memory. The present opinion article first summarizes findings in several electrophysiological studies that monitored the activity of individual neurons in the LEC with the focus on temporal features of their firing patterns. The survey has identified two distinct types, (1) drifting patterns during novel, open-ended experiences and (2) stable patterns during familiar, structured experiences (Figure 1). I then discuss their potential functions in the network dynamics supporting memory encoding and retrieval with several ideas for future investigations.