Chapter 21 Encoding-retrieval overlap in human episodic memory: A functional neuroimaging perspective
Introduction
Episodic memory is the collection of processes that together support the recollection of unique events (Tulving, 1983). By ‘unique event’, we are referring to events that are individuated by their contexts, such as where you parked your car today rather than yesterday. A key feature of episodic memories is the establishment of associations between the different elements of an event; in the example above, remembering where your car is currently parked depends on retrieval of associations between the act of parking the car, the location of the act, and when it occurred. Episodic memory can be contrasted with two other types of explicit (conscious) memory in which contextual associations play little or no role. Semantic memory supports general knowledge about the world that is acquired through repeated exposure to the same information in a variety of different contexts, such that memory for the information becomes contextually aspecific (Tulving, 1972). And memory based on a sense of familiarity can support simple recognition, recency, or frequency judgments, but provides no access to qualitative information about a prior event, and hence no specific information about where or when the event occurred (for review, see Yonelinas, 2002).
In the present paper, we bring together two frameworks — one rooted in experimental psychology and the other in neurobiology — that have each been highly influential in guiding ideas about how episodic memory operates. Both frameworks propose that there is an intimate relationship between the processes set in train when an event is experienced, and those that are engaged when it is later remembered. Although they derive from different experimental traditions, and are articulated at different levels of explanation, the two frameworks are highly complementary. Integrating the frameworks leads to an account of episodic memory encoding and retrieval that can be framed at the explanatory level of cognitive neuroscience. This account generates predictions about the neural correlates of episodic encoding and retrieval that can be tested in healthy humans with functional neuroimaging methods, notably, event-related functional magnetic resonance imaging (fMRI).
Below, we briefly describe the two frameworks and our attempt to integrate them into a single model. After noting some important caveats, we then describe recent findings from fMRI studies of episodic encoding and retrieval, and discuss how these findings can be understood in the context of the model.
Section snippets
Transfer-appropriate processing
For more than 40 years, experimental psychologists have investigated the relationship between the encoding and retrieval of episodic information, emphasizing the interdependency of these seemingly distinct mnemonic functions (e.g., Tulving and Thomson, 1973; Morris et al., 1977). One outcome of this line of research has been the principle of transfer-appropriate processing (TAP) (Morris et al., 1977). TAP is predicated on the twin assumptions that memories are represented in terms of the
Cortical reinstatement and episodic retrieval
The idea at the heart of TAP principle, namely that episodic retrieval involves the reinstatement of processes that were active at the time of encoding, is also found in several neurobiologically based models of memory retrieval (e.g., Alvarez and Squire, 1994; Rolls, 2000; Shastri, 2002; Norman and O’Reilly, 2003). According to such models, recollection of a recent event occurs when a pattern of cortical activity corresponding to the event is reinstated by activation of a hippocampally stored
The complementary nature of TAP and reinstatement theory
The principles of TAP and cortical reinstatement share several key concepts. These include the idea that memory retrieval involves the recapitulation of processes and representations that were active during encoding, and that the likelihood of successful retrieval is a function of the extent to which the processing engaged by a retrieval cue overlaps with that engaged at encoding.
Figure 1 illustrates one way in which these ideas can be schematized in terms of large-scale patterns of brain
Empirical findings
The foregoing caveats aside, the framework illustrated in Fig. 1 leads to a number of predictions about encoding- and retrieval-related neural activity that can be tested in functional neuroimaging studies in healthy humans. Below, we outline these predictions and discuss relevant fMRI data, focusing on recent findings from our laboratory.
Before moving on to the data, a brief methodological comment is in order. Several of the studies discussed below investigate the neural correlates of encoding
Concluding comments
The findings reviewed in the preceding three sections offer broad support for the framework illustrated in Fig. 1. The framework emphasizes the intimacy of the relationship between episodic encoding and retrieval that exists at both psychological and neural levels of analysis, and serves a useful role in integrating the two levels. As was discussed in “The complementary nature of TAP and reinstatement theory”, however, Fig. 1 offers what is at best a simplistic and incomplete account that
Acknowledgments
The research described in the chapter was supported by the Wellcome Trust, the Medical Research Council of the United Kingdom, and the National Institute of Mental Health (grant numbers R01-MH072966 and R01-MH074528).
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