Does reconsolidation occur in humans?

Reconsolidation of traumatic memories protocol compared to trauma-focussed cognitive behaviour therapy for post-traumatic stress disorder in UK military veterans: a randomised controlled feasibility trial

BACKGROUND

Post-traumatic stress disorder (PTSD) occurs more commonly in military veterans than the general population. Whilst current therapies are effective, up to half of veterans commencing treatment do not complete it. Reconsolidation of Traumatic Memories (RTM) protocol is a novel, easy to train, talking therapy with promising findings. We examine the feasibility of undertaking an efficacy trial of RTM in veterans.

METHODS

A parallel group, single-centre randomised controlled feasibility trial with a post-completion qualitative interview study. Sixty military veterans were randomised 2:1 to RTM (n = 35) or Trauma Focussed Cognitive Behaviour Therapy (CBT) (n = 25). We aimed to determine the rate of recruitment and retention, understand reasons for attrition, determine data quality and size of efficacy signal. We explored veterans' perceptions of experiences of joining the trial, the research procedures and therapy, and design improvements for future veteran studies. Military veterans with a diagnosis of PTSD or complex PTSD, and clinically significant symptoms, were recruited between January 2020 and June 2021. Primary outcome was feasibility using pre-determined progression criteria alongside PTSD symptoms, with depression, recovery, and rehabilitation as secondary outcomes. Data were collected at baseline, 6, 12, and 20 weeks. Interviews (n = 15) were conducted after 20 weeks. Both therapies were delivered by trained charity sector provider therapists.

RESULTS

Participants' mean age was 53 years, the mean baseline PTSD symptoms score assessed by the Post-traumatic Stress Checklist (PCL-5) was 57 (range 0-80). Fifty had complex PTSD and 39 had experienced ≥ 4 traumas. Data were analysed at 20 weeks for feasibility outcomes (n = 60) and mental health outcomes (n = 45). Seven of eight progression criteria were met. The RTM group experienced a mean 18-point reduction on the PCL-5. TFCBT group participants experienced a mean reduction of eight points. Forty-eight percent of the RTM group no longer met diagnostic criteria for PTSD compared to 16% in the TFCBT group. All veterans reported largely positive experiences of the therapy and research procedures and ways to improve them.

CONCLUSION

RTM therapy remains a promising psychological intervention for the treatment of PTSD, including complex PTSD, in military veterans. With specific strengthening, the research protocol is fit for purpose in delivering an efficacy trial.

TRIAL REGISTRATION

ISRCTN registration no 10314773 on 01.10.2019. Full trial protocol: available on request or downloadable at ISRCTN reg. no. 10314773.

Reactivating hippocampal-mediated memories during reconsolidation to disrupt fear

Memories are stored in the brain as cellular ensembles activated during learning and reactivated during retrieval. Using the Tet-tag system in mice, we label dorsal dentate gyrus neurons activated by positive, neutral or negative experiences with channelrhodopsin-2. Following fear-conditioning, these cells are artificially reactivated during fear memory recall. Optical stimulation of a competing positive memory is sufficient to update the memory during reconsolidation, thereby reducing conditioned fear acutely and enduringly. Moreover, mice demonstrate operant responding for reactivation of a positive memory, confirming its rewarding properties. These results show that interference from a rewarding experience can counteract negative affective states. While memory-updating, induced by memory reactivation, involves a relatively small set of neurons, we also find that activating a large population of randomly labeled dorsal dentate gyrus neurons is effective in promoting reconsolidation. Importantly, memory-updating is specific to the fear memory. These findings implicate the dorsal dentate gyrus as a potential therapeutic node for modulating memories to suppress fear.

Verbal manipulations of learning expectancy do not enhance reconsolidation

Objectives

Pharmacological studies using propranolol suggest that if reactivation signals that new information will be learned (i.e., there is an expectation for learning) reconsolidation can be enhanced. We examined if the verbal instructions to expect new learning will enhance reconsolidation of fear memories using the post-retrieval extinction paradigm.

Methods

On day one, participants (n = 48) underwent differential fear conditioning to two images (CS+ and CS-). On day two, participants were randomly assigned to one of three groups; groups one and two had their memory for the CS+ reactivated (i.e., a single presentation of the CS+) 10 minutes prior to extinction, whereas group three did not have their memory reactivated but went right to extinction (no reactivation group). One reactivation group was told that they would learn something new about the images (expectation for learning group), and the other group was told that they would not learn anything new (no expectation for learning group). On day three, return of fear was measured following reinstatement (i.e., four shocks). Fear potentiated startle (FPS) and skin conductance response (SCR) were measured throughout.

Results

There was evidence of fear acquisition for participants for SCR but not FPS. With regards to reconsolidation, SCR increased for the CS+ and CS-in all groups from the end of extinction to the beginning of re-extinction (i.e., return of fear). For FPS, post-hoc tests conducted on the sub-group of participants showing fear learning showed that FPS remained stable in the two reactivation groups, but increased to the CS+, but not the CS- in the no reactivation group.

Implications

These findings suggest that a verbal manipulation of the expectation for learning may not be salient enough to enhance reconsolidation. Results are discussed in relation to theories on differences in between SCR, as a measure of cognitive awareness, and FPS, as a measure of fear.

The day I told Karim Nader, "Don't do the study"

Karim Nader changed the course of memory research by reviving interest in the mostly forgotten topic of post-retrieval manipulations of memory. In this paper I summarize the events leading up to his ground-breaking study in my lab on so-called memory reconsolidation, and the effects of that study on the field.

Continuous theta-burst stimulation over the right dorsolateral prefrontal cortex disrupts fear memory reconsolidation in humans

Continuous theta-burst stimulation (cTBS), a non-invasive brain stimulation technique, can induce long-lasting changes in synaptic plasticity, vital for memory reconsolidation. For this study, a total of 170 participants completed four experiments by a randomized controlled design. Succeeding fear conditioning, the subjects received cTBS over the right dorsolateral prefrontal cortex (dlPFC) or vertex (control) with or without exposure to the conditioned stimulus to reactivate the original fear memory, and then underwent fear response tests. Compared with cTBS over the vertex and without memory reactivation, only cTBS over the right dlPFC after reactivation decreased the fear response for both recent and remote fear memories. This procedure was effective only during the reconsolidation window. The disruptive effect of cTBS over the right dlPFC on fear memory reconsolidation was delay-dependent. These findings demonstrate that cTBS time-dependently and delay-dependently prevents the return of fear and may have clinical potential for treating fear-related disorders.

Activation of CB1 pathway in the perirhinal cortex is necessary but not sufficient for destabilization of contextual fear memory in rats

According to the reconsolidation theory, memories can be modified through the destabilization-reconsolidation process. The rodent perirhinal cortex (PER; Brodmann areas 35 and 36) critically participates in the process of fear conditioning. Previous studies showed that some of the parahippocampal regions are critical for contextual fear memory reconsolidation. In our research, through a three-day paradigm of CFC, we showed that protein synthesis in PER of rats is required for memory reconsolidation, and activation of CB1 pathway is necessary but not sufficient in inducing memory destabilization. This result underlines parahippocampal regions in destabilization and reconsolidation process of fear memory besides amygdala and hippocampus.

Cathodal transcranial direct current stimulation on the prefrontal cortex applied after reactivation attenuates fear memories and prevent reinstatement after extinction

BACKGROUND

In the last decade, pharmacological strategies targeting reconsolidation after memory retrieval have shown promising efforts to attenuate persistent memories and overcome fear recovery. However, most reconsolidation inhibiting agents have not been approved for human testing. While non-invasive neuromodulation can be considered an alternative approach to pharmacological treatments, there is a lack of evidence about the efficacy of these technologies when modifying memory traces via reactivation/reconsolidation mechanism.

OBJECTIVE

In this study, we evaluate the effect of cathodal (c-tDCS) and anodal (a-DCS) transcranial direct current stimulation applied after memory reactivation and extinction in rats.

METHODS

Male Wistar rats were randomly assigned into three groups: one sham group, one anodal tDCS group, and one cathodal tDCS group (500 μA, 20 min). Reconsolidation and extinction of fear memories were evaluated using a contextual fear conditioning.

RESULTS

Our results showed that c-tDCS and a-tDCS after memory reactivation can attenuate mild fear memories. However, only c-tDCS stimulation prevented both fear expression under strong fear learning and fear recovery after a reinstatement protocol without modification of learning rate or extinction retrieval. Nevertheless, the remote memories were resistant to modification through this type of neuromodulation. Our results are discussed considering the interaction between intrinsic excitability promoted by learning and memory retrieval and the electric field applied during tDCS.

CONCLUSION

These results point out some of the boundary conditions influencing the efficacy of tDCS in fear attenuation and open new ways for the development of noninvasive interventions aimed to control fear-related disorders via reconsolidation.

Lack of Effect of Propranolol on the Reconsolidation of Conditioned Fear Memory due to a Failure to Engage Memory Destabilisation

The prospect of exploiting memory reconsolidation to treat mental health disorders has received great research interest, particularly following demonstrations that the β-adrenergic receptor antagonist propranolol, which is safe for use in humans, can disrupt the reconsolidation of pavlovian conditioned fear memories. However, recent studies have failed to replicate the effects of propranolol on fear memory reconsolidation, and have questioned whether treatments based upon reconsolidation blockade would be robust enough for clinical translation. It remains possible, though, that studies reporting no effect of propranolol on memory reconsolidation could be due to a failure to engage the memory destabilisation process, which is necessary for the memory to become susceptible to disruption with amnestic agents. Demonstrating that memory destabilisation has not been engaged is challenging when only using behavioural measures, but there are molecular correlates of memory destabilisation that can be used to determine whether memory lability has been induced. Here, we attempted to replicate the classic finding that systemic administration of propranolol disrupts the reconsolidation of a pavlovian auditory fear memory. Following a failure to replicate, we manipulated the parameters of the memory reactivation session to enhance prediction error in an attempt to overcome the boundary conditions of reconsolidation. On finding no disruption of memory despite these manipulations, we examined the expression of the post-synaptic density protein Shank in the basolateral amygdala. Degradation of Shank has been shown to correlate with the induction of memory lability, but we found no effect on Shank expression, consistent with the lack of observed behavioural effects.

Memory updating after retrieval: when new information is false or correct

We conducted three experiments testing the malleability of memory in incorporating new information following retrieval. All experiments used associative lists typical of the DRM paradigm [Deese, J. (1959). On the prediction of occurrence of particular verbal intrusions in immediate recall. Journal of Experimental Psychology, 58(1), 17-22; Roediger, H. L., & McDermott, K. B. (1995). Creating false memories: Remembering words not presented in lists. Journal of Experimental Psychology: Learning, Memory, and Cognition, 21(4), 803-814]. This paradigm enabled the evaluation of the integration of false information and correct information with the original information. In Experiment 1, participants studied DRM lists, and in a later phase either retrieved or restudied the lists and were presented with never-presented critical lures. The results of Experiment 1 showed that compared to restudy, retrieval enhanced the integration of subsequent false information, as measured by later recall in a follow-up test. In Experiments 2 and 3, after initial study, participants retrieved or studied incorrect information and received corrective feedback. The results showed that retrieval led to more error correction than restudy, when feedback was presented immediately. In general, this research suggests retrieval facilitates incorporation of new, related information, regardless of whether it is false or correct.

Changing memories by interference: the effect of emotional dimensions in reconsolidation of episodic memories

Episodes with an emotional component preoccupy memory formation and this advantage facilitates their preservation and mitigates the impact of interfering episodes. The present study examined the relation of the emotional dimensions of original and interfering episodes to the memory outcome, using a reconsolidation paradigm. In a between-subjects design, 102 healthy young adults were presented with an emotional or neutral image and learned either an emotional or neutral story, respectively (day 1). On day 2, experimental groups were presented with an image of the opposite emotionality, reactivated the original story, and learned a story of the opposite emotionality. On day 3, experimental and control groups were tested for their memory on target and filler clues of the original story and rated both stories for arousal and valence. Overall, there was evidence of interference on the long-term retention of target clues only for the neutral story (i.e. when the interfering story was emotional), and of filler clues for both types of stories. Moreover, individual target clue retention rates correlated with the arousal ratings for both the original neutral story and the interfering emotional story, while they were not related to arousal ratings for the original emotional story or the interfering neutral one.

Is Reconsolidation a General Property of Memory?

Memory reconsolidation holds great hope for memory modification approaches and clinical treatments of mental disorders associated with maladaptive memories. However, it remains controversial as to whether reconsolidation is a general property of all types of memory. Especially, discrepancies have been reported in research focusing on whether declarative memory undergoes reconsolidation, and whether old memories can be reorganized after retrieval. Here, we discuss how these inconsistent results can be reconciled and what information we need to uncover for the general use of reconsolidation.

Personality and Authenticity in Light of the Memory-Modifying Potential of Optogenetics

There has been a growing interest in research concerning memory modification technologies (MMTs) in recent years. Neuroscientists and psychologists are beginning to explore the prospect of controllable and intentional modification of human memory. One of the technologies with the greatest potential to this end is optogenetics-an invasive neuromodulation technique involving the use of light to control the activity of individual brain cells. It has recently shown the potential to modify specific long-term memories in animal models in ways not yet possible with other MMTs. As the therapeutic potential of optogenetics has already prompted approval of the first human trials, it is especially important and timely to consider the opportunities and dangers this technology may entail. In this article, we focus on possible consequences of optogenetics as an MMT by analyzing fundamental threats potentially associated with memory modifications: the potential disruption of personality and authenticity.

Easy to Make, Hard to Revise: Updating Spontaneous Trait Inferences in the Presence of Trait-Inconsistent Information

Previous research has shown that perceivers spontaneously form trait inferences from others' behaviors received at a single point in time. The present work examined the persistence of spontaneous trait inferences (STIs) in the presence of trait-inconsistent information about others. We hypothesized that STIs should be resistant to change over time and in the presence of new trait-inconsistent information due to perceivers forming and storing multiple STIs independently in memory. Consistently, Experiments 1a and 1b showed that initial STIs were not affected by new trait-inconsistent information. Experiments 2 and 3 revealed that STIs were persistent over 48 hours. Two experiments also tested memory reconsolidation as a possible mechanism of updating first impressions. While STIs were not substantially affected, spontaneous goal inferences (SGIs) were elevated among those with a better explicit memory of behaviors after learning trait-inconsistent information following a memory reactivation procedure. Implications of these findings on impression formation and updating processes are discussed.

Difficulties in retrieval multiplication facts: The case of interference to reconsolidation

OBJECTIVES

Many students have difficulties in retrieving multiplication facts from memory. The aim of the present study was to test the difficulty in retrieval of multiplication facts from the perspective of the reconsolidation of long-term memory phase, which has been found to be sensitive to interferences.

METHODS

Students learned multiplication facts and then received a reminder, which led to reactivation and reconsolidation. After the reminder, additional multiplication facts (interference) were learned and memory was tested.

RESULTS

Students who received both a reminder and interference during reconsolidation showed no significant improvement in retrieving multiplication facts from memory, whereas Students who received either a reminder or additional multiplication facts (interference) exhibited a better performance in retrieval.

CONCLUSIONS

These results indicate, for the first time, that the reconsolidation phase is sensitive to interferences in mathematical declarative memory content. The findings indicate additional possible causes for difficulties in retrieval of multiplication facts in class.

No persistent attenuation of fear memories in humans: A registered replication of the reactivation-extinction effect

It has been proposed that memory retrieval can destabilize consolidated memories, after which they need to be reconsolidated in order to be retained. The presentation of relevant information during memory reconsolidation could then result in the modification of a destabilized memory trace, by allowing the memory trace to be updated before being reconsolidated. In line with this idea, Schiller et al. (2010) have demonstrated that memory retrieval shortly before extinction training can prevent the later recovery of conditioned fear responding that is observed after regular extinction training. Those findings have been the subject of considerable controversy, due in part to theoretical reasons but also due to a number of failures to obtain similar results in conceptual replication attempts. Here, we report the results of a highly powered, direct, independent replication of the critical conditions of Schiller et al. (2010, Experiment 1). Due to misrepresentation of the exclusion criteria in the original Schiller et al. (2010) report, data collection was considerably delayed. When we eventually managed to attain our pre-registered sample size, we found that we could not observe any benefit of reactivation-extinction over regular extinction training in preventing recovery of conditioned fear. The results of the present study, along with the mixed findings in the literature and the misreporting in Schiller et al. (2010), give cause to question whether there is robust evidence that reactivation-extinction prevents the return of fear in humans.

Rescripting experimental trauma: Effects of imagery and writing as a way to reduce the development of intrusive memories

BACKGROUND AND OBJECTIVES

Imagery rescripting is an effective treatment strategy for trauma related disorders, but its underlying mechanisms are still largely unknown. The aim of the present study was to test whether a) imagery (versus writing) is essential in the process of rescripting, and b) rescripting affects emotional memories on an implicit level.

METHODS

Healthy participants were subjected to an experimental trauma ('trauma film'), and randomly allocated to four conditions: recall of film + Imagery Rescripting (ImRs), recall of film + Writing Rescripting (WRs), recall only (ImRE), or no recall + no manipulation (NM). Next, participants recorded intrusion frequency and distress during one week, after which they executed a visual interference task (VIT) including neutral and trauma film stills, to access implicit emotional memory.

RESULTS

Main findings were that ImRs and WRs resulted in fewer intrusions than NM, with no differences between both rescripting conditions. We did not find an effect on intrusion distress and the VIT.

LIMITATIONS

Stills in the VIT were distracted from all four film scenes, whereas rescripting was done on one scene only, possibly obscuring the effect. Also, an analogue sample was used, which may limit generalizability to clinical samples.

CONCLUSIONS

We replicated previous effects of ImRs on intrusion development. Furthermore, no superior effect of imagery as key modality for rescripting was found; writing seems a viable alternative. Measures for implicit emotional memory such as the VIT may have to be applied relatively soon after the experimental session (e.g., same day as the experimental session).

Memory engrams: Recalling the past and imagining the future

In 1904, Richard Semon introduced the term "engram" to describe the neural substrate for storing memories. An experience, Semon proposed, activates a subset of cells that undergo off-line, persistent chemical and/or physical changes to become an engram. Subsequent reactivation of this engram induces memory retrieval. Although Semon's contributions were largely ignored in his lifetime, new technologies that allow researchers to image and manipulate the brain at the level of individual neurons has reinvigorated engram research. We review recent progress in studying engrams, including an evaluation of evidence for the existence of engrams, the importance of intrinsic excitability and synaptic plasticity in engrams, and the lifetime of an engram. Together, these findings are beginning to define an engram as the basic unit of memory.

Impact of a reminder/extinction procedure on threat-conditioned pupil size and skin conductance responses

A reminder can render consolidated memory labile and susceptible to amnesic agents during a reconsolidation window. For the case of threat memory (also termed fear memory), it has been suggested that extinction training during this reconsolidation window has the same disruptive impact. This procedure could provide a powerful therapeutic principle for treatment of unwanted aversive memories. However, human research yielded contradictory results. Notably, all published positive replications quantified threat memory by conditioned skin conductance responses (SCR). Yet, other studies measuring SCR and/or fear-potentiated startle failed to observe an effect of a reminder/extinction procedure on the return of fear. Here we sought to shed light on this discrepancy by using a different autonomic response, namely, conditioned pupil dilation, in addition to SCR, in a replication of the original human study. N = 71 humans underwent a 3-d threat conditioning, reminder/extinction, and reinstatement, procedure with 2 CS+, of which one was reminded. Participants successfully learned the threat association on day 1, extinguished conditioned responding on day 2, and showed reinstatement on day 3. However, there was no difference in conditioned responding between the reminded and the nonreminded CS, neither in pupil size nor SCR. Thus, we found no evidence that a reminder trial before extinction prevents the return of threat-conditioned responding.

A computational model of systems memory consolidation and reconsolidation

In the mammalian brain, newly acquired memories depend on the hippocampus (HPC) for maintenance and recall, but over time, the neocortex takes over these functions, rendering memories HPC-independent. The process responsible for this transformation is called systems memory consolidation. Reactivation of a well-consolidated memory can trigger a temporary return to a HPC-dependent state, a phenomenon known as systems memory reconsolidation. The neural mechanisms underlying systems memory consolidation and reconsolidation are not well understood. Here, we propose a neural model based on well-documented mechanisms of synaptic plasticity and stability and describe a computational implementation that demonstrates the model's ability to account for a range of findings from the systems consolidation and reconsolidation literature. We derive several predictions from the computational model and suggest experiments that may test its validity.

Pharmacological interventions during the process of reconsolidation of aversive memories: A systematic review

Reconsolidation is the return of a memory to a transient state of lability, following memory consolidation, that can occur when memories are evoked. During the process of reconsolidation, memories may be modified by different means, including the administration of drugs, during a period called the "reconsolidation window”. This process has been widely studied in animals, but human studies are limited and include several methodological pitfalls. Our objective was to conducte a systematic review of the literature that utilizes pharmacological interventions during the process of reconsolidation of aversive memories in humans, with a critical analysis of the methodologies used. Searches were made in the electronic databases PubMed, Scopus, Web of Science and SciELO using the following search terms: (memory) AND (consolidation OR reconsolidation) AND (pharmacological manipulation OR pharmacological intervention). We found 294 references and ten (3.4%) were included in the review, based on preestablished eligibility criteria. All studies were randomized, double-blind clinical trials. The most commonly studied drug was propranolol. Two studies used a protocol involving autobiographical aversive memories, while in the remaining aversive memories were produced in the laboratory. The timing of pharmacological interventions is a controversial issue in the field, as drug activity must occur within the reconsolidation window. The small number of studies and some methodological difficulties of this type of research highlights the need for studies that individually evaluate some of the issues discussed, particularly the timing of pharmacological interventions and the duration of reconsolidation windows.

Cortisol suppression after memory reactivation impairs later memory performance

Experiencing stressful or traumatic events can result in disabling clinical symptoms of maladaptive emotional memory retrieval, which are only partly addressed by the currently proposed treatments. Cortisol modulation has been shown to affect emotional memory retrieval and potentially reconsolidation, offering an opportunity for developing more efficient treatments for disorders with an emotional memory component. Here, we investigated if cortisol suppression after reactivation of emotional memories weakens later memory thereof. Forty healthy young men were tested in a randomized, placebo controlled, double-blind, and between-subject design, assigned either to a cortisol suppression (metyrapone) group or a placebo group. Participants of both groups, were presented with two emotional stories at an encoding session (Day 1). One of the two stories was later reactivated and followed by metyrapone vs. placebo administration (Day 3). Memory for both stories was tested at a recognition memory session (Day 7). In the group undergoing cortisol suppression after memory reactivation memory performance was weaker compared to the placebo group, tested four days after reactivation. This study shows that cortisol suppression can weaken memory for past events, possibly by altering reconsolidation processes and thus exerting long-lasting weakening effects on the original memory.

Retrieval of retrained and reconsolidated memories are associated with a distinct neural network

Consolidated memories can persist from a single day to years, and persistence is improved by retraining or retrieval-mediated plasticity. One retrieval-based way to strengthen memory is the reconsolidation process. Strengthening occurs simply by the presentation of specific cues associated with the original learning. This enhancement function has a fundamental role in the maintenance of memory relevance in animals everyday life. In the present study, we made a step forward in the identification of brain correlates imprinted by the reconsolidation process studying the long-term neural consequences when the strengthened memory is stable again. To reach such a goal, we compared the retention of paired-associate memories that went through retraining process or were labilizated-reconsolidated. Using functional magnetic resonance imaging (fMRI), we studied the specific areas activated during retrieval and analyzed the functional connectivity of the whole brain associated with the event-related design. We used Graph Theory tools to analyze the global features of the network. We show that reconsolidated memories imprint a more locally efficient network that is better at exchanging information, compared with memories that were retrained or untreated. For the first time, we report a method to elucidate the neural footprints associated with a relevant function of memory reconsolidation.

Post-retrieval oxytocin facilitates next day extinction of threat memory in humans

RATIONALE

Memories can return to a labile state and become amenable to modification by pharmacological and behavioral manipulations after retrieval. This process may reduce the impact of aversive memories and provide a promising therapeutic technique for the treatment of anxiety disorders. A growing body of evidence suggests that the mammalian neuropeptide oxytocin (OT) plays a role in the regulation of emotional memories in animals. However, the effects of OT on threat memory in humans remain largely unknown.

OBJECTIVES

This study aimed to investigate the effects of OT administration following threat memory retrieval on subsequent memory expression in human participants.

METHODS

In a double-blind, randomized, placebo-controlled, between-subject design, 61 healthy human individuals completed a 3-day experiment. All the participants underwent threat conditioning on day 1. On day 2, the participants were randomized to receive an intranasal dose of OT (40 IU) or placebo after memory retrieval, or an intranasal dose of OT (40 IU) without retrieval. On day 3, the participants were tested for extinction and reinstatement.

RESULTS

On day 3, all groups showed equivalent stimulus discrimination during the early phase of extinction. However, the group that received OT following a memory reminder showed a greater decline in stimulus discrimination by the late phase of extinction relative to the two other groups.

CONCLUSIONS

The results indicate that OT did not block reconsolidation to prevent the return of threat memory but rather interacted with post-retrieval processes to facilitate next day extinction. The study provides novel preliminary evidence for the role of OT in human threat memory.

Attenuating Neural Threat Expression with Imagination

Imagination is an internal simulation of real-life events and a common treatment tool for anxiety disorders; however, the neural processes by which imagination exerts behavioral control are unclear. This investigation tests whether and how imagined exposures to a threatening stimulus, conditioned in the real world, influence neural and physiological manifestations of threat. We found that imagined and real extinction are equally effective in the reduction of threat-related neural patterns and physiological responses elicited upon re-exposure to real-world threatening cues. Network connectivity during the extinction phase showed that imagined, like real, extinction engaged the ventromedial prefrontal cortex (vmPFC) as a central hub. vmPFC, primary auditory cortex, and amygdala activation during imagined and real extinction were predictive of individual differences in extinction success. The nucleus accumbens, however, predicted extinction success in the imagined extinction group alone. We conclude that deliberate imagination can attenuate reactions to threat through perceptual and associative learning mechanisms.

Between an ugly truth and a perfect lie: Wiping off fearful memories using beta-adrenergic receptors antagonists

Psychiatric disorders such as anxiety, phobias, and post-traumatic stress disorder are considered of high global prevalence. Currently, a therapeutic approach to treat these disorders using beta-blockers, which antagonize the beta-adrenergic receptors (B1, B2, and B3) is being studied. This approach claims that beta-blockers, such as propranolol, inhibit fear memory reconsolidation. However, there are several studies refuting such claims by discrediting their experimental design and pointing out both the drugs pharmacokinetic properties and confounding factors. In this review, we explore the different effects of central beta-adrenergic agonists and antagonists on the fear memory consolidation providing mixed-evidence, limitations, and future directions.

Reconsolidation of human motor memory: From boundary conditions to behavioral interventions-How far are we from clinical applications?

The memory reconsolidation hypothesis states that a previously consolidated and stable memory can return to a temporary labile state after retrieved, requiring a new stabilization process. During the labile period, the memory trace is vulnerable to modification, which provides a potential therapeutic opportunity to weaken, updated or strengthen that memory. As such, reconsolidation has been the subject of numerous studies in different domains of human memory that seek strategies to treat post-traumatic disorders and erase or modify pathological memories. A few studies have also investigated the impairment effects of behavioral interferences on motor memory. However, very little has been researched and written about the possibility of using reconsolidation to enhance motor skill learning. Here, we present a critical review of the literature and trace possible applications for human motor memory reconsolidation. We discuss the boundary conditions and the mechanisms to trigger the reconsolidation process, as well as the effects of behavioral interventions in modifying the performance of motor skills.

Memory integration: An alternative to the consolidation/reconsolidation hypothesis

The original concept of consolidation considers that memory requires time to be fixed. Since 2000, a comparable protein-dependent re-stabilization phase, called reconsolidation, has been assumed to take place after memory retrieval. This consolidation/reconsolidation hypothesis, has dominated the literature for more than 50 years, despite compelling evidence that is inconsistent with it. In this review, we present an historical overview and explain how, despite serious criticisms, this hypothesis has persisted for decades and become accepted as a dogma. Based on both older and more recent evidence, we next propose the concept of memory integration which involves the linkage or embedding of new material into an already existing representation. We believe integration provides a viable explanation for retrograde amnesia in place of the consolidation/reconsolidation hypothesis. Integration can further be the basis for several major cases of memory alteration such as time dependent memory enhancement, interference, counter-conditioning, updating and other instances of memory malleability. In a final section we consider the implications this new concept may have for memory processes and its translational applications.

Memory and Forgetting

PURPOSE OF REVIEW

I summarize recent developments in understanding the phenomenology of memory in PTSD, describe the most prominent theoretical models, and outline new forms of treatment aimed at modifying the traumatic memory.

RECENT FINDINGS

Intrusive memories that have the quality of being relived in the present have been highlighted in ICD-11. Debate over whether trauma memories are disorganized has led to a distinction between global narratives that are usually well rehearsed and episodic memories of the most frightening moments when disruptions and fragmentation may occur. Attempts to prevent the initial consolidation of trauma memories have promise in prevention but face practical difficulties. Theoretical developments have led to a number of promising treatments for established PTSD including pre-retrieval propranolol and imagery rescripting. Research has suggested real possibilities to improve the prevention and treatment of PTSD by modifying trauma recall even though the theoretical basis for these interventions remains controversial.

Exploring the role of context on the existing evidence for reconsolidation of episodic memory

Recent research has provided evidence for memory modifications when a post-reactivation treatment (e.g., drugs, new learning) interferes with the memory re-stabilisation (reconsolidation) process. This finding contradicts the long-standing consolidation theory and has high practical and theoretical implications. With an object-learning paradigm, it was shown that episodic memory is highly susceptible to interfering material presented after its reactivation [Hupbach, A., Gomez, R., Hardt, O., & Nadel, L. (2007). Reconsolidation of episodic memories: A subtle reminder triggers integration of new information. Learning & Memory, 14, 47-53. doi: 10.1101/lm.365707 ]. The reactivation of a learned list (List 1) before a second learned list (List 2) led to intrusion errors from List 2 when trying to recall List 1, but not vice-versa. Their work has been widely cited and their findings have been explained according to reconsolidation theory. For the first time, we systematically explored the role of retrieval context as an alternative explanation for Hupbach's results. Our results showed that the intrusion effect occurs independently of the retrieval context (Experiment 1). Additionally, even when the intrusion rate probability is increased (i.e., List 1 memory test is performed in the List 2 learning context), the groups that did not reactivate the original list did not commit intrusion errors (Experiment 2). In sum, we found that the intrusion effect critically depends on the presence of reactivation, discarding alternative interpretations of the results.

Behavioral interventions to eliminate fear responses

Fear memory underlies anxiety-related disorders, including posttraumatic stress disorder (PTSD). PTSD is a fear-based disorder, characterized by difficulties in extinguishing the learned fear response and maintaining extinction. Currently, the first-line treatment for PTSD is exposure therapy, which forms an extinction memory to compete with the original fear memory. However, the extinguished fear often returns under numerous circumstances, suggesting that novel methods are needed to eliminate fear memory or facilitate extinction memory. This review discusses research that targeted extinction and reconsolidation to manipulate fear memory. Recent studies indicate that sleep is an active state that can regulate memory processes. We also discuss the influence of sleep on fear memory. For each manipulation, we briefly summarize the neural mechanisms that have been identified in human studies. Finally, we highlight potential limitations and future directions in the field to better translate existing interventions to clinical settings.

Behavioral reconsolidation interference with episodic memory within-subjects is elusive

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We assessed behavioral reconsolidation interference in episodic memory.

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Employed within-subjects paradigm measuring direct memory strength.

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Three experiments found no effects of reminder-potentiated interference.

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Null effects were substantiated by Bayesian analyses.

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Highlights boundary conditions for behavioral reconsolidation interference effects.

In studies of behavioral reconsolidation interference, reactivation of a consolidated memory using some form of reminder is followed by the presentation of new information that can cause interference with that memory. Under these conditions, the interference not only impairs retrieval by indirect processes such as cue interference, but supposedly disrupts the original memory trace directly. Almost all studies of behavioral reconsolidation interference in episodic memory in humans have employed between-subjects paradigms, and deduced reminder effects from intrusion errors. Such studies might introduce confounds arising, for example, from differences in retrieval strategies engendered by the pre-test treatments. We therefore set out to examine whether behavioral reconsolidation interference in episodic memory might be demonstrated within-subjects and by direct memory strength rather than intrusion errors. In three separate experiments, we attempted to disrupt reconsolidation of episodic object-picture memory using a reminder + retroactive interference manipulation. We applied the manipulation over three consecutive days, using a forced-choice recognition test without intrusions from interfering learning, keeping all other study and test parameters constant. No effects of reminder-potentiated interference were observed for measures of accuracy, response times, subjective expressions of recollection, or levels of confidence, as substantiated by Bayesian analyses. These results highlight the difficulty of observing clear behavioral reconsolidation interference effects within-subjects in human episodic memory, and provide some indications of what might be boundary conditions for its demonstration.

Dynamical system with plastic self-organized velocity field as an alternative conceptual model of a cognitive system

It is well known that architecturally the brain is a neural network, i.e. a collection of many relatively simple units coupled flexibly. However, it has been unclear how the possession of this architecture enables higher-level cognitive functions, which are unique to the brain. Here, we consider the brain from the viewpoint of dynamical systems theory and hypothesize that the unique feature of the brain, the self-organized plasticity of its architecture, could represent the means of enabling the self-organized plasticity of its velocity vector field. We propose that, conceptually, the principle of cognition could amount to the existence of appropriate rules governing self-organization of the velocity field of a dynamical system with an appropriate account of stimuli. To support this hypothesis, we propose a simple non-neuromorphic mathematical model with a plastic self-organized velocity field, which has no prototype in physical world. This system is shown to be capable of basic cognition, which is illustrated numerically and with musical data. Our conceptual model could provide an additional insight into the working principles of the brain. Moreover, hardware implementations of plastic velocity fields self-organizing according to various rules could pave the way to creating artificial intelligence of a novel type.

A reminder before extinction strengthens episodic memory via reconsolidation but fails to disrupt generalized threat responses

A reminder can temporarily renew flexibility of consolidated memories, referred to as reconsolidation. Pavlovian threat-conditioning studies suggest that a reminder can renew flexibility of threat responses but that episodic memories remain stable. In contrast, outside the threat-conditioning domain, studies testing memory for word lists or stories find that a reminder can renew flexibility of episodic memory. This discrepancy in findings leaves it unclear if episodic memories reconsolidate, or only Pavlovian responses. Here we unite the different approaches in the field and show that a reminder can retroactively strengthen episodic memory for Pavlovian threat-conditioned events, but that, in contrast to threat-conditioning studies with simple sensory stimuli, extinction after a reminder fails to prevent recovery of generalized threat responses. Our results indicate the episodic memories also reconsolidate, allowing strengthening of relevant memories. These findings also suggest that generalized threat responses and episodic memories are less susceptible to be modified by reminder-interventions procedures.

Extinction after fear memory reactivation fails to eliminate renewal in rats

Retrieving fear memories just prior to extinction has been reported to effectively erase fear memories and prevent fear relapse. The current study examined whether the type of retrieval procedure influences the ability of extinction to impair fear renewal, a form of relapse in which responding to a conditional stimulus (CS) returns outside of the extinction context. Rats first underwent Pavlovian fear conditioning with an auditory CS and footshock unconditional stimulus (US); freezing behavior served as the index of conditioned fear. Twenty-four hours later, the rats underwent a retrieval-extinction procedure. Specifically, 1h prior to extinction (45 CS-alone trials; 44 for rats receiving a CS reminder), fear memory was retrieved by either a single exposure to the CS alone, the US alone, a CS paired with the US, or exposure to the conditioning context itself. Over the next few days, conditional freezing to the extinguished CS was tested in the extinction and conditioning context in that order (i.e., an ABBA design). In the extinction context, rats that received a CS+US trial before extinction exhibited higher levels of conditional freezing than animals in all other groups, which did not differ from one another. In the renewal context, all groups showed renewal, and none of the reactivation procedures reduced renewal relative to a control group that did not receive a reactivation procedure prior to extinction. These data suggest retrieval-extinction procedures may have limited efficacy in preventing fear renewal.

Modification of episodic memories by novel learning: a failed replication study

Background: After reactivation, memories can become unstable and sensitive to modification before they are restored into long-term memory. Using behavioural manipulations, reactivated memories may be disrupted via the mechanism of interference (i.e. novel learning). In a laboratory study, Wichert et al. (2013a) showed that new learning after reactivation changed episodic memory, while new learning alone or reactivation alone did not. Objective: Given the potential clinical application of such a procedure in trauma-focused psychological treatments, such as CBT or EMDR, the aim of this study was to replicate Wichert et al. Method: On Day 1, participants (N = 96) viewed and recalled a series of emotional and non-emotional pictures. Then, participants were randomized to one of four groups. One week later, on Day 8, Group 1 reactivated the previously learned pictures and learned new pictures. To control for specific effects of reactivation or new learning, Group 2 only reactivated the previously learned pictures, and Group 3 only learned new pictures. Group 4 received no reactivation and no new learning. On Day 9, all groups indicated for each picture out of a series whether they had seen it on Day 1. Results: The data were analysed using Bayesian hypothesis testing, which allows for quantifying the evidence in favour of the alternative and the null hypothesis. In general, results showed that Group 1 recognized fewer pictures from Day 1 compared to Groups 2 and 4 on Day 9. However, the expected difference between new learning following reactivation (i.e. Group 1) and new learning alone (i.e. Group 3) was not substantially supported by the data for any of our dependent measures. Conclusions: We replicated some of the findings by Wichert et al., but did not find substantial support for the critical difference between new learning following reactivation and new learning alone.

An Update on Memory Reconsolidation Updating

The reactivation of a stored memory in the brain can make the memory transiently labile. During the time it takes for the memory to restabilize (reconsolidate) the memory can either be reduced by an amnesic agent or enhanced by memory enhancers. The change in memory expression is related to changes in the brain correlates of long-term memory. Many have suggested that such retrieval-induced plasticity is ideally placed to enable memories to be updated with new information. This hypothesis has been tested experimentally, with a translational perspective, by attempts to update maladaptive memories to reduce their problematic impact. We review here progress on reconsolidation updating studies, highlighting their translational exploitation and addressing recent challenges to the reconsolidation field.

Intrusions in episodic memory: reconsolidation or interference?

It would be profoundly important if reconsolidation research in animals and other memory domains generalized to human episodic memory. A 3-d-list-discrimination procedure, based on free recall of objects, with a contextual reminder cue (the testing room), has been thought to demonstrate reconsolidation of human episodic memory (as noted in a previous study). Our goal was to replicate the central result, a high intrusion rate during recall of the target list, and evaluate the reconsolidation account relative to an alternative account, based on state-dependent learning and interference. First, replication was not straightforward (Experiment 1). Second, using a very unique, highly salient context (Experiment 2), the method produced a qualitative replication, but it was small in magnitude. A critical assumption of the reconsolidation account, that the target list is reactivated and destabilized during re-exposure to the study context, was not supported (Experiment 3). Although troubling for the reconsolidation account, the findings can be easily accommodated by an alternative account that does not assume additional neurobiological processes underlying the destabilization of consolidated memories, instead explaining intrusion rates simply in terms of well-established cognitive effects, such as item-to-context binding and interference during retrieval.

Memory Reconsolidation Interference as an Emerging Treatment for Emotional Disorders: Strengths, Limitations, Challenges, and Opportunities

Experimental research on emotional memory reconsolidation interference, or the induction of amnesia for previously established emotional memory, has a long tradition, but the potential of that research for the development of novel interventions to treat psychological disorders has been recognized only recently. Here we provide an overview of basic research and clinical studies on emotional memory reconsolidation interference. We point out specific advantages of interventions based on memory reconsolidation interference over traditional treatment for emotional disorders. We also explain how findings from basic research suggest limitations and challenges to clinical translation that may help to understand why clinical trials have met with mixed success so far and how their success can be increased. In closing, we preview new intervention approaches beyond the induction of amnesia that the phenomenon of memory reconsolidation may afford for alleviating the burden imposed by emotional memories and comment on theoretical controversies regarding the nature of memory reconsolidation.

Translational Approaches Targeting Reconsolidation

Maladaptive learned responses and memories contribute to psychiatric disorders that constitute a significant socio-economic burden. Primary treatment methods teach patients to inhibit maladaptive responses, but do not get rid of the memory itself, which explains why many patients experience a return of symptoms even after initially successful treatment. This highlights the need to discover more persistent and robust techniques to diminish maladaptive learned behaviours. One potentially promising approach is to alter the original memory, as opposed to inhibiting it, by targeting memory reconsolidation. Recent research shows that reactivating an old memory results in a period of memory flexibility and requires restorage, or reconsolidation, for the memory to persist. This reconsolidation period allows a window for modification of a specific old memory. Renewal of memory flexibility following reactivation holds great clinical potential as it enables targeting reconsolidation and changing of specific learned responses and memories that contribute to maladaptive mental states and behaviours. Here, we will review translational research on non-human animals, healthy human subjects, and clinical populations aimed at altering memories by targeting reconsolidation using biological treatments (electrical stimulation, noradrenergic antagonists) or behavioural interference (reactivation-extinction paradigm). Both approaches have been used successfully to modify aversive and appetitive memories, yet effectiveness in treating clinical populations has been limited. We will discuss that memory flexibility depends on the type of memory tested and the brain regions that underlie specific types of memory. Further, when and how we can most effectively reactivate a memory and induce flexibility is largely unclear. Finally, the development of drugs that can target reconsolidation and are safe for use in humans would optimize cross-species translations. Increasing the understanding of the mechanism and limitations of memory flexibility upon reactivation should help optimize efficacy of treatments for psychiatric patients.

Does reactivation trigger episodic memory change? A meta-analysis

According to the reconsolidation hypothesis, long-term memories return to a plastic state upon their reactivation, leaving them vulnerable to interference effects and requiring re-storage processes or else these memories might be permanently lost. The present study used a meta-analytic approach to critically evaluate the evidence for reactivation-induced changes in human episodic memory. Results indicated that reactivation makes episodic memories susceptible to physiological and behavioral interference. When applied shortly after reactivation, interference manipulations altered the amount of information that could be retrieved from the original learning event. This effect was more pronounced for remote memories and memories of narrative structure. Additionally, new learning following reactivation reliably increased the number of intrusions from new information into the original memory. These findings support a dynamic view of long-term memory by showing that memories can be changed long after they were acquired.

Consolidation and reconsolidation are impaired by oral propranolol administered before but not after memory (re)activation in humans

Propranolol administered immediately after learning or after recall has been found to impair memory consolidation or reconsolidation (respectively) in animals, but less reliably so in humans. Since reconsolidation impairment has been proposed as a treatment for mental disorders that have at their core an emotional memory, it is desirable to understand how to reliably reduce the strength of pathogenic memories in humans. We postulated that since humans (unlike experimental animals) typically receive propranolol orally, this introduces a delay before this drug can exert its memory impairment effects, which may render it less effective. As a means to test this, in two double-blind placebo-controlled experiments, we examined the capacity of propranolol to impair consolidation and reconsolidation as a function of timing of ingestion in healthy subjects. In Experiment 1, (n=36), propranolol administered immediately after learning or recall failed to impair the consolidation or reconsolidation of the memory of a standardized slideshow with an accompanying emotional story. In Experiment 2 (n=50), propranolol given 60-75min before learning or recall successfully impaired memory consolidation and reconsolidation. These results suggest that it is possible to achieve reliable memory impairment in humans if propranolol is given before learning or before recall, but not after.

Reconsolidation and psychopathology: Moving towards reconsolidation-based treatments

Interfering with memory reconsolidation has valuable potential to be used as a treatment for maladaptive memories and psychiatric disorders. Numerous studies suggest that reconsolidation-based therapies may benefit psychiatric populations, but much remains unanswered. After reviewing the literature in clinical and healthy human populations, we discuss some of the major limitations to reconsolidation studies and clinical application. Finally, we provide recommendations for developing improved reconsolidation-based treatments, namely exploiting known boundary conditions and focusing on a novel unconditioned stimulus-retrieval paradigm.