Recent functional imaging work supports the view that item and relational memory depend upon distinct encoding operations within the medial temporal lobe. Specifically, emerging findings demonstrate ...that the level of engagement of perirhinal cortex predicts later memory for individual items, whereas the level of hippocampal processing correlates with later relational memory, or recovery of additional episodic details. Furthermore, recent functional magnetic resonance imaging evidence in humans suggests that medial temporal lobe cortical input structures, the perirhinal and posterior parahippocampal cortices, differentially participate in the encoding of objects and their context, providing domain-specific input to the hippocampus. Taken together, these data help to construct a working model of how distinct medial temporal lobe structures participate in episodic memory formation with domain-general relational binding mechanisms supported by the hippocampus and provide emerging evidence for domain-specificity within the perirhinal and parahippocampal cortices.
Everyday life unfolds continuously, yet we tend to remember past experiences as discrete event sequences or episodes. Although this phenomenon has been well documented, the neuromechanisms that ...support the transformation of continuous experience into distinct and memorable episodes remain unknown. Here, we show that changes in context, or event boundaries, elicit a burst of autonomic arousal, as indexed by pupil dilation. Event boundaries also lead to the segmentation of adjacent episodes in later memory, evidenced by changes in memory for the temporal duration, order, and perceptual details of recent event sequences. These subjective and objective changes in temporal memory are also related to distinct temporal features of pupil dilations to boundaries as well as to the temporal stability of more prolonged pupil-linked arousal states. Collectively, our findings suggest that pupil measures reflect both stability and change in ongoing mental context representations, which in turn shape the temporal structure of memory.
After experiences are encoded into memory, post-encoding reactivation mechanisms have been proposed to mediate long-term memory stabilization and transformation. Spontaneous reactivation of ...hippocampal representations, together with hippocampal–cortical interactions, are leading candidate mechanisms for promoting systems-level memory strengthening and reorganization. While the replay of spatial representations has been extensively studied in rodents, here we review recent fMRI work that provides evidence for spontaneous reactivation of nonspatial, episodic event representations in the human hippocampus and cortex, as well as for experience-dependent alterations in systems-level hippocampal connectivity. We focus on reactivation during awake post-encoding periods, relationships between reactivation and subsequent behavior, how reactivation is modulated by factors that influence consolidation, and the implications of persistent reactivation for biasing ongoing perception and cognition.
Recent human fMRI studies provide evidence for spontaneous memory-related reactivation and hippocampal interactions during awake post-encoding time periods.Post-encoding awake reactivation is modulated by factors that influence memory consolidation, such as salience and reward, and predicts behavioral memory integration and the reorganization of cortical memory representations.Reactivation of prior brain states may bias the way in which new information is experienced, perceived, and acted upon.Several open questions remain, such as how awake reactivation is related to ongoing conscious experience, which awake brain states may be optimal for supporting reactivation and memory retention, and how reactivation during sleep and wake may interact to ultimately support memory consolidation and retention.
Structured knowledge is thought to form, in part, through the extraction and representation of regularities across overlapping experiences. However, little is known about how consolidation processes ...may transform novel episodic memories to reflect such regularities. In a multi-day fMRI study, participants encoded trial-unique associations that shared features with other trials. Multi-variate pattern analyses were used to measure neural similarity across overlapping and non-overlapping memories during immediate and 1-week retrieval of these associations. We found that neural patterns in the hippocampus and medial prefrontal cortex represented the featural overlap across memories, but only after a week. Furthermore, after a week, the strength of a memory’s unique episodic reinstatement during retrieval was inversely related to its representation of overlap, suggesting a trade-off between the integration of related memories and recovery of episodic details. These findings suggest that consolidation-related changes in neural representations support the gradual organization of discrete episodes into structured knowledge.
•Patterns in mPFC and hippocampus represent featural overlap across remote memories•Hippocampal encoding patterns are reinstated during remote retrieval•Reinstatement is inversely related to the representation of overlap in hippocampus•Encoding-related changes in functional connectivity relate to measures of overlap
Using functional MRI in humans, Tompary et al. track time-dependent representational changes across overlapping and non-overlapping episodic memories. The authors demonstrate that neural patterns of memories in mPFC and hippocampus become restructured over time to represent overlap across memories.
Highlights • The hippocampus shows a forward prediction signal with sequence repetition. • Encoding novel sequences is facilitated by shared context. • Stability in hippocampal activation patterns ...across items relates to the successful encoding of temporal relationships between those items.
When our experience violates our predictions, it is adaptive to upregulate encoding of novel information, while down-weighting retrieval of erroneous memory predictions to promote an updated ...representation of the world. We asked whether mnemonic prediction errors promote hippocampal encoding versus retrieval states, as marked by distinct network connectivity between hippocampal subfields. During fMRI scanning, participants were cued to internally retrieve well-learned complex room-images and were then presented with either an identical or a modified image (0-4 changes). In the left hemisphere, we find that CA1-entorhinal connectivity increases, and CA1-CA3 connectivity decreases, with the number of changes. Further, in the left CA1, the similarity between activity patterns during cued-retrieval of the learned room and during the image is lower when the image includes changes, consistent with a prediction error signal in CA1. Our findings provide a mechanism by which mnemonic prediction errors may drive memory updating-by biasing hippocampal states.
Experiences unfold over time, but little is known about the mechanisms that support the formation of coherent episodic memories for temporally extended events. Recent work in animals has provided ...evidence for signals in hippocampus that could link events across temporal gaps; however, it is unknown whether and how such signals might be related to later memory for temporal information in humans. We measured patterns of fMRI BOLD activity as people encoded items that were separated in time and manipulated the presence of shared or distinct context across items. We found that hippocampal pattern similarity in the BOLD response across trials predicted later temporal memory decisions when context changed. By contrast, pattern similarity in lateral occipital cortex was related to memory only when context remained stable. These data provide evidence in humans that representational stability in hippocampus across time may be a mechanism for temporal memory organization.
•Context shifts influenced mnemonic judgments of temporal distance•Pattern similarity computed across time in hippocampus predicts temporal memory•Hippocampal pattern similarity is related to temporal memory across context shifts•Patterns in object-selective visual cortex are more stable within contexts
What mechanisms support episodic memory for temporal information? Ezzyat and Davachi show that similarity in patterns of activation across time differentially predicts temporal memory across and within contexts in hippocampus and visual cortex.
Throughout our lives, the actions we produce are often highly familiar and repetitive (e.g., commuting to work). However, layered upon these routine actions are novel, episodic experiences. ...Substantial research has shown that prior knowledge can facilitate learning of conceptually related new information. But despite the central role our behavior plays in real-world experience, it remains unclear how engagement in a familiar sequence of actions influences memory for unrelated, nonmotor information coincident with those actions. To investigate this, we had healthy young adults encode novel items while simultaneously following a sequence of actions (key presses) that was either predictable and well-learned or random. Across three experiments (N = 80 each), we found that temporal order memory, but not item memory, was significantly enhanced for novel items encoded while participants executed predictable compared with random action sequences. These results suggest that engaging in familiar behaviors during novel learning scaffolds within-event temporal memory, an essential feature of episodic experiences.
The transformation of new experiences into lasting memories is thought to be mediated by postencoding reactivation or the reexpression of activity patterns that characterize prior encoding ...experiences during subsequent offline periods. Although hippocampal reactivation has been well-described in the rodent, evidence for postencoding persistence of hippocampal encoding patterns has yet to be described in humans. Using functional MRI, we examined the persistence of multivoxel hippocampal encoding patterns into postencoding rest periods. To characterize activity patterns, we computed the pairwise multivoxel correlation structure (MVCS) across hippocampal voxels during two distinct encoding tasks as well as during pre- and postencoding rest periods. We found that the hippocampal MVCS for each encoding task was more similar to the MVCS during immediate postencoding rest periods compared with a preencoding, baseline rest period. Additionally, using a principal component decomposition approach, we found that the strongest encoding patterns showed evidence of preferential persistence into immediate postencoding rest periods. Finally, the extent to which the strongest encoding patterns showed evidence of preferential persistence into immediate postencoding rest significantly correlated with later memory for stimuli seen during encoding. Taken together, these results provide strong evidence for hippocampal reactivation in humans, which was measured by the persistence of hippocampal encoding patterns into immediate postencoding rest periods, and importantly, provide a possible link between this persistence and memory consolidation.
The idea of episodic memory implies the existence of a process that segments experience into episodes so that they can be stored in memory. It is therefore surprising that the link between event ...segmentation and the organization of experiences into episodes in memory has not been addressed. We found that after participants read narratives containing temporal event boundaries at varying locations in the narrative, their long-term associative memory for information across event boundaries was lower than their memory for information within an event. This suggests that event segmentation during encoding resulted in segmentation of those same events in memory. Further, functional imaging data revealed that, across participants, brain activity consistent with the ongoing integration of information within events correlated with this pattern of mnemonic segmentation. These data are the first to address the mechanisms that support the organization of experiences into episodes in long-term memory.
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