Remembering the past and planning for the future in rats

Abstract

A growing body of research suggests that rats represent and remember specific earlier events from the past. An important criterion for validating a rodent model of episodic memory is to establish that the content of the representation is about a specific event in the past rather than vague information about remoteness. Recent evidence suggests that rats may also represent events that are anticipated to occur in the future. An important capacity afforded by a representation of the future is the ability to plan for the occurrence of a future event. However, relatively little is known about the content of represented future events and the cognitive mechanisms that may support planning. This article reviews evidence that rats remember specific earlier events from the past, represent events that are anticipated to ccur in the future, and develops criteria for validating a rodent model of future planning. These criteria include representing a specific time in the future, the ability to temporarily disengage from a plan and reactivate the plan at an appropriate time in the future, and flexibility to deploy a plan in novel conditions.

Validation of a rodent model of source memory

“…Rats apparently remember the source by which information was acquired, in addition to other features of the episode (location, flavour and retrieval cue). Elsewhere we showed that source memory is hippocampal dependent [6] and that retrieval of episodic information can occur when encoding is incidental and memory assessment is unexpected [14,15].…”

Animal models of episodic memory

“…Zentall and colleagues have argued that when information is encoded for use in an expected memory test, explicitly encoded information may generate a planned action; thus, at the time of the test, the remembered action can occur successfully without remembering any earlier episodes. The central hypothesis of an animal model of episodic memory is that, at the moment of memory assessment, the animal remembers back in time to the event or episode (Crystal, 2013b(Crystal, , 2016a(Crystal, , 2016b; the focus on retrieving a memory of the earlier event is the key element that makes an animal model of episodic memory episodic. Therefore, carrying forward information that is needed at a future test while not specifically retrieving a memory of the earlier episode represents a serious threat to the episodicmemory hypothesis.…”

“…Therefore, carrying forward information that is needed at a future test while not specifically retrieving a memory of the earlier episode represents a serious threat to the episodicmemory hypothesis. Thus, it is necessary to rule out the hypothesis that accurate performance in the test is based on a planned action generated when information was explicitly encoded rather than a memory of the episode (Crystal, 2013b;Singer & Zentall, 2007;Zentall et al, 2001;Zentall et al, 2008;Zhou & Crystal, 2011). Notably, it is possible that animals may have solved previous tests of episodic memory by using learned semantic rules without remembering the episode.…”

“…This review advocates that it is profitable to focus on the content of episodic memories, rather than the subjective experiences that may accompany episodic memory. The central hypothesis of an animal model of episodic memory is that, at the moment of a memory assessment, the animal remembers back in time and retrieves a memory of the earlier event or episode (Crystal, 2013b(Crystal, , 2016a(Crystal, , 2016b). An important alternative explanation exists whenever the animal can solve the memory test without remembering back to the specific earlier event.…”

Declarative Memory

“…The most used tasks are the object recognition task, and variations thereof (Dere et al 2006;Ennaceur and Delacour 1988;Kesner et al 2008). Other examples are specific procedures in radial mazes (Crystal 2013), odor recognition (Eichenbaum et al 2010), and some procedures in the classic Morris water task (Zhang et al 2008). For all these test mentioned, the "what," "where," and "when" can be applied.…”