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Replicas of an aspect of an experienced event can serve as effective reminders, yet little is known about the neural basis of such reminding effects. Here we examined the neural activity underlying the memory-enhancing effect of reminders 1 week after encoding of naturalistic film clip events. We used fMRI to determine differences in network activity associated with recently reactivated memories relative to comparably aged, non-reactivated memories. Reminders were effective in facilitating overall retrieval of memory for film clips, in an all-or-none fashion. Prefrontal cortex and hippocampus were activated during both reminders and retrieval. Peak activation in ventro-lateral prefrontal cortex (vPFC) preceded peak activation in the right hippocampus during the reminders. For film clips that were successfully retrieved after 7 days, pre-retrieval reminders did not enhance the quality of the retrieved memory or the number of details retrieved, nor did they more strongly engage regions of the recollection network than did successful retrieval of a non-reminded film clip. These results suggest that reminders prior to retrieval are an effective means of boosting retrieval of otherwise inaccessible episodic events, and that the inability to recall certain events after a delay of a week largely reflects a retrieval deficit, rather than a storage deficit for this information. The results extend other evidence that vPFC drives activation of the hippocampus to facilitate memory retrieval and scene construction, and show that this facilitation also occurs when reminder cues precede successful retrieval attempts. The time course of vPFC-hippocampal activity during the reminder suggests that reminders may first engage schematic information meditated by vPFC followed by a recollection process mediated by the hippocampus., (© 2020 Wiley Periodicals LLC.)

There is considerable evidence from non-human animal studies that the anterior and posterior regions of the hippocampus have different anatomical connections and support different behavioural functions. Although there are some recent human studies using functional magnetic resonance imaging (fMRI) that have addressed this idea directly in the memory and spatial processing domains and provided support for it, there has been no broader meta-analysis of the fMRI literature to determine if there is consistent evidence for functional dissociations in anterior and posterior hippocampus across all of the different cognitive domains in which the hippocampus participates. The purpose of this review is to address this gap in our knowledge using three approaches. One approach involved PubMed searches to identify relevant fMRI papers reporting hippocampal activation during episodic encoding and retrieval, semantic retrieval, working memory, spatial navigation, simulation/scene construction, transitive inference, and social cognition tasks. The second was to use a large meta-analytic database (neurosynth) to find text terms and coactivation maps associated with the anterior and posterior hippocampal regions identified in the literature search. The third approach was to contrast the resting-state functional connectivity of the anterior and posterior hippocampal regions using a publicly available database that includes a large sample of adults. These three approaches provided converging evidence that not only are cognitive processes differently distributed along the hippocampal axis, but there also are distinct areas coactivated and functionally connected with the anterior and posterior segments. This anterior/posterior distinction involving multiple cognitive domains is consistent with the animal literature and provides strong support from fMRI for the idea of functional dissociations across the long axis of the hippocampus., (© 2019 Wiley Periodicals, Inc.)

The dynamic process of memory consolidation involves a reorganization of brain regions that support a memory trace over time, but exactly how the network reorganizes as the memory changes remains unclear. We present novel converging evidence from studies of animals (rats) and humans for the time-dependent reorganization and transformation of different types of memory as measured both by behavior and brain activation. We find that context-specific memories in rats, and naturalistic episodic memories in humans, lose precision over time and activity in the hippocampus decreases. If, however, the retrieved memories retain contextual or perceptual detail, the hippocampus is engaged similarly at recent and remote timepoints. As the interval between the timepoint increases, the medial prefrontal cortex is engaged increasingly during memory retrieval, regardless of the context or the amount of retrieved detail. Moreover, these hippocampal-frontal shifts are accompanied by corresponding changes in a network of cortical structures mediating perceptually-detailed as well as less precise, schematic memories. These findings provide cross-species evidence for the crucial interplay between hippocampus and neocortex that reflects changes in memory representation over time and underlies systems consolidation., (© 2018 Wiley Periodicals, Inc.)

Several recent studies have compared episodic and spatial memory in neuroimaging paradigms in order to understand better the contribution of the hippocampus to each of these tasks. In the present study, we build on previous findings showing common neural activation in default network areas during episodic and spatial memory tasks based on familiar, real-world environments (Hirshhorn et al. (2012) Neuropsychologia 50:3094-3106). Following previous demonstrations of the presence of functionally connected sub-networks within the default network, we performed seed-based functional connectivity analyses to determine how, depending on the task, the hippocampus and prefrontal cortex differentially couple with one another and with distinct whole-brain networks. We found evidence for a medial prefrontal-parietal network and a medial temporal lobe network, which were functionally connected to the prefrontal and hippocampal seeds, respectively, regardless of the nature of the memory task. However, these two networks were functionally connected with one another during the episodic memory task, but not during spatial memory tasks. Replicating previous reports of fractionation of the default network into stable sub-networks, this study also shows how these sub-networks may flexibly couple and uncouple with one another based on task demands. These findings support the hypothesis that episodic memory and spatial memory share a common medial temporal lobe-based neural substrate, with episodic memory recruiting additional prefrontal sub-networks., (© 2014 Wiley Periodicals, Inc.)

Preserved remote spatial memory in amnesic people with bilateral hippocampal damage, including the well-studied case K.C., challenges spatial theories, which assume that the hippocampus is needed to support all allocentric spatial representations, old or new. It remains possible, however, that residual hippocampal tissue is functional and contributes to successful performance. Here, we examine brain activity with fMRI during the retrieval of spatial information in K.C. and in healthy controls using landmark and route stimuli from a premorbidly familiar neighborhood that K.C. can navigate normally. In all participants, activity was found in the parahippocampal cortex, but not in the hippocampus itself, during all navigational tasks on which K.C. performs well, even though part of his hippocampus remains viable. The opposite pattern was observed on a house recognition task, which is inconsequential to navigation, and on which K.C. performed poorly. On that task, K.C. recruited the right hippocampus presumably because even "familiar" houses were treated as novel by him, whereas controls recruited occipitotemporal cortex, including parahippocampal cortex. The distinction between recent and remote memory, therefore, may apply as much to spatial theories of hippocampal function as it does to theories emphasizing the role of the hippocampus in other types of explicit memory., ((c) 2007 Wiley-Liss, Inc.)

The role of the hippocampus in recent spatial memory has been well documented in patients with damage to this structure, but there is now evidence that the hippocampus may not be needed for the storage and recovery of a spatial layout that was experienced long before injury. Such preservation may rely, instead, on a network of dissociable, extra-hippocampal regions implicated in topographical orientation. Using functional magnetic resonance imaging (fMRI), we investigated this hypothesis in healthy individuals with extensive experience navigating in a large-scale urban environment (downtown Toronto). Participants were scanned as they performed mental navigation tasks that emphasized different types of spatial representations. Tasks included proximity judgments, distance judgments, landmark sequencing, and blocked-route problem-solving. The following regions were engaged to varying degrees depending on the processing demands of each task: retrosplenial cortex, believed to be involved in assigning directional significance to locales within a relatively allocentric framework; medial and posterior parietal cortex, concerned with processing space within egocentric coordinates during imagined movement; and regions of prefrontal cortex, present in tasks heavily dependent on working memory. In a second, event-related experiment, a distinct area of inferotemporal cortex was revealed during identification of familiar landmarks relative to unknown buildings in addition to activation of many of those regions identified in the navigation tasks. This result suggests that familiar landmarks are strongly integrated with the spatial context in which they were experienced. Importantly, right medial temporal lobe activity was observed, its magnitude equivalent across all tasks, though the core of the activated region was in the parahippocampal gyrus, barely touching the hippocampus proper., (Copyright 2004 Wiley-Liss, Inc.)

The purpose of the current experiment was to examine the functional connectivity of the hippocampus during encoding in young and old adults, and the way in which this connectivity was related to recognition performance. Functional connectivity was defined as the correlation between activity in the hippocampus and activity in the rest of the brain, as measured by neuroimaging. During encoding of words and pictures of objects in young adults, hippocampal activity was correlated with activity in the ventral prefrontal and extrastriate regions, and increased activity in all these regions was associated with better recognition. In contrast, older adults showed correlations between hippocampal activity and the dorsolateral prefrontal and parietal regions, and positive correlations between activity in these regions and better memory performance. This ventral/dorsal distinction suggests a shift in the cognitive resources used with age from more perceptually based processes to those involved in executive and organizational functions. The results of this study provide evidence that aging is associated with alterations in hippocampal function, including how it is functionally connected with prefrontal cortex, and that these alterations have an impact on memory performance.