Mental time travel in the rat

Abstract

I outline the perspective that searching the contents of memory is a form of mental time travel (MTT) in non-humans that is relatively tractable because it focuses on the contents of memory. I propose that an animal model of MTT requires three elements: (i) the animal remembers multiple events using episodic memory, (ii) the order of events in time is included in the representation, and (iii) the sequence of events can be searched to find a target that occurred at a particular time. I review experiments suggesting that rats represent multiple items in episodic memory (element 1) in order of occurrence (element 2) and engage in memory replay to search representations in episodic memory in sequential order to find information at particular points in the sequence (element 3). The cognitive building blocks needed for MTT may be quite old in the evolutionary timescale.This article is part of the theme issue 'Elements of episodic memory: lessons from 40 years of research'.

Keywords: animal model; episodic memory; episodic-like memory; incidental encoding; mental time travel; unexpected question.

Figure 1.

Rats replay a stream of multiple episodic memories. Schematic depiction of (a) list encoding and memory assessments, (b) dissociation of episodic memory and memory-trace-strength accounts of list-memory performance, and (c) an interference test. Because the same time scale is used in (a) and (b), it may be noted that the second-last item in (b) occurred at the time of the fourth-last item in (a), and the fourth-last item in (b) occurred at the time of the eighth-last item in (a). Data from [18] showing (d) reliance on episodic memory rather than memory trace strength and overall high accuracy in list-memory tasks, including after a 60-min delay and interference, (e) equivalent performance in acquisition under varying memory loads, and (f) list-memory performance is impaired by inhibition of hippocampal neurons while sparing other aspects of memory. (d–f) Error bars represent 1 s.e.m. Adapted and reproduced from [18]. Copyright Elsevier Ltd. © 2018.

Figure 2.

Replay of episodic memories after incidental encoding of multiple events in an unexpected assessment of memory. Schematic depiction of (a) list encoding and memory assessment, (b) foraging for food (encountered below unscented lids on a radial maze), (c) a control condition that pits explicit encoding of an initial list in the trained list-encoding context (list 1) against stimulus generalization in a novel context (list 2) and (d) a critical test in which rats forage on the radial maze but encounter food below scented lids followed by a memory assessment to prompt replay of episodic memories. Data from [19] showing (e) failure of stimulus generalization (i.e. failure to automatically encode odours for the purpose of an upcoming test in a novel context), and (f,g) high accuracy when rats encountered odours while foraging in the radial maze and their memory for the order of encountered odours was unexpectedly assessed (f) immediately or (g) after a 15-min delay. (e–g) Error bars represent 1 s.e.m. Adapted and reproduced with permission from [13]. Copyright © 2024.