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The human hippocampus is a brain region that supports episodic and spatial memory. Recent experiments have drawn on animal research and computational modelling to reveal how the unique computations and representations of the hippocampus support episodic and spatial memory. Invasive electrophysiological recordings and non-invasive functional brain imaging have provided evidence for the rapid formation of hippocampal representations, as well as the ability of the hippocampus to both pattern-separate and pattern-complete input from the neocortex. Further, recent evidence has shown that hippocampal representations are in constant flux, undergoing a continual process of strengthening, weakening and altering. This research offers a glimpse into the highly plastic and flexible nature of the human hippocampal system in relation to episodic memory. [ABSTRACT FROM AUTHOR]

The hippocampal formation has been associated with a wide variety of functions including spatial navigation and planning, memory encoding and retrieval, relational processing, novelty detection, and imagination. These functions are dissimilar in terms of their behavioral consequences and modality of representation. Consequently, theoretical standpoints have focused on explaining the role of the hip-pocampal formation in terms of either its spatial or nonspatial functions. Contrary to this dichotomy, we propose that it is essential to look beyond these traditional boundaries between mnemonic and spatial functions and focus instead on the processes that these functions have in common. In this framework, we use electrophysiology data from the spatial domain to predict effects on the systems level, both in spatial and nonspatial domains. We initially outline the results of studies that have used findings from spatial navigation in rodents to predict the patterns of brain activity observable in people who are exploring virtual environments. We discuss how certain properties of space-defining neurons enable space to be represented as a mental map of interconnected locations, which are expressed at multiple spatial scales in separate modules in the hippocampal formation. We then suggest that memories are also organized in networks, characterized by mnemonic and temporal hierarchies. We finish by discussing how virtual-reality techniques can be used to create novel lifelike episodes allowing us to look at episodic memory processes while multivariate analysis tools can be used to explore the organizational structure of mnemonic networks. [ABSTRACT FROM AUTHOR]

How do we know where we are? Orientation in space is key to our daily existence as we follow familiar routes, navigate to a previous location, or just try to get home as quickly as possible. As well as being interesting in its own right, spatial cognition is also a useful model system in which to understand the neural bases of cognition and memory formation more generally. Spatial behavior offers potentially straightforward correlates of neuronal activity that can be studied similarly in adults and infants of both human and non-human animals. The neural mechanisms of spatial behavior can be realistically investigated in a well-controlled way with the aid of virtual reality technologies in humans and rodents. Virtual reality can thus help to bridge the gap between electrophysiological studies in rodents and brain imaging studies using functional magnetic resonance imaging in humans. Within this framework, this article aims to translate findings from the single cell level in rodents to understand the neural and systems level mechanisms of spatial cognition in the human brain. [ABSTRACT FROM PUBLISHER]

The hippocampus is crucial for episodic or declarative memory and the theta rhythm has been implicated in mnemonic processing, but the functional contribution of theta to memory remains the subject of intense speculation. Recent evidence suggests that the hippocampus might function as a network hub for volitional learning. In contrast to human experiments, electrophysiological recordings in the hippocampus of behaving rodents are dominated by theta oscillations reflecting volitional movement, which has been linked to spatial exploration and encoding. This literature makes the surprising crossspecies prediction that the human hippocampal theta rhythm supports memory by coordinating exploratory movements in the service of self-directed learning. We examined the links between theta, spatial exploration, and memory encoding by designing an interactive human spatial navigation paradigm combined with multimodal neuroimaging. We used both non-invasive whole-head Magnetoencephalography (MEG) to look at theta oscillations and Functional Magnetic Resonance Imaging (fMRI) to look at brain regions associated with volitional movement and learning. We found that theta power increases during the selfinitiation of virtual movement, additionally correlating with subsequent memory performance and environmental familiarity. Performance-related hippocampal theta increases were observed during a static pre-navigation retrieval phase, where planning for subsequent navigation occurred. Furthermore, periods of the task showing movement-related theta increases showed decreased fMRI activity in the parahippocampus and increased activity in the hippocampus and other brain regions that strikingly overlap with the previously observed volitional learning network (the reverse pattern was seen for stationary periods). These fMRI changes also correlated with participant's performance. Our findings suggest that the human hippocampal theta rhythm supports memory by coordinating exploratory movements in the service of self-directed learning. These findings directly extend the role of the hippocampus in spatial exploration in rodents to human memory and self-directed learning. [ABSTRACT FROM AUTHOR]

Planning spatial paths through our environment is an important part of everyday life and is supported by a neural system including the hippocampus and prefrontal cortex. Here we investigated the precise functional roles of the components of this system in humans by using fMRI as participants performed a simple goal-directed route-planning task. Participants had to choose the shorter of two routes to a goal in a visual scene that might contain a barrier blocking the most direct route, requiring a detour, or might be obscured by a curtain, requiring memory for the scene. The participant's start position was varied to parametrically manipulate their proximity to the goal and the difference in length of the two routes. Activity in medial prefrontal cortex, precuneus, and left posterior parietal cortex was associated with detour planning, regardless of difficulty, whereas activity in parahippocampal gyrus was associated with remembering the spatial layout of the visual scene. Activity in bilateral anterior hippocampal formation showed a strong increase the closer the start position was to the goal, together with medial prefrontal, medial and posterior parietal cortices. Our results are consistent with computational models in which goal proximity is used to guide subsequent navigation and with the association of anterior hippocampal areas with nonspatial functions such as arousal and reward expectancy. They illustrate how spatial and nonspatial functions combine within the anterior hippocampus, and how these functions interact with parahippocampal, parietal, and prefrontal areas in decision making and mnemonic function. [ABSTRACT FROM AUTHOR]

The hippocampus is crucial for both spatial navigation and episodic memory, suggesting that it provides a common function to both. Here we adapt a spatial paradigm, developed for rodents, for use with functional MRI in humans to show that activation of the right hippocampus predicts the use of an allocentric spatial representation, and activation of the left hippocampus predicts the use of a sequential egocentric representation. Both representations can be identified in hippocampal activity before their effect on behavior at subsequent choice-points. Our results suggest that, rather than providing a single common function, the two hippocampi provide complementary representations for navigation, concerning places on the right and temporal sequences on the left, both of which likely contribute to different aspects of episodic memory. [ABSTRACT FROM AUTHOR]

Abstract: It is a topic of current interest whether learning in humans relies on the acquisition of abstract rule knowledge (rule-based learning) or whether it depends on superficial item-specific information (instance-based learning). Here, we identified brain regions that mediate either of the two learning mechanisms by combining fMRI with an experimental protocol shown to be able to dissociate both learning mechanisms. Subjects had to learn object-position conjunctions in several trials and blocks. In a learning condition, either objects (Experiment 1) or positions (Experiment 2) were held constant within-blocks. In contrast to a control condition in which object–position conjunctions were trial-unique, a performance increase within and across-blocks was observed in the learning condition of both experiments. We hypothesized that within-block learning mainly relies on instance-based processes, whereas across-block learning might depend on rule-based mechanisms. A within-block parametric fMRI analysis revealed a learning-related increase of lateral prefrontal and striatal activity and a learning-related decrease of hippocampal activity in both experiments. By contrast, across-block learning was associated with an activation modulation in distinct prefrontal-striatal brain regions, but not in the hippocampus. These data indicate that hippocampal and prefrontal-striatal brain regions differentially contribute to instance-based and rule-based learning. [Copyright &y& Elsevier]

The article discusses the works regarding the conjunctive representations in the Hippocampus that allows acquisition of new memories. It mentions that work of O'Keefe and Nadel in 1978 and Eichenbaum in 2000 which demonstrates that hippocampus plays a vital role for episodic memory. It evaluates the work of Wood and colleagues in 1999 that uses rat models which exhibits that position is the most distinct correlate of place cell activity.

Maintaining an accurate model of the world relies on our ability to update memory representations in light of new information. Previous research on the integration of new information into memory mainly focused on the hippocampus. Here, we hypothesized that the angular gyrus, known to be involved in episodic memory and imagination, plays a pivotal role in the insight-driven reconfiguration of memory representations. To test this hypothesis, participants received continuous theta burst stimulation (cTBS) over the left angular gyrus or sham stimulation before gaining insight into the relationship between previously separate life-like animated events in a narrative-insight task. During this task, participants also underwent EEG recording and their memory for linked and non-linked events was assessed shortly thereafter. Our results show that cTBS to the angular gyrus decreased memory for the linking events and reduced the memory advantage for linked relative to non-linked events. At the neural level, cTBS targeting the angular gyrus reduced centro-temporal coupling with frontal regions and abolished insight-induced neural representational changes for events linked via imagination, indicating impaired memory reconfiguration. Further, the cTBS group showed representational changes for non-linked events that resembled the patterns observed in the sham group for the linked events, suggesting failed pruning of the narrative in memory. Together, our findings demonstrate a causal role of the left angular gyrus in insight-related memory reconfigurations. [ABSTRACT FROM AUTHOR]

Episodic memories are constructed from sequences of events. When recalling such amemory, we not only recall individual events, but we also retrieve information about how the sequence of events unfolded.Here, we focus on the role of the hippocampal--entorhinal region in processing and remembering sequences of events, which are thought to be stored in relational networks. We summarize evidence that temporal relations are a central organizational principle formemories in the hippocampus. Importantly, we incorporate novel insights from recent studies about the role of the adjacent entorhinal cortex in sequence memory. In rodents, the lateral entorhinal subregion carries temporal information during ongoing behavior. The human homologue is recruited during memory recall where its representations reflect the temporal relationships between events encountered in a sequence. We further introduce the idea that the hippocampal--entorhinal region might enable temporal scaling of sequence representations. Flexible changes of sequence progression speed could underlie the traversal of episodic memories and mental simulations at different paces. In conclusion, we describe how the entorhinal cortex and hippocampus contribute to remembering event sequences--a core component of episodic memory. [ABSTRACT FROM AUTHOR]

The hippocampal formation encodes maps of space and a key question in neuroscience is whether its spatial coding principles also provide a universal metric for the organization of nonspatial, conceptual information. Previous work demonstrated directional coding during navigation through a continuous stimulus feature space as well as mapping of distances in a feature space that was relevant for concept learning. Here we provide the first unambiguous evidence for a hippocampal representation of the actual concept space, by showing that the hippocampal distance signal selectively reflects the mapping of specifically conceptually relevant rather than of all feature dimensions. During fMRI scanning of 32 human participants (21 females), we presented everyday objects, which had beforehand been associated with specific values on three continuous feature dimensions. Crucially, only two dimensions were relevant to prior concept learning. We find that hippocampal responses to the objects reflect their relative distances in a space defined along conceptually relevant dimensions compared with distances in a space defined along all feature dimensions. These findings suggest that the hippocampus supports knowledge acquisition by dynamically encoding information in a space spanned along dimensions that are relevant in relation to define concepts. [ABSTRACT FROM AUTHOR]

A growing body of work is investigating the use of mental maps during decision-making. Here we discuss how decision-making organizes experiences according to an internal model of the current task, thereby structuring memory. Likewise, we consider how the structure of mental maps contributes to decision-making. [ABSTRACT FROM AUTHOR]

The brain encodes the statistical regularities of the environment in a task-specific yet flexible and generalizable format. Here, we seek to understand this process by bridging two parallel lines of research, one centered on sensorimotor timing, and the other on cognitive mapping in the hippocampal system. By combining functional magnetic resonance imaging (fMRI) with a fast-paced time-to-contact (TTC) estimation task, we found that the hippocampus signaled behavioral feedback received in each trial as well as performance improvements across trials along with reward-processing regions. Critically, it signaled performance improvements independent from the tested intervals, and its activity accounted for the trial-wise regression-to-the-mean biases in TTC estimation. This is in line with the idea that the hippocampus supports the rapid encoding of temporal context even on short time scales in a behavior-dependent manner. Our results emphasize the central role of the hippocampus in statistical learning and position it at the core of a brain-wide network updating sensorimotor representations in real time for flexible behavior. [ABSTRACT FROM AUTHOR]

Summary: Two recent imaging studies have shed new light on information representation in human parahippocampal cortex. Despite their different approaches, the two studies both support the view that this brain region represents space at an elementary level. [ABSTRACT FROM AUTHOR]

Summary Our memories are remarkably dynamic and allow us to reinterpret the past once new information comes to light. Gaining novel insights can lead to mental reorganization of previously unrelated events, thus linking them into narratives. The hippocampus and medial prefrontal cortex (mPFC) support integration of partially overlapping events, but the neural mechanisms underlying the reorganization of memories for the formation of coherent narratives remain elusive. Here, we combine fMRI with The Sims 3 videos of life-like animated events, which could either be integrated into narratives or not. We show that insight triggers the emergence of de novo mnemonic representations of the narratives and is associated with increased neural similarity between linked event representations in the posterior hippocampus, mPFC, and autobiographical-memory network. Simultaneously, events irrelevant to the newly established memory of the narrative were pruned out. This process was accompanied by increased neural dissimilarity between non-linked event representations in the posterior hippocampus and mPFC and was additionally signaled by a mismatch response in the anterior hippocampus. Our results demonstrate that insight leads to neural reconfiguration of representational networks within a memory space and have implications for knowledge acquisition in educational settings. [ABSTRACT FROM AUTHOR]

Memories, similar to the internal representation of space, can be recalled at different resolutions ranging from detailed events to more comprehensive, multi-event narratives. Single-cell recordings in rodents have suggested that different spatial scales are represented as a gradient along the hippocampal axis. We found that a similar organization holds for human episodic memory: memory representations systematically vary in scale along the hippocampal long axis, which may enable the formation of mnemonic hierarchies. [ABSTRACT FROM AUTHOR]

A key aspect of conceptual knowledge is that it can be flexibly applied at different levels of abstraction, implying a hierarchical organization. It is yet unclear how this hierarchical structure is acquired and represented in the brain. Here we investigate the computations underlying the acquisition and representation of the hierarchical structure of conceptual knowledge in the hippocampal-prefrontal system of 32 human participants (22 females). We assessed the hierarchical nature of learning during a novel tree-like categorization task via computational model comparisons. The winning model allowed to extract and quantify estimates for accumulation and updating of hierarchical compared with single-feature-based concepts from behavior. We find that mPFC tracks accumulation of hierarchical conceptual knowledge over time, and mPFC and hippocampus both support trial-to-trial updating. As a function of those learning parameters, mPFC and hippocampus further show connectivity changes to rostro-lateral PFC, which ultimately represented the hierarchical structure of the concept in the final stages of learning. Our results suggest that mPFC and hippocampus support the integration of accumulated evidence and instantaneous updates into hierarchical concept representations in rostro-lateral PFC. [ABSTRACT FROM AUTHOR]

• Episodic memory integration is often impaired in stress-related mental disorders. • Stress reduced medial temporal activity related to insight into linked events. • Stress abolished insight-driven increase in hippocampal pattern dissimilarity. • Stress-related neural changes were linked to subsequent memory. Memories are not stored in isolation. Insight into the relationship of initially unrelated events may trigger a flexible reconfiguration of the mnemonic representation of these events. Such representational changes allow the integration of events into coherent episodes and help to build up-to-date-models of the world around us. This process is, however, frequently impaired in stress-related mental disorders resulting in symptoms such as fragmented memories in PTSD. Here, we combined a real life-like narrative-insight task, in which participants learned how initially separate events are linked, with fMRI-based representational similarity analysis to test if and how acute stress interferes with the insight-driven reconfiguration of memories. Our results showed that stress reduced the activity of medial temporal and prefrontal areas when participants gained insight into the link between events. Moreover, stress abolished the insight-related increase in representational dissimilarity for linked events in the anterior part of the hippocampus as well as its association with measures of subsequent memory that we observed in non-stressed controls. However, memory performance, as assessed in a forced-choice recognition test, was even enhanced in the stress group. Our findings suggest that acute stress impedes the neural integration of events into coherent episodes but promotes long-term memory for these integrated narratives and may thus have implications for understanding memory distortions in stress-related mental disorders. [ABSTRACT FROM AUTHOR]

Narratives may provide a general context, unrestricted by space and time, which can be used to organize episodic memories into networks of related events. However, it is not clear how narrative contexts are represented in the brain. Here we test the novel hypothesis that the formation of narrative-based contextual representations in humans relies on the same hippocampal mechanisms that enable formation of spatiotemporal contexts in rodents. Participants watched a movie consisting of two interleaved narratives while we monitored their brain activity using fMRI. We used representational similarity analysis, a type of multivariate pattern analysis, which uses across-voxel correlations as a proxy for neural-pattern similarity, to examine whether the patterns of neural activity can be used to differentiate between narratives and recurring narrative elements, such as people and locations. We demonstrate that the neural activity patterns in the hippocampus differentiate between event nodes (people and locations) and narratives (different stories) and that these narrativecontext representations diverge gradually over time akin to remapping-induced spatial maps represented by rodent place cells. [ABSTRACT FROM AUTHOR]

Summary Integration of separate memories forms the basis of inferential reasoning—an essential cognitive process that enables complex behavior. Considerable evidence suggests that both hippocampus and medial prefrontal cortex (mPFC) play a crucial role in memory integration. Although previous studies indicate that theta oscillations facilitate memory processes, the electrophysiological mechanisms underlying memory integration remain elusive. To bridge this gap, we recorded magnetoencephalography data while participants performed an inference task and employed novel source reconstruction techniques to estimate oscillatory signals from the hippocampus. We found that hippocampal theta power during encoding predicts subsequent memory integration. Moreover, we observed increased theta coherence between hippocampus and mPFC. Our results suggest that integrated memory representations arise through hippocampal theta oscillations, possibly reflecting dynamic switching between encoding and retrieval states, and facilitating communication with mPFC. These findings have important implications for our understanding of memory-based decision making and knowledge acquisition. [ABSTRACT FROM AUTHOR]

The cortical reinstatement hypothesis of memory retrieval posits that content-specific cortical activity at encoding is reinstated at retrieval. Evidence for cortical reinstatement was found in higher-order sensory regions, reflecting reactivation of complex object-based information. However, it remains unclear whether the same detailed sensory, feature-based information perceived during encoding is subsequently reinstated in early sensory cortex and what the role of the hippocampus is in this process. In this study, we used a combination of visual psychophysics, functional neuroimaging, multivoxel pattern analysis, and a well controlled cued recall paradigm to address this issue. We found that the visual information human participants were retrieving could be predicted by the activation patterns in early visual cortex. Importantly, this reinstatement resembled the neural pattern elicited when participants viewed the visual stimuli passively, indicating shared representations between stimulus-driven activity and memory. Furthermore, hippocampal activity covaried with the strength of stimulus-specific cortical reinstatement on a trial-by-trial level during cued recall. These findings provide evidence for reinstatement of unique associative memories in early visual cortex and suggest that the hippocampus modulates the mnemonic strength of this reinstatement. [ABSTRACT FROM AUTHOR]

When we visualize scenes, either from our own past or invented, we impose a viewpoint for our "mind's eye" and we experience the resulting image as spatially coherent from that viewpoint. The hippocampus has been implicated in this process, but its precise contribution is unknown. We tested a specific hypothesis based on the spatial firing properties of neurons in the hippocampal formation of rats, that this region supports the construction of spatially coherent mental images by representing the locations of the environmental boundaries surrounding our viewpoint. Using functional magnetic resonance imaging, we show that hippocampal activation increases parametrically with the number of enclosing boundaries in the imagined scene. In contrast, hippocampal activity is not modulated by a nonspatial manipulation of scene complexity nor to increasing difficulty of imagining the scenes in general. Our findings identify a specific computational role for the hippocampus in mental imagery and episodic recollection. [ABSTRACT FROM AUTHOR]

The article reports that cognitive spaces provide a domaingeneral format for processing in the hippocampalentorhinal region, in line with its involvement beyond navigation and memory. It mentions that technological advances such as calcium imaging will clarify coding principles on the population level and facilitate the translation to human cognitive neuroscience.