Updating fearful memories with extinction training during reconsolidation: a human study using auditory aversive stimuli

Preventing fear return in humans: Music-based intervention during reactivation-extinction paradigm

In several research studies, the reactivation extinction paradigm did not effectively prevent the return of fear if administered without any intervention technique. Therefore, in this study, the authors hypothesized that playing music (high valence, low arousal) during the reconsolidation window may be a viable intervention technique for eliminating fear-related responses. A three-day auditory differential fear conditioning paradigm was used to establish fear conditioning. Participants were randomly assigned into three groups, i.e., one control group, standard extinction (SE), and two experimental groups, reactivation extinction Group (RE) and music reactivation extinction (MRE), of twenty participants in each group. Day 1 included the habituation and fear acquisition phases; on Day 2 (after 24 hours), the intervention was conducted, and re-extinction took place on Day 3. Skin conductance responses were used as the primary outcome measure. Results indicated that the MRE group was more effective in reducing fear response than the RE and SE groups in the re-extinction phase. Furthermore, there was no significant difference observed between SE and RE groups. This is the first study known to demonstrate the effectiveness of music intervention in preventing the return of fear in a healthy individual. Therefore, it might also be employed as an intervention strategy (non-pharmacological approach) for military veterans, in emotion regulation, those diagnosed with post-traumatic stress disorder, and those suffering from specific phobias.

Locus coeruleus integrity predicts ease of attaining and maintaining neural states of high attentiveness

The locus coeruleus (LC), a small subcortical structure in the brainstem, is the brain's principal source of norepinephrine. It plays a primary role in regulating stress, the sleep-wake cycle, and attention, and its degradation is associated with aging and neurodegenerative diseases associated with cognitive deficits (e.g., Parkinson's, Alzheimer's). Yet precisely how norepinephrine drives brain networks to support healthy cognitive function remains poorly understood - partly because LC's small size makes it difficult to study noninvasively in humans. Here, we characterized LC's influence on brain dynamics using a hidden Markov model fitted to functional neuroimaging data from healthy young adults across four attention-related brain networks and LC. We modulated LC activity using a behavioral paradigm and measured individual differences in LC magnetization transfer contrast. The model revealed five hidden states, including a stable state dominated by salience-network activity that occurred when subjects actively engaged with the task. LC magnetization transfer contrast correlated with this state's stability across experimental manipulations and with subjects' propensity to enter into and remain in this state. These results provide new insight into LC's role in driving spatiotemporal neural patterns associated with attention, and demonstrate that variation in LC integrity can explain individual differences in these patterns even in healthy young adults.

Making Leaps and Hitting Boundaries in Reconsolidation: Overcoming Boundary Conditions to Increase Clinical Translatability of Reconsolidation-based Therapies

Reconsolidation results in the restabilisation, and thus persistence, of a memory made labile by retrieval, and interfering with this process is thought to enable modification or weakening of the original trace. As such, reconsolidation-blockade has been a focus of research aiming to target the maladaptive memories underlying mental health disorders, including post-traumatic stress disorder and drug addiction. Current first-line therapies are not effective for all patients, and a substantial proportion of those for whom therapies are effective later relapse. A reconsolidation-based intervention would be invaluable as an alternative treatment for these conditions. However, the translation of reconsolidation-based therapies to the clinic presents a number of challenges, with arguably the greatest being the overcoming of the boundary conditions governing the opening of the reconsolidation window. These include factors such as the age and strength of memory, and can broadly be divided into two categories: intrinsic features of the targeted memory itself, and parameters of the reactivation procedure used. With maladaptive memory characteristics inevitably varying amongst individuals, manipulation of the other limitations imposed by procedural variables have been explored to circumvent the boundary conditions on reconsolidation. Although several apparently discrepant results remain to be reconciled and these limitations yet to be truly defined, many studies have produced successful results which encouragingly demonstrate that boundary conditions may be overcome using various proposed strategies to enable translation of a reconsolidation-based intervention to clinical use.

Effects of transcranial direct current stimulation on neural activity and functional connectivity during fear extinction

Background/Objective

Anxiety disorders are highly prevalent and negatively impact daily functioning and quality of life. Transcranial direct current stimulation (tDCS) targeting the dorsolateral prefrontal cortex (dlPFC), especially in the right hemisphere impacts extinction learning; however, the underlying neural mechanisms are elusive. Therefore, we aimed to investigate the effects of cathodal tDCS stimulation to the right dlPFC on neural activity and connectivity patterns during delayed fear extinction in healthy participants.

Methods

We conducted a two-day fear conditioning and extinction procedure. On the first day, we collected fear-related self-reports, clinical questionnaires, and skin conductance responses during fear acquisition. On the second day, participants in the tDCS group (n = 16) received 20-min offline tDCS before fMRI and then completed the fear extinction session during fMRI. Participants in the control group (n = 18) skipped tDCS and directly underwent fMRI to complete the fear extinction procedure. Whole-brain searchlight classification and resting-state functional connectivity analyses were performed.

Results

Whole-brain searchlight classification during fear extinction showed higher classification accuracy of threat and safe cues in the left anterior dorsal and ventral insulae and hippocampus in the tDCS group than in the control group. Functional connectivity derived from the insula with the dlPFC, ventromedial prefrontal cortex, and inferior parietal lobule was increased after tDCS.

Conclusion

tDCS over the right dlPFC may function as a primer for information exchange among distally connected areas, thereby increasing stimulus discrimination. The current study did not include a sham group, and one participant of the control group was not randomized. Therefore, to address potential allocation bias, findings should be confirmed in the future with a fully randomized and sham controlled study.

Reconsolidation and Fear Extinction: An Update

Fear memories can be updated behaviorally by delivering extinction trials during the reconsolidation window, which results in a persistent attenuation of fear memories (Monfils et al., Science 324:951-955, 2009). This safe and non-invasive paradigm, termed retrieval-extinction (or post-retrieval extinction), has also been found to be successful at preventing the return of fear in healthy fear conditioned humans (Schiller et al., Nature 463:49-53, 2010), and in the time since its discovery, there has been an explosion of research on the use of retrieval-extinction in fear memories in humans and other animals, some of which have found a long-term reduction in conditioned responding, and some who have not. These discrepant findings have raised concerns as to whether retrieval-extinction really results in updating of the original fear memory, or if it simply enhances extinction. We will first review the progress made on elucidating the cellular mechanisms underlying the fear attenuating effects of retrieval-extinction and how they differ from traditional extinction. Special attention will be paid to the molecular events necessary for retrieval-extinction to successfully occur and how these reconsolidated memories are represented in the brain. Next, we will examine the parameters that determine whether or not a memory will be updated via extinction during the reconsolidation window (also known as boundary conditions). These boundary conditions will also be discussed as possible explanations for discrepant findings of the retrieval-extinction effect. Then we will examine the factors that can determine whether an individual's fears will successfully be attenuated by retrieval-extinction. These individual differences include genetics, age, and psychopathology. Finally, we will discuss recent attempts to bring the retrieval-extinction paradigm from the bench to the bedside for the behavioral treatment of anxiety and trauma disorders.

Neuronal activity in the dorsal dentate gyrus during extinction regulates fear memory extinction and renewal

Memory extinction and renewal are major factors that limits the efficacy of exposure therapy. The dorsal dentate gyrus (dDG) plays a crucial role in spatial memory, and epigenetic modifications in the dDG play an important role in fear memory renewal. However, whether dDG activity regulates fear memory extinction and renewal remains unclear. In this study, we showed that an extinction procedure that prevents fear memory renewal (extinction within the reconsolidation window) leads to increased c-fos expression in the dDG. Chemicogenetic activation of dDG excitatory neurons during extinction training elevated fear memory extinction and prevented renewal, whereas inhibition of dDG excitatory neurons inhibited fear memory extinction. We also demonstrated that inhibiting fear engram cells (neurons active during fear acquisition) during extinction training inhibits fear memory extinction. Therefore, dDG activity during fear extinction plays an important role in fear memory extinction and renewal.

Highly Demand Working Memory Intervention Weakens a Reactivated Threat Memory and the Associated Cognitive Biases

Anxiety disorders are the most frequent type of mental disorder. Threat-conditioning memory plays a central role in anxiety disorders, impacting complex cognitive systems by modifying behavioral responses to fearful stimuli and inducing an overestimation of potential threats. Here, we analyzed the reminder-dependent amnesia on physiological responses, unconditioned stimulus (US) expectancy ratings, and measures of cognitive bias towards the threat of a threat-conditioning memory. Subjects received differential threat-conditioning. Twenty-four hours later, after reactivation of the memory of threat-conditioning, one group performed a high demand working memory task (HWM) and a second group a low demand working memory task (LWM). A third group only performed the HWM task. Retention of conditioned threat memory was tested on Day 3 in an extinction session followed by a reinstatement test. Tasks targeting stimulus representation, valuation, and attentional bias towards threat were performed. We show that the reminder-dependent intervention with an HWM weakened memory retention as expressed in skin conductance response (SCR) and faded the representation and valuation towards the threat, but it did not affect US expectancy or attentional bias. Our findings provide evidence for the experimental psychopathology approach opening the possibility to weaken both Threat conditioning memory and the systems associated with the maintenance of anxiety features.

The evidence for and against reactivation-induced memory updating in humans and nonhuman animals

Systematic investigation of reactivation-induced memory updating began in the 1960s, and a wave of research in this area followed the seminal articulation of "reconsolidation" theory in the early 2000s. Myriad studies indicate that memory reactivation can cause previously consolidated memories to become labile and sensitive to weakening, strengthening, or other forms of modification. However, from its nascent period to the present, the field has been beset by inconsistencies in researchers' abilities to replicate seemingly established effects. Here we review these many studies, synthesizing the human and nonhuman animal literature, and suggest that these failures-to-replicate reflect a highly complex and delicately balanced memory modification system, the substrates of which must be finely tuned to enable adaptive memory updating while limiting maladaptive, inaccurate modifications. A systematic approach to the entire body of evidence, integrating positive and null findings, will yield a comprehensive understanding of the complex and dynamic nature of long-term memory storage and the potential for harnessing modification processes to treat mental disorders driven by pervasive maladaptive memories.

Demarcating the boundary conditions of memory reconsolidation: An unsuccessful replication

Disrupting memory reconsolidation provides an opportunity to abruptly reduce the behavioural expression of fear memories with long-lasting effects. The success of a reconsolidation intervention is, however, not guaranteed as it strongly depends on the destabilization of the memory. Identifying the necessary conditions to trigger destabilization remains one of the critical challenges in the field. We aimed to replicate a study from our lab, showing that the occurrence of a prediction error (PE) during reactivation is necessary but not sufficient for destabilization. We tested the effectiveness of a reactivation procedure consisting of a single PE, compared to two control groups receiving no or multiple PEs. All participants received propranolol immediately after reactivation and were tested for fear retention 24 h later. In contrast to the original results, we found no evidence for a reconsolidation effect in the single PE group, but a straightforward interpretation of these results is complicated by the lack of differential fear retention in the control groups. Our results corroborate other failed reconsolidation studies and exemplify the complexity of experimentally investigating this process in humans. Thorough investigation of the interaction between learning and memory reactivation is essential to understand the inconsistencies in the literature and to improve reconsolidation interventions.

Can contingency rehearsal during the interval between a retrieval cue and extinction training change the effects of post-retrieval extinction?

Return of fear may be prevented by post-retrieval extinction (PRE), a procedure consisting of presenting a stimulus that was present during conditioning (retrieval cue) prior to extinction training. However, recent evidence suggests that there might be circumstances under which PRE is not effective to prevent the return of fear (boundary conditions), but some of these conditions remain unknown. We explored if rehearsing the CS, US or CS-US contingency during the interval between the retrieval cue and extinction training might change the effects of PRE. One day after differential fear conditioning, healthy human participants (n = 83) underwent either standard extinction (control condition, n = 31) or two different PRE procedures, one in which participants rehearsed the CS-US contingency during the interval between the retrieval cue and extinction (rehearsal condition, n = 25), or another in which they underwent a verbal fluency task directing their attention away from the experimental contingencies during this interval (nonrehearsal condition, n = 27). Return of fear in a reinstatement test was observed in both control and rehearsal conditions, whereas in the nonrehearsal condition there was a generalized increase in response to the CS+ and CS-. Differential response in the rehearsal condition had values slightly smaller than the control group with no significant differences from both control and nonrehearsal conditions. These results suggest that the overt behavior of participants during the interval between a retrieval cue and extinction training might change the effects of PRE in healthy human participants, but further manipulations of these variables are needed to confirm these findings.

Offline tDCS modulates prefrontal-cortical-subcortical-cerebellar fear pathways in delayed fear extinction

Transcranial direct current stimulation (tDCS) has been studied to enhance extinction-based treatments for anxiety disorders. However, the field shows conflicting results about its anxiolytic effect and only a few studies have observed the extinction of consolidated memories. We looked to study the effect of offline 1 mA tDCS over the right dorsolateral pre-frontal cortex across the fear pathways, in consolidated fear response during delayed extinction. Participants (N = 34 women) underwent in a two-day fear conditioning procedure. On day 1, participants were assigned to the control group (N = 18) or the tDCS group (N = 16) and went through a fear acquisition procedure. On day 2, the tDCS group received 20 min tDCS before extinction and while inside the MRI scanner. The control group completed the extinction procedure only. The tDCS session (for the tDCS group) and the fMRI scan (for both groups) were completed just on the second day. Univariate fMRI analysis showed stimulation-dependent activity during late extinction with the tDCS group showing decreased neural activity during the processing of threat cues (CS +) and increased activity during the processing of safety cues (CS -), in prefrontal, postcentral and paracentral regions, during late extinction. ROI to whole-brain psychophysiological interaction (PPI) analysis showed the tDCS effect on the connectivity between the left dorsolateral prefrontal cortex three cortical-amygdalo-hippocampal-cerebellar pathway clusters during the processing of the CS + in late extinction (TFCE corrected; p < 0.05). Increased neuronal activity during the processing of safety cues and stronger coupling during the processing of threat cues might be the mechanisms by which tDCS contributes to stimuli discrimination.

Reliability of web-based affective auditory stimulus presentation

Web-based experimental testing has seen exponential growth in psychology and cognitive neuroscience. However, paradigms involving affective auditory stimuli have yet to adapt to the online approach due to concerns about the lack of experimental control and other technical challenges. In this study, we assessed whether sounds commonly used to evoke affective responses in-lab can be used online. Using recent developments to increase sound presentation quality, we selected 15 commonly used sound stimuli and assessed their impact on valence and arousal states in a web-based experiment. Our results reveal good inter-rater and test-retest reliabilities, with results comparable to in-lab studies. Additionally, we compared a variety of previously used unpleasant stimuli, allowing us to identify the most aversive among these sounds. Our findings demonstrate that affective sounds can be reliably delivered through web-based platforms, which help facilitate the development of new auditory paradigms for affective online experiments.

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.

Unconditioned response to an aversive stimulus as predictor of response to conditioned fear and safety: A cross-species study

Safety signals predict the non-occurrence of an aversive event, thereby inhibiting fear responses. Previous research has shown that conditioned safety learning is impaired in patients suffering from post-traumatic stress disorder (PTSD). Using a translational approach, the present study aimed to investigate whether individual responses to an aversive unconditioned stimulus (US) in rats (basic science), non-traumatized (pre-clinical) or traumatized humans (clinical) predicts their response to a conditioned fear or safety stimulus. Using three different archival datasets, the unconditioned response (UCR) to the US during fear or safety conditioning was assessed in rats, non-traumatized humans, and trauma-exposed humans. The response to learned fear (CS+; context) and safety (CS-) was measured by the modulation of the startle response (rats, traumatized humans) or skin conductance response (non-traumatized humans). Our results showed that all groups with low UCR and those with high UCR from the rodent or non-traumatized human samples displayed lower fear response to the CS- than to the CS+ . Traumatized humans with high UCR showed similarly high responses to the CS+ and CS-. While all groups showed a positive association between the UCR and CS+ response, the UCR correlated positively with the CS- response in traumatized humans only. Our findings suggest that an elevated response to aversive stimuli predicts deficits in conditioned safety memory in those at risk for trauma-related disorders and confirms that impaired safety learning could be a valid biomarker for these diseases.

Reactivation during sleep with incomplete reminder cues rather than complete ones stabilizes long-term memory in humans

Reactivation by reminder cues labilizes memories during wakefulness, requiring reconsolidation to persist. In contrast, during sleep, cued reactivation seems to directly stabilize memories. In reconsolidation, incomplete reminders are more effective in reactivating memories than complete reminders by inducing a mismatch, i.e. a discrepancy between expected and actual events. Whether mismatch is likewise detected during sleep is unclear. Here we test whether cued reactivation during sleep is more effective for mismatch-inducing incomplete than complete reminders. We first establish that only incomplete but not complete reminders labilize memories during wakefulness. When complete or incomplete reminders are presented during 40-min sleep, both reminders are equally effective in stabilizing memories. However, when extending the retention interval for another 7 hours (following 40-min sleep), only incomplete but not complete reminders stabilize memories, regardless of the extension containing wakefulness or sleep. We propose that, during sleep, only incomplete reminders initiate long-term memory stabilization via mismatch detection.

Forcato et al. show that incomplete reminder cues rather than complete ones stabilize human memories during sleep. This study suggests that only incomplete reminders initiate long-term memory stabilization via mismatch detection during sleep.

Augmenting exposure therapy with pre-extinction fear memory reactivation and deepened extinction: A randomized controlled trial

BACKGROUND

Pre-extinction fear memory reactivation (PE-FMR) and deepened extinction (DE) enhance long-term extinction of shock-conditioned fear, and may also enhance long-term extinction of naturally acquired fear. Preliminary data suggest that PE-FMR may additionally boost the speed of fear reduction during exposure therapy.

DESIGN

Randomized controlled trial, factorial design.

METHODS

Participants with elevated fears of either spiders or snakes were randomized to (1) exposure therapy alone (n = 41), (2) exposure therapy + PE-FMR (n = 42), (3) exposure therapy + DE (n = 41), or (4) exposure therapy + PE-FMR + DE (n = 42). Participants were assessed at baseline, post-treatment, and one-week follow-up on subjective and behavioral indices of phobia. Because treatment length was tailored to speed of fear reduction, survival analyses were used to examine the speed of fear reduction during treatment.

RESULTS

DE did not improve clinical outcomes at post-treatment or follow-up, whereas PE-FMR produced more rapid fear reduction and was able to achive equivalent outcomes even when the duration of exposure therapy (tailored to speed of fear reduction) was shortened by an average of 21%.

CONCLUSIONS

Data suggest that PE-FMR is a promising strategy for reducing the overall duration of exposure-based therapies.

CLINICAL TRIAL REGISTRATION

(clinicaltrials.gov)NCT02160470.

Investigating the efficacy of the reminder-extinction procedure to disrupt contextual threat memories in humans using immersive Virtual Reality

Upon reactivation, consolidated memories can enter a temporary labile state and require restabilisation, known as reconsolidation. Interventions during this reconsolidation period can disrupt the reactivated memory. However, it is unclear whether different kinds of memory that depend on distinct brain regions all undergo reconsolidation. Evidence for reconsolidation originates from studies assessing amygdala-dependent memories using cue-conditioning paradigms in rodents, which were subsequently replicated in humans. Whilst studies providing evidence for reconsolidation of hippocampus-dependent memories in rodents have predominantly used context conditioning paradigms, studies in humans have used completely different paradigms such as tests for wordlists or stories. Here our objective was to bridge this paradigm gap between rodent and human studies probing reconsolidation of hippocampus-dependent memories. We modified a recently developed immersive Virtual Reality paradigm to test in humans whether contextual threat-conditioned memories can be disrupted by a reminder-extinction procedure that putatively targets reconsolidation. In contrast to our hypothesis, we found comparable recovery of contextual conditioned threat responses, and comparable retention of subjective measures of threat memory, episodic memory and exploration behaviour between the reminder-extinction and standard extinction groups. Our result provide no evidence that a reminder before extinction can prevent the return of context conditioned threat memories in humans.

Boundary conditions of post-retrieval extinction: A direct comparison of low and high partial reinforcement

Research has shown that a single presentation of the conditioned stimulus prior to extinction training can diminish conditioned responses. However, replication has proven difficult and appears to be limited by boundary conditions. Here we tested the boundary condition of memory strength by comparing the effect of reinforcement rate to assess its role in post-retrieval extinction. Eighty university students had undergone a three-day fear conditioning experiment in which two partial reinforcement schedules (40%, 80%) were applied. The findings indicated that both low and high partial reinforcement groups did not demonstrate recovery of conditioned responses after post-retrieval extinction. In contrast, both groups demonstrated significant recovery to standard extinction with significantly greater recovery in the 80% group relative to the 40% group. Additionally, we found that greater physiological arousal during memory retrieval significantly predicted recovery of fear at test phase. We conclude that when compared to a lower partial reinforcement schedule, a higher partial reinforcement resulted in the formation of a stronger memory as indicated by greater physiological arousal during memory reactivation and recovery of conditioned responses after standard extinction, but that it does not function as a boundary condition of post-retrieval extinction. These data are significant because it is the first study to investigate the effect of varying partial reinforcement schedules on fear recovery and add to the body of literature that continue to identify sources of failure in the application of post-retrieval extinction.

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.

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.

Navigating the garden of forking paths for data exclusions in fear conditioning research

In this report, we illustrate the considerable impact of researcher degrees of freedom with respect to exclusion of participants in paradigms with a learning element. We illustrate this empirically through case examples from human fear conditioning research, in which the exclusion of 'non-learners' and 'non-responders' is common - despite a lack of consensus on how to define these groups. We illustrate the substantial heterogeneity in exclusion criteria identified in a systematic literature search and highlight the potential problems and pitfalls of different definitions through case examples based on re-analyses of existing data sets. On the basis of these studies, we propose a consensus on evidence-based rather than idiosyncratic criteria, including clear guidelines on reporting details. Taken together, we illustrate how flexibility in data collection and analysis can be avoided, which will benefit the robustness and replicability of research findings and can be expected to be applicable to other fields of research that involve a learning element.

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.

From Extinction Learning to Anxiety Treatment: Mind the Gap

Laboratory models of extinction learning in animals and humans have the potential to illuminate methods for improving clinical treatment of fear-based clinical disorders. However, such translational research often neglects important differences between threat responses in animals and fear learning in humans, particularly as it relates to the treatment of clinical disorders. Specifically, the conscious experience of fear and anxiety, along with the capacity to deliberately engage top-down cognitive processes to modulate that experience, involves distinct brain circuitry and is measured and manipulated using different methods than typically used in laboratory research. This paper will identify how translational research that investigates methods of enhancing extinction learning can more effectively model such elements of human fear learning, and how doing so will enhance the relevance of this research to the treatment of fear-based psychological disorders.

A review of boundary conditions and variables involved in the prevention of return of fear after post-retrieval extinction

Experimental evidence suggests that the return of fear may be prevented by post-retrieval extinction (PRE), a procedure consisting of extinction training after the presentation of a retrieval cue. However, attempts to replicate these findings have yielded mixed results, with some studies showing diminished fear responses after PRE, whereas others show no effect on the return of fear following this procedure. The discrepancies across studies have been interpreted as evidence that there might be conditions under which PRE is not effective (boundary conditions), but these variables have yet to be fully described. We aimed to provide an overview of PRE in humans. We briefly present the theory and research that originated post-retrieval procedures with a focus on the experimental setup used in human studies. We continue with a compilation of possible experimental boundary conditions along with some questions for future research.

Implicit but not explicit extinction to threat-conditioned stimulus prevents spontaneous recovery of threat-potentiated startle responses in humans

INTRODUCTION

It has long been posited that threat learning operates and forms under an affective and a cognitive learning system that is supported by different brain circuits. A primary drawback in exposure-based therapies is the high rate of relapse that occurs when higher order areas fail to inhibit responses driven by the defensive circuit. It has been shown that implicit exposure of fearful stimuli leads to a long-lasting reduction in avoidance behavior in patients with phobia. Despite the potential benefits of this approach in the treatment of phobias and posttraumatic stress disorder, implicit extinction is still underinvestigated.

METHODS

Two groups of healthy participants were threat conditioned. The following day, extinction training was conducted using a stereoscope. One group of participants was explicitly exposed with the threat-conditioned image, while the other group was implicitly exposed using a continuous flash suppression (CFS) technique. On the third day, we tested the spontaneous recovery of defensive responses using explicit presentations of the images.

RESULTS

On the third day, we found that only the implicit extinction group showed reduced spontaneous recovery of defensive responses to the threat-conditioned stimulus, measured by threat-potentiated startle responses but not by the electrodermal activity.

CONCLUSION

Our results suggest that implicit extinction using CFS might facilitate the modulation of the affective component of fearful memories, attenuating its expression after 24 hr. The limitations of the CFS technique using threatful stimuli urge the development of new strategies to improve implicit presentations and circumvent such limitations. Our study encourages further investigations of implicit extinction as a potential therapeutic target to further advance exposure-based psychotherapies.

Memory boundaries: opening a window inspired by reconsolidation to treat anxiety, trauma-related, and addiction disorders

Pioneering research over the past two decades has shown that memories are far more malleable than we once thought, thereby highlighting the potential for new clinical avenues for treatment of psychopathology. We first briefly review the historical foundation of memory reconsolidation-a concept that refers to hypothetical processes that occur when a memory is retrieved and restored. Then, we provide an overview of the basic research on memory reconsolidation that has been done with humans and other animals, focusing on models of fear, anxiety-related disorders, and addiction, from the perspective that they all involve disorders of memory. This basic research has fuelled early stage developments of novel treatment techniques. More specifically, we consider behavioural interventions inspired by reconsolidation updating, namely retrieval-extinction techniques. We discuss the set of principles that would be needed for memory modifications within a putative reconsolidation time window, and review research that employs reconsolidation-based strategies with clinical populations. We conclude by highlighting current pitfalls and controversies surrounding the use of reconsolidation-based approaches, but end on an optimistic note for clinical research going forward. Despite the challenges, we believe that drawing on ideas from psychological science can help open up treatment innovation.

Avoidance Behavior Prevents Modification of Fear Memory During Reconsolidation

Several studies have revealed that fear recovery is prevented when extinction training is conducted after retrieval of a fear memory. Postretrieval extinction training is related to modification of memory during reconsolidation. Providing new information during reconsolidation can modify the original memory. We propose that avoidance behavior is a relevant factor that prevents subjects from obtaining new safety information during reconsolidation. Postretrieval extinction training without avoidance behavior reduced the fear response to conditioned stimulus and prevented spontaneous recovery in the current study, which corresponded with previous studies. Under the condition of postretrieval extinction training with avoidance behavior, the fear response was not reduced as much as it was in the condition without avoidance. It is possible that avoidance behavior prevents receiving new safety information during postretrieval extinction training.

Positive emotional induction interferes with the reconsolidation of negative autobiographical memories, in women only

After reactivation, a previously consolidated memory can enter into a labile state followed by a re-stabilization process defined as reconsolidation. The aim of this study was to explore whether an existing negative autobiographical memory can be modified by using a non-invasive interference (audiovisual positive preparation) after reactivation and to determine if this effect could be dependent on the reconsolidation process. We found that the presentation of a positive inductor after a negative autobiographical memory reactivation may lead to a change in the emotional information of the original trace and that such effect can be mediated by the reconsolidation process. The modification of the memory has been shown in women only. These results suggest that a positive audiovisual induction may play a potential role in psychotherapeutic techniques for the modification of dysfunctional autobiographical memories.

Exploring the boundaries of post-retrieval extinction in healthy and anxious individuals

Over a dozen studies have examined the efficacy of post-retrieval extinction (PRE) in healthy adults in the fear conditioning laboratory, with a recent meta-analysis reporting an overall small-moderate effect on attenuating the return of fear compared to standard extinction. The current study was designed to extend PRE effects to a mixed sample of healthy and anxious individuals, explore potential moderators, and examine the benefit of PRE for a memory conditioned over multiple days. Healthy (n = 49) and anxious (n = 43) adults received either one day of acquisition followed by PRE, one day of acquisition followed by extinction, or three days of acquisition followed by PRE. Comparing participants who received one day of acquisition followed by PRE or extinction, no significant effect of PRE was observed on differential skin conductance response reinstatement or reactivity to the conditioned stimulus alone. Anxiety symptoms did not moderate outcomes. There was no difference in return of fear for anxious participants who received three days of acquisition followed by PRE versus one day of acquisition followed by PRE. These results further highlight the variability of findings in the PRE literature and need for further examination of individual difference factors that may moderate PRE effects.

Is it possible to modify fear memories in humans with extinction training within a single day?

Extinction procedures have been used widely in the study of fear memories, and different positions have been adopted regarding the efficacy of such procedures and the mechanisms involved. It has been argued that extinction may interfere with the consolidation of the fear memory if the procedure is applied with the appropriate timing after acquisition. However, the opposite position is also held, that is, that the extinction does not achieve an elimination of the fear response. The aim of the present study is to test the short-term effects of immediate extinction in fear reduction when this extinction is preceded by a retrieval trial. For this, a procedure similar to that employed by Schiller et al. (Nature 463(7277): 49-53, 2010) was used, but in a single day and with white noise as an aversive unconditioned stimulus. The results indicate that a CS+ single retrieval trial before the extinction procedure after acquisition was more effective in fear reduction than standard immediate extinction.

Dnmt3a in the dorsal dentate gyrus is a key regulator of fear renewal

Renewal of extinguished fear memory in an altered context is widely believed to be a major limiting issue for exposure therapy in treating various psychiatric diseases. Effective prevention of fear renewal will significantly improve the efficacy of exposure therapy. DNA methyltransferase (DNMTs) mediated epigenetic processes play critical roles in long term memory, but little is known about their functions in fear memory extinction or renewal. Here we investigated whether DNMTs regulate fear renewal after extinction. We found that elevated Dnmt3a level in the dorsal dentate gyrus (dDG) of hippocampus was associated with the absence of fear renewal in an altered context after extinction training. Overexpression and knockdown of Dnmt3a in the dDG regulated the occurrence of fear renewal in a bi-directional manner. In addition, Dnmt3a overexpression was associated with elevated expression of c-Fos in the dDG during extinction training. Furthermore, we found that renewal of remote fear memory can be prevented, and the absence of renewal was concurrent with an elevated Dnmt3a level. Our results indicate that Dnmt3a in the dDG is a key regulator of fear renewal after extinction, and Dnmt3a may play a critical role in controlling fear memory return and thus has therapeutic values.

Multiple memory systems, multiple time points: how science can inform treatment to control the expression of unwanted emotional memories

Memories that have strong emotions associated with them are particularly resilient to forgetting. This is not necessarily problematic, however some aspects of memory can be. In particular, the involuntary expression of those memories, e.g. intrusive memories after trauma, are core to certain psychological disorders. Since the beginning of this century, research using animal models shows that it is possible to change the underlying memory, for example by interfering with its consolidation or reconsolidation. While the idea of targeting maladaptive memories is promising for the treatment of stress and anxiety disorders, a direct application of the procedures used in non-human animals to humans in clinical settings is not straightforward. In translational research, more attention needs to be paid to specifying what aspect of memory (i) can be modified and (ii) should be modified. This requires a clear conceptualization of what aspect of memory is being targeted, and how different memory expressions may map onto clinical symptoms. Furthermore, memory processes are dynamic, so procedural details concerning timing are crucial when implementing a treatment and when assessing its effectiveness. To target emotional memory in its full complexity, including its malleability, science cannot rely on a single method, species or paradigm. Rather, a constructive dialogue is needed between multiple levels of research, all the way 'from mice to mental health'.This article is part of a discussion meeting issue 'Of mice and mental health: facilitating dialogue between basic and clinical neuroscientists'.

Reconsolidation of Traumatic Memories for PTSD: A randomized controlled trial of 74 male veterans

Design: A randomized waitlist-controlled design (n = 74) examined the efficacy of Reconsolidation of Traumatic Memories (RTM) among male veterans with current-month flashbacks and nightmares. Volunteers were randomly assigned to immediate treatment (three 120-minute sessions of RTM), or to a 3-week waiting condition before receiving the RTM treatment. Blinded psychometricians evaluated the symptoms at intake, 2 weeks, and 6 weeks post. Wait-listed participants were re-evaluated and then treated. Sixty-five volunteers completed the treatment. Results: Of those treated, 46 (71%) lost DSM diagnosis for post-traumatic stress disorder (PTSD) by one of the following definitions: 42 persons (65%) were in complete remission (PTSD Symptom Scale Interview (PSS-I) ≤ 20 and DSM criteria not met). Four others (6%) lost the DSM diagnosis or were otherwise sub-clinical by dichotomous criteria (PSS-I < 20 and absence of flashbacks and nightmares) but non-ambiguous on the PTSD Checklist Military Version measures. Within-group RTM effect sizes (Hedges' g) for PSS-I score changes ranged from 1.45 to 2.3. The between-group comparison between the treatment group and the untreated controls was significant (p < .001) with an effect size equivalent to two standard deviations (g = 2.13; 95% CI [1.56, 2.70]). Patient satisfaction with the intervention was high. Conclusions: RTM shows promise as a brief, cost-effective intervention for PTSD characterized primarily by intrusive symptoms. Clinical or methodological significance of this article: The article provides evidence to support a fast (5 hours or fewer) robust intervention for PTSD characterized by intrusive symptoms including current-month flashbacks, nightmares, and accompanied by sympathetic arousal in response to trauma narratives. The intervention is well tolerated and has demonstrated efficacy up to one year.

A comparison of behavioral and pharmacological interventions to attenuate reactivated fear memories

Two experiments using rats in a contextual fear memory preparation compared two approaches to reduce conditioned fear: (1) pharmacological reconsolidation blockade and (2) reactivation-plus-extinction training. In Experiment 1, we explored different combinations of reactivation-plus-extinction parameters to reduce conditioned fear and attenuate reacquisition. In Experiment 2, memory reactivation was followed by extinction training or administration of midazolam (MDZ) (vs. vehicle) to reduce conditioned fear and attenuate spontaneous recovery. We found both treatments to be equally effective in both experiments. This study suggests that parameters leading to memory destabilization during reactivation are critical to observe long-lasting effects of MDZ or reactivation plus extinction.

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.

Targeting memory reconsolidation to prevent the return of fear in patients with fear of flying

BACKGROUND

When a memory is recalled, it may again exist in a labile state and stored information becomes amenable to change, a psychobiological process known as reconsolidation. Exposure therapy for anxiety disorders involves accessing a fear memory and modifying it with less fearful information. A preclinical study reported that providing a reminder of a fear memory 10 min prior to extinction training in humans decreased fear up to 1 year later (Schiller et al., 2010).

METHODS

For this pilot clinical study, we used virtual reality exposure therapy (VRE) for fear of flying (FoF) to determine if using a cue to reactivate the memory of the feared stimulus 10 min prior to exposure sessions leads to fewer anxiety-related behaviors and a more durable response compared to a neutral cue. FoF participants (N = 89) received four sessions of anxiety management training followed by four sessions of VRE. Participants were randomly assigned to receive an FoF cue (reactivation group) or a neutral cue (control group) prior to the VRE sessions. Heart rate (HR) and skin conductance levels (SCLs) were collected during posttreatment and 3-month follow-up assessments as objective markers of fear responding.

RESULTS

Treatment was effective and all clinical measures improved equally between groups at posttreatment with maintained gains through follow-ups. Significant differences were identified with regard to HR and SCL indices.

CONCLUSIONS

These results suggest that memory reactivation prior to exposure therapy did not have an impact on clinical measures but may enhance the effect of exposure therapy at the physiological level.

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.

Preventing the return of fear using reconsolidation updating and methylene blue is differentially dependent on extinction learning

Many factors account for how well individuals extinguish conditioned fears, such as genetic variability, learning capacity and conditions under which extinction training is administered. We predicted that memory-based interventions would be more effective to reduce the reinstatement of fear in subjects genetically predisposed to display more extinction learning. We tested this hypothesis in rats genetically selected for differences in fear extinction using two strategies: (1) attenuation of fear memory using post-retrieval extinction training, and (2) pharmacological enhancement of the extinction memory after extinction training by low-dose USP methylene blue (MB). Subjects selectively bred for divergent extinction phenotypes were fear conditioned to a tone stimulus and administered either standard extinction training or retrieval + extinction. Following extinction, subjects received injections of saline or MB. Both reconsolidation updating and MB administration showed beneficial effects in preventing fear reinstatement, but differed in the groups they targeted. Reconsolidation updating showed an overall effect in reducing fear reinstatement, whereas pharmacological memory enhancement using MB was an effective strategy, but only for individuals who were responsive to extinction.

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.

The computational nature of memory modification

Retrieving a memory can modify its influence on subsequent behavior. We develop a computational theory of memory modification, according to which modification of a memory trace occurs through classical associative learning, but which memory trace is eligible for modification depends on a structure learning mechanism that discovers the units of association by segmenting the stream of experience into statistically distinct clusters (latent causes). New memories are formed when the structure learning mechanism infers that a new latent cause underlies current sensory observations. By the same token, old memories are modified when old and new sensory observations are inferred to have been generated by the same latent cause. We derive this framework from probabilistic principles, and present a computational implementation. Simulations demonstrate that our model can reproduce the major experimental findings from studies of memory modification in the Pavlovian conditioning literature.

DOI:http://dx.doi.org/10.7554/eLife.23763.001

Our memories contain our expectations about the world that we can retrieve to make predictions about the future. For example, most people would expect a chocolate bar to taste good, because they have previously learned to associate chocolate with pleasure. When a surprising event occurs, such as tasting an unpalatable chocolate bar, the brain therefore faces a dilemma. Should it update the existing memory and overwrite the association between chocolate and pleasure? Or should it create an additional memory? In the latter case, the brain would form a new association between chocolate and displeasure that competes with, but does not overwrite, the original one between chocolate and pleasure.

Previous studies have shown that surprising events tend to create new memories unless the existing memory is briefly reactivated before the surprising event occurs. In other words, retrieving old memories makes them more malleable. Gershman et al. have now developed a computational model for how the brain decides whether to update an old memory or create a new one. The idea at the heart of the model is that the brain will attempt to infer what caused the surprising event. The reason the chocolate bar tastes unpalatable, for example, might be because it was old and had spoiled. Every time the brain infers a new possible cause for a surprising event, it will create an additional memory to store this new set of expectations. In the future we will know that spoiled chocolate bars taste bad.

However, if the brain cannot infer a new cause for the surprising event – because, for example, there appears to be nothing unusual about the unpalatable chocolate bar – it will instead opt to update the existing memory. The next time we buy a chocolate bar, we will have slightly lower expectations about how good it will taste. The dilemma of whether to update an existing memory or create a new one thus boils down to the question: is the surprising event the consequence of a new cause or an old one? This theory implies that retrieving a memory nudges the brain to infer that its associated cause is once again active and, since this is an old cause, it means that the memory will be eligible for updating.

Many experiments have been performed on the topic of modifying memories, but this is the first computational model that offers a unifying explanation for the results. The next step is to work out how to apply the model, which is phrased in abstract terms, to networks of neurons that are more biologically realistic.

DOI:http://dx.doi.org/10.7554/eLife.23763.002

Use of a Brief Fear Memory Reactivation Procedure for Enhancing Exposure Therapy

We tested postretrieval extinction as an augmentation strategy for enhancing in vivo exposure therapy for naturally acquired pathological fear. Participants displaying marked phobic responding to either spiders or snakes were randomized to receive a standard one session in vivo exposure therapy protocol under one of two conditions. The experimental group (RFM-EXP) completed a 10-s fear reactivation procedure 30 min prior to initiating exposure therapy. Controls (EXP-RFM) completed the reactivation procedure after completing exposure therapy. Expected and peak fear during confrontation with live spiders or snakes were collected at pretreatment, posttreatment, and 1-month follow-up. RFM-EXP participants displayed significantly lower phobic responding at the 1-month follow-up relative to EXP-RFM controls. Unexpectedly, RFM-EXP participants showed more rapid fear attenuation during exposure relative to controls. Results provide preliminary support for further investigation of this exposure augmentation strategy across a wider range of anxiety-related disorders.

Effect of a Brief Memory Updating Intervention on Smoking Behavior: A Randomized Clinical Trial

IMPORTANCE

Recent research on addiction-related memory processes suggests that protracted extinction training following brief cue-elicited memory retrieval (ie, retrieval-extinction [R-E] training) can attenuate/eradicate the ability of cues to elicit learned behaviors. One study reported that cue-elicited craving among detoxified heroin addicts was substantially attenuated following R-E training and through 6-month follow-up.

OBJECTIVE

To build on these impressive findings by examining whether R-E training could attenuate smoking-related craving and behavior.

DESIGN, SETTING, AND PARTICIPANTS

This prospective, mixed-design, human laboratory randomized clinical trial took place between December 2013 and September 2015. Participants were recruited in Charleston, South Carolina. Study sessions took place at the Medical University of South Carolina. The participants were 168 screened volunteer smokers, of whom 88 were randomized; 72 of these 88 participants (81.8%) attended all the follow-up sessions through 1 month. The primary eligibility criteria were current nicotine dependence (DSM criteria), smoking 10 or more cigarettes per day, and a willingness to attempt smoking cessation.

INTERVENTIONS

Participants were randomly assigned to receive either smoking-related memory retrieval followed by extinction training (the R-E group) or nonsmoking-related retrieval followed by extinction training (the NR-E group).

MAIN OUTCOMES AND MEASURES

Primary outcomes were cue-elicited craving and physiological responding to familiar and novel cues in the R-E group vs the NR-E group over a 1-month follow-up period. Secondary outcomes were smoking-related behaviors.

RESULTS

A total of 44 participants were randomly assigned to the R-E group (mean age, 48.3 years; 72.7% male); a total of 44 participants were randomly assigned to the NR-E group, with 43 attending at least 1 training session (mean age, 46.7 years; 55.8% male). The mean craving response to both familiar and novel smoking cues was significantly lower for participants in the R-E group than for participants in the NR-E group at 1-month follow-up (for both cue types: t1225 = 2.1, P = .04, d = 0.44, and Δ = 0.47 [95% CI, 0.04-0.90]). The mean numbers of cigarettes smoked per day at 2 weeks and 1-month were significantly lower for the R-E group than for the NR-E group (treatment main effect: F1,68 = 5.4, P = .02, d = 0.50, and Δ = 2.4 [95% CI, 0.4-4.5]). Significant differences in physiological responses, urine cotinine level, number of days abstinent, lapse, and relapse were not observed between groups (all between P = .06 and .75).

CONCLUSIONS AND RELEVANCE

Retrieval-extinction training substantially attenuated craving to both familiar and novel smoking cues and reduced the number of cigarettes smoked per day by participants 1 month after treatment relative to extinction training alone. Between-group differences were not observed for physiological responses, cotinine level, number of days abstinent, relapse, or lapse. In summary, R-E training is a brief behavioral treatment that targets smoking-related memories and has the potential to enhance relapse prevention.

TRIAL REGISTRATION

clinicaltrials.gov Identifier: NCT02154685.

Extinction during reconsolidation eliminates recovery of fear conditioned to fear-irrelevant and fear-relevant stimuli

Extant literature suggests that extinction training delivered during the memory reconsolidation period is superior to traditional extinction training in the reduction of fear recovery, as it targets the original fear memory trace. At present it is debated whether different types of fear memories are differentially sensitive to behavioral manipulations of reconsolidation. Here, we examined post-reconsolidation recovery of fear as a function of conditioned stimulus (CS) fear-relevance, using the unconditioned stimulus (US) to reactivate and destabilize conditioned fear memories. Participants (N = 56; 25 male; M = 24.39 years, SD = 7.71) in the US-reactivation and control group underwent differential fear conditioning to fear-relevant (spiders/snakes) and fear-irrelevant (geometric shapes) CSs on Day 1. On Day 2, participants received either reminded (US-reactivation) or non-reminded extinction training. Tests of fear recovery, conducted 24 h later, revealed recovery of differential electrodermal responding to both classes of CSs in the control group, but not in the US-reactivation group. These findings indicate that the US reactivation-extinction procedure eliminated recovery of extinguished responding not only to fear-irrelevant, but also to fear-relevant CSs. Contrasting previous reports, our findings show that post-reconsolidation recovery of conditioned responding is not a function of CS fear-relevance and that persistent reduction of fear, conditioned to fear-relevant CSs, can be achieved through behavioral manipulations of reconsolidation.

Recent developments in the behavioural and pharmacological enhancement of extinction of drug seeking

One of the principal barriers to overcoming addiction is the propensity to relapse, even after months or years of abstinence. Relapse can be precipitated by cues and contexts associated with drug use; thus, decreasing the conditioned properties of these cues and contexts may assist in preventing relapse. The predictive power of drug cues and contexts can be reduced by repeatedly presenting them in the absence of the drug reinforcer, a process known as extinction. The potential of extinction to limit relapse has generated considerable interest and research over the past few decades. While pre-clinical animal models suggest extinction learning assists relapse prevention, treatment efficacy is often lacking when extinction learning principles are translated into clinical trials. Conklin and Tiffany (Addiction, 2002) suggest the lack of efficacy in clinical practice may be due to limited translation of procedures demonstrated through animal research and propose several methodological improvements to enhance extinction learning for drug addiction. This review will examine recent advances in the behavioural and pharmacological manipulation of extinction learning, based on research from pre-clinical models. In addition, the translation of pre-clinical findings-both those suggested by Conklin and Tiffany () and novel demonstrations from the past 13 years-into clinical trials and the efficacy of these methods in reducing craving and relapse, where available, will be discussed. Finally, we highlight areas where promising pre-clinical models have not yet been integrated into current clinical practice but, if applied, could improve upon existing behavioural and pharmacological methods.

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.