Item, context and relational episodic encoding in humans

Distinct cortisol effects on item and associative memory across memory phases

Our daily lives are filled with stressful situations, which powerfully shape the way we form, consolidate, and retrieve episodic memories. As such, stress hormones affect different memory phases of both individual items and their associations, whether they are neutral or emotional. However, an interplay between all these factors in our memory of stressful events is still poorly understood. To address this conundrum, we employed a within-subject, double-blind, placebo-controlled design with exogenous cortisol administration (10 mg hydrocortisone) to affect different memory phases (pre-encoding, post-encoding, pre-retrieval). Our participants encoded neutral and emotional noun - image pairs. After a 24 h delay, we tested their memory for individual items (nouns) and their associations (nouns - objects). While accounting for baseline (no stress) memory performance, we found divergent cortisol effects on item and associative memory, depending on affected memory phase and on emotionality of memoranda. While post-encoding cortisol administration enhanced item memory, pre-encoding, and pre-retrieval cortisol administration impaired item memory. Similarly, pre-encoding cortisol administration impaired associative memory, but only for neutral stimuli. Moreover, we observed that both salivary cortisol levels and emotionality of memoranda modulated item and associative memory performance. These findings highlight a complex interplay of how stress hormone cortisol, throughout all memory phases, differently modulates item and associative memory of neutral and emotional events.

The ubiquity of episodic-like memory during infancy

Considerable progress has been made in understanding early memory development. However, much of this research pre-dates contemporary theories of memory systems in the mature brain. This review provides a refresher on these conceptual frameworks and proposes a common theoretical foundation for reconciling adult and infant studies. This foundation enables a critical analysis of infant studies that have directly tested memory and suggests that they may not capture the full nature and extent of episodic memory abilities in infancy. The analysis is extended to infant studies that are ostensibly focused on cognitive domains other than memory and finds that many such tasks require episodic-like memory. Thus, there may be substantially more evidence for episodic-like memory in infants than previously recognized.

Hippocampal-entorhinal cognitive maps and cortical motor system represent action plans and their outcomes

Efficiently interacting with the environment requires weighing and selecting among multiple alternative actions based on their associated outcomes. However, the neural mechanisms underlying these processes are still debated. We show that forming relations between arbitrary action-outcome associations involve building a cognitive map. Using an immersive virtual reality paradigm, participants learned 2D abstract motor action-outcome associations and later compared action combinations while their brain activity was monitored with fMRI. We observe a hexadirectional modulation of the activity in entorhinal cortex while participants compared different action plans. Furthermore, hippocampal activity scales with the 2D similarity between outcomes of these action plans. Conversely, the supplementary motor area represents individual actions, showing a stronger response to overlapping action plans. Crucially, the connectivity between hippocampus and supplementary motor area is modulated by the similarity between the action plans, suggesting their complementary roles in action evaluation. These findings provide evidence for the role of cognitive maps in action selection, challenging classical models of memory taxonomy and its neural bases.

The superior memory effect of insightful learning: The role of associative novelty

Previous research has confirmed the advantage of insightful learning in item memory, but its impact on associative memory, particularly the role of novelty, remains unclear. This study aims to investigate the role of novelty in insightful learning on associative memory and metamemory using a Chinese logogriph learning-testing paradigm. Participants were asked to judge whether the "Midi" word/character (the answer) matched the "Mimian" phrase (the problem) in both the high- and low-novelty conditions. Then they took immediate (2 min later) and delayed (24 h later) recognition tests to identify whether the "Mimian"-"Midi" (phrase-character) pairs were previously presented (old) or not (new), and assessed their retrospective confidences. Results showed that individuals had higher recognition rates for high- (vs. low-) novelty logogriphs, and higher retrospective confidence for recognized pairs. This research indicates that novelty enhances both associative memory and the confidence judgment of metamemory monitoring in insightful learning.

Finding the sweet spot of memory modification: An fMRI study on episodic prediction error strength and type

Previous research has highlighted the critical role of prediction errors (PEs) in signaling the need to adapt memory representations in response to unexpected changes in the environment. Yet, the influence of PE type and strength on memory remains underexplored. In this study, participants encoded naturalistic dialogues prior to undergoing fMRI scanning. During the fMRI session, they listened to dialogues that had been modified in their surface or gist, to varying extents. As expected, our findings revealed robust activation in the inferior frontal gyrus for all PEs. Notably, gist modifications elicited additional activations within the episodic memory network, including the hippocampus. A post-fMRI recognition test demonstrated that surface modifications had no significant impact on memory. Conversely, weak gist changes impaired memory for the original content and hindered learning of the modification. These weak gist changes also triggered activation in the parahippocampal cortex. These results underscore the importance of both the type and strength of PEs in shaping brain activations and memory outcomes, highlighting their complex interplay in cognitive processes.

A vectorial code for semantics in human hippocampus

As we listen to speech, our brains actively compute the meanings of individual words. Inspired by the success of large language models (LLMs), we hypothesized that the brain employs vectorial coding principles, such that meaning is reflected in distributed activity of single neurons. We recorded responses of hundreds of neurons in the human hippocampus, which has a well-established role in semantic coding, while participants listened to narrative speech. We find encoding of contextual word meaning in the simultaneous activity of neurons whose individual selectivities span multiple unrelated semantic categories. Like embedding vectors in semantic models, distance between neural population responses correlates with semantic distance; however, this effect was only observed in contextual embedding models (like BERT) and was reversed in non-contextual embedding models (like Word2Vec), suggesting that the semantic distance effect depends critically on contextualization. Moreover, for the subset of highly semantically similar words, even contextual embedders showed an inverse correlation between semantic and neural distances; we attribute this pattern to the noise-mitigating benefits of contrastive coding. Finally, in further support for the critical role of context, we find that neural response variance increases with lexical polysemy. Ultimately, these results support the hypothesis that semantic coding in the hippocampus follows vectorial principles.

Hippocampal encoding of memories in human infants

Humans lack memories for specific events from the first few years of life. We investigated the mechanistic basis of this infantile amnesia by scanning the brains of awake infants with functional magnetic resonance imaging while they performed a subsequent memory task. Greater activity in the hippocampus during the viewing of previously unseen photographs was related to later memory-based looking behavior beginning around 1 year of age, suggesting that the capacity to encode individual memories comes online during infancy. The availability of encoding mechanisms for episodic memory during a period of human life that is later lost from our autobiographical record implies that postencoding mechanisms, whereby memories from infancy become inaccessible for retrieval, may be more responsible for infantile amnesia.

Evidence for shallow cognitive maps in Schizophrenia

Individuals with schizophrenia can have marked deficits in goal-directed decision making. Prominent theories differ in whether schizophrenia (SZ) affects the ability to exert cognitive control or the motivation to exert control. An alternative explanation is that schizophrenia negatively impacts the formation of cognitive maps, the internal representations of the way the world is structured, necessary for the formation of effective action plans. That is, deficits in decision-making could arise when goal-directed control and motivation are intact but used to plan over ill-formed maps. We tested the hypothesis that individuals with SZ are impaired in constructing cognitive maps. We combine a behavioral representational similarity analysis technique with a sequential decision-making task. This enables us to examine how relationships between choice options change when individuals with SZ and healthy age-matched controls build a cognitive map of the task structure. Our results indicate that SZ affects how people represent the structure of the task, focusing more on simpler visual features and less on abstract, higher-order, planning-relevant features. At the same time, we find that individuals with SZ were able to display similar performance on this task compared with controls, emphasizing the need for a distinction between cognitive map formation and changes in goal-directed control in understanding cognitive deficits in schizophrenia.

Contractions in human cerebellar-cortical manifold structure underlie motor reinforcement learning

How the brain learns new motor commands through reinforcement involves distributed neural circuits beyond known frontal-striatal pathways, yet a comprehensive understanding of this broader neural architecture remains elusive. Here, using human functional MRI (N = 46, 27 females) and manifold learning techniques, we identified a low-dimensional neural space that captured the dynamic changes in whole-brain functional organization during a reward-based trajectory learning task. By quantifying participants' learning rates through an Actor-Critic model, we discovered that periods of accelerated learning were characterized by significant manifold contractions across multiple brain regions, including areas of limbic and hippocampal cortex, as well as the cerebellum. This contraction reflected enhanced network integration, with notably stronger connectivity between several of these regions and the sensorimotor cerebellum correlating with higher learning rates. These findings challenge the traditional view of the cerebellum as solely involved in error-based learning, supporting the emerging view that it coordinates with other brain regions during reinforcement learning.Significance Statement This study reveals how distributed brain systems, including the cerebellum and hippocampus, alter their functional connectivity to support motor learning through reinforcement. Using advanced manifold learning techniques on functional MRI data, we examined changes in regional connectivity during reward-based learning and their relationship to learning rate. For several brain regions, we found that periods of heightened learning were associated with increased cerebellar connectivity, suggesting a key role for the cerebellum in reward-based motor learning. These findings challenge the traditional view of the cerebellum as solely involved in supervised (error-based) learning and add to a growing rodent literature supporting a role for cerebellar circuits in reward-driven learning.

Gamma-frequency transcranial alternating current stimulation over the left posterior parietal cortex enhances the long-term retention of associative memory

Transcranial alternating current stimulation (tACS) has been reported to improve associative memory (AM) by modulating the frequency of neural oscillations in the brain; however, whether gamma-frequency (> 30 Hz) tACS in the left posterior parietal lobe (PPC) can enhance memory retention in AM remains unclear. This study aimed to investigate whether memory retention in AM could be improved after gamma-frequency tACS of the left PPC. We used a randomly assigned, double-blind, repeated-measures, sham-control design, in which 28 healthy adult participants were assigned to receive a single 20-min session of gamma-frequency (60 Hz) tACS or sham stimulation. The memory learning task consisted of studying and testing 50 unrelated word pairs three times on day 1. The number of correct responses in the cued recall task was measured at three time points: days 1, 7, and 28. The results revealed a significant difference in the number of correct responses between the interventions on day 7 and day 28. These data suggest that gamma-frequency tACS stimulation of the left PPC enhances the long-term retention of AM in healthy adults.

Contextual Influence on Pattern Separation During Encoding

Pattern separation is considered a crucial process that allows us to distinguish among the highly similar and overlapping experiences that constitute our episodic memory. Not only do different episodes share common features, but it is often the case that they share the context in which they occurred. While there have been a great number of studies investigating pattern separation and its behavioral counterpart, a process known as mnemonic discrimination, surprisingly, research exploring the influence of context on pattern separation or mnemonic discrimination has been less common. The available evidence shows that similar items with similar contexts led to a failure in pattern separation due to high similarity that triggers overlap between events. On the other hand, others have shown that pattern separation can take place even under these conditions, allowing humans to distinguish between events with similar items and contexts, as different hippocampal subfields would play complementary roles in enabling both pattern separation and pattern completion. In the present study, we were interested in testing how stability in context influenced pattern separation. Despite the fact that pattern separation is by definition an encoding computation, the existing literature has focused on the retrieval phase. Here, we used a subsequent memory paradigm in which we manipulated the similarity of context during the encoding of visual objects selected from diverse categories. Thus, we manipulated the encoded context of each object category (four items within a category), so that some categories had the same context and others had a different context. This approach allowed us to test not only the items presented but also to include the conditions that entail the greatest demand on pattern separation. After a 20 min period, participants performed a visual mnemonic discrimination task in which they had to differentiate between old, similar, and new items by providing one of the three options for each tested item. Similarly to previous studies, we found no interaction between judgments and contexts, and participants were able to discriminate between old and lure items at the behavioral level in both conditions. Moreover, when averaging the ERPs of all the items presented within a category, a significant SME emerged between hits and new misses, but not between hits and old false alarms or similar false alarms. These results suggest that item recognition emerges from the interaction with subsequently encoded information, and not just between item memory strength and retrieval processes.

Tell me why: the missing w in episodic memory's what, where, and when

Endel Tulving defined episodic memory as consisting of a spatiotemporal context. It enables us to recollect personal experiences of people, things, places, and situations. In other words, it is made up of what, where, and when components. However, this definition does not include arguably the most important aspect of episodic memory: the why. Understanding why we remember has important implications to better understand how our memory system works and as a potential target of intervention for memory impairment. The intrinsic and extrinsic factors related to why some experiences are better remembered than others have been widely investigated but largely independently studied. How these factors interact with one another to drive an event to become a lasting memory is still unknown. This review summarizes research examining the why of episodic memory, where we aim to uncover the factors that drive core features of our memory. We discuss the concept of episodic memory examining the what, where, and when, and how the why is essential to each of these key components of episodic memory. Furthermore, we discuss the neural mechanisms known to support our rich episodic memories and how a why signal may provide critical modulatory impact on neural activity and communication. Finally, we discuss the individual differences that may further drive why we remember certain experiences over others. A better understanding of these elements, and how we experience memory in daily life, can elucidate why we remember what we remember, providing important insight into the overarching goal of our memory system.

Lack of context modulation in human single neuron responses in the medial temporal lobe

In subjects implanted with intracranial electrodes, we use two different stories involving the same person (or place) to evaluate whether and to what extent context modulates human single-neuron responses. Nearly all neurons (97% during encoding and 100% during recall) initially responding to a person/place do not modulate their response with context. Likewise, nearly none (<1%) of the initially non-responsive neurons show conjunctive coding, responding to particular persons/places in a particular context during the tasks. In line with these findings, taking all neurons together it is possible to decode the person/place being depicted in each story, but not the particular story. Moreover, the neurons show consistent results across encoding and recall of the stories and during passive viewing of pictures. These results suggest a context invariant, non-conjunctive coding of memories at the single-neuron level in the human hippocampus and amygdala, in contrast to what has been described in other species.

Changes in the level of unitization moderate the impact of unitization on associative memory and its underlying processing

The viewpoint that unitization provides a possibility of increasing the contribution of familiarity to associative memory has been widely accepted, but its effects on associative memory and recollection remain controversial. The current study aims to explain these mixed results by considering a potential moderator: changes in the level of unitization from encoding to retrieval phases. During the encoding phase, participants learned the related and unrelated picture pairs (i.e., high vs. low levels of unitization). Subsequently, they needed to distinguish between the intact and rearranged pairs during retrieval, where, in these rearranged pairs, the level of unitization from encoding to retrieval phases may or may not change. Meanwhile, the scalp electroencephalographic activity (EEG) was recorded. The results showed a significant familiarity-related FN400 old/new effect for related picture pairs alone, which supported the above viewpoint. However, its impact on the associative memory and recollection-related LPC old/new effects varied with the level of unitization changes-specifically, under the unchanged conditions. Although related pairs elicited significant FN400 and LPC old/new effects, the differences in these old/new effects and associative memory between the related and unrelated picture pairs were not significant. Conversely, under the changed conditions, related picture pairs not only elicited significantly larger FN400 and LPC old/new effects but also improved associative memory more than unrelated picture pairs. These findings not only clarify some of the inconsistencies in the literature concerning the impact of unitization on associative memory but also suggest that unitization affects the contributions of familiarity and recollection to associative memory differently, its effectiveness varying with the level of unitization changes.

Event Segmentation Promotes the Reorganization of Emotional Memory

Event boundaries help structure the content of episodic memories by segmenting continuous experiences into discrete events. Event boundaries may also serve to preserve meaningful information within an event, thereby actively separating important memories from interfering representations imposed by past and future events. Here, we tested the hypothesis that event boundaries organize emotional memory based on changing dynamics as events unfold. We developed a novel threat-reversal learning task whereby participants encoded trial-unique exemplars from two semantic categories across three phases: preconditioning, fear acquisition, and reversal. Shock contingencies were established for one category during acquisition (CS+) and then switched to the other during reversal (CS-). Importantly, reversal was either separated by a perceptible event boundary (Experiment 1) or occurred immediately after acquisition, with no perceptible context shift (Experiment 2). In a surprise recognition memory test the next day, memory performance tracked the learning contingencies from encoding in Experiment 1, such that participants selectively recognized more threat-associated CS+ exemplars from before (retroactive) and during acquisition, but this pattern reversed toward CS- exemplars encoded during reversal. By contrast, participants with continuous encoding-without a boundary between conditioning and reversal-exhibited undifferentiated memory for exemplars from both categories encoded before acquisition and after reversal. Further analyses highlight nuanced effects of event boundaries on reversing conditioned fear, updating mnemonic generalization, and emotional biasing of temporal source memory. These findings suggest that event boundaries provide anchor points to organize memory for distinctly meaningful information, thereby adaptively structuring memory based on the content of our experiences.

Hippocampus supports long-term maintenance of language representations: Evidence of impaired collocation knowledge in amnesia

Traditional systems consolidation theories of memory suggest that the role of the hippocampus in maintaining memory representations diminishes over time, with learned information eventually becoming fully independent of the hippocampus. Knowledge of collocations in one's native (L1) language are acquired during development and are solidly acquired by adulthood. Remote semantic knowledge of collocations might therefore be expected to be resistant to hippocampal pathology. Patients with hippocampal damage and severe anterograde amnesia completed two tasks testing English collocation knowledge originally designed for use with English language learners. Patients with hippocampal damage demonstrated impairments in recognition of common English collocations, despite a lifetime of language experience (including postsecondary education) prior to sustaining this damage. These results suggest the hippocampus contributes to the long-term maintenance of linguistic representations and provides a challenge to traditional consolidation views of memory and an extension of newer theories to include a role for the hippocampus in maintaining semantic memory.

The Multiple Dimensions of Familiarity: From Representations to Phenomenology

This article focuses on familiarity, the form of memory allowing humans to recognize stimuli that have been encountered before. We aim to emphasize its complex nature which includes representational and phenomenological dimensions. The former implies that its neural correlates depend on the type and complexity of the cue stimulus, as different classes of stimuli are represented in distributed ventral visual and medial temporal regions. The second dimension relates to the subjective feeling of familiarity, which results from a fluency signal that is attributed to past encounters with the stimulus. We review mnemonic and non-mnemonic sources of fluency that can induce a sense of familiarity, as well as cases where fluency is not attributed to memory, among which the phenomenological experience of déjà-vu. Across these two dimensions, we highlight key questions to be answered by future studies to improve our understanding of the underpinnings of this form of memory and contribute to building an integrative neurocognitive model of familiarity. Essential to this aim is the clarification of the computational, cognitive, and neural mechanisms involved, namely global matching, fluency attribution, and sharpening. Furthermore, future research is needed to unravel the relationships between these mechanisms. We argue that to achieve these goals, researchers must use appropriate behavioral paradigms and clearly define which dimension of familiarity they investigate.

Positive affect amplifies integration within episodic memories in the laboratory and the real world

Emotional events hold a privileged place in our memories, differing in accuracy and structure from memories for neutral experiences. Although much work has focused on the pronounced differences in memory for negative experiences, there is growing evidence that positive events may lead to more holistic, or integrated, memories. However, it is unclear whether these affect-driven changes in memory structure, which have been found in highly controlled laboratory environments, extend to real-world episodic memories. We ran experiments that assessed memory for experiences created in the laboratory (Experiment 1) and, using smartphones, memories for everyday experiences (Experiment 2). We complement these design innovations with a novel analysis approach to model memory accuracy and integration in both settings. Consistent with past findings, emotional events were subjectively remembered more strongly. These studies also revealed that features of more positive events were indeed more integrated within memory, both in the laboratory and the real world. These effects were specific to participants' emotional responses to the events during encoding rather than general emotional states at the time of retrieval, and reflected a general increase in integration between multiple memory features. Together, these results demonstrate robust differences in memory for positive events, introduce a novel measure of memory integration, and highlight the importance of assessing the impact of emotion on memory beyond the laboratory.

Neural Correlates of Narrative Reading Development: A Comparative fMRI Study of Adults and Children Using Time-Locked Inter-Subject Correlation Analyses

The naturalistic paradigm and analytical methods present new approaches that are particularly suitable for research concentrating on narrative reading development. We analyzed fMRI data from 44 adults and 42 children engaged in story reading using time-locked inter-subject correlation (ISC), inter-subject representation similarity analysis (IS-RSA), and inter-subject functional correlation (ISFC). The ISC results indicated that for both children and adults, narrative reading recruited not only traditional reading areas but also regions that are sensitive to long-time-scale information, such as the medial prefrontal cortex and hippocampus, which increased involvement from children to adults. The results of the IS-RSA indicated that during narrative reading, children exhibited greater uniqueness in neural patterns, while adults demonstrated greater similarity. The analysis of reading-level subgroups with the ISC and ISFC reveals differences in narrative reading development that span from children to adults, especially for regions sensitive to long-time-scale semantic processing. These results indicate that the maturity and experience play a crucial role in narrative reading development.

Physical activity indexed using table tennis skills modulates the neural dynamics of involuntary retrieval of negative memories

Memory intrusion is a characteristic of posttraumatic stress disorder manifesting as involuntary flashbacks of negative events. Interference of memory reconsolidation using cognitive tasks has been employed as a noninvasive therapy to prevent subsequent intrusive retrieval. The present study aims to test whether physical activity, with its cognitive demands and unique physiological effects, may provide a novel practice to reduce later involuntary retrieval via the reconsolidation mechanism. In addition, the study investigates the EEG representation of neural function in interpreting the interplay of intrusion and recognition. Eighty-seven participants were tested on successive sessions comprised encoding (Day 0), reconsolidation (24-hr) and priming retrieval (Day 7) in a between-subject design with random assignment to 3 different groups: whole-body exercise, sensorimotor engagement and sitting groups. Of the key results, when involuntary retrieval was subsequently triggered by relevant stimuli, reduced subjective recognition was observed, and working memory maintenance was shortened, indicated by shorter Negative Slow Wave duration. The study implicates the potential neurophysiological mechanism of cognitive and behavioral interventions, specifically those aimed at reducing intrusion frequency through the reconsolidation mechanism; these are proposed to facilitate accelerated recovery from involuntary memories.

Partially dissociable roles of the orbitofrontal cortex and dorsal hippocampus in context-dependent hierarchical associations

Reward cues are often ambiguous; what is good in one context is not necessarily good in another. To solve this ambiguity, animals form hierarchical associations in which the context gates the retrieval of appropriate cue-evoked memories. These hierarchical associations regulate cue-elicited behavior and influence subsequent learning, promoting the inference of context-dependency. The orbitofrontal cortex (OFC) and dorsal hippocampus (DH) are both proposed to encode a "cognitive map" encompassing hierarchical, context-dependent associations. However, OFC- and DH-specific contributions to the different functional properties of hierarchical associations remain controversial. Using chemogenetic inactivation in rats, we show that the OFC is essential to both properties of hierarchical associations (performance regulation and learning bias). In contrast, DH's role appears limited to the contextual learning bias conferred by hierarchical associations. This work establishes the OFC as a critical orchestrator of hierarchical associations and provides insights into the extended circuits mediating the functional properties of these associations.

Threat Impairs the Organization of Memory Around Motivational Context

Previous work highlighted a critical role for top-down goals in shifting memory organization, namely, through studying the downstream influences of event segmentation and task switching on free recall. Here, we extend these frameworks into the realm of motivation, by comparing how threat motivation influences memory organization by capturing free recall dynamics. In Study 1, we manipulated individuals' motivation to successfully encode information by the threat of exposure to aversive sounds for forgetting. In Study 2, we conducted a parallel study manipulating motivation via instruction rather than threat, allowing us to examine changes directly related to threat motivation. Our findings showed that motivation to avoid threat broadly enhances memory for items presented within a threatening context, regardless of whether items were directly associated with the threat or not. Concurrently, these memory enhancements coincide with a decrease in the organization of memory around motivationally relevant features. These results highlight the importance of considering motivational valence when conceptualizing memory organization within adaptive memory frameworks.

Preclinical investigation of the effect of stress on the binding of [18F]F13640, a 5-HT1A radiopharmaceutical

BACKGROUND

[18F]F13640 is a new PET radiopharmaceutical for brain molecular imaging of serotonin 5-HT1A receptors. Since we intend to use this radiopharmaceutical in psychiatric studies, it is crucial to establish possible sensitivity modification of 5-HT1A receptors availability during an acute stress exposure. In this study, we first assessed the cerebrometabolic effects of a new animal model of stress with [18F]FDG and then proceeded to test for effects of this model on the cerebral binding of [18F]F13640, a 5-HT1A receptors PET radiopharmaceutical.

METHODS

Four groups of male Sprague-Dawley were used to identify the optimal model: "stressed group" (n = 10), "post-traumatic stress disorder (PTSD) group" (n = 9) and "restraint group" (n = 8), compared with a control group (n = 8). All rats performed neuroimaging [18F]FDG μPET-CT to decipher which model was the most appropriate to test effects of stress on radiotracer binding. Subsequently, a group of rats (n = 10) underwent two PET imaging acquisitions (baseline and PTSD condition) using the PET radiopharmaceutical [18F]F13640 to assess influence of stress on its binding. Voxel-based analysis was performed to assess [18F]FDG or [18F]F13640 changes.

RESULTS

In [18F]FDG experiments, the PTSD group showed a pattern of cerebrometabolic activation in various brain regions previously implicated in stress (amygdala, perirhinal cortex, olfactory bulb and caudate). [18F]F13640 PET scans showed increased radiotracer binding in the PTSD condition in caudate nucleus and brainstem.

CONCLUSIONS

The present study demonstrated stress-induced cerebrometabolic activation or inhibition of various brain regions involved in stress model. Applying this model to our radiotracer, [18F]F13640 showed few influence of stress on its binding. This will enable to rule out any confounding effect of stress during imaging studies.

Medial Temporal Lobe Damage Impairs Temporal Integration in Episodic Memory

Although the role of the medial temporal lobe (MTL) and the hippocampus in episodic memory is well established, there is emerging evidence that these regions play a broader role in cognition, specifically in temporal processing. However, despite strong evidence that the hippocampus plays a critical role in sequential processing, the involvement of the MTL in timing per se is poorly understood. In the present study, we investigated whether patients with MTL damage exhibit differential performance on a temporal distance memory task. Critically, we manipulated context shifts, or boundaries, which have been shown to interfere with associative binding, leading to increases in subjective temporal distance. We predicted that patients with MTL damage would show impaired binding across boundaries and thus fail to show temporal expansion. Consistent with this hypothesis, unilateral patients failed to show a temporal expansion effect, and bilateral patients actually exhibited the reverse effect, suggesting a critical role for the MTL in binding temporal information across boundaries. Furthermore, patients were impaired overall on both the temporal distance memory task and recognition memory, but not on an independent, short-timescale temporal perception task. Interestingly, temporal distance performance could be independently predicted by performance on recognition memory and the short temporal perception task. Together, these data suggest that distinct mnemonic and temporal processes may influence long interval temporal memory and that damage to the MTL may impair the ability to integrate episodic and temporal information in memory.

Opposing effects of pre-encoding stress on neural substrates of item and emotional contextual source memory retrieval

Although the mediating role of the stress hormone systems in memory for single- especially emotional- events is well-stablished, less is known about the influence of stress on memory for associated contextual information (source memory). Here, we investigated the impact of acute stress on the neural underpinnings of emotional contextual source memory. Participants underwent a stress or a control manipulation before they encoded objects paired with pleasant, neutral, or unpleasant backgrounds. One week later, item and contextual source memory were tested. Acute stress modulated the neural signature of item and contextual source memory in an opposite fashion: stressed participants showed larger activation in the precuneus and the medial prefrontal cortex (mPFC) during the retrieval of items, while the retrieval of contextual unpleasant information was associated with lower activation in the angular gyrus (AG) and mPFC. Furthermore, as revealed by cross-region representational similarity analyses, stress also reduced the memory reinstatement of the previously encoded visual cortex representations of object/unpleasant background pairings in the AG and mPFC. These results suggest that pre-encoding stress induction increases the activity of memory-related regions for single items but reduces the activity of these regions during the retrieval of contextual unpleasant information. Our findings provide new insights into the dissociative effects of stress on item and contextual source memory which could have clinical relevance for stress-related disorders.

Recall as a Window into Hippocampally Defined Events

We experience the present as a continuous stream of information, but often experience the past in parcels of unique events or episodes. Decades of research have helped to articulate how we perform this event segmentation in the moment, as well as how events and their boundaries influence what we later remember. More recently, neuroscientific research has suggested that the hippocampus plays a role at critical moments during event formation alongside its established role in enabling subsequent recall. Here, we review and explore the relationship between event processing and recall with the perspective that it can be uniquely characterized by the contributions of the hippocampus and its interactions with the rest of the brain. Specifically, we highlight a growing number of empirical studies suggesting that the hippocampus is important for processing events that have just ended, bridging the gap between the prior and current event, and influencing the contents and trajectories of recalled information. We also catalogue and summarize the multifaceted sets of findings concerning how recall is influenced by event structure. Lastly, we discuss several exciting directions for future research and how our understanding of events might be enriched by characterizing them in terms of the operations of different regions of the brain.

Suppressing the Maintenance of Information in Working Memory Alters Long-term Memory Traces

The sensory recruitment hypothesis conceptualizes information in working memory as being activated representations of information in long-term memory. Accordingly, changes made to an item in working memory would be expected to influence its subsequent retention. Here, we tested the hypothesis that suppressing information from working memory, which can reduce short-term access to that information, may also alter its long-term neural representation. We obtained fMRI data (n = 25; 13 female / 12 male participants) while participants completed a working memory removal task with scene images as stimuli, followed by a final surprise recognition test of the examined items. We applied a multivariate pattern analysis to the data to quantify the engagement of suppression on each trial, to track the contents of working memory during suppression, and to assess representational changes afterward. Our analysis confirms previous reports that suppression of information in working memory involves focused attention to target and remove unwanted information. Furthermore, our findings provide new evidence that even a single dose of suppression of an item in working memory can (if engaged with sufficient strength) produce lasting changes in its neural representation, particularly weakening the unique, item-specific features, which leads to forgetting. Our study sheds light on the underlying mechanisms that contribute to the suppression of unwanted thoughts and highlights the dynamic interplay between working memory and long-term memory.

Scene construction processes in the anterior hippocampus during temporal episodic memory retrieval

Although the hippocampus has been implicated in both the temporal organization of memories and association of scene elements, some theoretical accounts posit that the role of the hippocampus in episodic memory is largely atemporal. In this study, we set out to explore this discrepancy by identifying hippocampal activity patterns related to scene construction while participants performed a temporal order memory task. Participants in the fMRI scanner were shown a sequence of photographs, each consisting of a central object and a contextual background scene. On each retrieval trial, participants were shown a pair of the original photographs (FULL), objects from the scenes without the background (OBJ), or background contexts without the main foreground object (BACK). In the temporal order judgment (TOJ) task, participants judged the temporal order of the pair of scenes; in the Viewing trials, two identical scenes were shown without any task. First, we found that the anterior hippocampus-particularly the CA1 and subiculum-showed similar patterns of activation between the BACK and OBJ conditions, suggesting that scene construction occurred spontaneously during both TOJ and Viewing. Furthermore, neural markers of scene construction in the anterior hippocampus did not apply to incorrect trials, showing that successful temporal memory retrieval was functionally linked to scene construction. In the cortex, time-processing areas, such as the supplementary motor area and the precuneus, and scene-processing areas, such as the parahippocampal cortex, were activated and functionally connected with the hippocampus. Together, these results support the view that the hippocampus is concurrently involved in scene construction and temporal organization of memory and propose a model of hippocampal episodic memory that takes both processes into account.

Pronouns reactivate conceptual representations in human hippocampal neurons

During discourse comprehension, every new word adds to an evolving representation of meaning that accumulates over consecutive sentences and constrains the next words. To minimize repetition and utterance length, languages use pronouns, like the word "she," to refer to nouns and phrases that were previously introduced. It has been suggested that language comprehension requires that pronouns activate the same neuronal representations as the nouns themselves. We recorded from individual neurons in the human hippocampus during a reading task. Cells that were selective to a particular noun were later reactivated by pronouns that refer to the cells' preferred noun. These results imply that concept cells contribute to a rapid and dynamic semantic memory network that is recruited during language comprehension.

The brain hierarchically represents the past and future during multistep anticipation

Memory for temporal structure enables both planning of future events and retrospection of past events. We investigated how the brain flexibly represents extended temporal sequences into the past and future during anticipation. Participants learned sequences of environments in immersive virtual reality. Pairs of sequences had the same environments in a different order, enabling context-specific learning. During fMRI, participants anticipated upcoming environments multiple steps into the future in a given sequence. Temporal structure was represented in the hippocampus and across higher-order visual regions (1) bidirectionally, with graded representations into the past and future and (2) hierarchically, with further events into the past and future represented in successively more anterior brain regions. In hippocampus, these bidirectional representations were context-specific, and suppression of far-away environments predicted response time costs in anticipation. Together, this work sheds light on how we flexibly represent sequential structure to enable planning over multiple timescales.

Lifespan differences in hippocampal subregion connectivity patterns during movie watching

Age-related episodic memory decline is attributed to functional alternations in the hippocampus. Less clear is how aging affects the functional connections of the hippocampus to the rest of the brain during episodic memory processing. We examined fMRI data from the CamCAN dataset, in which a large cohort of participants watched a movie (N = 643; 18-88 years), a proxy for naturalistic episodic memory encoding. We examined connectivity profiles across the lifespan both within the hippocampus (anterior, posterior), and between the hippocampal subregions and cortical networks. Aging was associated with reductions in contralateral (left, right) but not ipsilateral (anterior, posterior) hippocampal subregion connectivity. Aging was primarily associated with increased coupling between the anterior hippocampus and regions affiliated with Control, Dorsal Attention and Default Mode networks, yet decreased coupling between the posterior hippocampus and a selection of these regions. Differences in age-related hippocampal-cortical, but not within-hippocampus circuitry selectively predicted worse memory performance. Our findings comprehensively characterize hippocampal functional topography in relation to cognition in older age, suggesting that shifts in cortico-hippocampal connectivity may be sensitive markers of age-related episodic memory decline.

SSTE: Syllable-Specific Temporal Encoding to FORCE-learn audio sequences with an associative memory approach

The circuitry and pathways in the brains of humans and other species have long inspired researchers and system designers to develop accurate and efficient systems capable of solving real-world problems and responding in real-time. We propose the Syllable-Specific Temporal Encoding (SSTE) to learn vocal sequences in a reservoir of Izhikevich neurons, by forming associations between exclusive input activities and their corresponding syllables in the sequence. Our model converts the audio signals to cochleograms using the CAR-FAC model to simulate a brain-like auditory learning and memorization process. The reservoir is trained using a hardware-friendly approach to FORCE learning. Reservoir computing could yield associative memory dynamics with far less computational complexity compared to RNNs. The SSTE-based learning enables competent accuracy and stable recall of spatiotemporal sequences with fewer reservoir inputs compared with existing encodings in the literature for similar purpose, offering resource savings. The encoding points to syllable onsets and allows recalling from a desired point in the sequence, making it particularly suitable for recalling subsets of long vocal sequences. The SSTE demonstrates the capability of learning new signals without forgetting previously memorized sequences and displays robustness against occasional noise, a characteristic of real-world scenarios. The components of this model are configured to improve resource consumption and computational intensity, addressing some of the cost-efficiency issues that might arise in future implementations aiming for compactness and real-time, low-power operation. Overall, this model proposes a brain-inspired pattern generation network for vocal sequences that can be extended with other bio-inspired computations to explore their potentials for brain-like auditory perception. Future designs could inspire from this model to implement embedded devices that learn vocal sequences and recall them as needed in real-time. Such systems could acquire language and speech, operate as artificial assistants, and transcribe text to speech, in the presence of natural noise and corruption on audio data.

Temporal organization of narrative recall is present but attenuated in adults with hippocampal amnesia

Studies of the impact of brain injury on memory processes often focus on the quantity and episodic richness of those recollections. Here, we argue that the organization of one's recollections offers critical insights into the impact of brain injury on functional memory. It is well-established in studies of word list memory that free recall of unrelated words exhibits a clear temporal organization. This temporal contiguity effect refers to the fact that the order in which word lists are recalled reflects the original presentation order. Little is known, however, about the organization of recall for semantically rich materials, nor how recall organization is impacted by hippocampal damage and memory impairment. The present research is the first study, to our knowledge, of temporal organization in semantically rich narratives in three groups: (1) Adults with bilateral hippocampal damage and severe declarative memory impairment, (2) adults with bilateral ventromedial prefrontal cortex (vmPFC) damage and no memory impairment, and (3) demographically matched non-brain-injured comparison participants. We find that although the narrative recall of adults with bilateral hippocampal damage reflected the temporal order in which those narratives were experienced above chance levels, their temporal contiguity effect was significantly attenuated relative to comparison groups. In contrast, individuals with vmPFC damage did not differ from non-brain-injured comparison participants in temporal contiguity. This pattern of group differences yields insights into the cognitive and neural systems that support the use of temporal organization in recall. These data provide evidence that the retrieval of temporal context in narrative recall is hippocampal-dependent, whereas damage to the vmPFC does not impair the temporal organization of narrative recall. This evidence of limited but demonstrable organization of memory in participants with hippocampal damage and amnesia speaks to the power of narrative structures in supporting meaningfully organized recall despite memory impairment.

Contribution of the lateral occipital and parahippocampal cortices to pattern separation of objects and contexts

Contextual features are integral to episodic memories; yet, we know little about context effects on pattern separation, a hippocampal function promoting orthogonalization of overlapping memory representations. Recent studies suggested that various extrahippocampal brain regions support pattern separation; however, the specific role of the parahippocampal cortex-a region involved in context representation-in pattern separation has not yet been studied. Here, we investigated the contribution of the parahippocampal cortex (specifically, the parahippocampal place area) to context reinstatement effects on mnemonic discrimination, using functional magnetic resonance imaging. During scanning, participants saw object images on unique context scenes, followed by a recognition task involving the repetitions of encoded objects or visually similar lures on either their original context or a lure context. Context reinstatement at retrieval improved item recognition but hindered mnemonic discrimination. Crucially, our region of interest analyses of the parahippocampal place area and an object-selective visual area, the lateral occipital cortex indicated that while during successful mnemonic decisions parahippocampal place area activity decreased for old contexts compared to lure contexts irrespective of object novelty, lateral occipital cortex activity differentiated between old and lure objects exclusively. These results imply that pattern separation of contextual and item-specific memory features may be differentially aided by scene and object-selective cortical areas.

Melatonin attenuates scopolamine-induced cognitive dysfunction through SIRT1/IRE1α/XBP1 pathway

BACKGROUND

The prevalence of dementia around the world is increasing, and these patients are more likely to have cognitive impairments, mood and anxiety disorders (depression, anxiety, and panic disorder), and attention deficit disorders over their lifetime. Previous studies have proven that melatonin could improve memory loss, but its specific mechanism is still confused.

METHODS

In this study, we used in vivo and in vitro models to examine the neuroprotective effect of melatonin on scopolamine (SCOP)-induced cognitive dysfunction. The behavioral tests were performed. 18F-FDG PET imaging was used to assess the metabolism of the brain. Protein expressions were determined through kit detection, Western blot, and immunofluorescence. Nissl staining was conducted to reflect neurodegeneration. MTT assay and RNAi transfection were applied to perform the in vitro experiments.

RESULTS

We found that melatonin could ameliorate SCOP-induced cognitive dysfunction and relieve anxious-like behaviors or HT22 cell damage. 18F-FDG PET-CT results showed that melatonin could improve cerebral glucose uptake in SCOP-treated mice. Melatonin restored the cholinergic function, increased the expressions of neurotrophic factors, and ameliorated oxidative stress in the brain of SCOP-treated mice. In addition, melatonin upregulated the expression of silent information regulator 1 (SIRT1), which further relieved endoplasmic reticulum (ER) stress by decreasing the expression of phosphorylate inositol-requiring enzyme (p-IRE1α) and its downstream, X-box binding protein 1 (XBP1).

CONCLUSIONS

These results indicated that melatonin could ameliorate SCOP-induced cognitive dysfunction through the SIRT1/IRE1α/XBP1 pathway. SIRT1 might be the critical target of melatonin in the treatment of dementia.

Age differences in generalization, memory specificity, and their overnight fate in childhood

Memory enables generalization to new situations, and memory specificity that preserves individual episodes. This study investigated generalization, memory specificity, and their overnight fate in 141 4- to 8-year-olds (computerized memory game; 71 females, tested 2020-2021 in Germany). The results replicated age effects in generalization and memory specificity, and a contingency of generalization on object conceptual properties and interobject semantic proximity. Age effects were stronger in generalization than in memory specificity, and generalization was more closely linked to the explicit regularity knowledge in older than in younger children. After an overnight delay, older children retained more generalized and specific memories and showed greater gains but only in generalization. These findings reveal distinct age differences in generalization and memory specificity across childhood.

Spindle-locked ripples mediate memory reactivation during human NREM sleep

Memory consolidation relies in part on the reactivation of previous experiences during sleep. The precise interplay of sleep-related oscillations (slow oscillations, spindles and ripples) is thought to coordinate the information flow between relevant brain areas, with ripples mediating memory reactivation. However, in humans empirical evidence for a role of ripples in memory reactivation is lacking. Here, we investigated the relevance of sleep oscillations and specifically ripples for memory reactivation during human sleep using targeted memory reactivation. Intracranial electrophysiology in epilepsy patients and scalp EEG in healthy participants revealed that elevated levels of slow oscillation - spindle activity coincided with the read-out of experimentally induced memory reactivation. Importantly, spindle-locked ripples recorded intracranially from the medial temporal lobe were found to be correlated with the identification of memory reactivation during non-rapid eye movement sleep. Our findings establish ripples as key-oscillation for sleep-related memory reactivation in humans and emphasize the importance of the coordinated interplay of the cardinal sleep oscillations.

Improving associative memory in younger and older adults with unitization: evidence from meta-analysis and behavioral studies

Introduction

The finding that familiarity can support associative memory by unitizing the to -be-learned items into a novel representation has been widely accepted, but its effects on overall performance of associative memory and recollection are still controversial.

Methods

The current study aims to elucidate these discrepancies by identifying potential moderating factors through a combined approach of meta-analysis and behavioral experiment.

Results

Results consistently showed that changes in the level of unitization and age groups were two important moderators. Specifically, unitization enhanced younger and older adults' associative memory and its supporting processes (i.e., familiarity and recollection) when the level of unitization between studied and rearranged pairs was changed. However, when this level remained constant, unitization exhibited no impact on associative memory and familiarity in younger adults, but showed an enhanced effect in older adults. Furthermore, results revealed a marked group difference between younger and older adults in associative memory when the unitization level of noncompound words remained unaltered. Upon breaking this condition, the group difference was reduced by enhancing familiarity or recollection.

Discussion

These findings not only clarify some of the inconsistencies in the literature concerning the impact of unitization on associative memory, but also suggest that unitization is a beneficial strategy for reducing group difference in associative memory, with its effectiveness varying according to the level of unitization changes.

Integrating and fragmenting memories under stress and alcohol

Stress can powerfully influence the way we form memories, particularly the extent to which they are integrated or situated within an underlying spatiotemporal and broader knowledge architecture. These different representations in turn have significant consequences for the way we use these memories to guide later behavior. Puzzlingly, although stress has historically been argued to promote fragmentation, leading to disjoint memory representations, more recent work suggests that stress can also facilitate memory binding and integration. Understanding the circumstances under which stress fosters integration will be key to resolving this discrepancy and unpacking the mechanisms by which stress can shape later behavior. Here, we examine memory integration at multiple levels: linking together the content of an individual experience, threading associations between related but distinct events, and binding an experience into a pre-existing schema or sense of causal structure. We discuss neural and cognitive mechanisms underlying each form of integration as well as findings regarding how stress, aversive learning, and negative affect can modulate each. In this analysis, we uncover that stress can indeed promote each level of integration. We also show how memory integration may apply to understanding effects of alcohol, highlighting extant clinical and preclinical findings and opportunities for further investigation. Finally, we consider the implications of integration and fragmentation for later memory-guided behavior, and the importance of understanding which type of memory representation is potentiated in order to design appropriate interventions.

The effects of mnemonic variability and spacing on memory over multiple timescales

The memory benefit that arises from distributing learning over time rather than in consecutive sessions is one of the most robust effects in cognitive psychology. While prior work has mainly focused on repeated exposures to the same information, in the real world, mnemonic content is dynamic, with some pieces of information staying stable while others vary. Thus, open questions remain about the efficacy of the spacing effect in the face of variability in the mnemonic content. Here, in two experiments, we investigated the contributions of mnemonic variability and the timescale of spacing intervals, ranging from seconds to days, to long-term memory. For item memory, both mnemonic variability and spacing intervals were beneficial for memory; however, mnemonic variability was greater at shorter spacing intervals. In contrast, for associative memory, repetition rather than mnemonic variability was beneficial for memory, and spacing benefits only emerged in the absence of mnemonic variability. These results highlight a critical role for mnemonic variability and the timescale of spacing intervals in the spacing effect, bringing this classic memory paradigm into more ecologically valid contexts.

The beneficial effects of concept definition and interactive imagery tasks on associative memory: Evidence from event-related potentials

It is widely accepted that familiarity can support associative memory when the to-be-remember items are unitized into a new representation. However, there has been relatively little attention devoted to investigating the effects of different unitization manipulations on associative memory. The present study aimed to address this gap by examining the effects of varying levels of unitization through three tasks: Concept definition, interactive imagery, and sentence frame tasks. The behavioral results revealed that associative memory was significantly enhanced in the interactive imagery task compared to the sentence frame task. However, no significant differences were found between the sentence frame and concept definition tasks, or between the concept definition and interactive imagery tasks. In terms of the neural correlates, the event-related potential (ERP) results revealed that the sentence frame task only elicited a significant recollection-related LPC old/new effect, while the concept definition task only elicited a significant familiarity-related FN400 old/new effect. However, the interactive representation task elicited both of these distinct effects. These findings suggest that both the concept definition and interactive imagery tasks can enhance familiarity for supporting associative memory, but their beneficial effects on associative memory or LPC old/new effects may be different.

Memory functioning after hippocampal removal: Does side matter?

To address the memory functioning after medial temporal lobe (MTL) surgery for refractory epilepsy and relationships with the side of the hippocampal removal, 22 patients with pharmaco-resistant epilepsy who had undergone MTL resection (10 right/12 left) at the Salpêtrière Hospital were compared with 21 matched healthy controls. We designed a specific neuropsychological binding memory test that specifically addressed hippocampal cortex functioning, and left-right material-specific lateralization. Our results showed that both left and right mesial temporal lobe removal cause a severe memory impairment, for both verbal and visual material. The removal of left medial temporal lobe causes worse memory impairment than the right removal regardless of the stimuli type (verbal or visual) questioning the theory of the hippocampal material-specific lateralization. The present study provided new evidence for the role of both hippocampus and surrounding cortices in memory-binding whatever the material type and also suggested that a left MTL removal is more deleterious for both verbal and visual episodic memory in comparison with right MTL removal.

Evidence for shallow cognitive maps in schizophrenia

Individuals with schizophrenia can have marked deficits in goal-directed decision making. Prominent theories differ in whether schizophrenia (SZ) affects the ability to exert cognitive control, or the motivation to exert control. An alternative explanation is that schizophrenia negatively impacts the formation of cognitive maps, the internal representations of the way the world is structured, necessary for the formation of effective action plans. That is, deficits in decision-making could also arise when goal-directed control and motivation are intact, but used to plan over ill-formed maps. Here, we test the hypothesis that individuals with SZ are impaired in the construction of cognitive maps. We combine a behavioral representational similarity analysis technique with a sequential decision-making task. This enables us to examine how relationships between choice options change when individuals with SZ and healthy age-matched controls build a cognitive map of the task structure. Our results indicate that SZ affects how people represent the structure of the task, focusing more on simpler visual features and less on abstract, higher-order, planning-relevant features. At the same time, we find that SZ were able to display similar performance on this task compared to controls, emphasizing the need for a distinction between cognitive map formation and changes in goal-directed control in understanding cognitive deficits in schizophrenia.

Subcortical contributions to the sense of body ownership

The sense of body ownership (i.e. the feeling that our body or its parts belong to us) plays a key role in bodily self-consciousness and is believed to stem from multisensory integration. Experimental paradigms such as the rubber hand illusion have been developed to allow the controlled manipulation of body ownership in laboratory settings, providing effective tools for investigating malleability in the sense of body ownership and the boundaries that distinguish self from other. Neuroimaging studies of body ownership converge on the involvement of several cortical regions, including the premotor cortex and posterior parietal cortex. However, relatively less attention has been paid to subcortical structures that may also contribute to body ownership perception, such as the cerebellum and putamen. Here, on the basis of neuroimaging and neuropsychological observations, we provide an overview of relevant subcortical regions and consider their potential role in generating and maintaining a sense of ownership over the body. We also suggest novel avenues for future research targeting the role of subcortical regions in making sense of the body as our own.

Option similarity modulates the link between choice and memory

Choices made in everyday life are highly variable. Sometimes, you may find yourself choosing between two similar options (e.g., breakfast foods to eat) and other times between two dissimilar options (e.g., what to buy with a gift certificate). The goal of the present study was to understand how the similarity of choice options affects our ability to remember what we choose and what we did not choose. We hypothesized that choosing between similar as compared to dissimilar options would evoke a comparison-based strategy (evaluating options with respect to one another), fostering a relational form of encoding and leading to better memory for both the chosen and unchosen options. In Experiment 1, participants reported their strategy when choosing between pairs of similar or dissimilar options, revealing that participants were more likely to use a comparison-based strategy when faced with similar options. In Experiment 2, we tested memory after participants made choices between similar or dissimilar options, finding improved memory for both chosen and unchosen options from the similar compared to dissimilar choice trials. In Experiment 3, we examined strategy use when choosing between pairs of similar or dissimilar options and memory for these options. Replicating and extending the results of the first two experiments, we found that participants were more likely to use a comparison-based strategy when choosing between similar than dissimilar options, and that the positive effect of similarity on memory was stronger for unchosen than chosen options when controlling for strategy use. We interpret our results as evidence that option similarity impacts the mnemonic processes used during choice, altering what we encode and ultimately remember about our choices.

The emergence of task-relevant representations in a nonlinear decision-making task

This paper describes the relationship between performance in a decision-making task and the emergence of task-relevant representations. Participants learnt two tasks in which the appropriate response depended on multiple relevant stimuli and the underlying stimulus-outcome associations were governed by a latent feature that participants could discover. We divided participants into good and bad performers based on their overall classification rate and computed behavioural accuracy for each feature value. We found that participants with better performance had a better representation of the latent feature space. We then used representation similarity analysis on Electroencephalographic (EEG) data to identify when these representations emerge. We were able to decode task-relevant representations in a time window emerging 700 ms after stimulus presentation, but only for participants with good task performance. Our findings suggest that, in order to make good decisions, it is necessary to create and extract a low-dimensional representation of the task at hand.

Long-term, multi-event surprise correlates with enhanced autobiographical memory

Neurobiological and psychological models of learning emphasize the importance of prediction errors (surprises) for memory formation. This relationship has been shown for individual momentary surprising events; however, it is less clear whether surprise that unfolds across multiple events and timescales is also linked with better memory of those events. We asked basketball fans about their most positive and negative autobiographical memories of individual plays, games and seasons, allowing surprise measurements spanning seconds, hours and months. We used advanced analytics on National Basketball Association play-by-play data and betting odds spanning 17 seasons, more than 22,000 games and more than 5.6 million plays to compute and align the estimated surprise value of each memory. We found that surprising events were associated with better recall of positive memories on the scale of seconds and months and negative memories across all three timescales. Game and season memories could not be explained by surprise at shorter timescales, suggesting that long-term, multi-event surprise correlates with memory. These results expand notions of surprise in models of learning and reinforce its relevance in real-world domains.

Neural patterns differentiate traumatic from sad autobiographical memories in PTSD

For people with post-traumatic stress disorder (PTSD), recall of traumatic memories often displays as intrusions that differ profoundly from processing of 'regular' negative memories. These mnemonic features fueled theories speculating a unique cognitive state linked with traumatic memories. Yet, to date, little empirical evidence supports this view. Here we examined neural activity of patients with PTSD who were listening to narratives depicting their own memories. An intersubject representational similarity analysis of cross-subject semantic content and neural patterns revealed a differentiation in hippocampal representation by narrative type: semantically similar, sad autobiographical memories elicited similar neural representations across participants. By contrast, within the same individuals, semantically similar trauma memories were not represented similarly. Furthermore, we were able to decode memory type from hippocampal multivoxel patterns. Finally, individual symptom severity modulated semantic representation of the traumatic narratives in the posterior cingulate cortex. Taken together, these findings suggest that traumatic memories are an alternative cognitive entity that deviates from memory per se.

Neuropsychological predictions on involuntary autobiographical memory and déjà vu

I strongly support Barzykowski and Moulin in their proposal that common retrieval mechanisms can lead to distinct phenomenological memory experiences. I emphasize the importance of one of these mechanisms, namely the attribution system. Neuropsychological studies should help clarifying the role of these retrieval mechanisms, notably in cases of medial temporal-lobe lesions and cases of dementia.

Hippocampal Mechanisms Support Cortisol-Induced Memory Enhancements

Stress can powerfully influence episodic memory, often enhancing memory encoding for emotionally salient information. These stress-induced memory enhancements stand at odds with demonstrations that stress and the stress-related hormone cortisol can negatively affect the hippocampus, a brain region important for episodic memory encoding. To resolve this apparent conflict and determine whether and how the hippocampus supports memory encoding under cortisol, we combined behavioral assays of associative memory, high-resolution fMRI, and pharmacological manipulation of cortisol in a within-participant, double-blinded procedure (in both sexes). Behaviorally, hydrocortisone promoted the encoding of subjectively arousing, positive associative memories. Neurally, hydrocortisone led to enhanced functional connectivity between hippocampal subregions, which predicted subsequent memory enhancements for emotional associations. Cortisol also modified the relationship between hippocampal representations and associative memory: whereas hippocampal signatures of distinctiveness predicted memory under placebo, relative integration predicted memory under cortisol. Together, these data provide novel evidence that the human hippocampus contains the necessary machinery to support emotional associative memory enhancements under cortisol.SIGNIFICANCE STATEMENT Our daily lives are filled with stressful events, which powerfully shape the way we form episodic memories. For example, stress and stress-related hormones can enhance our memory for emotional events. However, the mechanisms underlying these memory benefits are unclear. In the current study, we combined functional neuroimaging, behavioral tests of memory, and double-blind, placebo-controlled hydrocortisone administration to uncover the effects of the stress-related hormone cortisol on the function of the human hippocampus, a brain region important for episodic memory. We identified novel ways in which cortisol can enhance hippocampal function to promote emotional memories, highlighting the adaptive role of cortisol in shaping memory formation.