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This Special Feature explores the various purposes served by sleep, describing current attempts to understand how the many functions of sleep are instantiated in neural circuits and cognitive structures. Our feature reflects current experts' opinions about, and insights into, the dynamic processes of sleep. In the last few decades, technological advances have supported the updated view that sleep plays an active role in both cognition and health. However, these roles are far from understood. This collection of articles evaluates the dynamic nature of sleep, how it evolves across the lifespan, becomes a competitive arena for memory systems through the influence of the autonomic system, supports the consolidation and integration of new memories, and how lucid dreams might originate. This set of papers highlights new approaches and insights that will lay the groundwork to eventually understand the full range of functions supported by sleep.

Memory reconsolidation refers to the phenomenon whereby a previously consolidated memory, i.e., one that is resistant to interference or disruption, becomes labile due to reactivation, initiating a short window during which that memory can be modified. With a wide range of potential clinical and educational applications, reconsolidation has been demonstrated across multiple domains and timescales, including fear, motor, and episodic memory. This chapter seeks to clarify the psychological processes involved in reconsolidation, making connections to underlying physiological mechanisms, with the goal of providing a framework for understanding why and when reconsolidation takes place. Drawing on reviews of both human and relevant animal studies, this chapter highlights the importance of both context and predictions for determining whether new experiences give rise to new learning or the updating of previously-learned associations.

It is well established that in most species, the hippocampus shows extensive postnatal development. This delayed maturation has a number of implications, which can be thought of in three categories. First, the late maturation has the direct effect of depriving the developing organism of at least some of the functions of the hippocampus, in particular place learning, context coding and in humans, episodic memory. Second, such learning that does occur very early in life, prior to hippocampal maturation, will largely bear the imprint and properties of those brain systems that, unlike the hippocampus, are fully functional early in life. Third, the active state of development of hippocampus in the first weeks and months of life render this structure susceptible to disruption by environmental and/or chromosomal factors. In this article, I discuss my efforts, with many colleagues over the past 40 years, to understand each of these implications.

Mild cognitive impairment (MCI) during aging is often a harbinger of Alzheimer’s disease, and, therefore, early intervention to preserve cognitive abilities before the MCI symptoms become medically refractory is particularly critical. Functional MRI–guided transcranial magnetic stimulation is a promising approach for modulating hippocampal functional connectivity and enhancing memory in healthy adults. Here, we extend these previous findings to individuals with MCI and leverage theta burst stimulation (TBS) and white matter tractography derived from diffusion-weighted MRI to target the hippocampus. Our preliminary findings suggested that TBS could be used to improve associative memory performance and increase resting-state functional connectivity of the hippocampus and other brain regions, including the occipital fusiform, frontal orbital cortex, putamen, posterior parahippocampal gyrus, and temporal pole, along the inferior longitudinal fasciculus in MCI. Although the sample size is small, these results shed light on how TBS propagates from the superficial cortex around the parietal lobe to the hippocampus.

Processes occurring during sleep contribute critically to the stabilization of new learning for long-term retention. Previously consolidated memory traces can be reactivated rendering memories labile again, and vulnerable to disruption or alteration. Across the phases of reactivation, modification, and re-consolidation, processing during sleep may play an essential role in restabilizing the transformed memory. We discuss recent research assessing the impact of sleep on reactivated memories potentially undergoing reconsolidation. We further evaluate targeted memory reactivation, an intervention that can directly engineer reconsolidation during sleep. Although sleep may play a role in restabilizing newly-modified memories, much remains to be explored before we fully understand the supporting mechanisms.

The question of why our conceptions of space and time are intertwined with memory in the hippocampal formation is at the forefront of much current theorizing about this brain system. In this article I argue that animals bridge spatial and temporal gaps through the creation of internal models that allow them to act on the basis of things that exist in a distant place and/or existed at a different time. The hippocampal formation plays a critical role in these processes by stitching together spatiotemporally disparate entities and events. It does this by 1) constructing cognitive maps that represent extended spatial contexts, incorporating and linking aspects of an environment that may never have been experienced together; 2) creating neural trajectories that link the parts of an event, whether they occur in close temporal proximity or not, enabling the construction of event representations even when elements of that event were experienced at quite different times; and 3) using these maps and trajectories to simulate possible futures. As a function of these hippocampally driven processes, our subjective sense of both space and time are interwoven constructions of the mind, much as the philosopher Immanuel Kant postulated.

Tracking moment-to-moment change in input, and detecting change sufficient to require altering behavior is crucial to survival. We discuss how the brain evaluates change over time, focusing on hippocampus and its role in tracking context. We leverage the anatomy and physiology of the hippocampal longitudinal axis, re-entrant loops, and amorphous networks, to account for stimulus equivalence and the updating of an organism’s sense of its context. Place cells play a central role in tracking contextual continuities and discontinuities across multiple scales, a capacity beyond current models of pattern separation and completion. This perspective highlights the critical role of the hippocampus in both spatial cognition and episodic memory: tracking change and detecting boundaries separating one context, or episode, from another.

Tracking moment-to-moment change in input and detecting change sufficient to require altering behavior is crucial to survival. Here, we discuss how the brain evaluates change over time, focusing on the hippocampus and its role in tracking context. We leverage the anatomy and physiology of the hippocampal longitudinal axis, re-entrant loops, and amorphous networks to account for stimulus equivalence and the updating of an organism's sense of its context. Place cells have a central role in tracking contextual continuities and discontinuities across multiple scales, a capacity beyond current models of pattern separation and completion. This perspective highlights the critical role of the hippocampus in both spatial cognition and episodic memory: tracking change and detecting boundaries separating one context, or episode, from another.

The model of enduring change in psychotherapy featuring memory reconsolidation and emotional arousal was based on recent neuroscientific advances that were presented originally from a predominantly psychological perspective. This chapter translates the components and processes of the model into evidence-based neural systems terms. This neural circuitry model highlights what is known and not known and where new research is most urgently needed. The authors then consider the research agenda, emphasizing what they consider to be the most important knowledge gaps. The basic science research agenda focuses on a variety of topics pertaining to memory and memory reconsolidation as well as interactions between emotion and memory. The clinical science research agenda focuses on the most pressing issues pertaining to the processes and mechanisms contributing to enduring change in psychotherapy. The potential exists to develop a new taxonomy of clinical interventions based on what problems are being targeted, how intractable they are, and how long-lasting the intervention needs to be.

This chapter provides an overview of current thinking about certain aspects of memory, in particular, those most relevant in the clinic. It briefly sketches some early history and then discusses those features of memory that are important in understanding what is going on when one changes one’s mind. Major conclusions include (a) there are multiple forms of memory, governed by unique rules—some forms are relatively easy to change; others, less so; (b) reactivating a memory can enable change in that memory, but the conditions governing this remain unclear; (c) memory and emotion interact, affecting the likelihood of a memory being formed, or reformed, after reactivation; and (d) prediction is at the core of what all forms of memory do.

The application of computational neuroscience models to mental disorders has given rise to the emerging field of computational psychiatry. To date, however, there has been limited application of this approach to understanding the change process in psychotherapy. This chapter reviews leading approaches in computational neuroscience: predictive coding, active inference, and reinforcement learning. We then provide examples of how these complimentary approaches can be used to model a range of clinical phenomena and associated clinical interventions, including those associated with emotional awareness, specific phobia, maladaptive self-related beliefs, maladaptive repetitive behavior patterns, and the role of re-experiencing negative affect in the therapeutic process. The authors illustrate how this perspective can provide additional insights into the nature of the types of memories (cast as parameters in computational models) that maintain psychopathology, how they may be instantiated in the brain, and how new experiences in psychotherapy can alter/update these memories in a manner that can be quantitatively modeled. The authors conclude that the computational perspective represents a unique level of description that compliments that of the integrated memory model in a synergistic and informative manner.

The hippocampus undergoes substantial postnatal maturation. This brain structure is central to episodic memory and allocentric spatial mapping, and infants lack these cognitive capacities until 18 months of age or older. The still-maturing hippocampus is especially sensitive to influences from environmental and genetic factors, helping us understand the patterns of developmental change associated with this region and its associated cognitive processes.

Many of the foundational theoretical ideas in the field of learning and memory are traced to Donald Hebb. Examination of these ideas and their evolution suggest that Karl Lashley might have significantly influenced their development. Here, we discuss the relationship between Hebb and Lashley, and the parallels between them. Many now investigating the neurobiological basis of memory may be unaware both of Hebb’s original descriptions, and the likely substantial contributions of Lashley. Many of their concerns remain with us today, and by clarifying the history we hope to strengthen the foundations of our field.

Sleep-dependent memory processing is dependent on several factors at learning, including emotion, encoding strength, and knowledge of future relevance. Recent work documents the role of curiosity on learning, showing that memory associated with high-curiosity encoding states is retained better and that this effect may be driven by activity within the dopaminergic circuit. Here, we examined whether this curiosity effect was enhanced by or dependent on sleep-related consolidation. Participants learned the answers to trivia questions that they had previously rated on a curiosity scale, and they were shown faces between each question and answer presentation. Memory for these answers and faces was tested either immediately or after a 12-hour delay containing sleep or wakefulness, and polysomnography data was collected for a subset of the sleep participants. Although the curiosity effect for both the answers and incidentally-learned faces was replicated in immediate tests and after the 12-hour delay, the effect was not impacted by the presence of sleep in either case, nor did the effect show a relationship with total sleep time or time in slow-wave sleep. This study suggests that curiosity may be a learning factor that is not subsequently affected by sleep-dependent memory consolidation, but more work ought to examine the role of sleep on curiosity-driven memory in other contexts.

Targeting memories during sleep opens powerful and innovative ways to influence the mind. We used targeted memory reactivation (TMR), which to date has been shown to strengthen learned episodes, to instead induce forgetting (TMR-Forget). Participants were first trained to associate the act of forgetting with an auditory forget tone. In a second, separate, task they learned object-sound-location pairings. Shortly thereafter, some of the object sounds were played during slow wave sleep, paired with the forget tone to induce forgetting. One week later, participants demonstrated lower recall of reactivated versus non-reactivated objects and impaired recognition memory and lowered confidence for the spatial location of the reactivated objects they failed to spontaneously recall. The ability to target specific episodic memories for forgetting during sleep has implications for developing novel therapeutic techniques for psychological disorders such as PTSD and phobias.

Episodic memories, when reactivated, can be modified or updated by new learning. Since such dynamic memory processes remain largely unexplored in psychiatric disorders, we examined the impact of depression on episodic memory updating. Unipolar and bipolar depression patients, and age/education matched controls, first learned a set of objects (List-1). Two days later, participants in all three groups were either reminded of the first learning session or not followed by the learning of a new set of objects (List-2). Forty-eight hours later, List-1 recall was impaired in unipolar and bipolar patients compared to control participants. Further, as expected, control participants who received a reminder spontaneously recalled items from List-2 during recall of List-1, indicative of an updated List-1 memory. Such spontaneous intrusions were also seen in the unipolar and bipolar patients that received the reminder, suggesting that memory updating was unaffected in these two patient groups despite impaired recall of List 1. Unexpectedly, we observed a trend towards higher intrusions, albeit statistically insignificant, not only in the reminder but also in the no-reminder subgroups of bipolar patients. We probed this further in a second cohort by testing recall of List-2, which was also impaired in both depression groups. Again bipolar patients showed intrusions, but this time in the reverse order from List-1 into List-2, independent of a reminder. Taken together, despite impaired recall, updating of episodic memories was intact and unidirectional in unipolar depression. In contrast, indiscriminate updating, as evidenced by bidirectional interference between episodic memories, was seen in bipolar depression. These findings reveal a novel distinction between unipolar versus bipolar depression using a reactivation-dependent memory updating paradigm.

The journal Hippocampus has passed the milestone of 25 years of publications on the topic of a highly studied brain structure, and its closely associated brain areas. In a recent celebration of this event, a Boston memory group invited 16 speakers to address the question of progress in understanding the hippocampus that has been achieved. Here we present a summary of these talks organized as progress on four main themes: (1) Understanding the hippocampus in terms of its interactions with multiple cortical areas within the medial temporal lobe memory system, (2) understanding the relationship between memory and spatial information processing functions of the hippocampal region, (3) understanding the role of temporal organization in spatial and memory processing by the hippocampus, and (4) understanding how the hippocampus integrates related events into networks of memories. © 2016 Wiley Periodicals, Inc.

Despite the high prevalence of stress exposure healthy adaptation or resilience is a common response. Theoretical work and recent empirical evidence suggest that a robust reward system, in part, supports healthy adaptation by preserving positive emotions even under exceptionally stressful circumstances. We tested this prediction by examining empirical relations among behavioral and self-reported measures of sensitivity to reward, trait resilience, and measures of affect in the context of experimentally induced stress. Using a quasi-experimental design we obtained measures of sensitivity to reward (self-report and behavioral), as well as affective and physiological responses to experimental psychosocial stress in a sample of 140 healthy college-age participants. We used regression-based moderation and mediational models to assess associations among sensitivity to reward, affect in the context of stress, and trait resilience and found that an interaction between exposure to experimental stress and self-reported sensitivity to reward predicted positive affect following experimental procedure. Participants with high sensitivity to reward reported higher positive affect following stress. Moreover, positive affect during or after stress mediated the relation between sensitivity to reward and trait resilience. Consistent with the prediction that a robust reward system serves as a protective factor against stress-related negative outcomes, our results found predictive associations among sensitivity to reward, positive affect, and resilience.

Event related potentials (ERPs) can help to determine the cognitive and neural processes underlying memory functions and are often used to study populations with severe memory impairment. In healthy adults, memory is typically assessed with active tasks, while in patient studies passive memory paradigms are generally used. In this study we examined whether active and passive continuous object recognition tasks measure the same underlying memory process in typically developing (TD) adults and in individuals with Down syndrome (DS), a population with known hippocampal impairment. We further explored how ERPs in these tasks relate to behavioral measures of memory. Data-driven analysis techniques revealed large differences in old-new effects in the active versus passive task in TD adults, but no difference between these tasks in DS. The group with DS required additional processing in the active task in comparison to the TD group in two ways. First, the old-new effect started 150 ms later. Second, more repetitions were required to show the old-new effect. In the group with DS, performance on a behavioral measure of object-location memory was related to ERP measures across both tasks. In total, our results suggest that active and passive ERP memory measures do not differ in DS and likely reflect the use of implicit memory, but not explicit processing, on both tasks. Our findings highlight the need for a greater understanding of the comparison between active and passive ERP paradigms before they are inferred to measure similar functions across populations (e.g., infants or intellectual disability).

The last decade has seen dramatic technological and conceptual changes in research on episodic memory and the brain. New technologies, and increased use of more naturalistic observations, have enabled investigators to delve deeply into the structures that mediate episodic memory, particularly the hippocampus, and to track functional and structural interactions among brain regions that support it. Conceptually, episodic memory is increasingly being viewed as subject to lifelong transformations that are reflected in the neural substrates that mediate it. In keeping with this dynamic perspective, research on episodic memory (and the hippocampus) has infiltrated domains, from perception to language and from empathy to problem solving, that were once considered outside its boundaries. Using the component process model as a framework, and focusing on the hippocampus, its subfields, and specialization along its longitudinal axis, along with its interaction with other brain regions, we consider these new developments and their implications for the organization of episodic memory and its contribution to functions in other domains.

Recent work suggests that memory representations may guide basic perceptual functions, such as figure-ground perception. In three studies we assessed top-down contributions to figure-ground perception in typical development and in two developmental disorders: Down syndrome (DS) and autism (ASD). We investigated how figure-ground segregation is modulated by high-level cues (i.e., memory representations) and low-level cues (i.e., convexity and surface integration). Study 1 results showed that both high-level and low-level contributions to figure-ground perception are functional by the age of 4 years. In Study 2, individuals with DS exhibited intact figure-ground segregation based on low level cues when compared with mental age–matched participants, but they showed attenuated effects of high-level memory cues on figure-ground assignment. In Study 3, individuals with ASD showed intact effects of both high-level and low-level cues on figure-ground perception, counter to previous suggestions that high-level influences on perception are usually impaired in ASD.

The hippocampus serves a critical function in memory, navigation, and cognition. Nature Neuroscience asked John Lisman to lead a group of researchers in a dialog on shared and distinct viewpoints on the hippocampus.

Episodic memories (in humans) and event-like memories (in non-human animals) require the hippocampus for some time after acquisition, but at remote points seem to depend more on cortical areas instead. Systems consolidation refers to the process that promotes this reorganization of memory. Various theoretical frameworks accounting for this process have been proposed, but clear evidence favoring one or another of these positions has been lacking. Addressing this issue, a recent study deployed some of the most advanced neurobiological technologies - optogenetics and calcium imaging - and provided high resolution, precise observations regarding brain systems involved in recent and remote contextual fear memories. We critically review these findings within their historical context and conclude that they do not resolve the debate concerning systems consolidation. This is because the relevant question concerning the quality of memory at recent and remote time points has not been answered: Does the memory reorganization taking place during systems consolidation result in changes to the content of memory?

This chapter describes conditions under which people remember traumatic events: most quite clearly, some barely at all, while a minority remembers such events pathologically well. The account outlined here is based on the central premise that acute and chronic stressors, acting through the sympathetic-adrenal-medullary (SAM) and hypothalamic-pituitary-adrenal (HPA) systems, modulate brain systems to change the ways in which experience is transduced, distributed, stored, retrieved and integrated, reaggregated, and used. We start with a discussion of contextualized remembering, separating the concept of context into two distinct entities: settings and situations. We argue that a person's perception of setting and situation is filtered through individual differences reflecting genetic predispositions (traits), acquired characteristics (habits), and acute motivational states. We then outline remembering under normal circumstances. We then disambiguate the concepts of fear, stress, and trauma. Fear, we claim, is a state triggered through extant traits, states, and acquired characteristics mediated through the SAM axis. Once triggered, fear can bleed into stress, mediated through the HPA axis, which, under specific circumstances, bleeds into trauma. We discuss the neural bases of settings, situations, fear, and stress and discuss the representational structures the brain generates under both normal and stressful conditions. These considerations offer a way of understanding the nature of memory for trauma and suggest a resolution of recent debates in the trauma memory literature.

In rodents representations of environmental positions follow a granularity gradient along the hippocampal and entorhinal anterior–posterior axis; with fine-grained representations most posteriorly. To investigate if such a gradient exists in humans, functional magnetic resonance imaging data were acquired during virtual environmental learning of the objects' positions and the association between the objects and room geometry. The Objects-room geometry binding led to increased activation throughout the hippocampus and in the posterior entorhinal cortex. Within subject comparisons related specifically to the level of spatial granularity of the object position encoding showed that activation in the posterior and intermediate hippocampus was highest for fine-grained and medium-grained representations, respectively. In addition, the level of fine granularity in the objects' positions encoded between subjects correlated with posterior hippocampal activation. For the anterior hippocampus increased activation was observed for coarse-grained representations as compared to failed encoding. Activation in anterior hippocampus correlated with the number of environments in which the objects positions were remembered when permitting a coarse representation of positions. In the entorhinal cortex, activation in the posterior part correlated with level of fine granularity for the objects' positions encoded between subjects, and activation in the posterior and intermediate entorhinal cortex increased for medium-grained representations. This demonstrates directly that positional granularity is represented in a graded manner along the anterior–posterior axis of the human hippocampus, and to some extent entorhinal cortex, with most fine-grained positional representations posteriorly. © 2014 Wiley Periodicals, Inc.

Stress at encoding affects memory processes, typically enhancing, or preserving, memory for emotional information. These effects have interesting implications for eyewitness accounts, which in real-world contexts typically involve encoding an aversive event under stressful conditions followed by potential exposure to misinformation. The present study investigated memory for a negative event encoded under stress and subsequent misinformation endorsement. Healthy young adults participated in a between- groups design with three experimental sessions conducted 48 h apart. Session one consisted of a psycho- social stress induction (or control task) followed by incidental encoding of a negative slideshow. During session two, participants were asked questions about the slideshow, during which a random subgroup was exposed to misinformation. Memory for the slideshow was tested during the third session. Assess- ment of memory accuracy across stress and no-stress groups revealed that stress induced just prior to encoding led to significantly better memory for the slideshow overall. The classic misinformation effect was also observed - participants exposed to misinformation were significantly more likely to endorse false information during memory testing. In the stress group, however, memory accuracy and misinfor- mation effects were moderated by arousal experienced during encoding of the negative event. Misin- formed-stress group participants who reported that the negative slideshow elicited high arousal during encoding were less likely to endorse misinformation for the most aversive phase of the story. Fur- thermore, these individuals showed better memory for components of the aversive slideshow phase that had been directly misinformed. Results from the current study provide evidence that stress and high sub- jective arousal elicited by a negative event act concomitantly during encoding to enhance emotional memory such that the most aversive aspects of the event are well remembered and subsequently more resistant to misinformation effects.

Spatial context supports memory retrieval in adults. To understand the development of these effects, context effects on object recognition were tested in neurotypical children ages 3 years to adulthood (n 3–6 years = 34, n 10–16 years = 32, n college age = 22) and individuals with Down syndrome (DS) ages 10–29 years (n = 21). Participants engaged in an object recognition task; objects were presented in scenes and either remained in that same scene or were removed at test. In some groups (

The hippocampus has frequently been viewed as a core long-term memory structure, separate from brain structures mediating other cognitive functions such as working memory and perception. Much the same is true of other medial temporal lobe (MTL) structures such as the parahippocampal gyrus and the perirhinal cortex. Recent evidence suggests, instead, that these latter structures are also important for certain perceptual functions, leading many to propose a representational account of MTL that defines its functions in terms of what is being represented, rather than how long this representation lasts, or which psychological function is being invoked. We discuss a common framework within which perception and memory are seen as involving many, if not most, of the structures in the ventral representational stream, critically dependent on extensive feedforward and feedback processes. A variety of perceptual and memorial phenomena are discussed within this framework, and it is concluded that the hippocampus is in many ways like its neighbors in the MTL, yet has some distinct properties that account for its unique role in episodic memory.

Videos were presented depicting events such as baking cookies or cleaning a fish tank. Periodically, the video paused and an instruction to Remember (R) or Forget (F) the preceding video segment was presented; the video then resumed. Participants later responded more accurately to cued-recall questions (E1) and to true/false statements (E2–5) regarding R segments than F segments. This difference was larger for specific information (the woman added 3 cups of flour) than for general information (the woman added flour). Participants were also slower to detect visual probes presented following F instructions compared to those presented following R instructions. These findings suggest that intentional forgetting is an effortful process that can be performed even on segments of otherwise continuous events and that the result is a relatively impoverished representation of the unwanted information in memory.

In a novel event method directed forgetting task, instructions to Remember (R) or Forget (F) were integrated throughout the presentation of four videos depicting common events (e.g., baking cookies). Participants responded more accurately to cued recall questions (E1) and true/false statements (E2-4) regarding R segments than F segments. This was true even when forced to attend to F segments by virtue of having to perform concurrent discrimination (E2) or conceptual segmentation (E3) tasks. The final experiment (E5) demonstrated a larger R >F difference for specific true/false statements (the woman added three cups of flour) than for general true/false statements (the woman added flour) suggesting that participants likely encoded and retained at least a general representation of the events they had intended to forget, even though this representation was not as specific as the representation of events they had intended to remember.

Although the biological bases of forgetting remain obscure, the consensus among cognitive psychologists emphasizes interference processes, rejecting decay in accounting for memory loss. In contrast to this view, recent advances in understanding the neurobiology of long-term memory maintenance lead us to propose that a brain-wide well-regulated decay process, occurring mostly during sleep, systematically removes selected memories. Down-regulation of this decay process can increase the life expectancy of a memory and may eventually prevent its loss. Memory interference usually occurs during certain active processing phases, such as encoding and retrieval, and will be stronger in brain areas with minimal sensory integration and less pattern separation. In areas with efficient pattern separation, such as the hippocampus, interference-driven forgetting will be minimal, and, consequently, decay will cause most forgetting.

When a consolidated memory is reactivated, it becomes labile and modifiable. Recently, updating of reactivated episodic memory was demonstrated by Hupbach et al. (Learn Mem 14:47–53, 2007). Memory updating involves two vital processes—reactivation followed by reconsolidation. Here, we explored effects of psychosocial stress on episodic memory updating. Based on prior research, we hypothesized that stress before reactivation or stress before reconsolidation would impair memory updating. Participants learned a set of objects (list 1) on day 1. On day 2, some participants were reminded of list 1 before learning a second set of objects (list 2). Memory for list 1 was tested on day 3. Stress was administered either before reactivation of list 1 on day 2 (exp 1) or before reconsolidation of list 1, i.e., after reactivation and learning list 2 on day 2 (exp 2). Memory updating involves the incorporation of list 2 items into list 1 memory, contingent upon the reactivation of list 1 memory. In exp 1, the reminder groups had higher intrusions than the no-reminder groups, but contrary to our predictions, stress did not reduce this reminder effect. Stress effects were, however, found in exp 2: the reminder group that was stressed after reactivation and new learning showed fewer intrusions than the control reminder group. The findings suggest that stress before reactivation does not impair memory updating but stress at the onset of reconsolidation can. Timing may determine the effects of stress on memory processing.

In this paper, we investigate the process by which new experiences reactivate and potentially update old memories. Such memory reconsolidation appears dependent on the extent to which current experience deviates from what is predicted by the reactivated memory (i.e. prediction error). If prediction error is low, the reactivated memory is likely to be updated with new information. If it is high, however, a new, separate, memory is more likely to be formed. The temporal parietal junction TPJ has been shown across a broad range of content areas (attention, social cognition, decision making and episodic memory) to be sensitive to the degree to which current information violates the observer's expectations - in other words, prediction error. In the current paper, we investigate whether the level of TPJ activation during encoding predicts if the encoded information will be used to form a new memory or update a previous memory. We find that high TPJ activation predicts new memory formation. In a secondary analysis, we examine whether reactivation strength - which we assume leads to a strong memory-based prediction - mediates the likelihood that a given individual will use new information to form a new memory rather than update a previous memory. Individuals who strongly reactivate previous memories are less likely to update them than individuals who weakly reactivate them. We interpret this outcome as indicating that strong predictions lead to high prediction error, which favors new memory formation rather than updating of a previous memory.

Memory formation is a highly dynamic process. In this review we discuss traditional views of memory and offer some ideas about the nature of memory formation and transformation. We argue that memory traces are transformed over time in a number of ways, but that understanding these transformations requires careful analysis of the various representations and linkages that result from an experience. These transformations can involve: (1) the selective strengthening of only some, but not all, traces as a function of synaptic rescaling, or some other process that can result in selective survival of some traces; (2) the integration (or assimilation) of new information into existing knowledge stores; (3) the establishment of new linkages within existing knowledge stores; and (4) the up-dating of an existing episodic memory. We relate these ideas to our own work on reconsolidation to provide some grounding to our speculations that we hope will spark some new thinking in an area that is in need of transformation.

There is strong evidence that reactivation of a memory returns it to a labile state, initiating a restabilization process termed reconsolidation, which allows for updating of the memory. In this study we investigated reactivation-dependent updating using a new positively motivated spatial task in rodents that was designed specifically to model a human list-learning paradigm. On Day 1, rats were trained to run to three feeders (List 1) for rewards. On Day 2, rats were trained to run to three different feeders (List 2) in either the same (Reminder condition) or a different (No Reminder condition) experimental context than on Day 1. On Day 3, rats were cued to recall List 1. Rats in the Reminder condition made significantly more visits to List 2 feeders (intrusions) during List 1 recall than rats in the No Reminder condition, indicating that the reminder triggered reactivation and allowed integration of List 2 items into List 1. This reminder effect was selective for the reactivated List 1 memory, as no intrusions occurred when List 2 was recalled on Day 3. No intrusions occurred when retrieval took place in a different context from the one used at encoding, indicating that the expression of the updated memory is dependent upon the retrieval context. Finally, the level of intrusions was highest when retrieval took place immediately after List 2 learning, and generally declined when retrieval occurred 1–4 h later, indicating that the List 2 memory competed with short-term retrieval of List 1. These results demonstrate the dynamic nature of memory over time and the impact of environmental context at different stages of memory processing.

Background Children with Down syndrome (DS) suffer from sleep problems, including sleep mainte- nance problems, as well as snoring, and other symptoms of disordered breathing. To examine sleep in DS, we gave parents a questionnaire assess- ing their child's sleep.