Biology subjects

Hasselmo, M. E.

Publications and source records attributed to Hasselmo, M. E..

At least 19 recordsLinked to original sources

bioRxiv2025

Relative phase of membrane potential theta oscillations between individual hippocampal neurons code space

The timing of spikes dictates a neurons impact on downstream circuits and behavior, and spike timing is determined by the membrane potential (Vm). However, due to technical challenges, it has been impossible to analyze the relative timing of Vm dynamics between neurons during behavior. Using large scale membrane voltage imaging, we simultaneously recorded Vm from many individual hippocampal neurons in animals engaged in a virtual spatial task. We found that relative phase of Vm theta oscillations across neurons exhibit gradual or discrete shifts depending on spatial position. This finding extends beyond previous studies showing Vm dynamics in single neurons or spiking activity in multiple neurons, revealing previously unknown evidence for consistent coding of space by spike-independent relative phase of Vm theta dynamics between neurons.

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bioRxiv2025

Granularity of thalamic head direction cells

Head direction signaling is fundamental for spatial orientation and navigation. The anterodorsal nucleus of the thalamus (ADn) contains a high density of head direction (HD) cells that process sensorimotor inputs for subsequent synaptic integration in postsynaptic cortical areas. We tested the hypothesis that individual HD cells show differences in their firing patterns and connectivity by recording and juxtacellularly labeling single HD cells in subregions of the ADn in awake mice during passive rotation. We identified HD cells that exhibited different response profiles to light, sound, and movement. We also identified a mediolateral gradient of calretinin-expressing (CR+) ADn cells, with CR+ HD cells having narrower tuning widths, lower maximal firing rates, and different intrinsic properties compared to CR-cells. Axons of labeled HD cells could be followed to the retrosplenial cortex, with collaterals innervating the thalamic reticular nucleus (type I cells); others additionally innervated the dorsomedial striatum (type II cells). Most medial CR+ cells preferentially projected to ventral retrohippocampal regions. Surprisingly, we also identified a subpopulation of medial CR+ cells with twisted dendrites and descending axons that avoided the thalamic reticular nucleus, termed tortuosa HD cells (type III cells). We conclude that HD cells of the mouse ADn comprise distinct cell types, providing parallel head-direction-modulated sensorimotor messages to synaptic target neurons within the head direction network.

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bioRxiv2025

Modeling presynaptic inhibition by the amyloid precursor protein demonstrates one potential mechanism for preventing runaway synaptic modification in Alzheimers disease

INTRODUCTIONPrevious simulations of Hebbian associative memory models inspired the malignant synaptic growth hypothesis of Alzheimers disease (AD), which suggests that cognitive impairments arise due to runaway synaptic modification resulting from poor separation between encoding and retrieval. METHODSWe computationally model presynaptic inhibition by the recently identified interaction of soluble amyloid precursor protein (sAPP) with the {gamma}-aminobutyric acid type B receptor (GABABR) as one potential biological mechanism which can enhance separation between encoding and retrieval. RESULTSSimulations predict that the dual effect of sAPP on long-term potentiation and presynaptic inhibition of glutamatergic synapses maintains effective associative memory function and prevents runaway synaptic modification. Moreover, computational modeling predicts that sAPP, which interacts with the 1a isoform of GABABR, is more effective at stabilizing associative memory than the GABABR agonist Baclofen. DISCUSSIONMolecular mechanisms that enhance presynaptic inhibition, such as sAPP-GABABR1a signaling, are potential therapeutic targets for preventing cognitive impairments in AD.

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bioRxiv2025

Cortical dissociation of spatial reference frames during place navigation

Animals rely on both sensory perception and memory when navigating relative to learned allocentric locations. Incoming sensory stimuli, which arrive from an egocentric perspective, must be integrated into an allocentric reference frame to allow neural computations that direct an animal toward a learned goal. This egocentric-allocentric spatial transformation has been proposed to involve projections from the rodent postrhinal cortex (POR), which receives strong visual input, to the medial entorhinal cortex (MEC), which contains allocentric spatial cell types such as grid and border cells. A major step toward understanding this transformation is to identify how POR and MEC spatial representations differ during place navigation, which is currently unknown. To answer this question, we recorded single neurons from POR and MEC as rats engaged in a navigation task that required them to repeatedly visit a learned uncued allocentric location in an open field arena to receive a randomly scattered food reward. While neurons in both regions displayed strong tuning to the spatial structure of the environment, neither showed bias toward the goal location despite strongly biased behavior. Critically, when local visual landmarks were manipulated to place the visual scene in conflict with the learned location, POR neurons adjusted their tuning preferences to follow the visual landmarks, while MEC neurons remained in register with the true global reference frame. These findings reveal a strong dissociation between POR and MEC spatial reference frames during place navigation and raise questions regarding the mechanisms underlying integration of POR egocentric signals into the MEC allocentric spatial map.

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bioRxiv2025

How do egocentric boundary cells depend upon the coordinate system of environmental features?

Neurons in the retrosplenial (RSC) (Alexander et al 2020a, LaChance & Hasselmo 2024) and postrhinal cortex (POR) respond to environmental boundaries and configurations using egocentric coordinates relative to an animals current position. Neurons in these structures and adjacent structures also respond to spatial dimensions of self-motion such as running velocity (Carstensen et al 2021, Robinson et al 2023). Experimental and modeling data suggest that these responses could be essential for guiding behaviors such as obstacle avoidance and goal-directed navigation (Erdem & Hasselmo 2012, Erdem & Hasselmo 2014). However, these findings still leave the unanswered question: What specific features, and in what coordinate frames, drive these egocentric neural responses? Here we present models of the potential circuit mechanisms generating egocentric responses in RSC. One model posits that neurons encode internal representations of barriers in head-centered coordinates, defined by distance and angle, which are modulated by running velocity to enable trajectory planning and obstacle avoidance. We contrast this with a complementary hypothesis in which neurons respond to retinotopic features--such as the top, bottom, or edges of walls, which may serve as precursors to head-centered representations. Additional hypotheses include trajectory-based forward scanning (e.g., ray tracing) for barrier detection or comparing optic flow across the visual field. These hypotheses generate complementary modeling predictions about how changes in environmental parameters could alter the neural responses of egocentric boundary cells that are presented here.

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bioRxiv2024

Pathological tau alters head direction signaling and induces spatial disorientation

Spatial disorientation, an early symptom of dementia, is emerging as an early and reliable cognitive biomarker predicting future memory problems associated with Alzheimers disease, but the underlying neural mechanisms have yet to be fully defined. The anterodorsal thalamic nucleus (ADn) exhibits early and selective vulnerability to pathological misfolded forms of tau, a major hallmark of Alzheimers disease and ageing. The ADn contains a high density of head direction (HD) cells, which contribute to spatial navigation and orientation. Hence, their disruption may contribute to spatial disorientation. To test this, we virally expressed human mutant tau (htau) in the ADn of adult mice. HD-tau mice were defined by phosphorylated and oligomeric forms of htau in ADn somata and in axon terminals in postsynaptic target regions. Compared to controls, HD-tau mice exhibited increased looping behavior during spatial learning, and made a greater number of head turns during memory recall, indicative of spatial disorientation. Using in vivo extracellular recordings, we identified htau-expressing ADn cells and found a lower proportion of HD cells in the ADn from HD-tau mice, along with reduced directionality and altered burst firing. These findings provide evidence that expression of pathological human tau can alter HD signaling, leading to impairments in spatial orientation.

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bioRxiv2024

Distinct cortical spatial representations learned along disparate visual pathways

Recent experimental studies have discovered diverse spatial properties, such as head direction tuning and egocentric tuning, of neurons in the postrhinal cortex (POR) and revealed how the POR spatial representation is distinct from the retrosplenial cortex (RSC). However, how these spatial properties of POR neurons emerge is unknown, and the cause of distinct cortical spatial representations is also unclear. Here, we build a learning model of POR based on the pathway from the superior colliculus (SC) that has been shown to have motion processing within the visual input. Our designed SC-POR model demonstrates that diverse spatial properties of POR neurons can emerge from a learning process based on visual input that incorporates motion processing. Moreover, combining SC-POR model with our previously proposed V1-RSC model, we show that distinct cortical spatial representations in POR and RSC can be learnt along disparate visual pathways (originating in SC and V1), suggesting that the varying features encoded in different visual pathways contribute to the distinct spatial properties in downstream cortical areas. Conflict of interest statementThe authors declare no competing financial interests.

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bioRxiv2024

Neurovascular Impulse Response Function (IRF) during spontaneous activity differentially reflects intrinsic neuromodulation across cortical regions

Ascending neuromodulatory projections from deep brain nuclei generate internal brain states that differentially engage specific neuronal cell types. Because neurovascular coupling is cell-type specific and neuromodulatory transmitters have vasoactive properties, we hypothesized that the impulse response function (IRF) linking spontaneous neuronal activity with hemodynamics would depend on neuromodulation. To test this hypothesis, we used optical imaging to measure (1) release of neuromodulatory transmitters norepinephrine (NE) or acetylcholine (ACh), (2) Ca2+ activity of local cortical neurons, and (3) changes in hemoglobin concentration and oxygenation across the dorsal surface of cerebral cortex during spontaneous neuronal activity in awake mice. A canonical convolution model with a global, stationary IRF (i.e., the convolution kernel) describing evolution of total hemoglobin (HbT, reflective of dilation dynamics) with respect to Ca2+, resulted in a poor fit to the data. However, the HbT time-course was well predicted, pixel-by-pixel, by a weighted sum of Ca2+ and NE time-courses. Consistent with this result, modeling HbT as a weighted sum of stationary Ca2+ - and NE-specific IRFs (IRFCa2+ and IRFNE) convolved with the respective time-courses dramatically improved the fit compared to the global IRF. IRFCa2+ and IRFNE, estimated from the data, were positive and negative, respectively. In contrast to NE, ACh was largely redundant with Ca2+ and therefore did not improve HbT estimation. Because NE covaried with arousal, we observed instances of the diminished hemodynamic coherence between cortical regions during high arousal despite coherent behavior of the underlying neuronal Ca2+ activity. We conclude that while neurovascular coupling with respect to neuronal Ca2+ is a dynamic and seemingly complex phenomenon, hemodynamic fluctuations can be captured by a simple linear model with stationary IRFs with respect to the underlying dilatory and constrictive forces. In the current study, these forces were captured by the positive IRFCa2+ (dilation) and negative IRFNE (constriction). Without accounting for NE neuromodulation and the associated vasoconstriction, diminished hemodynamic coherence, commonly referred to as "functional (dys)connectivity" in BOLD fMRI studies, can be falsely interpreted as neuronal desynchronizations.

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bioRxiv2023

Optogenetic Silencing of Medial Septal GABAergic Neurons Disrupts Grid Cell Spatial and Temporal Coding in the Medial Entorhinal Cortex

The hippocampus and medial entorhinal cortex (MEC) form a cognitive map that facilitates spatial navigation. As part of this map, MEC grid cells fire in a repeating hexagonal pattern across an environment. This grid pattern relies on inputs from the medial septum (MS). The MS, and specifically its GABAergic neurons, are essential for theta rhythm oscillations in the entorhinal-hippocampal network, however, it is unknown if this subpopulation is also essential for grid cell function. To investigate this, we used optogenetics to inhibit MS-GABAergic neurons during grid cell recordings. We found that MS-GABAergic inhibition disrupted grid cell spatial periodicity both during optogenetic inhibition and during short 30-second recovery periods. Longer recovery periods of 60 seconds between the optogenetic inhibition periods allowed for the recovery of grid cell spatial firing. Grid cell temporal coding was also disrupted, as observed by a significant attenuation of theta phase precession. Together, these results demonstrate that MS-GABAergic neurons are critical for grid cell spatial and temporal coding in the MEC.

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bioRxiv2023

Hippocampal remapping induced by new behavior is mediated by spatial context

The hippocampus plays a central role in episodic memory and spatial navigation. Hippocampal neurons form unique representational codes in different spatial environments, which may provide a neural substrate for context that can trigger memory recall or enable performance of context-guided memory tasks. However, new learning often occurs in a familiar location, requiring that locations representation to be updated without erasing the previously existing memory representations that may be adaptive again in the future. To study how new learning affects a previously acquired spatial memory representation, we trained mice to perform two plus maze tasks across nine days in the sequence Turn Right 1 - Go East - Turn Right 2 (three days each), while we used single-photon calcium imaging to record the activity of hundreds of neurons in dorsal CA1. One cohort of mice performed the entire experiment on the same maze (One-Maze), while the second cohort performed the Go East task on a unique maze (Two-Maze). We hypothesized that CA1 representations in One-Maze mice would exhibit more change in the spatial patterns of neuronal activity on the maze from Turn Right 1 to Turn Right 2 than would be seen in Two-Maze mice. Indeed, changes in single unit activity and in the population code were larger in the One-Maze group. We further show evidence that Two-Maze mice utilize a separate neural representation for each maze environment. Finally, we found that remapping across the two Turn Right epochs did not involve an erasure of the representation for the first Turn Right experience, as many neurons in mice from both groups maintained Turn Right-associated patterns of activity even after performing the Go East rule. These results demonstrate that hippocampal activity patterns remap in response to new learning, that remapping is greater when experiences occur in the same spatial context, and that throughout remapping information from each experience is preserved. Significance StatementThe hippocampus plays a central role in self-localization and the consolidation of new experiences into long term memory. The activity of hippocampal place cells tracks an animals spatial location and upcoming navigational decisions, providing, at the ensemble level, unique patterns of activity for experiences that occur in the same physical location. Many studies have demonstrated the existence of divergent patterns at short time scales and how remapping can orthogonalize distinct experiences learned simultaneously. Here, we expand on this knowledge using the power of single-photon calcium imaging to track how new learning affects previously existing spatial memories either in the same or different environments over long periods of time. We observe patterns of hippocampal neural activity in mice during performance of two different rules either in the same environment or in different environments. We find that performing a new behavioral rule in the same environment as a previous rule causes significantly more remapping of hippocampal activity associated with the first rule than observed in mice that perform the two rules in separate environments. However, this remapping does not wholly destabilize memory for the first rule, as many neurons in both groups of mice maintain spatial activity patterns specific to the first rule. These results provide an important step forward in understanding the function of the hippocampus in memory by dramatically expanding the temporal scale over which changes to memory are measured.

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bioRxiv2022

Time or distance: predictive coding of Hippocampal cells

The discovery of place cells within the hippocampus has pointed to the importance of the hippocampus for navigation. The more recent discovery of hippocampal time cells has broadened the perspective of encoding in the hippocampus. An alternative hypothesis to the existence of time cells is based on the notion that hippocampal cells deduce location by integrating travelled distance ("path integration"). According to this alternate hypothesis, time cells, which fire at particular times when animals are running on a treadmill without changing location, actually encode accumulated distance on the treadmill. To examine this hypothesis, Kraus et al.1 performed treadmill experiments in which animals either ran for a fixed time or a fixed distance with varying velocities. Two distinct coding modes of hippocampal principal cells were found. Some cells encoded travelled distance and others elapsed time, thus refuting the notion that all hippocampal cells were performing path integration. Using the data from these experiments, we asked whether the two populations depended on the type of task the rats were engaged in. We show that the type of experiment determined the cells encoding, such that in fixed-distance experiments distance-encoding cells dominated, while on fixed-time experiments time-encoding cells dominated. These results suggest that the cells encoding contains a predictive element, dependent on the important variables of the experiment.

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bioRxiv2022

Composition of a spatial engram in mouse dentate gyrus

The dentate gyrus (DG) of hippocampus is hypothesized to act as a pattern separator that distinguishes between similar input patterns during memory formation and retrieval. Sparse ensembles of DG cells associated with learning and memory, i.e. engrams, have been labeled and manipulated to recall novel context memories. Functional studies of DG cell activity have demonstrated the spatial specificity and stability of DG cells during navigation. To reconcile how the DG contributes to separating global context as well as individual navigational routes, we trained mice to perform a delayed-non-match-to-position (DNMP) T-maze task and labeled DG neurons during performance of this task on a novel T-maze. The following day, mice navigated a second environment: the same T-maze, the same T-maze with one route permanently blocked but still visible, or a novel open field. We found that the degree of engram reactivation across days differed based on the traversal of maze routes, such that mice traversing only one arm had higher ensemble overlap than chance but less overlap than mice running the full two-route task. Mice experiencing the open field had similar ensemble sizes to the other groups but only chance-level ensemble reactivation. Ensemble overlap differences could not be explained by behavioral variability across groups, nor did behavioral metrics correlate to degree of ensemble reactivation. Together, these results support the hypothesis that DG contributes to spatial navigation memory and that partially non-overlapping ensembles encode different routes within the context of different environments. HighlightsO_LIImmediate-early-gene labeling strategy revealed spatial navigation ensembles in DG C_LIO_LISub-ensembles encode separate maze routes within a larger task context C_LIO_LIEnsemble reactivation does not correlate with behavioral variables C_LI

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bioRxiv2022

Temporal dynamics of cholinergic activity in the septo-hippocampal system

Cholinergic projection neurons in the medial septum and diagonal band of Broca are the major source of cholinergic modulation of hippocampal circuit functions that support neural coding of location and running speed. Changes in cholinergic modulation are known to correlate with changes in brain states, cognitive functions, and behavior. However, whether cholinergic modulation can change fast enough to serve as a potential speed signal in hippocampal and parahippocampal cortices and whether the temporal dynamics in such a signal depend on the presence of visual cues remain unknown. In this study, we use a fiber-photometric approach to quantify the temporal dynamics of cholinergic activity in freely moving mice as a function of the animals running speed and visual cues. We show that the population activity of cholinergic neurons in the medial septum and diagonal band of Broca changes fast enough to be aligned well with changes in the animals running speed and is strongly and linearly correlated to the logarithm of the animals running speed. Intriguingly, the cholinergic modulation remains strongly and linearly correlated to the speed of the animals neck movements during periods of stationary activity. Furthermore, we show that cholinergic modulation is unaltered during darkness. Lastly, we identify rearing, a discrete behavior where the mouse stands on its hindlimbs to scan the environment from an elevated perspective, is associated with higher cholinergic activity than expected from neck movements on the horizontal plane alone. Taken together, these data show that temporal dynamics in the cholinergic modulation of hippocampal circuits are fast enough to provide a potential running speed signal in real-time. Moreover, the data show that cholinergic modulation is primarily a function of the logarithm of the animals movement speed, both during locomotion and during stationary activity, with no significant interaction with visual inputs. These data advance our understanding of temporal dynamics in cholinergic modulation of hippocampal circuits and their functions in the context of neural coding of location and running speed.

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bioRxiv2022

Learning the Vector Coding of Egocentric Boundary Cells from Visual Data

The use of spatial maps to navigate through the world requires a complex ongoing transformation of egocentric views of the environment into position within the allocentric map. Recent research has discovered neurons in retrosplenial cortex and other structures that could mediate the transformation from egocentric views to allocentric views. These egocentric boundary cells respond to the egocentric direction and distance of barriers relative to an animals point of view. This egocentric coding based on the visual features of barriers would seem to require complex dynamics of cortical interactions. However, computational models presented here show that egocentric boundary cells can be generated with a remarkably simple synaptic learning rule that forms a sparse representation of visual input as an animal explores the environment. Simulation of this simple sparse synaptic modification generates a population of egocentric boundary cells with distributions of direction and distance coding that strikingly resemble those observed within the retrosplenial cortex. This provides a framework for understanding the properties of neuronal populations in the retrosplenial cortex that may be essential for interfacing egocentric sensory information with allocentric spatial maps of the world formed by neurons in downstream areas including the grid cells in entorhinal cortex and place cells in the hippocampus.

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bioRxiv2021

A compressed representation of spatial distance in the rodent hippocampus

Principal cells in the rodent hippocampus often fire in response to traversal through a specific spatial location (place cells), as well as elapsed time during an imposed temporal delay or after stimulus offset (time cells). Sequences of time cells unfold rapidly at first, with many time cells with narrow time fields. As the triggering event recedes into the past, time cells are fewer and have broader fields. This means that the representation of time in the hippocampus is compressed with greater resolution for time points near the present. Using tetrode recordings we measured individual CA1 units while rats traveled along a track that could be changed in length. Consistent with previous results, most place cells coded for distance from the starting point of the trajectory. Critically, place cells became less numerous and showed gradually widening fields with distance from the starting location. These results suggest that as the animal leaves a landmark, the hippocampal place code forms a compressed representation of distance from the starting location. The representation of time and space in the hippocampus have similar properties suggesting that they arise from similar computational mechanisms. Significance StatementThe hippocampus represents relationships between events in time and space. It has been hypothesized that temporal and spatial relationships are the result of a common computational mechanism. Previous work has shown that the representation of time in the hippocampus is compressed, with less neural resolution for more temporally remote events, consistent with the observation that temporal memory is worse for events further in the past. This paper shows an analogous result for spatial relationships. Place cells coded for distance from the start of a journey. As distance increased, place fields became broader and less numerous, showing a decrease in spatial resolution. This result suggests a unified coding scheme for the dimensions of time and space in the rodent hippocampus.

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bioRxiv2020

Adaptive integration of self-motion and goals in posterior parietal cortex

Animals engage in a variety of navigational behaviors that require different regimes of behavioral control. In the wild, rats readily switch between foraging and more complex behaviors such as chase, wherein they pursue other rats or small prey. These tasks require vastly different tracking of multiple behaviorally-significant variables including self-motion state. It is unknown whether changes in navigational context flexibly modulate the encoding of these variables. To explore this possibility, we compared self-motion processing in the multisensory posterior parietal cortex while rats performed alternating blocks of free foraging and visual target pursuit. Animals performed the pursuit task and demonstrated predictive processing by anticipating target trajectories and intercepting them. Relative to free exploration, pursuit sessions yielded greater proportions of parietal cortex neurons with reliable sensitivity to self-motion. Multiplicative gain modulation was observed during pursuit which increased the dynamic range of tuning and led to enhanced decoding accuracy of self-motion state. We found that self-motion sensitivity in parietal cortex was history-dependent regardless of behavioral context but that the temporal window of self-motion tracking was extended during target pursuit. Finally, many self-motion sensitive neurons conjunctively tracked the position of the visual target relative to the animal in egocentric coordinates, thus providing a potential coding mechanism for the observed gain changes to self-motion signals. We conclude that posterior parietal cortex dynamically integrates behaviorally-relevant information in response to ongoing task demands.

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bioRxiv2020

Effects of visual inputs on neural dynamics for coding of location and running speed in medial entorhinal cortex.

Neuronal representations of spatial location and movement speed in the medial entorhinal cortex during the "active" theta state of the brain are important for memory-guided navigation and rely on visual inputs. However, little is known about how visual inputs change neural dynamics as a function of running speed and time. By manipulating visual inputs in mice, we demonstrate that changes in spatial stability of grid cell firing as a function of time correlate with changes in a proposed speed signal by local field potential theta frequency. In contrast, visual inputs do not affect the speed modulation of firing rates. Moreover, we provide evidence that sensory inputs other than visual inputs can support grid cell firing, though less accurately, in complete darkness. Finally, changes in spatial accuracy of grid cell firing on a 10-s time scale suggest that grid cell firing is a function of velocity signals integrated over past time.

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bioRxiv2019

Hippocampal spatial memory representations in mice are heterogeneously stable

The population of hippocampal neurons actively coding space continually changes across days as mice repeatedly perform tasks. Many hippocampal place cells become inactive while other previously silent neurons become active, challenging the belief that stable behaviors and memory representations are supported by stable patterns of neural activity. Active cell replacement may disambiguate unique episodes that contain overlapping memory cues, and could contribute to reorganization of memory representations. How active cell replacement affects the evolution of representations of different behaviors within a single task is unknown. We trained mice to perform a Delayed Non-Match to Place (DNMP) task over multiple weeks, and performed calcium imaging in area CA1 of the dorsal hippocampus using head-mounted miniature microscopes. Cells active on the central stem of the maze "split" their calcium activity according to the animals upcoming turn direction (left or right), the current task phase (study or test), or both task dimensions, even while spatial cues remained unchanged. We found that different splitter neuron populations were replaced at unequal rates, resulting in an increasing number of cells modulated by turn direction and a decreasing number of cells with combined modulation by both turn direction and task phase. Despite continual reorganization, the ensemble code stably segregated these task dimensions. These results show that hippocampal memories can heterogeneously reorganize even while behavior is unchanging. Significance statementSingle photon calcium imaging using head-mounted miniature microscopes in freely moving animals, has enabled researchers to measure the long term stability of hippocampal pyramidal cells during repeated behaviors. Previous studies have demonstrated instability of neural circuit components including dendritic spines and axonal boutons. It is now known that single units in the neuronal population exhibiting behaviorally relevant activity eventually become inactive and that previously silent neurons can quickly acquire task-relevant activity. The function of such population dynamics is unknown. We show here that population dynamics differ for cells coding distinct task dimensions, suggesting such dynamics are part of a mechanism for latent memory reorganization. These results add to a growing body of work showing that maintenance of episodic memory is an ongoing and dynamic process.

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