Search

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. [ABSTRACT FROM AUTHOR]

Prior knowledge and long‐term memory can guide our attention to facilitate search for and detection of subtle targets embedded in a complex scene. A number of neuropsychological and experimental studies have investigated this effect, yet results in the field remain mixed, as there is a lack of consensus regarding the neural correlates thought to support memory‐guided attention. The purpose of this systematic review and meta‐analysis was to identify a common set of brain structures involved in memory‐guided attention. Statistical analyses were computed on functional magnetic resonance imaging (fMRI) studies that presented participants with a task that required them to detect a target or a change embedded in repeated and novel complex visual displays. After a systematic search, 10 fMRI studies met the selection criteria and were included in the analysis. The results yielded four significant clusters. Activity in right inferior parietal (Brodmann area [BA] 9) and right superior parietal (BA 7) lobes suggests involvement of a fronto‐parietal attention network, while activity in left mid‐cingulate cortex (BA 23) and right middle frontal gyrus (BA 10) suggests involvement of a fronto‐parietal control network. These findings are consistent with the notion that fronto‐parietal circuits are important for interfacing retrieved memories with attentional systems to guide search. This article is categorized under:Psychology > MemoryPsychology > LearningPsychology > Attention [ABSTRACT FROM AUTHOR]

A number of theories of hippocampal function have placed spatial context at the center of richly recollected memories, but the subjective and objective ways that spatial context underlies the recollection of single words has been largely overlooked and underexplained. In this study, we conducted three experiments to investigate the involvement of spatial context in the recollection of single words. In all three experiments, participants encoded single words with varying features such as location and color. The subjective experience of recollection was measured using remember/know judgments and participant self‐report of the types of information they recollected about the words. Objectively, recollection was measured using source memory judgments for both spatial and non‐spatial features associated with the words. Our results provide evidence that spatial context frequently accompanies the recollection of single, isolated words, reviving discussions on the role of the hippocampus in spatial and detailed recollection. [ABSTRACT FROM AUTHOR]

Memory deficits in aging are characterized by impaired hippocampus‐mediated relational binding—the formation of links between items in memory. By reducing reliance on relational binding, unitization of two items into one concept enhances associative recognition among older adults. Can a similar enhancement be obtained when probing memory with recall? This question has yet to be examined, because recall has been assumed to rely predominantly on relational binding. Inspired by recent evidence challenging this assumption, we investigated individual differences in older adults' recall of unitized and nonunitized associations. Compared with successfully aging individuals, older adults with mild memory deficits, typically mediated by the hippocampus, were impaired in recall of paired‐associates in a task which relies on relational binding (study: "PLAY–TUNNEL"; test: PLAY–T?). In stark contrast, the two groups showed similar performance when items were unitized into a novel compound word (study: "LOVEGIGGLE"; test: LOVEG?). Thus, boosting nonrelational aspects of recall enhances associative memory among aging individuals with subtle memory impairments to comparable levels as successfully aging older adults. [ABSTRACT FROM AUTHOR]

It is well‐established that whether the information will be remembered or not depends on the extent to which the learning context is reinstated during post‐encoding rest and/or at retrieval. It has yet to be determined, however, if the fundamental importance of contextual reinstatement to memory extends to periods of spontaneous neurocognitive activity prior to learning. We thus asked whether memory performance can be predicted by the extent to which spontaneous pre‐encoding neural patterns resemble patterns elicited during encoding. Individuals studied and retrieved lists of words while undergoing fMRI‐scanning. Multivoxel hippocampal patterns during resting periods prior to encoding resembled hippocampal patterns at encoding most strongly for items that were subsequently remembered. Furthermore, across subjects, the magnitude of similarity correlated with a behavioral measure of episodic recall. The results indicate that the neural context before learning is an important determinant of memory. [ABSTRACT FROM AUTHOR]

As London taxi drivers acquire "the knowledge" and develop a detailed cognitive map of London, their posterior hippocampi (pHPC) gradually increase in volume, reflecting an increasing pHPC/aHPC volume ratio. In the mnemonic domain, greater pHPC/aHPC volume ratios in young adults have been found to relate to better recollection ability, indicating that the balance between pHPC and aHPC volumes might be reflective of cross‐domain individual differences. Here, we examined participants' self‐reported use of cognitive map‐based navigational strategies in relation to their pHPC/aHPC hippocampal volume ratio. We find that greater reported cognitive map use was related to significantly greater posterior, relative to anterior, hippocampal volume in two separate samples of young adults. Further, greater reported cognitive map usage correlated with better performance on a self‐initiated navigation task. Together, these data help to advance our understanding of differences between aHPC and pHPC and the greater role of pHPC in spatial mapping. [ABSTRACT FROM AUTHOR]

The hippocampus supports flexible decision‐making through memory integration: bridging across episodes and inferring associations between stimuli that were never presented together ('associative inference'). A pre‐requisite for memory integration is flexible representations of the relationships between stimuli within episodes (AB) but also of the constituent units (A,B). Here we investigated whether the hippocampus is required for parsing experienced episodes into their constituents to infer their re‐combined within‐episode associations ('dissociative inference'). In three experiments male rats were trained on an appetitive conditioning task using compound auditory stimuli (AB+, BA+, CD−, DC−). At test either the compound or individual stimuli were presented as well as new stimuli. Rats with hippocampal lesions acquired and retained the compound discriminations as well as controls. Single constituent stimuli (A, B, C, D) were presented for the first time at test, so the only value with which they could be associated was the one from the compound to which they belonged. Controls inferred constituent tones' corresponding values while hippocampal rats did not, treating them as merely familiar stimuli with no associated value. This finding held whether compound training occurred before or after hippocampal lesions, suggesting that hippocampus‐dependent inferential processes more likely occur at retrieval. The findings extend recent discoveries about the role of the hippocampus in intrinsic value representation, demonstrating hippocampal contributions to allocating value from primary rewards to individual stimuli. Importantly, we discovered that dissociative inferences serve to restructure or reparse patterns of directly acquired associations when animals are faced with environmental changes and need to extract relevant information from a multiplex memory. The hippocampus is critical for this fundamental flexible use of associations. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

For memory to be efficient and useful during recall, problem-solving, and planning, retrieval must be compressed in time. Evidence from rodents suggests that neural compression during replay of spatial memories varies widely, with a range of compression ratios reported from 6:1 to 64:1. Anecdotal evidence suggests that similar compression occurs during mental navigation in humans: we recall how to get from one place to another countless times almost every day of our lives, and this recall never takes as long as physically travelling those routes would take. In this experiment we sought to determine whether this behavioural compression could be measured during mental navigation in humans (spatial memory replay), and which factors might affect the compression of such spatial memories. To this end, thirty participants mentally navigated routes between two landmarks, which varied in length and number of turns, as we measured replay times and recorded ratings of familiarity, detail, and presence. A multi-level model was used to determine which factors were associated with variation in compression. Route length and number of turns emerged from this model as significantly correlated with compression, such that longer routes were more compressed while compression was attenuated as the number of turns in a route increased. This suggests that compression during recall may be affected by specific features of a route, especially those that may act to segment the space or event being represented. [ABSTRACT FROM AUTHOR]

ABSTRACT 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. [ABSTRACT FROM AUTHOR]

The current ubiquity of functional neuroimaging studies, and the importance they have had in elucidating brain function, obscures the fact that much of what we know about brain-behavior relationships derives largely from the study of single- and multiple-patient cases. A major goal of the present review is to describe how single cases continue to uniquely and critically contribute to cognitive neuroscience theory. With several recent examples from the literature, we demonstrate that single cases can both challenge accepted dogma and generate hypotheses and theories that steer the field in new directions. We discuss recent findings from case studies that specify critical functions of the hippocampus in episodic memory and recollection, and clarify its role in nonmnemonic abilities. Although we focus on the hippocampus, we discuss other regions and the occurrence of new associative learning, as well as the involvement of the ventromedial prefrontal and parietal cortices in memory encoding and retrieval. We also describe ways of dealing with the shortcomings of case studies, and emphasize the partnership of patient and neuroimaging methods in constraining neurocognitive models of memory. [ABSTRACT FROM AUTHOR]

ABSTRACT Perceptual richness, a defining feature of episodic memory, emerges from the reliving of multimodal sensory experiences. Although the importance of the medial temporal lobe (MTL) to episodic memory retrieval is well documented, the features that determine its engagement are not well characterized. The current study assessed the relationship between MTL function and episodic memory's perceptual richness. We designed a laboratory memory task meant to capture the complexity of memory for life episodes, while manipulating memory's perceptual content. Participants encoded laboratory episodes with rich (film clips) and impoverished (written narratives) perceptual content that were matched for other characteristics such as personal significance, emotionality and story content. At retrieval, participants were probed to describe the stories' perceptual features and storyline. Participants also recalled autobiographical memories (AMs) in a comparison condition. We compared the performance of patients with unilateral medial temporal lobe epilepsy (mTLE) and healthy controls to assess how damage to the MTL affects retrieval in these conditions. We observed an overall decrease in detail count in the mTLE group, along with a disproportionate deficit in perceptual details that was most acute in the AM and the perceptually enriched film clip conditions. Our results suggest that the impaired sense of reliving the past that accompanies MTL insult is mediated by a paucity of perceptual episodic memory details. We also introduce a new protocol that successfully mimics naturalistic memories while benefiting from the experimental control provided by using laboratory stimuli. © 2014 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

A growing body of evidence suggests that the hippocampus contributes to performance (or is implicated) in non-memory domains from perception to problem solving. In a previous study we found that hippocampal contribution to exemplar generation in a fluency task was determined jointly by the open-endedness of the task and its ability to elicit episodic memories (Sheldon and Moscovitch (2012) Hippocampus 22:1451-1466). In the current study, we extend these observations by exploring the role of the hippocampus in generative, goal-directed open-ended thought in patients with medial temporal lobe (MTL) amnesia on a free association task (think of words as they come to mind). Patients and control participants were asked to associate freely for one minute to cue words that varied in the open-endedness of the responses they elicited (greater for low- than high-frequency words), and in the ease with which episodic memories were evoked (greater for high imageable than low imageable words). As predicted, MTL amnesia patients generated fewer words than control participants when cues were highly imageable and low in frequency, but performed equivalently to them in the other conditions. These results support our prediction that the hippocampus contributes to free association, and possibly more generally to other generative tasks that are open-ended, creative, or that elicit the use of contextual and likely episodic memories in order to derive relevant information. © 2013 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Rats were administered contextual fear conditioning and trained on a water-maze, spatial memory task 28 days or 24 h before undergoing hippocampal lesion or control surgery. When tested postoperatively on both tasks, rats with hippocampal lesions exhibited retrograde amnesia for spatial memory at both delays but temporally graded retrograde amnesia for the contextual fear response. In demonstrating both types of retrograde amnesia in the same animals, the results parallel similar observations in human amnesics with hippocampal damage and provide compelling evidence that the nature of the task and the type of information being accessed are crucial factors in determining the pattern of retrograde memory loss associated with hippocampal damage. The results are interpreted as consistent with our transformation hypothesis (Winocur et al. (2010a) Neuropsychologia 48:2339-2356; Winocur and Moscovitch (2011) J Int Neuropsychol Soc 17:766-780) and at variance with standard consolidation theory and other theoretical models of memory. © 2013 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Recent investigations have shown that the medial temporal lobe (MTL), a region thought to be exclusive to episodic memory, can also influence performance on tests of semantic memory. The present study examined further the nature of MTL contributions to semantic memory tasks by tracking MTL activation as participants performed category fluency, a traditional test of semantic retrieval. For categories that were inherently autobiographical (e.g. names of friends), the MTLs were activated throughout the time period in which items were generated, consistent with the MTLs role in retrieving autobiographical memories. For categories that could not benefit from autobiographical or spatial/context information (e.g. governmental offices), the MTL was not implicated at any time point. For categories for which both prototypical and episodically-related information exists (e.g. kitchen utensils), there was more robust MTL activity for the open-ended, late generation periods compared with the more well-defined, early item generation time periods. We interpret these results as suggesting that early in the generation phase, responses are based on well-rehearsed prototypical knowledge whereas later performance relies more on open-ended strategies, such as deriving exemplars from personally relevant contextual information (e.g. imagining one's own kitchen). These findings and interpretation were consistent with the results of an initial, separate behavioral study (Expt 1), that used the distinctiveness of responses as a measure of open-endedness across the generation phase: Response distinctiveness corresponded to the predicted open-endedness of the various tasks at early and late phases. Overall, this is consistent with the view that as generation of semantic information becomes open-ended, it recruits processes from other domains, such as episodic memory, to support performance. © 2011 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

The hippocampus is crucial for episodic autobiographical memory retrieval. Functional neuroimaging evidence suggests that it is similarly engaged in recent and remote retrieval when memories are matched on vividness and personal importance. Far fewer studies have investigated the nature of hippocampal-neocortical coactivation in relation to memory remoteness. The purpose of this study was to examine hippocampal activity and functional connectivity as a function of memory age. Unlike most studies of autobiographical memory, we included autobiographical memories formed in the days and weeks before scanning, in addition to truly remote memories on the order of months and years. Like previous studies, we found that the hippocampus was active bilaterally regardless of memory age, with anterior activity increasing up to 1 yr and then decreasing, and with posterior activity being less sensitive to memory age. More importantly, hippocampal functional connectivity varied with memory age. Retrieving recent memories (≤1 yr) showed a late coactivation of the hippocampus and areas of the autobiographical memory network, whereas retrieving remote memories (10 yrs) showed an early negative coactivation of the hippocampus and left inferior frontal gyrus followed by a positive coactivation with anterior cingulate. This finding may reflect that the hippocampus is more strongly integrated with the autobiographical memory network for recent than for remote memories, and that more effort is required to recover remote memories. © 2011 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Functional magnetic resonance imaging (MRI) was used to investigate the hypothesis that memory for a large-scale environment is initially dependent on the hippocampus but is later supported by extra-hippocampal structures (e.g., precuneus, posterior parahippocampal cortex, and lingual gyrus) once the environment is well-learned. Participants were scanned during mental navigation tasks initially when they were newly arrived to the city of Toronto, and later after having lived and navigated within the city for 1 yr. In the first session, activation was observed in the right hippocampus, left precuneus, and postcentral gyrus. The second session revealed activation in the caudate and lateral temporal cortex, but not in the right hippocampus; additional activation was instead observed in the posterior parahippocampal cortex, lingual gyrus, and precuneus. These findings suggest that the right hippocampus is required for the acquisition of new spatial information but is not needed to represent this information when the environment is highly familiar. © 2011 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

We examined hippocampal contribution to remote spatial memory in older adults by correlating their performance on tests sensitive to hippocampal damage with their description of routes they traversed many times or only once, and with their map-like knowledge of downtown Toronto. We found that performance on table-top tests of spatial location (Smith and Milner () Neuropsychologia 19:781-793) and on paired-associate learning, and the number of Internal Details on the Autobiographical Interview (Levine et al., () Psychol Aging 17:677-689), all correlated significantly with the number and type of perceptual details used in describing routes one has traversed, but not with map-like knowledge of Toronto. No significant correlations were found with performance on tests of frontal function (WCST, phonemic fluency, and backward digit span). We conclude that the hippocampus is implicated in vivid re-experiencing of a familiar route, but not with map-like knowledge of a large-scale environment. These findings are interpreted as consistent with Multiple Trace Theory's prediction that it is the degree of detail of a retrieved memory that is crucially dependent on the hippocampus. © 2010 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

In previous work, we showed that adult rats that were reared socially for 3 months in a complex (village) environment retained allocentric spatial memory for that environment following hippocampal lesions (Winocur et al., () Nat Neurosci 8:273-275). In the present series of experiments, we showed that 3 months of postoperative rearing did not confer the same benefits (Experiment 1), although hippocampal groups, with or without rearing experience, exhibited spatial learning after extensive training (Experiments 1 and 2). Experiment 3 showed that as little as 2 weeks of preoperative rearing in the village was sufficient to retain acquired spatial memories after hippocampal lesions. Probe testing revealed that, although rats with hippocampal lesions exhibited remarkably good memory for preoperatively learned locations in the village, they were impaired when changes in task demands required flexible use of existing spatial representations. In a direct test of flexibility (Experiment 4), preoperatively reared rats were administered a blocked-routes task in the original learning environment, in which on designated trials, a barrier was placed across one of the direct paths to the goal compartment. On encountering the barrier, control rats consistently selected the next most direct route, whereas rats with hippocampal lesions, despite using spatial strategies, made more errors and took longer to find the goal. The present results confirm that allocentric spatial memories can survive hippocampal damage but they are schematic in nature and less cohesive than those associated with cognitive maps in intact brains. As well, there was evidence that, although different processes are involved in their formation, the schematic memories that were acquired preoperatively and survived hippocampal lesions are essentially the same as those laboriously formed postoperatively after extensive training. © 2010 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]

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. © 2007 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]

Contextual fear conditioning has become a benchmark measure for hippocampal function, even though several studies report successful acquisition in hippocampal-damaged rodents. The current study examined whether environmental complexity may account for these discrepancies. We directly compared single-session contextual fear conditioning in rats in a simple vs. complex environment. Hippocampal lesions led to reduced fear conditioning in both contexts, as measured by freezing, but the effect was significantly greater in the complex context. As well, lesions led to generalized fear when the complex context was paired with shock, but not when the simple context was paired. We suggest that the representation of the simple context formed by rats with hippocampal lesions was adequate to support associative learning, but the representation of the complex context, which depended to a greater extent on relational learning, was not. The results were interpreted as consistent with theories of hippocampal function that emphasize its role in integrating multiple stimulus elements in a memory trace. © 2007 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]

Lesions restricted to the hippocampal formation and/or extended hippocampal system (hippocampal formation, fornix, mammillary bodies, and anterior thalamic nuclei) can disrupt conscious recollection in anterograde amnesia, while leaving familiarity-based memory relatively intact. Familiarity may be supported by extra-hippocampal medial temporal lobe (MTL) structures. Within-task dissociations in recognition memory best exemplify this distinction in anterograde amnesia. The authors report for the first time comparable dissociations within recognition memory in retrograde amnesia. An amnesic patient (A.D.) with bilateral fornix and septal nuclei lesions failed to recognize details pertaining to personal past events only when recollection was required, during recognition of episodic details. His intact recognition of generic and semantic details pertaining to the same events was ascribed to intact familiarity processes. Recollective processes in the controls were reflected by asymmetrical Receiver's Operating Characteristic curves, whereas the patient's Receiver's Operating Characteristic was symmetrical, suggesting that his inferior recognition performance on episodic details was reliant on familiarity processes. Anterograde and retrograde memories were equally affected, with no temporal gradient for retrograde memories. By comparison, another amnesic person (K.C.) with extensive MTL damage (involving extra-hippocampal MTL structures in addition to hippocampal and fornix lesions) had very poor recognition and no recollection of either episodic or generic/semantic details. These data suggest that the extended hippocampal system is required to support recollection for both anterograde and retrograde memories, regardless of their age. © 2006 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]

We review lesion and neuroimaging evidence on the role of the hippocampus, and other structures, in retention and retrieval of recent and remote memories. We examine episodic, semantic and spatial memory, and show that important distinctions exist among different types of these memories and the structures that mediate them. We argue that retention and retrieval of detailed, vivid autobiographical memories depend on the hippocampal system no matter how long ago they were acquired. Semantic memories, on the other hand, benefit from hippocampal contribution for some time before they can be retrieved independently of the hippocampus. Even semantic memories, however, can have episodic elements associated with them that continue to depend on the hippocampus. Likewise, we distinguish between experientially detailed spatial memories (akin to episodic memory) and more schematic memories (akin to semantic memory) that are sufficient for navigation but not for re-experiencing the environment in which they were acquired. Like their episodic and semantic counterparts, the former type of spatial memory is dependent on the hippocampus no matter how long ago it was acquired, whereas the latter can survive independently of the hippocampus and is represented in extra-hippocampal structures. In short, the evidence reviewed suggests strongly that the function of the hippocampus (and possibly that of related limbic structures) is to help encode, retain, and retrieve experiences, no matter how long ago the events comprising the experience occurred, and no matter whether the memories are episodic or spatial. We conclude that the evidence favours a multiple trace theory (MTT) of memory over two other models: (1) traditional consolidation models which posit that the hippocampus is a time-limited memory structure for all forms of memory; and (2) versions of cognitive map theory which posit that the hippocampus is needed for representing all forms of allocentric space in memory. [ABSTRACT FROM AUTHOR]

The representation of autobiographical memory is distributed over a network of brain structures, with the medial temporal lobe (MTL) at its epicenter. Some believe that, over time, all memories become independent of their MTL component ('consolidation theories'). Others have suggested that this is true only of semantic memory, while episodic aspects of autobiographical memories are dependent on the MTL for as long as they exist, such as multiple trace theory (MTT). In the present study, the volumes of 28 brain regions, including the MTL, and their relation to autobiographical memory were investigated in a group of patients with Alzheimer's disease with varying degrees of retrograde memory loss as assessed by the Autobiographical Memory Interview (AMI). We used the multivariate analysis method of partial least squares (PLS) to assess patterns of atrophy that can lead to retrograde amnesia. We found that different aspects of autobiographical memory were associated with different patterns of tissue loss. Personal semantics were related to a pattern of bilateral anterior and posterior lateral temporal cortex degeneration, more pronounced on the left, as well as right frontal degeneration. Autobiographical event memory ('episodic') was associated with combined atrophy in bilateral MTL and anterior lateral temporal neocortex, more pronounced on the right. This pattern was invariant for memories from childhood, early adulthood, and recent memories, in line with the predictions of MTT, suggesting that MTL tissue is crucial for retrieval of episodic memories regardless of their age. © 2005 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]

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. © 2004 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]

Recent neuroimaging studies report preferential hippocampal engagement during autobiographical memory (AM) retrieval. Although the basis of this preferential activation remains unclear, it may be related to the temporal specificity, recency, or recollective qualities of AMs, such as detail, emotionality, and personal significance. Typically, however, these variables are confounded, and thus we sought to investigate the contributions of each to hippocampal activation during AM retrieval. We conducted an event-related functional magnetic resonance imaging (fMRI) study in which participants retrieved temporally specific AMs and general, repeated AMs, and rated each for level of detail, emotion, or personal significance. These ratings, as well as the recency of AMs, were used in parametric modulation analyses to identify brain regions that correlated positively with ratings, independent of recency, and vice versa. Retrieval of AMs activated a number of regions, including the hippocampus. No differences in hippocampal activation were evident between specific and general AM retrieval, suggesting that temporal specificity, on its own, is not a key modulator of hippocampal activation. Activation of the left hippocampus during specific AM retrieval did vary with the level of detail, personal significance, and at a subthreshold level, emotionality, when the effect of recency was covaried out. Further, during general AM retrieval, all three recollective qualities modulated activity in the right hippocampus. Although the recency of specific AMs modulated hippocampal activation bilaterally, this effect dissipated in the left hippocampus when detail or emotionality was included as a covariate, and was no longer present in either hippocampus when personal significance was taken into account. Our results suggest that recollective qualities are important predictors of hippocampal engagement during AM retrieval independent of factors such as recency. These findings are consistent with theories of hippocampal function that emphasize its role in the recollection of multifaceted autobiographical experiences. © 2004 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]

Discusses the influence of memory processes on tests of symbolic representation. Parallel performance of amnesiacs and infants on object search tasks consistent with the hypothesis that memory deficit, rather than defective object concept plays a major role in the AB error; Mnemonic precedence in amnesic patients.

Cover legend: This cover image is based on the Research Article Changes in patterns of neural activity underlie a time‐dependent transformation of memory in rats and humans by Melanie J. Sekeres et al., DOI: 10.1002/hipo.23009. [ABSTRACT FROM AUTHOR]