Handbook of Episodic Memory

Ekrem Dere, Alexander Easton, Lynn Nadel, Joseph P Huston, Ekrem Dere, Alexander Easton, Lynn Nadel, Joe P Huston

• Publisher: Elsevier Science
• Year: 2008
• ISBN:
  9780080932361
• Topic: Psicología
• Subtopic: Neuropsicología

Episodic memory is the name of the kind of memory that records personal experiences instead of the mere remembering of impersonal facts and rules. This type of memory is extremely sensitive to ageing and disease so an understanding of the mechanisms of episodic memory might lead to the development of therapies suited to improve memory in some patient populations. Episodic memory is unique in that it includes an aspect of self-awareness and helps us to remember who we are in terms of what we did and what we have been passed through and what we should do in the future.

This book brings together a renowned team of contributors from the fields of cognitive psychology, neuropsychology and behavioural and molecular neuroscience. It provides a detailed and comprehensive overview of recent developments in understanding human episodic memory and animal episodic-like memory in terms of concepts, methods, mechanisms, neurobiology and pathology. The work presented within this book will have a profound effect on the direction that future research in this topic will take.

• The first and most current comprehensive handbook on what we know about episodic memory, the memory of events, time, place, and emotion, and a key feature of awareness and consciousness
• Articles summarize our understanding of the mechanisms of episodic memory as well as surveying the neurobiology of epsidodic memory in patients, animal studies and functional imaging work
• Includes 34 heavily illustrated chapters in two sections by the leading scientists in the field

Chapter 1.1 Perspectives on episodic and semantic memory retrieval

Lee Ryan, Siobhan Hoscheidt, Lynn Nadel^*

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Department of Psychology and McKnight Brain Institute, University of Arizona, Tucson, AZ, USA

^* Corresponding author.

E-mail address: [email protected]

Abstract

Episodic or autobiographical recollection involves re-experiencing a past event that is specific in time and place, while semantic recollection is concerned with facts and general knowledge about the world. Several prominent memory theories posit that the hippocampus differentiates between these two types of memories, mediating episodic, but not semantic, retrieval. In this chapter we explore a different view of hippocampus, one that emphasizes a singular response of the hippocampus during memory encoding and retrieval of both episodic and semantic memories, based on an amalgam of two existing theories of hippocampal function, multiple trace theory (MTT; Nadel & Moscovitch, 1997), and cognitive map theory (O'Keefe & Nadel, 1978). We review neuropsychological and neuroimaging literature suggesting that both semantic and episodic memory retrieval engages the hippocampus, at least within the normally functioning brain. We then describe an updated version of MTT that incorporates these new findings. Finally, we explore the notion that differences in the role the hippocampus plays in these forms of memory reflect two critical factors - the nature of the information being retrieved, and the requirements of the retrieval task.

Episodic and semantic memory seem, at least phenomenologically, quite different from one another. Episodic or autobiographical recollection involves thinking about a past event – it is personal, emotional, populated with players and specific places, imbued with detail, and it often has relevance to our sense of self and the meaning of our lives. Semantic recollection, on the other hand, has to do with knowledge – it is factual, and typically devoid of emotion or reference to time, place, and self. While semantic knowledge conveys meanings, it is rarely the kind of personal meaning embodied in autobiographical and episodic memories. This distinction, as outlined by Tulving (1983), focused originally on the different types of information processed by the two systems, unique spatial-temporal contexts for episodic memory, and facts and concepts for semantic memory. More recently, Tulving (2002, 2005) has emphasized that the critical distinction is not so much the type of information being processed, but instead that episodic memory allows the rememberer to have the conscious experience, or autonoesis, of being mentally present once again within the spatial-temporal context of the original experience – the phenomenal experience of remembering. Of course, this ability presupposes that the individual can retrieve the spatial-temporal context in which the to-be-remembered event occurred. Thus, spatial-temporal context remains a critical component of episodic memory.

Given the significant differences across these two memory types, it is not unreasonable to assume that they are mediated by separate and relatively independent systems, not only at the level of cognition, but also the brain. This idea was bolstered early on by the finding that amnesia resulting from damage to the medial temporal lobe (MTL) combined severe deficits in episodic memory retrieval with an apparent ability to access previously acquired world knowledge, facts, and skills. Given the brain damage observed in H.M. and other MTL amnesics, it seemed plausible that the hippocampus differentiated between these two types of memories, mediating episodic, but not semantic, retrieval.

The hippocampal role in retrieval has traditionally been viewed as temporary, lasting only until a process of memory consolidation (cf., McGaugh, 2002) transferred responsibility for retrieval to extrahippocampal (presumably neocortical) circuits. This has become known as the standard theory of consolidation (Squire, 1992). A more recent alternative view, multiple trace theory (MTT; Nadel and Moscovitch, 1997, 1998) addressed the question of what happens to episodic memories over time and, in contrast to standard consolidation theory, postulated an ongoing role of hippocampus in autobiographical memory retrieval. Based on an extensive review of the amnesia literature, MTT proposed that episodic memory retrieval would elicit a new encoding, leading to an expanded representation of that memory within the hippocampus itself. Early computational models suggested that such an assumption could plausibly account for the facts of retrograde amnesia (e.g., Nadel et al., 2000).

The debate over whether the hippocampus is utilized in retrieval of well-consolidated episodic memories has been largely resolved. fMRI studies have shown repeatedly, and in various ways, that even very remote event memories activate the hippocampus. Neuroimaging studies have shown consistent activation in MTL structures during retrieval of recent and very remote autobiographical memories (Ryan et al., 2001; Maguire et al., 2001a; Gilboa et al., 2004; Rekkas and Constable, 2005). Even those studies reporting a time-dependent gradient have shown that the activity appears to be related to aspects of the memories such as vividness or the amount of detail retrieved, rather than how recently the event was experienced (Addis et al., 2004) Additionally, it appears that amnesics are not normal in autobiographical memory retrieval for very old events as was once assumed (Cipolotti et al., 2001; Steinvorth et al., 2005; but see Kirwan et al., 2008). Although they may be able to access the general facts of a particular past event (e.g., that their wedding happened in 1972 in Toronto), their recollections are generally sparse and lacking in detail; they are unable to reconstruct a cohesive description of these types of events that normal individuals would produce naturally (Moscovitch et al., 2006). And, consistent with Tulving's (1983, 2005) predictions, they appear equally unable to imagine themselves within the context of a future event (Klein et al., 2002; Hassabis et al., 2007). The preponderance of evidence supports the view that the hippocampus plays a lasting role in the retrieval of episodic memories (see the chapter by Moscovitch et al., this volume, for further discussion of issues surrounding episodic memory consolidation).

In contrast, debate continues regarding whether the hippocampus is critical for the retrieval of semantic memories, including personal semantics and world knowledge. Much of the evidence on both sides of this debate comes from patients with MTL damage. Squire and others (Squire and Zola, 1998; Lou and Niki, 2002; Manns et al., 2003; Squire et al., 2004) emphasize that at least some amnesics appear to have significant deficits in semantic memory retrieval, even for well-established world knowledge. However, semantic memory impairment tends to be extensive only when the damage extends beyond the hippocampus to other MTL and neocortical structures (Schmolck et al., 2002) and can reach the same level of deficit as autobiographical memory loss, or even exceed it, in some patients (Bayley et al., 2003; Bayley and Squire, 2005). Alternatively, a recent review of the patient literature (Moscovitch et al., 2006) concluded that retrograde amnesia for semantic memory is either spared completely or confined to a period of about 10 years prior to the head injury, providing that the damage is limited primarily to the hippocampal formation.

While the extent of hippocampal involvement in retrieval remains controversial, there is little doubt that acquisition of new episodic and semantic memories is impaired by damage to the hippocampus, at least when the injury is acquired in adulthood. Anterograde amnesia remains a defining feature of the amnesic disorder (Milner, 2005; Keane and Verfaellie, 2006). Cases of developmental amnesia caused by hippocampal damage early in life are interesting because of the remarkable amount of semantic information that these individuals acquire despite profound deficits in episodic memory (de Haan et al., 2006), although their acquisition of new knowledge is not completely normal (Vicari et al., 2007). Adult-onset amnesics too can learn new semantic information, but it is clearly a very inefficient learning process and the resulting knowledge is inflexible and does not easily generalize to other contexts (Baddeley and Wilson, 1986; Glisky et al., 1986; Wilson and Baddeley, 1988). Thus, the hippocampus appears to play an important role in the acquisition, but not retrieval, of semantic memories, while participating in both the acquisition of new episodes and their subsequent retrieval throughout the lifetime of the rememberer.

In this chapter we explore a different view of hippocampus, one that emphasizes a singular response of the hippocampus during memory encoding and retrieval of both episodic and semantic memories. This view is an amalgam of fundamental assumptions drawn from cognitive map theory (CMT; O’Keefe and Nadel, 1978), and MTT (Nadel and Moscovitch, 1997). CMT assumes that the hippocampus is preferentially involved in the processing of spatial contexts and spatial relations. MTT assumes that inputs to the MTL automatically engage hippocampal networks, whether the information involved is semantic or episodic, resulting in activation in cortical networks related to the input. While CMT placed special emphasis on the role of the hippocampus in episodic memory, here we will assume a preferential role of the hippocampus in the processing of all spatial content, whether episodic or semantic.

In what follows we review some of the literature suggesting that semantic and episodic memory are interactive and that retrieval of either engages the hippocampus, at least within the intact brain. We describe an updated version of MTT that incorporates these new findings. Finally, we explore the notion that differences in the role the hippocampus plays in these forms of memory reflect two critical factors – the nature of the information being retrieved, and the requirements of the retrieval task.

I MTT: Episodic retrieval and beyond

MTT posits that information representing the spatial context of an event is encoded in an ensemble of neurons within the hippocampus, and that this ensemble trace acts as a pointer to the various features of the event represented in cortical regions. Contrary to the standard view of consolidation (e.g., Squire and Alvarez, 1995), MTT assumes that the hippocampal trace remains relevant over time, as it represents a critical component of the memory for any episode – namely, the precise details of the spatial-temporal configural context within which the event transpired. Expansion of this hippocampal trace over repeated reactivations increases the likelihood that the complete episode can be successfully retrieved given only a partial cue from the original event.

Assuming that retrieval of even quite old episodic memories engages the hippocampus, it is worth pursuing the idea that the same might be true for semantic knowledge gained at various times in the past, particularly when the information refers to spatial locations or spatial relations. We, along with Moscovitch (1995), have suggested that the hippocampus automatically binds portions of an event to related information that has been previously sto...