Reversible information flow across the medial temporal lobe: the hippocampus links cortical modules during memory retrieval

Intracranial EEGs evidenced visual object processing in the human medial temporal lobe subregions

The perirhinal cortex (PRC) and parahippocampal cortex (PHC) are core regions along the visual dual-stream. The specific functional roles of the PRC and PHC and their interactions with the downstream hippocampus cortex (HPC) are crucial for understanding visual memory. Our research used human intracranial EEGs to study the neural mechanism of the PRC, PHC, and HPC in visual object encoding. Single-regional function analyses found evidence that the PRC, PHC, and HPC are activated ∼100 ms within the broad-gamma band and that the PRC was more strongly activated than either the PHC or the HPC after an object stimulus. Inter-regional analyses showed strong bidirectional interactions of the PRC with both the PHC and HPC in the low-frequency band, whereas the interactions between the PHC and HPC were not significant. These findings demonstrated the core role of the PRC in encoding visual object information and supported the hypothesis of PRC-HPC-ventral object pathway. The recruitment of the PHC and its interaction with the PRC in visual object encoding also provide new insights beyond the traditional dorsal-stream hypothesis.

Embodiment in episodic memory through premotor-hippocampal coupling

Episodic memory (EM) allows us to remember and relive past events and experiences and has been linked to cortical-hippocampal reinstatement of encoding activity. While EM is fundamental to establish a sense of self across time, this claim and its link to the sense of agency (SoA), based on bodily signals, has not been tested experimentally. Using real-time sensorimotor stimulation, immersive virtual reality, and fMRI we manipulated the SoA and report stronger hippocampal reinstatement for scenes encoded under preserved SoA, reflecting recall performance in a recognition task. We link SoA to EM showing that hippocampal reinstatement is coupled with reinstatement in premotor cortex, a key SoA region. We extend these findings in a severe amnesic patient whose memory lacked the normal dependency on the SoA. Premotor-hippocampal coupling in EM describes how a key aspect of the bodily self at encoding is neurally reinstated during the retrieval of past episodes, enabling a sense of self across time.

A third kind of episodic memory: Context familiarity is a distinct process from item familiarity and recollection

Episodic memory is accounted for with two processes: 'familiarity' when generally recognizing an item and 'recollection' when retrieving the full contextual details bound with the item. Paradoxically, people sometimes report contextual information as familiar but without recollecting details, which is not easily accounted for by existing theories. We tested a combination of item recognition confidence and source memory, focusing upon 'item-only hits with source unknown' ('item familiarity'), 'low-confidence hits with correct source memory' ('context familiarity'), and 'high-confidence hits with correct source memory' ('recollection'). Results across multiple within-subjects (trial-wise) and between subjects (individual variability) levels indicated these were behaviorally and physiologically distinct. Behaviorally, a crossover interaction was evident in response times, with context familiarity being slower than each condition during item recognition, but faster during source memory. Electrophysiologically, a Condition x Time x Location triple dissociation was evident in event-related potentials (ERPs), which was then independently replicated. Context familiarity exhibited an independent negative central effect from 800-1200 ms, differentiated from positive ERPs for item-familiarity (400 to 600 ms) and recollection (600 to 900 ms). These three conditions thus reflect mutually exclusive, fundamentally different processes of episodic memory. Context familiarity is a third distinct process of episodic memory.

Significance statement/Summary

Memory for past events is widely believed to operate through two different processes: one called 'recollection' when retrieving confident, specific details of a memory, and another called 'familiarity' when only having an unsure but conscious awareness that an item was experienced before. When people successfully retrieve details such as the source or context of a prior event, it has been assumed to reflect recollection. We demonstrate that familiarity of context is functionally distinct from familiarity of items and recollection and offer a new trivariate model. The three memory responses were differentiated across multiple behavioral and brain wave measures. What has traditionally been thought to be two kinds of memory processes are actually three, becoming evident when using sensitive enough multi-measures. Results are independently replicated across studies from different labs. These data reveal that context familiarity is a third process of human episodic memory.

The locus of recognition memory signals in human cortex depends on the complexity of the memory representations

According to a "Swiss Army Knife" model of the brain, cognitive functions such as episodic memory and face perception map onto distinct neural substrates. In contrast, representational accounts propose that each brain region is best explained not by which specialized function it performs, but by the type of information it represents with its neural firing. In a functional magnetic resonance imaging study, we asked whether the neural signals supporting recognition memory fall mandatorily within the medial temporal lobes (MTL), traditionally thought the seat of declarative memory, or whether these signals shift within cortex according to the content of the memory. Participants studied objects and scenes that were unique conjunctions of pre-defined visual features. Next, we tested recognition memory in a task that required mnemonic discrimination of both simple features and complex conjunctions. Feature memory signals were strongest in posterior visual regions, declining with anterior progression toward the MTL, while conjunction memory signals followed the opposite pattern. Moreover, feature memory signals correlated with feature memory discrimination performance most strongly in posterior visual regions, whereas conjunction memory signals correlated with conjunction memory discrimination most strongly in anterior sites. Thus, recognition memory signals shifted with changes in memory content, in line with representational accounts.

Disruptions in network plasticity precede deficits in memory following inhibition of retinoid signaling

Retinoic acid, the active metabolite of vitamin A, is important for vertebrate cognition and hippocampal plasticity, but few studies have examined its role in invertebrate learning and memory, and its actions in the invertebrate central nervous system are currently unknown. Using the mollusc Lymnaea stagnalis, we examined operant conditioning of the respiratory behavior, controlled by a well-defined central pattern generator (CPG), and used citral to inhibit retinoic acid signaling. Both citral- and vehicle-treated animals showed normal learning, but citral-treated animals failed to exhibit long-term memory at 24 h. Cohorts of citral- or vehicle-treated animals were dissected into semi-intact preparations, either 1 h after training, or after the memory test 24 h later. Simultaneous electrophysiological recordings from the CPG pacemaker cell (right pedal dorsal 1; RPeD1) and an identified motorneuron (VI) were made while monitoring respiratory activity (pneumostome opening). Activity of the CPG pneumostome opener interneuron (input 3 interneuron; IP3) was also monitored indirectly. Vehicle-treated conditioned preparations showed significant changes in network parameters immediately after learning, such as reduced motorneuron bursting activity (from IP3 input), delayed pneumostome opening, and decoupling of coincident IP3 input within the network. However, citral-treated preparations failed to exhibit these network changes and more closely resembled naïve preparations. Importantly, these citral-induced differences were manifested immediately after training and before any overt changes in the behavioral response (memory impairment). These studies shed light on where and when retinoid signaling might affect a central pattern-generating network to promote memory formation during conditioning of a homeostatic behavior.NEW & NOTEWORTHY We provide novel evidence for how conditioning-induced changes in a CPG network are disrupted when retinoid signaling is inhibited. Inhibition of retinoic acid signaling prevents long-term memory formation following operant conditioning, but has no effect on learning. Simultaneous electrophysiological and behavioral analyses indicate network changes immediately following learning, but these changes are prevented with inhibition of retinoid signaling, before any overt changes in behavior. These data suggest sites for retinoid actions during memory formation.

Patterns of episodic content and specificity predicting subjective memory vividness

The ability to remember and internally represent events is often accompanied by a subjective sense of "vividness". Vividness measures are frequently used to evaluate the experience of remembering and imagining events, yet little research has considered the objective attributes of event memories that underlie this subjective judgment, and individual differences in this mapping. Here, we tested how the content and specificity of event memories support subjectively vivid recollection. Over three experiments, participants encoded events containing a theme word and three distinct elements - a person, a place, and an object. In a memory test, memory for event elements was assessed at two levels of specificity - semantic gist (names) and perceptual details (lure discrimination). We found a strong correspondence between memory vividness and memory for gist information that did not vary by which elements were contained in memory. There was a smaller, additive benefit of remembering specific perceptual details on vividness, which, in one study, was driven by memory for place details. Moreover, we found individual differences in the relationship between memory vividness and objective memory attributes primarily along the specificity dimension, such that one cluster of participants used perceptual detail to inform memory vividness whereas another cluster was more driven by gist information. Therefore, while gist memory appears to drive vividness on average, there were idiosyncrasies in this pattern across participants. When assessing subjective ratings of memory and imagination, research should consider how these ratings map onto objective memory attributes in the context of their study design and population.

Category-specific memory encoding in the medial temporal lobe and beyond: the role of reward

The medial temporal lobe (MTL), including the hippocampus (HC), perirhinal cortex (PRC), and parahippocampal cortex (PHC), is central to memory formation. Reward enhances memory through interplay between the HC and substantia nigra/ventral tegmental area (SNVTA). While the SNVTA also innervates the MTL cortex and amygdala (AMY), their role in reward-enhanced memory is unclear. Prior research suggests category specificity in the MTL cortex, with the PRC and PHC processing object and scene memory, respectively. It is unknown, however, whether reward modulates category-specific memory processes. Furthermore, no study has demonstrated clear category specificity in the MTL for encoding processes contributing to subsequent recognition memory. To address these questions, we had 39 healthy volunteers (27 for all memory-based analyses) undergo functional magnetic resonance imaging while performing an incidental encoding task pairing objects or scenes with high or low reward, followed by a next-day recognition test. Behaviorally, high reward preferably enhanced object memory. Neural activity in the PRC and PHC reflected successful encoding of objects and scenes, respectively. Importantly, AMY encoding effects were selective for high-reward objects, with a similar pattern in the PRC. The SNVTA and HC showed no clear evidence of successful encoding. This behavioral and neural asymmetry may be conveyed through an anterior-temporal memory system, including the AMY and PRC, potentially in interplay with the ventromedial prefrontal cortex.

Brief memory reactivations induce learning in the numeric domain

Learning of arithmetic facts such as the multiplication table requires time-consuming, repeated practice. In light of evidence indicating that reactivation of encoded memories can modulate learning and memory processes at the synaptic, system and behavioral levels, we asked whether brief memory reactivations can induce human learning in the numeric domain. Adult participants performed a number-fact retrieval task in which they learned arbitrary numeric facts. Following encoding and a baseline test, 3 passive, brief reactivation sessions of only 40 s each were conducted on separate days. Learning was evaluated in a retest session. Results showed reactivations induced learning, with improved performance at retest relative to baseline test. Furthermore, performance was superior compared to a control group performing test-retest sessions without reactivations, who showed significant memory deterioration. A standard practice group completed active-retrieval sessions on 3 separate days, and showed significant learning gains. Interestingly, while these gains were higher than those of the reactivations group, subjects showing reactivation-induced learning were characterized by superior efficiency relative to standard practice subjects, with higher rate of improvement per practice time. A follow-up long-term retention experiment showed that 30 days following initial practice, weekly brief reactivations reduced forgetting, with participants performing superior to controls undergoing the same initial practice without reactivations. Overall, the results demonstrate that brief passive reactivations induce efficient learning and reduce forgetting within a numerical context. Time-efficient practice in the numeric domain carries implications for enhancement of learning strategies in daily-life settings.

Category-sensitive incidental reinstatement in medial temporal lobe subregions during word recognition

During associative retrieval, the brain reinstates neural representations that were present during encoding. The human medial temporal lobe (MTL), with its subregions hippocampus (HC), perirhinal cortex (PRC), and parahippocampal cortex (PHC), plays a central role in neural reinstatement. Previous studies have given compelling evidence for reinstatement in the MTL during explicitly instructed associative retrieval. High-confident recognition may be similarly accompanied by recollection of associated information from the encoding context. It is unclear, however, whether high-confident recognition memory elicits reinstatement in the MTL even in the absence of an explicit instruction to retrieve associated information. Here, we addressed this open question using high-resolution fMRI. Twenty-eight male and female human volunteers engaged in a recognition memory task for words that they had previously encoded together with faces and scenes. Using complementary univariate and multivariate approaches, we show that MTL subregions including the PRC, PHC, and HC differentially reinstate category-sensitive representations during high-confident word recognition, even though no explicit instruction to retrieve the associated category was given. This constitutes novel evidence that high-confident recognition memory is accompanied by incidental reinstatement of associated category information in MTL subregions, and supports a functional model of the MTL that emphasizes content-sensitive representations during both encoding and retrieval.

Brain activity patterns underlying memory confidence

The primary aim of this review is to examine the brain activity patterns that are related to subjectively perceived memory confidence. We focus on the main brain regions involved in episodic memory: the medial temporal lobe (MTL), prefrontal cortex (PFC), and posterior parietal cortex (PPC), and relate activity in their subregions to memory confidence. How this brain activity in both the encoding and retrieval phase is related to (subsequent) memory confidence ratings will be discussed. Specifically, encoding related activity in MTL regions and ventrolateral PFC mainly shows a positive linear increase with subsequent memory confidence, while dorsolateral and ventromedial PFC activity show mixed patterns. In addition, encoding-related PPC activity seems to only have indirect effects on memory confidence ratings. Activity during retrieval in both the hippocampus and parahippocampal cortex increases with memory confidence, especially during high-confident recognition. Retrieval-related activity in the PFC and PPC show mixed relationships with memory confidence, likely related to post-retrieval monitoring and attentional processes, respectively. In this review, these MTL, PFC, and PPC activity patterns are examined in detail and related to their functional roles in memory processes. This insight into brain activity that underlies memory confidence is important for our understanding of brain-behaviour relations and memory-guided decision making.

The hippocampus as the switchboard between perception and memory

Adaptive memory recall requires a rapid and flexible switch from external perceptual reminders to internal mnemonic representations. However, owing to the limited temporal or spatial resolution of brain imaging modalities used in isolation, the hippocampal-cortical dynamics supporting this process remain unknown. We thus employed an object-scene cued recall paradigm across two studies, including intracranial electroencephalography (iEEG) and high-density scalp EEG. First, a sustained increase in hippocampal high gamma power (55 to 110 Hz) emerged 500 ms after cue onset and distinguished successful vs. unsuccessful recall. This increase in gamma power for successful recall was followed by a decrease in hippocampal alpha power (8 to 12 Hz). Intriguingly, the hippocampal gamma power increase marked the moment at which extrahippocampal activation patterns shifted from perceptual cue toward mnemonic target representations. In parallel, source-localized EEG alpha power revealed that the recall signal progresses from hippocampus to posterior parietal cortex and then to medial prefrontal cortex. Together, these results identify the hippocampus as the switchboard between perception and memory and elucidate the ensuing hippocampal-cortical dynamics supporting the recall process.

Brain Mechanisms Underlying the Subjective Experience of Remembering

The ability to remember events in vivid, multisensory detail is a significant part of human experience, allowing us to relive previous encounters and providing us with the store of memories that shape our identity. Recent research has sought to understand the subjective experience of remembering, that is, what it feels like to have a memory. Such remembering involves reactivating sensory-perceptual features of an event and the thoughts and feelings we had when the event occurred, integrating them into a conscious first-person experience. It allows us to reflect on the content of our memories and to understand and make judgments about them, such as distinguishing events that actually occurred from those we might have imagined or been told about. In this review, we consider recent evidence from functional neuroimaging in healthy participants and studies of neurological and psychiatric conditions, which is shedding new light on how we subjectively experience remembering. Expected final online publication date for the Annual Review of Psychology, Volume 73 is January 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Neuromodulation of Memory Formation and Extinction

Memory retrieval is mediated by discharges of acetylcholine, glutamate, gammaaminobutyric acid, norepinephrine, and serotonin/5-hydroxytryptamine circuits. These projections and memory interact through engram circuits, neurobiological traces of memory. Increased excitability in engram circuits of the medial prefrontal cortex and hippocampus results in remote and recent memory retrievals, respectively. However, due to degenerated neurotransmitter projections, the excitability state of engram circuits is decreased in the patient with dementia; and thus, acquired- memory cannot be retrieved by natural cues. Here, we suggest that artificial neuropharmacological stimulations of the acquired-memory with an excitation potential higher than a natural cue can excite engram circuits in the medial prefrontal cortex, which results in the retrieval of lost memories in dementia. The neuropharmacological foundations of engram cell-mediated memory retrieval strategy in severe dementia, in line with this has also been explained. We particularly highlighted the close interactions between periaqueductal gray, locus coeruleus, raphe nuclei, and medial prefrontal cortex and basolateral amygdala as treatment targets for memory loss. Furthermore, the engram circuits projecting raphe nuclei, locus coeruleus, and pontomesencephalic tegmentum complex could be significant targets of memory editing and memory formation in the absence of experience, and a well-defined study of the neural events underlying the interaction of brain stem and memory will be relevant for such developments. We anticipate our perspective to be a starting point for more sophisticated in vivo models for neuropharmacological modulations of memory retrieval in Alzheimer's dementia.

Functional Specialization of the Medial Temporal Lobes in Human Recognition Memory: Dissociating Effects of Hippocampal versus Parahippocampal Damage

A central debate in the systems neuroscience of memory concerns whether different medial temporal lobe (MTL) structures support different processes in recognition memory. Using two recognition memory paradigms, we tested a rare patient (MH) with a perirhinal lesion that appeared to spare the hippocampus. Consistent with a similar previous case, MH showed impaired familiarity and preserved recollection. When compared with patients with hippocampal lesions appearing to spare perirhinal cortex, MH showed greater impairment on familiarity and less on recollection. Nevertheless, the hippocampal patients also showed impaired familiarity compared with healthy controls. However, when replacing this traditional categorization of patients with analyses relating memory performance to continuous measures of damage across patients, hippocampal volume uniquely predicted recollection, whereas parahippocampal, rather than perirhinal, volume uniquely predicted familiarity. We consider whether the familiarity impairment in MH and our patients with hippocampal lesions arises from "subthreshold" damage to parahippocampal cortex (PHC). Our data provide the most compelling neuropsychological support yet for dual-process models of recognition memory, whereby recollection and familiarity depend on different MTL structures, and may support a role for PHC in familiarity. Our study highlights the value of supplementing single-case studies with examinations of continuous brain-behavior relationships across larger patient groups.

Moment-by-moment tracking of naturalistic learning and its underlying hippocampo-cortical interactions

Humans form lasting memories of stimuli that were only encountered once. This naturally occurs when listening to a story, however it remains unclear how and when memories are stored and retrieved during story-listening. Here, we first confirm in behavioral experiments that participants can learn about the structure of a story after a single exposure and are able to recall upcoming words when the story is presented again. We then track mnemonic information in high frequency activity (70–200 Hz) as patients undergoing electrocorticographic recordings listen twice to the same story. We demonstrate predictive recall of upcoming information through neural responses in auditory processing regions. This neural measure correlates with behavioral measures of event segmentation and learning. Event boundaries are linked to information flow from cortex to hippocampus. When listening for a second time, information flow from hippocampus to cortex precedes moments of predictive recall. These results provide insight on a fine-grained temporal scale into how episodic memory encoding and retrieval work under naturalistic conditions.

When listening to a story, humans learn about its structure and content. Here the authors reveal the neural processes behind episodic memory and predictive recall at a fine temporal scale in this naturalistic setting

Features of intracranial interictal epileptiform discharges associated with memory encoding

OBJECTIVE

Interictal epileptiform discharges (IEDs) were shown to be associated with cognitive impairment in persons with epilepsy. Previous studies indicated that IED rate, location, timing, and spatial relation to the seizure onset zone could predict an IED's impact on memory encoding and retrieval if they occurred in lateral temporal, mesial temporal, or parietal regions. In this study, we explore the influence that other IED properties (e.g., amplitude, duration, white matter classification) have on memory performance. We were specifically interested in investigating the influence that lateral temporal IEDs have on memory encoding.

METHODS

Two hundred sixty-one subjects with medication-refractory epilepsy undergoing intracranial electroencephalographic monitoring performed multiple sessions of a delayed free-recall task (n = 671). Generalized linear mixed models were utilized to examine the relationship between IED properties and memory performance.

RESULTS

We found that increased IED rate, IEDs propagating in white matter, and IEDs localized to the left middle temporal region were associated with poorer memory performance. For lateral temporal IEDs, we observed a significant interaction between IED white matter categorization and amplitude, where IEDs with an increased amplitude and white matter propagation were associated with reduced memory performance. Additionally, changes in alpha power after an IED showed a significant positive correlation with memory performance.

SIGNIFICANCE

Our results suggest that IED properties may be useful for predicting the impact an IED has on memory encoding. We provide an essential step toward understanding pathological versus potentially beneficial interictal epileptiform activity.

Distinct patterns of hippocampal activity associated with color and spatial source memory

The hippocampus is known to be involved in source memory across a wide variety of stimuli and source types. Thus, source memory activity in the hippocampus is thought to be domain-general such that different types of source information are similarly processed in the hippocampus. However, there is some evidence of domain-specificity for spatial and temporal source information. The current fMRI study aimed to determine whether patterns of activity in the hippocampus differed for two types of visual source information: spatial location and background color. Participants completed three runs of a spatial memory task and three runs of a color memory task. During the study phase, 32 line drawings of common objects and animals were presented to either the left or right of fixation for the spatial memory task or on either a red or green background for the color memory task. During the test phase of both tasks, 48 object word labels were presented in the center of the screen and participants classified the corresponding item as old and previously on the "left"/on a "green" background, old and previously on the "right"/on a "red" background, or "new." Two analysis methods were employed to assess whether hippocampal activity differed between the two source types: a general linear model analysis and a classification-based searchlight multivoxel pattern analysis (MVPA). The searchlight MVPA revealed that activity associated with spatial memory and color memory could be classified with above-chance accuracy in a region of the right anterior hippocampus, and a follow-up analysis revealed that there was a significant effect of memory accuracy. These results indicate that different types of source memory are represented by distinct patterns of activity in the hippocampus.

Forgetting across a hierarchy of episodic representations

Rich episodic experiences are represented in a hierarchical manner across a diverse network of brain regions, and as such, the way in which episodes are forgotten is likely to be similarly diverse. Using novel experimental approaches and statistical modelling, recent research has suggested that item-based representations, such as ones related to the colour and shape of an object, fragment over time, whereas higher-order event-based representations may be forgotten in a more 'holistic' uniform manner. We propose a framework that reconciles these findings, where complex episodes are represented in a hierarchical manner, from individual items, to small-scale events, to large-scale episodic narratives. Each level in the hierarchy is represented in distinct brain regions, from the perirhinal cortex, to posterior hippocampus, to anterior hippocampus and ventromedial prefrontal cortex. Critically, forgetting may be underpinned by different mechanisms at each level in the hierarchy, leading to different patterns of behaviour.

Mechanisms of memory: An intermediate level of analysis and organization

Research in the last five years has made great strides toward mechanistic explanations of how the brain enables memory. This progress builds upon decades of research from two complementary strands: a Levels of Analysis approach and a Levels of Organization approach. We review how research in cognitive psychology and cognitive neuroscience under these two approaches has recently converged on mechanistic, brain-based theories, couched at the optimal level for explaining cognitive phenomena - the intermediate level. Furthermore, novel empirical and data analysis techniques are now providing ways to test these theories' predictions, a crucial step in unraveling the mechanisms of memory.

Reactivated Spatial Context Guides Episodic Recall

The medial temporal lobe (MTL) is known as the locus of spatial coding and episodic memory, but the interaction between these cognitive domains as well as the extent to which they rely on common neurophysiological mechanisms is poorly understood. Here, we use intracranial electroencephalography and a hybrid spatial-episodic memory task (29 subjects, 15 female) to determine how spatial information is dynamically reactivated in subregions of the human MTL and how this reactivation guides recall of episodic information. Our results implicate theta oscillations across the MTL as a common neurophysiological substrate for spatial coding in navigation and episodic recall. We further show that our index of retrieved spatial context is high in the hippocampus (HC) in an early time window preceding recall. Closer to recall, it decreases in the HC and increases in the parahippocampal gyrus. Finally, we demonstrate that hippocampal theta phase modulates parahippocampal gamma amplitude during retrieval of spatial context, suggesting a role for cross-frequency coupling in coding and transmitting retrieved spatial information.SIGNIFICANCE STATEMENT By recording from the human medial temporal lobe (MTL) while subjects recall items experienced in a virtual environment, we establish a direct relation between the strength of theta activity during memory search and the extent to which memories are organized by their spatial locations. We thereby pinpoint a role for theta oscillations in accessing the "cognitive map" during episodic retrieval and further highlight the dynamic interplay of hippocampus and extrahippocampal MTL in representing retrieved spatial context. Our results provide an important step toward a unified theory of MTL function encompassing its role in spatial navigation and episodic memory.

Hippocampal Engagement during Recall Depends on Memory Content

The hippocampus is considered pivotal to recall, allowing retrieval of information not available in the immediate environment. In contrast, neocortex is thought to signal familiarity, contributing to recall only when called upon by the hippocampus. However, this view is not compatible with representational accounts of memory, which reject the mapping of cognitive processes onto brain regions. According to representational accounts, the hippocampus is not engaged by recall per se, rather it is engaged whenever hippocampal representations are required. To test whether hippocampus is engaged by recall when hippocampal representations are not required, we used functional imaging and a non-associative recall task, with images (objects, scenes) studied in isolation, and image patches as cues. As predicted by a representational account, hippocampal activation was modulated by the content of the recalled memory, increasing during recall of scenes-which are known to be processed by hippocampus-but not during recall of objects. Object recall instead engaged neocortical regions known to be involved in object-processing. Further supporting the representational account, effective connectivity analyses revealed that changes in functional activation during recall were driven by increased information flow from neocortical sites, rather than by the spreading of recall-related activation from hippocampus back to neocortex.

The neurobiological foundation of memory retrieval

Memory retrieval involves the interaction between external sensory or internally generated cues and stored memory traces (or engrams) in a process termed 'ecphory'. While ecphory has been examined in human cognitive neuroscience research, its neurobiological foundation is less understood. To the extent that ecphory involves 'reawakening' of engrams, leveraging recently developed technologies that can identify and manipulate engrams in rodents provides a fertile avenue for examining retrieval at the level of neuronal ensembles. Here we evaluate emerging neuroscientific research of this type, using cognitive theory as a guiding principle to organize and interpret initial findings. Our Review highlights the critical interaction between engrams and retrieval cues (environmental or artificial) for memory accessibility and retrieval success. These findings also highlight the intimate relationship between the mechanisms important in forming engrams and those important in their recovery, as captured in the cognitive notion of 'encoding specificity'. Finally, we identify several questions that currently remain unanswered.

Content Tuning in the Medial Temporal Lobe Cortex: Voxels that Perceive, Retrieve

How do we recall vivid details from our past based only on sparse cues? Research suggests that the phenomenological reinstatement of past experiences is accompanied by neural reinstatement of the original percept. This process critically depends on the medial temporal lobe (MTL). Within the MTL, perirhinal cortex (PRC) and parahippocampal cortex (PHC) are thought to support encoding and recall of objects and scenes, respectively, with the hippocampus (HC) serving as a content-independent hub. If the fidelity of recall indeed arises from neural reinstatement of perceptual activity, then successful recall should preferentially draw upon those neural populations within content-sensitive MTL cortex that are tuned to the same content during perception. We tested this hypothesis by having eighteen human participants undergo functional MRI (fMRI) while they encoded and recalled objects and scenes paired with words. Critically, recall was cued with the words only. While HC distinguished successful from unsuccessful recall of both objects and scenes, PRC and PHC were preferentially engaged during successful versus unsuccessful object and scene recall, respectively. Importantly, within PRC and PHC, this content-sensitive recall was predicted by content tuning during perception: Across PRC voxels, we observed a positive relationship between object tuning during perception and successful object recall, while across PHC voxels, we observed a positive relationship between scene tuning during perception and successful scene recall. Our results thus highlight content-based roles of MTL cortical regions for episodic memory and reveal a direct mapping between content-specific tuning during perception and successful recall.

Holistic Recollection via Pattern Completion Involves Hippocampal Subfield CA3

Episodic memories typically comprise multiple elements. A defining characteristic of episodic retrieval is holistic recollection, i.e., comprehensive recall of the elements a memorized event encompasses. A recent study implicated activity in the human hippocampus with holistic recollection of multi-element events based on cues (Horner et al., 2015). Here, we obtained ultra-high resolution functional neuroimaging data at 7 tesla in 30 younger adults (12 female) using the same paradigm. In accordance with anatomically inspired computational models and animal research, we found that metabolic activity in hippocampal subfield CA3 (but less pronounced in dentate gyrus) correlated with this form of mnemonic pattern completion across participants. Our study provides the first evidence in humans for a strong involvement of hippocampal subfield CA3 in holistic recollection via pattern completion.

SIGNIFICANCE STATEMENT Memories of daily events usually involve multiple elements, although a single element can be sufficient to prompt recollection of the whole event. Such holistic recollection is thought to require reactivation of brain activity representing the full event from one event element (“pattern completion”). Computational and animal models suggest that mnemonic pattern completion is accomplished in a specific subregion of the hippocampus called CA3, but empirical evidence in humans was lacking. Here, we leverage the ultra-high resolution of 7 tesla neuroimaging to provide first evidence for a strong involvement of the human CA3 in holistic recollection of multi-element events via pattern completion.

A Roadmap for Understanding Memory: Decomposing Cognitive Processes into Operations and Representations

Thanks to patients Phineas Gage and Henry Molaison, we have long known that behavioral control depends on the frontal lobes, whereas declarative memory depends on the medial temporal lobes (MTL). For decades, cognitive functions-behavioral control, declarative memory-have served as labels for characterizing the division of labor in cortex. This approach has made enormous contributions to understanding how the brain enables the mind, providing a systems-level explanation of brain function that constrains lower-level investigations of neural mechanism. Today, the approach has evolved such that functional labels are often applied to brain networks rather than focal brain regions. Furthermore, the labels have diversified to include both broadly-defined cognitive functions (declarative memory, visual perception) and more circumscribed mental processes (recollection, familiarity, priming). We ask whether a process-a high-level mental phenomenon corresponding to an introspectively-identifiable cognitive event-is the most productive label for dissecting memory. For example, recollection conflates a neurocomputational operation (pattern completion-based retrieval) with a class of representational content (associative, high-dimensional memories). Because a full theory of memory must identify operations and representations separately, and specify how they interact, we argue that processes like recollection constitute inadequate labels for characterizing neural mechanisms. Instead, we advocate considering the component operations and representations of processes like recollection in isolation. For the organization of memory, the evidence suggests that pattern completion is recapitulated widely across the ventral visual stream and MTL, but the division of labor between sites within this pathway can be explained by representational content.

Coordinated representational reinstatement in the human hippocampus and lateral temporal cortex during episodic memory retrieval

Theoretical models of episodic memory have proposed that retrieval depends on interactions between the hippocampus and neocortex, where hippocampal reinstatement of item-context associations drives neocortical reinstatement of item information. Here, we simultaneously recorded intracranial EEG from hippocampus and lateral temporal cortex (LTC) of epilepsy patients who performed a virtual reality spatial navigation task. We extracted stimulus-specific representations of both item and item-context associations from the time-frequency patterns of activity in hippocampus and LTC. Our results revealed a double dissociation of representational reinstatement across time and space: an early reinstatement of item-context associations in hippocampus preceded a later reinstatement of item information in LTC. Importantly, reinstatement levels in hippocampus and LTC were correlated across trials, and the quality of LTC reinstatement was predicted by the magnitude of phase synchronization between hippocampus and LTC. These findings confirm that episodic memory retrieval in humans relies on coordinated representational interactions within a hippocampal-neocortical network.

Episodic memory retrieval is hypothesized to rely on hippocampal reinstatement of item-context associations which drives reinstatement of item information in cortex. Here, the authors confirm this sequence of events, using iEEG recordings from the human hippocampus and lateral temporal cortex.

Recollection in the human hippocampal-entorhinal cell circuitry

Imagine how flicking through your photo album and seeing a picture of a beach sunset brings back fond memories of a tasty cocktail you had that night. Computational models suggest that upon receiving a partial memory cue (‘beach’), neurons in the hippocampus coordinate reinstatement of associated memories (‘cocktail’) in cortical target sites. Here, using human single neuron recordings, we show that hippocampal firing rates are elevated from ~ 500–1500 ms after cue onset during successful associative retrieval. Concurrently, the retrieved target object can be decoded from population spike patterns in adjacent entorhinal cortex (EC), with hippocampal firing preceding EC spikes and predicting the fidelity of EC object reinstatement. Prior to orchestrating reinstatement, a separate population of hippocampal neurons distinguishes different scene cues (buildings vs. landscapes). These results elucidate the hippocampal-entorhinal circuit dynamics for memory recall and reconcile disparate views on the role of the hippocampus in scene processing vs. associative memory.

The hippocampus is involved both in episodic memory recall and scene processing. Here, the authors show that hippocampal neurons first process scene cues before coordinating memory-guided pattern completion in adjacent entorhinal cortex.

Cortico-hippocampal network connections support the multidimensional quality of episodic memory

Episodic memories reflect a bound representation of multimodal features that can be reinstated with varying precision. Yet little is known about how brain networks involved in memory, including the hippocampus and posterior-medial (PM) and anterior-temporal (AT) systems, interact to support the quality and content of recollection. Participants learned color, spatial, and emotion associations of objects, later reconstructing the visual features using a continuous color spectrum and 360-degree panorama scenes. Behaviorally, dependencies in memory were observed for the gist but not precision of event associations. Supporting this integration, hippocampus, AT, and PM regions showed increased connectivity and reduced modularity during retrieval compared to encoding. These inter-network connections tracked a multidimensional, objective measure of memory quality. Moreover, distinct patterns of connectivity tracked item color and spatial memory precision. These findings demonstrate how hippocampal-cortical connections reconfigure during episodic retrieval, and how such dynamic interactions might flexibly support the multidimensional quality of remembered events.

Multielement Episodic Encoding in Young and Older Adults

Previous research on age-related associative memory deficits has generally focused on memory for single associations. However, our real-world experiences contain a multitude of details that must be effectively integrated and encoded into coherent representations to facilitate subsequent retrieval of the event as a whole. How aging interferes with the processes necessary for multielement encoding is still unknown. We investigated this issue in the current fMRI study. While undergoing scanning, young and older adults were presented with an occupation and an object and were asked to judge how likely the two were to interact, either in general or within the context of a given scene. After scanning, participants completed recognition tasks for the occupation-object pairs and the sources/contexts with which the pairs were studied. Using multivariate behavioral partial least squares analyses, we identified a set of regions including anterior pFC and medial-temporal lobes whose activity was beneficial to subsequent memory for the pairs and sources in young adults but detrimental in older adults. An additional behavioral partial least squares analysis found that, although both groups recruited anterior pFC areas to support context memory performance, only in the young did this activity appear to reflect integration of the occupation, object, and scene features. This was also consistent with behavioral results, which found that young adults showed greater conditional dependence between pair and context memory compared with older adults. Together, these findings suggest that binding and/or retrieving multiple details as an integrated whole becomes increasingly difficult with age.

The dynamic contribution of the high-level visual cortex to imagery and perception

Mental imagery and visual perception rely on the same content-dependent brain areas in the high-level visual cortex (HVC). However, little is known about dynamic mechanisms in these areas during imagery and perception. Here we disentangled local and inter-regional dynamic mechanisms underlying imagery and perception in the HVC and the hippocampus (HC), a key region for memory retrieval during imagery. Nineteen healthy participants watched or imagined a familiar scene or face during fMRI acquisition. The neural code for familiar landmarks and faces was distributed across the HVC and the HC, although with a different representational structure, and generalized across imagery and perception. However, different regional adaptation effects and inter-regional functional couplings were detected for faces and landmarks during imagery and perception. The left PPA showed opposite adaptation effects, with activity suppression following repeated observation of landmarks, but enhancement following repeated imagery of landmarks. Also, functional coupling between content-dependent brain areas of the HVC and HC changed as a function of task and content. These findings provide important information about the dynamic networks underlying imagery and perception in the HVC and shed some light upon the thin line between imagery and perception which has characterized the neuropsychological debates on mental imagery.

Evidence that neural information flow is reversed between object perception and object reconstruction from memory

Remembering is a reconstructive process, yet little is known about how the reconstruction of a memory unfolds in time in the human brain. Here, we used reaction times and EEG time-series decoding to test the hypothesis that the information flow is reversed when an event is reconstructed from memory, compared to when the same event is initially being perceived. Across three experiments, we found highly consistent evidence supporting such a reversed stream. When seeing an object, low-level perceptual features were discriminated faster behaviourally, and could be decoded from brain activity earlier, than high-level conceptual features. This pattern reversed during associative memory recall, with reaction times and brain activity patterns now indicating that conceptual information was reconstructed more rapidly than perceptual details. Our findings support a neurobiologically plausible model of human memory, suggesting that memory retrieval is a hierarchical, multi-layered process that prioritises semantically meaningful information over perceptual details.

Improving the integrative memory model by integrating the temporal dynamics of memory

Despite highlighting the role of the attribution system and proposing a coherent large-scale architecture of declarative memory, the integrative memory model would be more “integrative” if the temporal dynamics of the interactions between its components was clarified. This is necessary to make predictions in patients with brain injury and hypothesize dissociations.

Frequency-specific noninvasive modulation of memory retrieval and its relationship with hippocampal network connectivity

Episodic memory is thought to rely on interactions of the hippocampus with other regions of the distributed hippocampal-cortical network (HCN) via interregional activity synchrony in the theta frequency band. We sought to causally test this hypothesis using network-targeted transcranial magnetic stimulation. Healthy human participants completed four experimental sessions, each involving a different stimulation pattern delivered to the same individualized parietal cortex location of the HCN for all sessions. There were three active stimulation conditions, including continuous theta-burst stimulation, intermittent theta-burst stimulation, and beta-frequency (20-Hz) repetitive stimulation, and one sham condition. Resting-state fMRI and episodic memory testing were used to assess the impact of stimulation on hippocampal fMRI connectivity related to retrieval success. We hypothesized that theta-burst stimulation conditions would most strongly influence hippocampal-HCN fMRI connectivity and retrieval, given the hypothesized relevance of theta-band activity for HCN memory function. Continuous theta-burst stimulation improved item retrieval success relative to sham and relative to beta-frequency stimulation, whereas intermittent theta-burst stimulation led to numerical but nonsignificant item retrieval improvement. Mean hippocampal fMRI connectivity did not vary for any stimulation conditions, whereas individual differences in retrieval improvements due to continuous theta-burst stimulation were associated with corresponding increases in fMRI connectivity between the hippocampus and other HCN locations. No such memory-related connectivity effects were identified for the other stimulation conditions, indicating that only continuous theta-burst stimulation affected memory-related hippocampal-HCN connectivity. Furthermore, these effects were specific to the targeted HCN, with no significant memory-related fMRI connectivity effects for two distinct control brain networks. These findings support a causal role for fMRI connectivity of the hippocampus with the HCN in episodic memory retrieval and indicate that contributions of this network to retrieval are particularly sensitive to continuous theta-burst noninvasive stimulation.

Scale-Free Dynamics of the Mouse Wakefulness and Sleep Electroencephalogram Quantified Using Wavelet-Leaders

Scale-free analysis of brain activity reveals a complexity of synchronous neuronal firing which is different from that assessed using classic rhythmic quantifications such as spectral analysis of the electroencephalogram (EEG). In humans, scale-free activity of the EEG depends on the behavioral state and reflects cognitive processes. We aimed to verify if fractal patterns of the mouse EEG also show variations with behavioral states and topography, and to identify molecular determinants of brain scale-free activity using the ‘multifractal formalism’ (Wavelet-Leaders). We found that scale-free activity was more anti-persistent (i.e., more different between time scales) during wakefulness, less anti-persistent (i.e., less different between time scales) during non-rapid eye movement sleep, and generally intermediate during rapid eye movement sleep. The scale-invariance of the frontal/motor cerebral cortex was generally more anti-persistent than that of the posterior cortex, and scale-invariance during wakefulness was strongly modulated by time of day and the absence of the synaptic protein Neuroligin-1. Our results expose that the complexity of the scale-free pattern of organized neuronal firing depends on behavioral state in mice, and that patterns expressed during wakefulness are modulated by one synaptic component.

Hippocampal-targeted Theta-burst Stimulation Enhances Associative Memory Formation

The hippocampus plays a critical role in episodic memory, among other cognitive functions. However, few tools exist to causally manipulate hippocampal function in healthy human participants. Recent work has targeted hippocampal-cortical networks by performing TMS to a region interconnected with the hippocampus, posterior inferior parietal cortex (pIPC). Such hippocampal-targeted TMS enhances associative memory and influences hippocampal functional connectivity. However, it is currently unknown which stages of mnemonic processing (encoding or retrieval) are affected by hippocampal-targeted TMS. Here, we examined whether hippocampal-targeted TMS influences the initial encoding of associations (vs. items) into memory. To selectively influence encoding and not retrieval, we performed continuous theta-burst TMS before participants encoded object-location associations and assessed memory after the direct effect of stimulation dissipated. Relative to control TMS and baseline memory, pIPC TMS enhanced associative memory success and confidence. Item memory was unaffected, demonstrating a selective influence on associative versus item memory. The strength of hippocampal-pIPC functional connectivity predicted TMS-related memory benefits, which was mediated by parahippocampal and retrosplenial cortices. Our findings indicate that hippocampal-targeted TMS can specifically modulate the encoding of new associations into memory without directly influencing retrieval processes and suggest that the ability to influence associative memory may be related to the fidelity of hippocampal TMS targeting. These results support the notion that pIPC TMS may serve as a potential tool for manipulating hippocampal function in healthy participants. Nonetheless, future work combining hippocampal-targeted continuous theta-burst TMS with neuroimaging is needed to better understand the neural basis of TMS-induced memory changes.

What does the functional organization of cortico-hippocampal networks tell us about the functional organization of memory?

Historically, research on the cognitive processes that support human memory proceeded, to a large extent, independently of research on the neural basis of memory. Accumulating evidence from neuroimaging, however, has enabled the field to develop a broader and more integrative perspective. Here, we briefly outline how advances in cognitive neuroscience can potentially shed light on concepts and controversies in human memory research. We argue that research on the functional properties of cortico-hippocampal networks informs us about how memories might be organized in the brain, which, in turn, helps to reconcile seemingly disparate perspectives in cognitive psychology. Finally, we discuss several open questions and directions for future research.

Dynamic Neural Network Reconfiguration During the Generation and Reinstatement of Mnemonic Representations

Mnemonic representations allow humans to re-experience the past or simulate future scenarios by integrating episodic features from memory. Theoretical models posit that mnemonic representations require dynamic processing between neural indexes in the hippocampus and areas of the cortex providing specialized information processing. However, it remains unknown whether global and local network topology varies as information is encoded into a mnemonic representation and subsequently reinstated. Here, we investigated the dynamic nature of memory networks while a representation of a virtual city is generated and reinstated during mental simulations. We find that the brain reconfigures from a state of heightened integration when encoding demands are highest, to a state of localized processing once representations are formed. This reconfiguration is associated with changes in hippocampal centrality at the intra- and inter-module level, decreasing its role as a connector hub between modules and within a hippocampal neighborhood as encoding demands lessen. During mental simulations, we found increased levels of hippocampal centrality within its local neighborhood coupled with decreased functional interactions between other regions of the neighborhood during highly vivid simulations, suggesting that information flow vis-à-vis the hippocampus is critical for high fidelity recapitulation of mnemonic representations.

Spatial scaffold effects in event memory and imagination

Spatial context is a defining feature of episodic memories, which are often characterized as being events occurring in specific spatiotemporal contexts. In this review, I summarize research suggesting a common neural basis for episodic and spatial memory and relate this to the role of spatial context in episodic memory. I review evidence that spatial context serves as a scaffold for episodic memory and imagination, in terms of both behavioral and neural effects demonstrating a dependence of episodic memory on spatial representations. These effects are mediated by a posterior-medial set of neocortical regions, including the parahippocampal cortex, retrosplenial cortex, posterior cingulate cortex, precuneus, and angular gyrus, which interact with the hippocampus to represent spatial context in remembered and imagined events. I highlight questions and areas that require further research, including differentiation of hippocampal function along its long axis and subfields, and how these areas interact with the posterior-medial network. This article is categorized under: Psychology > Memory Neuroscience > Cognition.

Hippocampal Contributions to the Large-Scale Episodic Memory Network Predict Vivid Visual Memories

A common approach in memory research is to isolate the function(s) of individual brain regions, such as the hippocampus, without addressing how those regions interact with the larger network. To investigate the properties of the hippocampus embedded within large-scale networks, we used functional magnetic resonance imaging and graph theory to characterize complex hippocampal interactions during the active retrieval of vivid versus dim visual memories. The study yielded 4 main findings. First, the right hippocampus displayed greater communication efficiency with the network (shorter path length) and became a more convergent structure for information integration (higher centrality measures) for vivid than dim memories. Second, vivid minus dim differences in our graph theory measures of interest were greater in magnitude for the right hippocampus than for any other region in the 90-region network. Moreover, the right hippocampus significantly reorganized its set of direct connections from dim to vivid memory retrieval. Finally, beyond the hippocampus, communication throughout the whole-brain network was more efficient (shorter global path length) for vivid than dim memories. In sum, our findings illustrate how multivariate network analyses can be used to investigate the roles of specific regions within the large-scale network, while also accounting for global network changes.

Response to 'Hippocampus as a wormhole: gateway to consciousness'

Reply to: Behrendt R-P. Hippocampus as a wormhole: gateway to consciousness. WIREs Cogn Sci 2017, e1446. doi: 10.1002/wcs.1446.

The hippocampus is particularly important for building associations across stimulus domains

The medial temporal lobe (MTL) is critical for binding together different attributes that together form memory for prior episodes, but whether it is preferentially involved in supporting specific types of associations is a topic of much debate. Some have argued that the MTL, specifically the hippocampus, may be specialized for binding information from different stimulus domains (e.g., linking visual and auditory stimuli). In the current study, we examined the role of the MTL in memory for associations within- vs. across-domains. Patients with either selective hippocampal lesions or more extensive MTL lesions studied pairs of items within the same stimulus domain (i.e., image-image or sound-sound pairs) or across different domains (i.e., image-sound pairs). Associative memory was subsequently tested by having participants discriminate between previously studied and rearranged pairs. Compared to healthy controls, the patients were significantly more impaired in the across-domain condition than the within-domain conditions. Similar deficits were observed for patients with hippocampal lesions and those with more extensive MTL lesions, suggesting that the hippocampus itself is particularly important for binding associations across stimulus domains.

Neural Oscillations and a Nascent Corticohippocampal Theory of Reference

The ability to use words to refer to the world is vital to the communicative power of human language. In particular, the anaphoric use of words to refer to previously mentioned concepts (antecedents) allows dialogue to be coherent and meaningful. Psycholinguistic theory posits that anaphor comprehension involves reactivating a memory representation of the antecedent. Whereas this implies the involvement of recognition memory or the mnemonic subroutines by which people distinguish old from new, the neural processes for reference resolution are largely unknown. Here, we report time–frequency analysis of four EEG experiments to reveal the increased coupling of functional neural systems associated with referentially coherent expressions compared with referentially problematic expressions. Despite varying in modality, language, and type of referential expression, all experiments showed larger gamma-band power for referentially coherent expressions compared with referentially problematic expressions. Beamformer analysis in high-density Experiment 4 localized the gamma-band increase to posterior parietal cortex around 400–600 msec after anaphor onset and to frontotemporal cortex around 500–1000 msec. We argue that the observed gamma-band power increases reflect successful referential binding and resolution, which links incoming information to antecedents through an interaction between the brain's recognition memory networks and frontotemporal language network. We integrate these findings with previous results from patient and neuroimaging studies, and we outline a nascent corticohippocampal theory of reference.

Material Specificity Drives Medial Temporal Lobe Familiarity But Not Hippocampal Recollection

The specific role of the perirhinal (PRC), entorhinal (ERC) and parahippocampal cortices (PHC) in supporting familiarity‐based recognition remains unknown. An fMRI study explored whether these medial temporal lobe (MTL) structures responded in the same way or differentially to familiarity as a function of stimulus type at recognition. A secondary aim was to explore whether the hippocampus responds in the same way to equally strong familiarity and recollection and whether this is influenced by the kind of stimulus involved. Univariate and multivariate analyses revealed that familiarity responses in the PRC, ERC, PHC and the amygdala are material‐specific. Specifically, the PRC and ERC selectively responded to object familiarity, while the PHC responded to both object and scene familiarity. The amygdala only responded to familiarity memory for faces. The hippocampus did not respond to stimulus familiarity for any of the three types of stimuli, but it did respond to recollection for all three types of stimuli. This was true even when recollection was contrasted to equally accurate familiarity. Overall, the findings suggest that the role of the MTL neocortices and the amygdala in familiarity‐based recognition depends on the kind of stimulus in memory, whereas the role of the hippocampus in recollection is independent of the type of cuing stimulus. © 2016 The Authors Hippocampus Published by Wiley Periodicals, Inc.

Hippocampal structure predicts cortical indices of reactivation of related items

One of the key components of relational memory is the ability to bind together the constituent elements of a memory experience, and this ability is thought to be supported by the hippocampus. Previously we had shown that these relational bindings can be used to reactivate the cortical processors of an absent item in the presence of a relationally bound associate (Walker et al., 2014). Specifically, we recorded the event-related optical signal (EROS) when presenting the scene of a face-scene pair during a preview period immediately preceding a test display, and demonstrated reactivation of a face-processing cortical area (the superior temporal sulcus, STS) for scenes that had been previously paired with faces, relative to scenes that had not. Here we combined the EROS measures during the same preview paradigm with anatomical estimates of hippocampal integrity (structural MRI measures of hippocampal volume and diffusion tensor imaging measures of mean fractional anisotropy and diffusivity) to provide evidence that the hippocampus is mediating this reactivation phenomenon. The study was run in a sample of older adults aged 55-87, taking advantage of the high amount of hippocampal variability present in aging. We replicated the functional reactivation of STS during the preview period, specific to scenes previously paired with faces. Crucially, we also found that this phenomenon is correlated with structural hippocampus integrity. Both STS reactivation and hippocampal structure predicted subsequent recognition performance. These data support the theory that relational memory is sustained by an interaction between hippocampal and cortical sensory processing regions, and that these functions may be at the basis of episodic memory changes in normal aging.

Hippocampal pattern completion is linked to gamma power increases and alpha power decreases during recollection

How do we retrieve vivid memories upon encountering a simple cue? Computational models suggest that this feat is accomplished by pattern completion processes involving the hippocampus. However, empirical evidence for hippocampal pattern completion and its underlying mechanisms has remained elusive. Here, we recorded direct intracranial EEG as human participants performed an associative memory task. For each study (encoding) and test (retrieval) event, we derived time-frequency resolved representational patterns in the hippocampus and compared the extent of pattern reinstatement for different mnemonic outcomes. Results show that successful associative recognition (AR) yields enhanced event-specific reinstatement of encoding patterns compared to non-associative item recognition (IR). Moreover, we found that gamma power (50–90 Hz) increases – in conjunction with alpha power (8–12 Hz) decreases not only distinguish AR from IR, but also correlate with the level of hippocampal reinstatement. These results link single-shot hippocampal pattern completion to episodic recollection and reveal how oscillatory dynamics in the gamma and alpha bands orchestrate these mnemonic processes.

DOI:http://dx.doi.org/10.7554/eLife.17397.001