Episodic-like memory in animals: psychological criteria, neural mechanisms and the value of episodic-like tasks to investigate animal models of neurodegenerative disease

Abstract

The question of whether any non-human species displays episodic memory is controversial. Associative accounts of animal learning recognize that behaviour can change in response to single events but this does not imply that animals need or are later able to recall representations of unique events at a di¡erent time and place. The lack of language is also relevant, being the usual medium for communicating about the world, but whether it is critical for the capacity to represent and recall events is a separate matter. One reason for suspecting that certain animals possess an episodic-like memory system is that a variety of learning and memory tasks have been developed that, even though they do not meet the strict criteria required for episodic memory, have an`episodic-like' character. These include certain one-trial learning tasks, scene-speci¢c discrimination learning, multiple reversal learning, delayed matching and nonmatching tasks and, most recently, tasks demanding recollection of`what, where and when' an event happened. Another reason is that the neuronal architecture of brain areas thought to be involved in episodic memory (including the hippocampal formation) are substantially similar in mammals and, arguably, all vertebrates. Third, our developing understanding of activity-dependent synaptic plasticity (which is a candidate neuronal mechanism for encoding memory traces) suggests that its expression re£ects certain physiological characteristics that are ideal components of a neuronal episodic memory system. These include the apparently digital character of synaptic change at individual terminals and the variable persistence of potentiation accounted for by the synaptic tag hypothesis. A further value of studying episodic-like memory in animals is the opportunity it a¡ords to model certain kinds of neurodegenerative disease that, in humans, a¡ect episodic memory. An example is recent work on a transgenic mouse that over-expresses a mutation of human amyloid precursor protein (APP) that occurs in familial Alzheimer's disease, under the control of platelet derived (PD) growth factor promoter (the PDAPP mouse). A striking age-and amyloid plaque-related de¢cit is seen using a task in which the mice have to keep changing their memory representation of the world rather than learn a single fact.

Faster forgetting contributes to impaired spatial memory in the PDAPP mouse: Deficit in memory retrieval associated with increased sensitivity to interference?

“…In contrast, by the time five separate spatial locations had been trained in a single context, the mice would have been in a position where their memory retrieval mechanism would need to successfully distinguish between five separate long-term memory traces and somehow identify which was the appropriate memory to recall that day-a process that would entail contextual pattern separation. This appears to be a difficulty for PDAPP mice (Savonenko et al 2005), with episodic-like tasks (such as serial reversal) being particularly sensitive for revealing such a deficit (Morris 2001). The process of encoding highly similar but distinct episodes as separate memory traces is thought to require neural activity and plasticity in the dentate gyrus (DG).…”

Hippocampal place cells, context, and episodic memory

“…In the following discussion, we propose that place cells participate in episodic memory processes by providing a neural representation of the context, which is a necessary component of episodic memories. Before doing so, it is important to address the question of whether nonhuman animals possess the capacity for episodic memory (for excellent discussions of this issue see Aggleton and Pearce, 2001;Morris, 2001). Tulving (2002) proposed that a key feature of episodic memory is ''conscious recollection'' that is experienced as a form of ''mental time travel.''…”

Hippocampal and neocortical interactions during context discrimination: Electrophysiological evidence from the rat

“…This proposal is based in part on past suggestions that hippocampal-dependent memory involves associations of spatial, and temporal features (e.g. O' Keefe and Nadel, 1978;Mizumori et al, 2000;Redish et al, 2000;Burgess et al, 2001;Eichenbaum and Cohen, 2001;Morris, 2001;O'Reilly and Rudy, 2001;Buzsáki, 2005). At slight variance with many of these proposals, Mizumori (2007) suggest that the default mode of hippocampal processing is to continually integrate memory-guided perceptions of sensory, movement, and motivational information, or memory (M), within a spatial (S) reference framework as a function of time (T; Fig.…”