Brain networks underlying episodic memory retrieval
Highlights
► Recollection depends on a content-insensitive network centered on the hippocampus. ► The recollection network comprises both medial temporal and neocortical regions. ► The network interacts with cortical regions that represent retrieved content.
Introduction
Episodic memory — consciously accessible memory for unique events — allows us to represent past experiences and to flexibly employ these representations in service of current and future goals [1]. The present review focuses on recent human fMRI findings relevant to the functional neuroanatomy of successful episodic memory retrieval. The majority of the reviewed studies took as their starting point a ‘dual-process’ model of memory [2, 3]. These models posit that a retrieval cue (such as a recognition memory test item) can elicit two qualitatively distinct kinds of mnemonic information: a multi-dimensional recollection signal that provides information about qualitative aspects of a prior event, including its context, and a scalar familiarity signal that can support simple judgments of prior occurrence. From this perspective, identifying the neural bases of episodic retrieval requires experimental designs that permit recollection-driven and familiarity-driven memory to be dissociated (Box 1). Current evidence suggests that the distinction between recollection and familiarity holds both within the MTL and at the level of the cerebral cortex, where a network of regions that appears to be preferentially engaged during successful recollection can be identified.
Section snippets
Memory signals within the MTL
The MTL — the hippocampus and surrounding perirhinal, entorhinal and parahippocampal cortices — has long been recognized as a key brain area supporting episodic memory. Reminiscent of electrophysiological findings in primates [4], fMRI studies have reported that perirhinal activity covaries inversely with the familiarity of recognition memory test items (e.g., [5]). These fMRI results are consistent with evidence from animal lesion studies [6] and a human single-case study [7] that suggest a
Cortical recollection effects
In addition to enhancement of hippocampal and parahippocampal activity, successful recollection is characteristically associated with engagement of several cortical regions, including retrosplenial/posterior cingulate cortex (BA 23/29/30/31), ventral posterior parietal cortex centered on the angular gyrus (BA 39) and mPFC (BA 10/32) [11]. Because of the density of its connections with the hippocampus and the memory impairments that accompany lesions to the region, retrosplenial cortex has been
A general recollection network?
As it was just reviewed, recollection-sensitive fMRI effects have consistently been identified in the hippocampus, parahippocampal, retrosplenial/posterior cingulate and lateral parietal cortices, and mPFC (Figure 2). The robustness of these effects in the face of wide variation in test materials and procedures for operationalizing recollection have led to the proposal that the regions constitute a content-independent network engaged whenever a retrieval cue elicits recollection [43, 44]. In
Content-selective recollection effects
According to an influential class of models (e.g., [49, 50]), a key role of the hippocampus is to store non-overlapping representations of the distributed patterns of cortical activity elicited when different events are encoded. When an effective retrieval cue is present, the appropriate hippocampal representation is reactivated, causing the reinstatement of the original pattern of activity in the cortex and the event to be ‘re-experienced’. Thus, successful recollection should be associated
Summary and open questions
Recollection of a prior experience is associated with engagement of a general network, centered on the hippocampus, in concert with cortical regions that, collectively, represent the contents of recollection. Among the many questions raised by this framework, three questions stand out. First, what are the specific functional roles of the different regions comprising this network? Second, how does the network interact with content-sensitive regions thought to represent the contents of
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgment
Preparation of this article and some of the research described in it was supported by NIMH grant 5R01MH072966.
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