Co-activation of the amygdala, hippocampus and inferior frontal gyrus during autobiographical memory retrieval

Functional MRI was used to investigate the role of medial temporal lobe and inferior frontal lobe regions in autobiographical recall. Prior to scanning, participants generated cue words for 50 autobiographical memories and rated their phenomenological properties using our autobiographical memory questionnaire (AMQ). During scanning, the cue words were presented and participants pressed a button when they retrieved the associated memory. The autobiographical retrieval task was interleaved in an event-related design with a semantic retrieval task (category generation). Region-of-interest analyses showed greater activation of the amygdala, hippocampus, and right inferior frontal gyrus during autobiographical retrieval relative to semantic retrieval. In addition, the left inferior frontal gyrus showed a more prolonged duration of activation in the semantic retrieval condition. A targeted correlational analysis revealed pronounced functional connectivity among the amygdala, hippocampus, and right inferior frontal gyrus during autobiographical retrieval but not during semantic retrieval. These results support theories of autobiographical memory that hypothesize co-activation of frontotemporal areas during recollection of episodes from the personal past.     

Autobiographical memory in semantic dementia: New insights from two patients using fMRI

Episodic autobiographical memory (EAM) consists of personal events embedded within a specific spatiotemporal context. Patients with semantic dementia (SD) generally show preserved recent EAMs, but a controversy remains concerning their ability to retrieve remote ones. Only one fMRI study examined remote autobiographical memory in SD through a longitudinal case study (Maguire, Kumaran, Hassabis, & Kopelman, 2010). Here, we propose a cross-sectional study to test the hippocampo-neocortical up-regulation hypothesis, through a multimodal approach (gray matter volume, activation, connectivity analyses), directly comparing recent and remote autobiographical memory retrieval and collecting data to asses phenomelogical re-experiencing. EAM retrieval recruits a distributed network of brain regions, notably the hippocampus which is shown to be atrophied in SD, although some studies report no hippocampal atrophy in SD. Using fMRI, we examined recent and remote EAM retrieval in two SD patients with different profiles of hippocampal atrophy, compared to 12 healthy elders (HE). JPL presented severe bilateral hippocampal atrophy, while EP showed sparing of both hippocampi. Behaviourally, JPL was impaired at retrieving EAMs from both life periods and showed poorer use of visual mental imagery than HE, while EP retrieved memories which were as episodic as those of HE for both periods and relied on greater use of visual mental imagery than HE. Neuroimaging results showed that, for JPL, hyperactivations of the residual hippocampal tissue and of frontal, lateral temporal, occipital and parietal cortices did not efficiently compensate his autobiographical memory deficit. EP however presented hyperactivations in similar neocortical regions which appeared to be more efficient in compensating for atrophy elsewhere, since EP's EAM retrieval was preserved. Functional connectivity analyses focusing on the hippocampus showed how the residual hippocampal activity was connected to other brain areas. For JPL, recent autobiographical retrieval was associated with connectivity between the posterior hippocampus and middle occipital gyrus, while for EP, connectivity was detected between the anterior hippocampus and numerous regions (medial temporal, occipital, temporal, frontal, parietal) for both recent and remote periods. These findings suggest that intensification of hippocampal atrophy in SD strongly affects both recent and remote autobiographical recollection. Up-regulation of neocortical regions and functional hippocampal–neocortical connectivity within the autobiographical network may be insufficient to compensate the lifelong episodic memory deficit for patients with extensive hippocampal atrophy.     

Recapitulating emotional context: activity of amygdala, hippocampus and fusiform cortex during recollection and familiarity

The amygdala is thought to enhance long-term memory for emotionally arousing events by modulating memory formation and storage in the hippocampus and in neocortical areas. Recent animal studies have raised the possibility that cooperativity between amygdala and hippocampus contributes to the retrieval of fear memories. The functional contributions of the amygdala to the retrieval of emotional memories in humans are less well known. Here, in a functional magnetic resonance imaging experiment, 20 healthy subjects studied neutral words in the context of a fearful or a neutral human face. In a subsequent test, they made 'remember' (conscious recollection of the study context), 'know' (familiarity in the absence of conscious recollection) and 'new' judgements on the studied and newly presented neutral words, in the absence of face stimuli. At test, bilateral amygdala, hippocampus and fusiform face area (FFA) were more strongly activated during recollection than during familiarity. Higher activity for fearful than for neutral study context was found in bilateral FFA during recollection but not during familiarity. This difference recapitulated higher activity for fearful than for neutral context in the FFA during study. These data suggest that the amygdalae and hippocampi contribute to the retrieval of emotion-laden context memories by coordinating the reactivation of stored representations in neocortical areas, such as the FFA. However, there also was a recapitulation of emotional study context in the right amygdala during familiarity only, which might therefore be related to affective implicit memory.     

The perceptual richness of complex memory episodes is compromised by medial temporal lobe damage

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.     

Patterns of hippocampal–neocortical interactions in the retrieval of episodic autobiographical memories across the entire life‐span of aged adults

We previously demonstrated that episodic autobiographical memories (EAMs) rely on a network of brain regions comprising the medial temporal lobe (MTL) and distributed neocortical regions regardless of their remoteness. The findings supported the model of memory consolidation, which proposes a permanent role of MTL during EAM retrieval (multiple‐trace theory or MTT) rather than a temporary role (standard model). Our present aim was to expand the results by examining the interactions between the MTL and neocortical regions (or MTL–neocortical links) during EAM retrieval with varying retention intervals. We used an experimental paradigm specially designed to engage aged participants in the recollection of EAMs, extracted from five different time‐periods, covering their whole life‐span, in order to examine correlations between activation in the MTL and neocortical regions. The nature of the memories was checked at debriefing by means of behavioral measures to control the degree of episodicity and properties of memories. Targeted correlational analyses carried out on the MTL, frontal, lateral temporal, and posterior regions revealed strong links between the MTL and neocortex during the retrieval of both recent and remote EAMs, challenging the standard model of memory consolidation and supporting MTT instead. Further confirmation was given by results showing that activation in the left and right hippocampi significantly correlated during the retrieval of both recent and remote memories. Correlations among extra‐MTL neocortical regions also emerged for all time‐periods, confirming the critical role of the prefrontal, temporal (lateral temporal cortex and temporal pole), precuneus, and posterior cingulate regions in EAM retrieval. Overall, this paper emphasizes the role of a bilateral network of MTL and neocortical areas whose activation correlate during the recollection of rich phenomenological recent and remote EAMs. © 2009 Wiley‐Liss, Inc.     

Patterns of hippocampal-cortical interaction dissociate temporal lobe memory subsystems

A distributed network of brain regions supports memory retrieval in humans, but little is known about the functional interactions between areas within this system. During functional magnetic resonance imaging (fMRI), subjects retrieved real-world memories: autobiographical events, public events, autobiographical facts, and general knowledge. A common memory retrieval network was found to support all memory types. However, examination of the correlations (i.e., effective connectivity) between the activity of brain regions within the temporal lobe revealed significant changes dependent on the type of memory being retrieved. Medially, effective connectivity between the parahippocampal cortex and hippocampus increased for recollection of autobiographical events relative to other memory types. Laterally, effective connectivity between the middle temporal gyrus and temporal pole increased during retrieval of general knowledge and public events. The memory types that dissociate the common system into its subsystems correspond to those that typically distinguish between patients at initial phases of Alzheimer's disease or semantic dementia. This approach, therefore, opens the door to new lines of research into memory degeneration, capitalizing on the functional integration of different memory-involved regions. Indeed, the ability to examine interregional interactions may have important diagnostic and prognostic implications.     

Hippocampal hemispheric and long‐axis differentiation of stimulus content during episodic memory encoding and retrieval: An activation likelihood estimation meta‐analysis

While there is ample evidence that the hippocampus is functionally heterogeneous along its longitudinal axis, there is still no consensus regarding its exact organization. Whereas spatial memory tasks frequently engage the posterior hippocampus, the regions engaged during episodic memory are more varying and may depend on the specific nature of the stimuli. Here, we investigate the effect of stimulus content on the location of hippocampal recruitment during episodic memory encoding and retrieval of pictorial and verbal material with a meta‐analysis approach, using activation likelihood estimation and restricting the analysis to the hippocampus. Verbal material was associated with left‐lateralized anterior activation, compared to pictorial material that recruited a more posterior aspect of the hippocampus, primarily within the right hemisphere. This effect held for encoding of both single items and item–item associations but was less clear during retrieval. The findings lend further support to a functional subdivision of the hippocampus along its longitudinal axis and indicate that the content of episodic memories is one factor that determines the location of hippocampal recruitment. © 2015 Wiley Periodicals, Inc.     

Anterior Cingulate Cortex Signals the Need to Control Intrusive Thoughts during Motivated Forgetting

How do people limit awareness of unwanted memories? When such memories intrude, a control process engages the right DLPFC (rDLPFC) to inhibit hippocampal activity and stop retrieval. It remains unknown how the need for control is detected, and whether control operates proactively to prevent unwelcome memories from being retrieved, or responds reactively, to counteract intrusions. We hypothesized that dorsal ACC (dACC) detects the emergence of an unwanted trace in awareness and transmits the need for inhibitory control to rDLPFC. During a memory suppression task, we measured in humans (both sexes) trial-by-trial variations in the theta power and N2 amplitude of dACC, two EEG markers that are thought to reflect the need for control. With simultaneous EEG-fMRI recordings, we tracked interactions among dACC, rDLPFC, and hippocampus during suppression. We found a clear role of dACC in detecting the need for memory control and upregulating prefrontal inhibition. Importantly, we identified distinct early (300–450 ms) and late (500–700 ms) dACC contributions, suggesting both proactive control before recollection and reactive control in response to intrusions. Stronger early activity was associated with reduced hippocampal activity and diminished BOLD signal in dACC and rDLPFC, suggesting that preempting retrieval reduced overall control demands. In the later window, dACC activity was larger, and effective connectivity analyses revealed robust communication from dACC to rDLPFC and from rDLPFC to hippocampus, which are tied to successful forgetting. Together, our findings support a model in which dACC detects the emergence of unwanted content, triggering top-down inhibitory control, and in which rDLPFC countermands intruding thoughts that penetrate awareness.SIGNIFICANCE STATEMENT Preventing unwanted memories from coming to mind is an adaptive ability of humans. This ability relies on inhibitory control processes in the prefrontal cortex to modulate hippocampal retrieval processes. How and when reminders to unwelcome memories come to trigger prefrontal control mechanisms remains unknown. Here we acquired neuroimaging data with both high spatial and temporal resolution as participants suppressed specific memories. We found that the anterior cingulate cortex detects the need for memory control, responding both proactively to early warning signals about unwelcome content and reactively to intrusive thoughts themselves. When unwanted traces emerge in awareness, anterior cingulate communicates with prefrontal cortex and triggers top-down inhibitory control over the hippocampus through specific neural oscillatory networks.     

Associations between sleep and episodic memory updating

Prior research shows that contextual reminders can reactivate hippocampal links to previously consolidated memories, rendering them susceptible to being updated with new information which then is reconsolidated. Studies implicate sleep in the reconsolidation of reactivated memories, but it is unknown what role sleep plays in updating of a previously consolidated trace with new information. We tracked participants' sleep during an episodic reconsolidation paradigm, first with actigraphy (Experiment 1) then with polysomnography (Experiment 2). Our paradigm involved two learning sessions and a retrieval session, each separated by 48 hr. We reminded participants of the first learning experience immediately prior to the second, which led them to update the earlier memory with elements of the later experience. In Experiment 1, less sleep after Session 1 and more sleep after Session 2 are associated with increased updating. In Experiment 2, N2 sleep spindles (SSs) after the reminder and new learning are associated with more updating, but primarily when spindle activity after Session 1 is low. Thus, total sleep time and N2 SSs contribute to sleep‐dependent updating of episodic memory. This outcome is consistent with other work connecting SS activity to the integration of novel information into existing knowledge structures, extended here with the study of how variations in sleep over successive nights contribute to this process. We discuss some possible roles of spindles in the decontextualization of hippocampal memory over time. Although much work addresses the role of sleep in the consolidation of new memories, this work uniquely addresses the contribution of sleep to the updating of a previously consolidated trace with new information.     

Hippocampal activity during recognition memory co-varies with the accuracy and confidence of source memory judgments

It has been proposed that the hippocampus selectively supports retrieval of contextual associations, but an alternative view holds that the hippocampus supports strong memories regardless of whether they contain contextual information. We employed a memory test that combined the 'Remember/Know' and source memory procedures, which allowed test items to be segregated both by memory strength (recognition accuracy) and, separately, by the quality of the contextual information that could be retrieved (indexed by the accuracy/confidence of a source memory judgment). As measured by fMRI, retrieval-related hippocampal activity tracked the quality of retrieved contextual information and not memory strength. These findings are consistent with the proposal that the hippocampus supports contextual recollection rather than recognition memory more generally.     

Memory consolidation and the hippocampus: further evidence from studies of autobiographical memory in semantic dementia and frontal variant frontotemporal dementia.

Studies of autobiographical memory in semantic dementia have found relative preservation of memories for recent rather than remote events. As semantic dementia is associated with progressive atrophy to temporal neocortex, with early asymmetric sparing of the hippocampus, this neuropsychological pattern suggests that the hippocampal complex plays a role in the acquisition and retrieval of recent memories, but is not necessary for the recall of older episodic events. In an alternative view of memory consolidation, however, the hippocampus plays a role in the retrieval of all autobiographical memories, regardless of the age of the memory [Curr. Opin. Neurobiol. 7(1997)217]. This 'multiple trace theory' predicts that patients with semantic dementia should show no effects of time in their autobiographical recall. In this article, we ask whether it is possible to reconcile the data from semantic dementia with the multiple trace theory by investigating whether the time-dependent pattern of autobiographical retrieval seen in the disease is due to (i) patients showing this effect being exceptional in their presentation; and/or (ii) patients with semantic dementia exhibiting impaired strategic retrieval from concomitant frontal damage. A series of experiments in patients with semantic dementia, the frontal variant of frontotemporal dementia and Alzheimer's disease clearly demonstrates that neither of these two factors can explain the documented effect of time seen in semantic dementia. Nonetheless, we discuss how damage to semantic knowledge could result in an autobiographical memory deficit and suggest that data from semantic dementia may be consistent with both views of hippocampal involvement in long-term memory.     

Neural bases of autobiographical support for episodic recollection of faces

Incidental retrieval of autobiographical knowledge can provide rich contextual support for episodic recollection of a recent event. We examined the neural bases of these two processes by performing fMRI scanning during a recognition memory test for faces that were unfamiliar, famous, or personally known. The presence of pre‐experimental knowledge of a face was incidental to the task, but nonetheless resulted in improved performance. Two distinct networks of activation were associated with correct recollection of a face's prior presentation (recollection hits vs. correct rejections) on one hand, and with pre‐experimental knowledge about it (famous or personally known vs. unfamiliar faces) on the other. The former included mid/posterior cingulate cortex, precuneus, and ventral striatum. The latter included bilateral hippocampus, retrosplenial, and ventromedial prefrontal cortices. Anterior and medial thalamic activations showed an interaction between both effects, driven by increased activation for recollection of unfamiliar faces. When recollecting the presentation of a famous or personally known face, hippocampal activation increased with participants' ratings of how well they felt they knew the person shown. Ventromedial prefrontal cortex showed significantly greater activation for personally known than famous faces. Our results indicate a dissociation between the involvement of retrosplenial vs. mid/posterior cingulate and precuneus in memory tasks. They also indicate that, during recognition memory experiments, the hippocampus supports incidental retrieval of pre‐experimental knowledge about the stimuli presented. This type of knowledge likely underlies the additional recollection found for prior presentation of well known stimuli compared with novel ones and may link hippocampal activation at encoding to subsequent memory performance more generally. © 2009 Wiley‐Liss, Inc.     

Hippocampal contributions to serial‐order memory

Our memories form a record not only of our experiences, but also of their temporal structure. Although memory for the temporal structure of experience likely relies on multiple neural systems, numerous studies have implicated the hippocampus in the encoding and retrieval of temporal information. This review evaluates the literature on hippocampal contributions to human serial‐order memory from the perspective of three cognitive theories: associative chaining theory, positional‐coding theory and retrieved‐context theory. Evaluating neural findings through the lens of cognitive theories enables us to draw more incisive conclusions about the relations between brain and behavior.     

Hippocampal complex and retrieval of recent and very remote autobiographical memories: evidence from functional magnetic resonance imaging in neurologically intact people.

It has been argued that the role of the hippocampus in memory is time-limited: during a period of memory consolidation, other brain regions such as the neocortex are said to acquire the ability to support memory retention and retrieval on their own. An alternative view is that retention and retrieval of memory for autobiographical episodes depend on the hippocampal complex, regardless of the age of the memory. We examined the participation of the hippocampal complex in a functional magnetic resonance imaging (fMRI) study in which participants were asked to recollect autobiographical events that occurred either within the last 4 years or more than 20 years ago. We found equivalent levels of hippocampal activation in both conditions in all participants (N = 10). In addition, activation in neocortical regions did not differ as a function of the age of the memory, even though most of the recent memories recalled were less than 2 years old and the remote memories more than 35 years old. The results support the notion that the hippocampal complex participates in retention and recovery of even very old autobiographical memories, and place boundary conditions on theories of memory consolidation.     

Muscarinic (M1) cholinergic receptor activation within the dorsal hippocampus promotes destabilization of strongly encoded object location memories

Following the initial consolidation process, memories can become reactivated by exposure to a reminder of the original learning event. This can lead to the memory becoming destabilized and vulnerable to disruption or other forms of modification. The memory must then undergo the protein-synthesis dependent process of reconsolidation in order to be retained. However, older and/or stronger memories resist destabilization, but can become labile when reactivated in the presence of salient novelty. We have implicated the neurotransmitter acetylcholine, acting at M₁ muscarinic cholinergic receptors (mAChRs) within perirhinal cortex (PRh), in novelty-induced destabilization of remote object memories. It remains unclear, however, whether mAChRs are involved in destabilization of other forms of memory. We hypothesized that the role of M₁ mAChRs previously demonstrated for PRh-dependent object memory would extend to hippocampus-dependent spatial memory. Using the object location (OL) task, which relies on the dorsal hippocampus (dHPC), we showed that (a) reactivation-dependent reconsolidation of OL memories requires protein synthesis within the dHPC; (b) destabilization of relatively weak OL memories depends on M₁ mAChR activation within the dHPC; (c) salient novelty during reactivation promotes destabilization of resistant strongly encoded OL memories; (d) novelty-induced destabilization of strong OL memories requires activation of mAChRs within the dHPC; and (e) M₁ mAChR activation within the dHPC in the absence of novelty during memory reactivation mimics the effect of novelty, destabilizing strongly encoded OL memories. These results implicate ACh acting at M₁ mAChRs in the destabilization of dHPC-dependent spatial memories, demonstrating generalizability of this cholinergic function beyond memory for object identity. These findings therefore enhance our understanding of the dynamics of long-term memory storage and suggest implications for the treatment of human conditions such as Alzheimer's disease and aging, which are characterized by behavioral and mnemonic inflexibility.     

Differential modulation of a common memory retrieval network revealed by positron emission tomography

Functional neuroimaging is uniquely placed to examine the dynamic nature of normal human memory, the distributed brain networks that support it, and how they are modulated. Memory has traditionally been classified into context-specific memories personally experienced ("episodic memory") and impersonal non-context-specific memories ("semantic memory"). However, we suggest that another useful distinction is whether events are personally relevant or not. Typically the factors of personal relevance and temporal context are confounded, and it is as yet not clear the precise influence of either on how memories are stored or retrieved. Here we focus on the retrieval of real-world memories unconfounding personal relevance and temporal context during positron emission tomography (PET) scanning. Memories differed along two dimensions: They were personally relevant (or not) and had temporal specificity (or not). Recollection of each of the resultant four memory subtypes-autobiographical events, public events, autobiographical facts, and general knowledge-was associated with activation of a common network of brain regions. Within this system, however, enhanced activity was observed for retrieval of personally relevant, time-specific memories in left hippocampus, medial prefrontal cortex, and left temporal pole. Bilateral temporoparietal junctions were activated preferentially for personal memories, regardless of time specificity. Finally, left parahippocampal gyrus, left anterolateral temporal cortex, and posterior cingulate cortex were involved in memory retrieval irrespective of person or time. Our findings suggest that specializations in memory retrieval result from associations between subsets of regions within a common network. We believe that these findings throw new light on an old debate surrounding episodic and declarative theories of memory and the precise involvement of the hippocampus.     

CBP‐Dependent memory consolidation in the prefrontal cortex supports object‐location learning

Recognition of an object's location in space is supported by hippocampus‐dependent recollection. Converging evidence strongly suggests that the interplay between the prefrontal cortex and hippocampus is critical for spatial memory. Lesion, pharmacological, and genetic studies have been successful in dissecting the role of plasticity in the hippocampal circuit in a variety of neural processes relevant to spatial memory, including memory for the location of objects. However, prefrontal mechanisms underlying spatial memory are less well understood. Here, we show that an acute hypofunction of the cyclic‐AMP regulatory element binding protein (CREB) Binding Protein (CBP) histone acetyltransferase (HAT) in the medial prefrontal cortex (mPFC) results in delay‐dependent disruption of object‐location memory. These data suggest that mechanisms involving CBP HAT‐mediated lysine acetylation of nuclear proteins support selectively long‐term encoding in the mPFC circuits. Evidence from the object‐location task suggests that long‐term memory encoding within the mPFC complements hippocampus‐dependent spatial memory mechanisms and may be critical for broader network integration of information necessary for an assessment of subtle spatial differences to guide appropriate behavioral response during retrieval of spatial memories. © 2015 Wiley Periodicals, Inc.     

A single‐trace dual‐process model of episodic memory: A novel computational account of familiarity and recollection

Dual‐process theories of episodic memory state that retrieval is contingent on two independent processes: familiarity (providing a sense of oldness) and recollection (recovering events and their context). A variety of studies have reported distinct neural signatures for familiarity and recollection, supporting dual‐process theory. One outstanding question is whether these signatures reflect the activation of distinct memory traces or the operation of different retrieval mechanisms on a single memory trace. We present a computational model that uses a single neuronal network to store memory traces, but two distinct and independent retrieval processes access the memory. The model is capable of performing familiarity and recollection‐based discrimination between old and new patterns, demonstrating that dual‐process models need not to rely on multiple independent memory traces, but can use a single trace. Importantly, our putative familiarity and recollection processes exhibit distinct characteristics analogous to those found in empirical data; they diverge in capacity and sensitivity to sparse and correlated patterns, exhibit distinct ROC curves, and account for performance on both item and associative recognition tests. The demonstration that a single‐trace, dual‐process model can account for a range of empirical findings highlights the importance of distinguishing between neuronal processes and the neuronal representations on which they operate. © 2009 Wiley‐Liss, Inc.     

Encoding and retrieval along the long axis of the hippocampus and their relationships with dorsal attention and default mode networks: The HERNET model

The encoding of sensory input is intertwined with external attention, whereas retrieval is intrinsically related to internal attention. This study proposes a model in which the encoding of sensory input involves mainly the anterior hippocampus and the external attention network, whereas retrieval, the posterior hippocampus and the internal attention network. This model is referred to as the HERNET (hippocampal encoding/retrieval and network) model. Functional neuroimaging studies have identified two intrinsic large‐scale networks closely associated with external and internal attention, respectively. The dorsal attention network activates during any externally oriented mental activity, whereas the default mode network shows increased activity during internally oriented mental activity. Therefore, the HERNET model may predict the activation of the anterior hippocampus and the dorsal attention network during the encoding and activation of the posterior hippocampus and the default mode network during retrieval. To test this prediction, this study provides a meta‐analysis of three memory‐imaging paradigms: subsequent memory, laboratory‐based recollection, and autobiographical memory retrieval. The meta‐analysis included 167 individual studies and 2,856 participants. The results provide support for the HERNET model and suggest that the anterior–posterior gradient of encoding and retrieval includes amygdala regions. More broadly, humans continuously oscillate between external and internal attention and thus between encoding and retrieval processes. These oscillations may involve repetitive and spontaneous activity switching between the anterior hippocampus/dorsal attention network and the posterior hippocampus/default mode network. © 2014 Wiley Periodicals, Inc.     

Different reactivation procedures enable or prevent episodic memory updating

The present study asked whether the specific method of reactivation modulates the impact of new learning on reactivated episodic memories. The study consisted of three sessions that were spaced 48 hr apart. It used an ABAC paradigm that allowed for the simultaneous assessment of retroactive interference (RI: reduced A–B recall after A–C learning) and intrusions from C into A–B recall. In Session 1, participants learned a list of paired‐associates A–B. In Session 2, memory for A–B was reactivated or not and then participants either learned a second list of paired‐associates A–C or completed a control task. Three different reminder conditions were compared to a no‐reminder condition: a test condition, in which participants were asked to recall B in response to A, a restudy condition, in which A–B pairs were presented again for study, and a cue‐word only reminder condition, in which A cues were presented in an unrelated rating task. In Session 3, recall of A–B was tested. Moderate or indirect reactivation of A–B (presentation of cue‐words only) resulted in high RI effects and intrusion rates, whereas strong and direct reactivation (test and restudy) drastically reduced these effects. We suggest that direct reactivation of A–B before A–C learning strengthens memory and draws attention to list differences, thereby enhancing list segregation, and reducing memory updating.