The findings from fMRI studies of episodic encoding and retrieval are remarkably congruent with the two major theoretical frameworks – TAP and cortical reinstate- ment – outlined at the beginning of the chapter. In keeping with those frameworks, there is little evidence for a specialized encoding network at the cortical level: instead, the enhancement of cortical activity associated with successful encoding (subsequent memory effects) is found in some of the same regions that are engaged by the online processing demands of the study event, and these effects are accompanied by enhanced activity in the hippocampus. By contrast, successful recollection is associated with engagement not only of the hippocampus and a subset of the cortical regions that were active during encoding, but also with other regions – including ventral posterior parietal cortex – where activity is enhanced regardless of the nature of the retrieved content.
These findings shed significant light on the cognitive neuroscience of encoding, retrieval, and their interrelationship, while leaving many questions unresolved.
Perhaps the most intriguing of these concerns how the information represented in distributed patterns of “reinstated” cortical activity becomes bound into a coherent, consciously accessible representation of a prior experience. We conjec- ture that the answer to this question will be more apparent when there is a fuller understanding of the functions of the generic recollection network illustrated in Figure 5.6.
Acknowledgments
Preparation of this chapter, and much of the research described in it, was supported by NIMH grants R01MH072966 and R01MH074528.
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Medial Temporal Lobe
Subregional Function in Human Episodic Memory
Insights from High‐Resolution fMRI
Jackson C. Liang and Alison R. Preston
6
Introduction
Episodic memory fundamentally shapes human behavior, allowing us to draw upon past experience to inform current decisions and make predictions about upcoming events. For decades, research has documented the critical role of the medial temporal lobe (MTL) in episodic memory (Eichenbaum and Cohen, 2001). While the link between MTL function and episodic memory is beyond debate, a recent focus has centered on characterizing the contributions of specific MTL substructures to episodic memory formation and retrieval (see Chapter 5).
The MTL (Figure 6.1a) comprises a heterogeneous group of structures, each with a unique cellular organization and pattern of anatomical connectivity. The subregions of the MTL include the hippocampus, which itself comprises the dentate gyrus (DG), the cornu ammonis (CA) fields, and the subiculum, as well as the surrounding ento- rhinal (ERC), perirhinal (PRC), and parahippocampal (PHC) cortices (Figure 6.1b).
Several theoretical perspectives propose that anatomical differences between MTL subregions give rise to unique functional roles in episodic memory (Davachi, 2006;
Diana, Yonelinas, and Ranganath, 2007; McClelland, McNaughton, and O’Reilly, 1995; O’Reilly and Rudy, 2001). However, testing how MTL subregions contribute to human memory function poses a unique challenge for cognitive neuroscience research. Individuals with MTL lesions typically have damage that affects several MTL subregions, spanning both hippocampus and surrounding MTL cortices. Even those individuals with restricted hippocampal lesions have damage to multiple hippocampal subregions. Therefore, while the neuropsychological study of MTL patients has taught us a great deal about the essential nature of the region for episodic memory, it is limited in its ability to discern the functional roles of individual human MTL subregions.
Similarly, because MTL subregions are relatively small and adjacent, standard approaches to functional magnetic resonance imaging (fMRI) that use voxel dimensions greater than 3 mm cannot resolve signal originating from a particular MTL subfield. Testing
the predictions of anatomically based models of MTL function in the human brain thus requires a spatial resolution beyond the limits of neuropsychological study and standard functional neuroimaging methods.
Over the last decade, implementation of high‐resolution functional magnetic reso- nance imaging (hr‐fMRI) has opened the door for investigation of MTL subregional function in humans. In this chapter, we review the technical aspects of hr‐fMRI as applied to the study of the human MTL and discuss two core topics that have domi- nated research in this area: (1) functional dissociations between hippocampus and surrounding MTL cortices based on episodic memory content, and (2) functional distinctions between the components of the hippocampal circuit. We also discuss new multivariate pattern‐information analysis techniques, which examine distributed pat- terns of activation in contrast to average responses pooled across an entire region.
Such techniques, when combined with hr‐fMRI, have the power to provide new insights into the function of MTL subregions. We end by discussing challenges for hr‐fMRI of the human MTL and suggest future directions that could improve our ability to answer questions about the role of this region in episodic memory.