Can Transcranial Direct Current Stimulation Augment Extinction of Conditioned Fear?

The effect of transcranial direct current and magnetic stimulation on fear extinction and return of fear: A meta-analysis and systematic review

BACKGROUND

We conducted a meta-analysis and qualitative review on the randomized controlled trials investigating the effects of transcranial direct current stimulation and transcranial magnetic stimulation on fear extinction and the return of fear in non-primate animals and humans.

METHODS

The meta-analysis was conducted by searching PubMed, Web of science, PsycINFO, and Cochrane Library and extracting fear response in the active and sham groups in the randomized controlled trials. The pooled effect size was quantified by Hedges' g using a three-level meta-analytic model in R.

RESULTS

We identified 18 articles on the tDCS effect and 5 articles on the TMS effect, with 466 animal subjects and 621 human subjects. Our findings show that tDCS of the prefrontal cortex significantly inhibit fear retrieval in animal models (Hedges' g = -0.50). In human studies, TMS targeting the dorsolateral/ventromedial prefrontal cortex has an inhibiting effect on the return of fear (Hedges' g = -0.24).

LIMITATIONS

The limited number of studies and the heterogeneous designs of the selected studies made cross-study and cross-species comparison difficult.

CONCLUSIONS

Our findings shed light on the optimal non-invasive brain stimulation protocols for targeting the neural circuitry of threat extinction in humans.

'Nip it in the bud': Low-frequency rTMS of the prefrontal cortex disrupts threat memory consolidation in humans

It is still unclear how the human brain consolidates aversive (e.g., traumatic) memories and whether this process can be disrupted. We hypothesized that the dorsolateral prefrontal cortex (dlPFC) is crucially involved in threat memory consolidation. To test this, we used low-frequency repetitive transcranial magnetic stimulation (LF-rTMS) within the memory stabilization time window to disrupt the expression of threat memory. We combined a differential threat-conditioning paradigm with LF-rTMS targeting the dlPFC in the critical condition, and occipital cortex stimulation, delayed dlPFC stimulation, and sham stimulation as control conditions. In the critical condition, defensive reactions to threat were reduced immediately after brain stimulation, and 1 h and 24 h later. In stark contrast, no decrease was observed in the control conditions, thus showing both the anatomical and temporal specificity of our intervention. We provide causal evidence that selectively targeting the dlPFC within the early consolidation period prevents the persistence and return of conditioned responses. Furthermore, memory disruption lasted longer than the inhibitory window created by our TMS protocol, which suggests that we influenced dlPFC neural activity and hampered the underlying, time-dependent consolidation process. These results provide important insights for future clinical applications aimed at interfering with the consolidation of aversive, threat-related memories.

Medial anterior prefrontal cortex stimulation downregulates implicit reactions to threats and prevents the return of fear

Downregulating emotional overreactions toward threats is fundamental for developing treatments for anxiety and post-traumatic disorders. The prefrontal cortex (PFC) is critical for top-down modulatory processes, and despite previous studies adopting repetitive transcranial magnetic stimulation (rTMS) over this region provided encouraging results in enhancing extinction, no studies have hitherto explored the effects of stimulating the medial anterior PFC (aPFC, encompassing the Brodmann area 10) on threat memory and generalization. Here we showed that rTMS over the aPFC applied before threat memory retrieval immediately decreases implicit reactions to learned and novel stimuli in humans. These effects enduringly persisted 1 week later in the absence of rTMS. No effects were detected on explicit recognition. Critically, rTMS over the aPFC resulted in a more pronounced reduction of defensive responses compared to rTMS targeting the dorsolateral PFC. These findings reveal a previously unexplored prefrontal region, the modulation of which can efficiently and durably inhibit implicit reactions to learned threats. This represents a significant advancement toward the long-term deactivation of exaggerated responses to threats.

Combined EEG-tDCS approach in resting state to reduce comorbid anxiety and depressive symptoms in affective disorders: A sham-controlled pilot study

Continuous challenges have been imposed on mental health science by Anxiety and Depression disorders as the most prevalent and debilitating psychiatric conditions worldwide. Pharmacologic and cognitive behavioral therapies, either alone or in combination, have been considered as the first-line therapies, however, resistant symptomatology is prevalent in comorbid conditions with symptoms remaining after interventions. The demand for new therapeutic solutions has given space to the development of non-invasive brain stimulation techniques (NIBS), and the transmagnetic direct current stimulation (tDCS) has been reported as a safe and well-tolerated technique for the treatment of several mental health conditions, including Anxiety and Depression disorders. Relying on quantitative electroencephalography(qEEG)- tDCS approach, the current study aims to inspect the effect of tDCS intervention on patients who suffer from anxiety-depression comorbidity, in particular, the impact of tDCS intervention on qEEG spectral power activity and resting-state connectivity organization during eyes closed and eyes open protocols. QEEG data were acquired from eight patients suffering from moderate to severe anxiety-depression comorbid symptoms along with poor coping skills to manage stress and negative affect. Twelve control subjects allocated in the control group exhibiting low to moderate symptoms in both anxiety and depression conditions went also through the qEEG data acquisition. In addition, a sham-controlled study was conducted, and the patient group went through resting-state qEEG-tDCS neuromodulation once a week for ten weeks. Various-stage qEEG recordings were performed to inspect the efficacy of tDCS treatment during the modulation of brain regions involved in the regulation of affective responses. Our results demonstrated that after tDCS neuromodulation, the patients' groups exhibited decreased absolute power abnormalities over the left anterior cingulate cortex and reduced abnormal activity in the alpha band over posterior regions; improved functional connectivity indexes; decreased anxiety and depressive scores while positive affect score was improved. Besides the promising improvements, our study did not find a significant tDCS effect on perceived stress and negative affect scores. Consistently, significant differences in absolute spectral power over the left anterior cingulate cortex were detected among the patient group, as compared to the controls, as expected. Therefore, our study offers preliminary data to understand the neuroplasticity changes that potentially result from the manipulation of cortical excitability during affective regulation protocols followed by the consequent decrease of comorbid anxiety and depressive symptomatology. The pilot study was followed by prospective registration with ChiCTR2200062142.

Virtual Reality and Transcranial Direct Current Stimulation for Posttraumatic Stress Disorder: A Randomized Clinical Trial

Importance

Posttraumatic stress disorder (PTSD) is a common psychiatric disorder that is particularly difficult to treat in military veterans. Noninvasive brain stimulation has significant potential as a novel treatment to reduce PTSD symptoms.

Objective

To test whether active transcranial direct current stimulation (tDCS) plus virtual reality (VR) is superior to sham tDCS plus VR for warzone-related PTSD.

Design, Setting, and Participants

This double-blind randomized clinical trial was conducted among US military veterans enrolled from April 2018 to May 2023 at a secondary care Department of Veterans Affairs hospital and included 1- and 3-month follow-up visits. Participants included US military veterans with chronic PTSD and warzone-related exposure, recruited via referral and advertisement. Patients in psychiatric treatment had to be on a stable regimen for at least 6 weeks to be eligible for enrollment. Data were analyzed from May to September 2023.

Intervention

Participants were randomly assigned to receive 2-mA anodal tDCS or sham tDCS targeted to the ventromedial prefrontal cortex, during six 25-minute sessions of standardized warzone VR exposure, delivered over 2 to 3 weeks.

Main Outcomes and Measures

The co-primary outcomes were self-reported PTSD symptoms, measured via the PTSD checklist for DSM-5 (PCL-5), alongside quality of life. Other outcomes included psychophysiological arousal, clinician-assessed PTSD, depression, and social/occupational function.

Results

A total of 54 participants (mean [SD] age, 45.7 [10.5] years; 51 [94%] males) were assessed, including 26 in the active tDCS group and 28 in the sham tDCS group. Participants in the active tDCS group reported a superior reduction in self-reported PTSD symptom severity at 1 month (t = -2.27, P = .02; Cohen d = -0.82). There were no significant differences in quality of life between active and sham tDCS groups. Active tDCS significantly accelerated psychophysiological habituation to VR events between sessions compared with sham tDCS (F5,7689.8 = 4.65; P < .001). Adverse effects were consistent with the known safety profile of the corresponding interventions.

Conclusions and Relevance

These findings suggest that combined tDCS plus VR may be a promising strategy for PTSD reduction and underscore the innovative potential of these combined technologies.

Trial Registration

ClinicalTrials.gov Identifier: NCT03372460.

Good moments to stimulate the brain - A randomized controlled double-blinded study on anodal transcranial direct current stimulation of the ventromedial prefrontal cortex on two different time points in a two-day fear conditioning paradigm

It is assumed that extinction learning is a suitable model for understanding the mechanisms underlying exposure therapy. Furthermore, there is evidence that non-invasive brain stimulation (NIBS) can elevate extinction learning by enhancing frontal brain activity and therefore NIBS can augment symptom reduction during exposure therapy in phobias. But, the underlying processes are still not well established. Open questions arise from NIBS time points and electrode placement, among others. Therefore, we investigated in a 2-day fear conditioning experiment, whether anodal transcranial direct current stimulation (tDCS) of the ventromedial prefrontal cortex (vmPFC) modulates either fear memory consolidation or dampened fear reaction during fear extinction. Sixty-six healthy participants were randomly assigned either to a group that received tDCS after fear acquisition (and before fear memory consolidation), to a group that received tDCS directly before fear extinction, or to a control group that never received active stimulation (sham). Differential skin conductance response (SCR) to CS+ vs. CS- was significantly decreased in both tDCS-groups compared to sham group. Our region of interest, the vmPFC, was stimulated best focally with a lateral anode position and a cathode on the contralateral side. But this comes along with a slightly lateral stimulation of vmPFC depending on whether anode is placed left or right. To avoid unintended effects of stimulated sides the two electrode montages (anode left or right) were mirror-inverted which led to differential effects in SCR and electrocortical (mainly late positive potential [LPP]) data in our exploratory analyses. Results indicated that tDCS-timing is relevant for fear reactions via disturbed fear memory consolidation as well as fear expression, and this depends on whether vmPFC is stimulated with either left- or right-sided anode electrode montage. Electrocortical data can shed more light on the underlying neural correlates and exaggerated LPP seems to be associated with disturbed fear memory consolidation and dampened SCR to CS+ vs. CS-, but solely in the right anode electrode montage. Further open questions addressing where and when to stimulate the prefrontal brain in the course of augmenting fear extinction are raised.

Efficacy of tDCS to enhance virtual reality exposure therapy response in acrophobia: A randomized controlled trial

[BACKGROUND]: Virtual reality exposure therapy (VRET) has been recognized as an effective treatment for specific phobias and has the potential to overcome the limitations of traditional exposure therapy. The pursuit of non-invasive brain stimulation provides a practical means of augmenting VRET. Transcranial direct current stimulation (tDCS), a non-invasive neuromodulation technique, stimulates the medial prefrontal cortex (mPFC), with the potential to enhance the effects of exposure therapy. Therefore, we conducted a randomized controlled trial to examine whether tDCS enhanced the effects of VRET in acrophobia. [METHOD]: This study recruited 64 college students with significant fear of height (based on the Acrophobia Questionnaire, AQ). Finally, 61 participants were randomly allocated to the tDCS active-stimulated group (n = 30) or the sham-stimulated group (n = 31). After stimulation, VRET was conducted, and clinical indices were recorded. The AQ was used as the first primary outcome, and Subjective Units of Distress (SUDS) and the Heights Interpretation Questionnaire (HIQ) were used as secondary outcomes. [RESULT]: There was a significant reduction in psychometric and behavioral anxiety measurements from pre to post treatment as indicated by main effects for the factor time (AQ-Anxiety: F (2.60) = 139.55, p < 0.001, η2 = 0.83; AQ-Avoidance: F (2.60) = 53.73, p < 0.001, η2 = 0.69; HIQ: F (2.60) = 128.12, p < 0.001, η2 = 0.81; STAI-Y-S: F (2.60) = 15.44, p < 0.001, η2 = 0.34; BAI: F (2.60) = 73.81, p < 0.001, η2 = 0.71). Compared with the sham-stimulated group, the reduction of AQ-Anxiety and SUDS in the first exposure trial (F (2,60) = 8.56, p = 0.001, η2 = 0.23; t = 2.34, p = 0.024, d = 0.61) was significantly faster in the active group. At follow-up, there was also a further reduction in AQ anxiety and avoidance (Anxiety: M = 56.51 ± 27.19; main effect time F (1,60) = 25.16, p < 0.001, η2 = 0.35; Avoidance: M = 12.57 ± 7.97; main effect time F (1,60) = 31.40, p < 0.001, η2 = 0.45) without interaction time*group (Anxiety: F (1.60) = 0.12, p = 0.740, η2 = 0.00; Avoidance: F (1.60) = 0.64, p = 0.430, η2 = 0.02). [CONCLUSION]: Results could be explained tDCS could accelerate the effects of VRET on acrophobia by stimulating mPFC, indicating that tDCS may be used as an enhancement technique for exposure therapy for specific phobias.

Transcranial focused ultrasound of the amygdala modulates fear network activation and connectivity

BACKGROUND

Current noninvasive brain stimulation methods are incapable of directly modulating subcortical brain regions critically involved in psychiatric disorders. Transcranial Focused Ultrasound (tFUS) is a newer form of noninvasive stimulation that could modulate the amygdala, a subcortical region implicated in fear.

OBJECTIVE

We investigated the effects of active and sham tFUS of the amygdala on fear circuit activation, skin conductance responses (SCR), and self-reported anxiety during a fear-inducing task. We also investigated amygdala tFUS' effects on amygdala-fear circuit resting-state functional connectivity.

METHODS

Thirty healthy individuals were randomized in this double-blinded study to active or sham tFUS of the left amygdala. We collected fMRI scans, SCR, and self-reported anxiety during a fear-inducing task (participants viewed red or green circles which indicated the risk of receiving an aversive stimulus), as well as resting-state scans, before and after tFUS.

RESULTS

Compared to sham tFUS, active tFUS was associated with decreased (pre to post tFUS) blood-oxygen-level-dependent fMRI activation in the amygdala (F(1,25) = 4.86, p = 0.04, η2 = 0.16) during the fear task, and lower hippocampal (F(1,27) = 4.41, p = 0.05, η2 = 0.14), and dorsal anterior cingulate cortex (F(1,27) = 6.26, p = 0.02; η2 = 0.19) activation during the post tFUS fear task. The decrease in amygdala activation was correlated with decreased subjective anxiety (r = 0.62, p = 0.03). There was no group effect in SCR changes from pre to post tFUS (F(1,23) = 0.85, p = 0.37). The active tFUS group also showed decreased amygdala-insula (F(1,28) = 4.98, p = 0.03) and amygdala-hippocampal (F(1,28) = 7.14, p = 0.01) rsFC, and increased amygdala-ventromedial prefrontal cortex (F(1,28) = 3.52, p = 0.05) resting-state functional connectivity.

CONCLUSIONS

tFUS can change functional connectivity and brain region activation associated with decreased anxiety. Future studies should investigate tFUS' therapeutic potential for individuals with clinical levels of anxiety.

Frontopolar multifocal transcranial direct current stimulation reduces conditioned fear reactivity during extinction training: A pilot randomized controlled trial

Exposure-based therapies for anxiety and related disorders are believed to depend on fear extinction learning and corresponding changes in extinction circuitry. Frontopolar multifocal transcranial direct current stimulation (tDCS) has been shown to improve therapeutic safety learning during in vivo exposure and may modulate functional connectivity of networks implicated in fear processing and inhibition. A pilot randomized controlled trial was completed to determine the effects of frontopolar tDCS on extinction learning and memory. Community volunteers (n = 35) completed a 3-day fear extinction paradigm with measurement of electrodermal activity. Participants were randomized (single-blind) to 20-min of sham (n = 17, 30 s. ramp in/out) or active (n = 18) frontopolar (anode over Fpz, 10-10 EEG) multifocal tDCS (20-min, 1.5 mA) prior to extinction training. Mixed ANOVAs revealed a significant group*trial effect on skin conductance response (SCR) to the conditioned stimulus (CS + ) during extinction training (p = 0.007, Cohen's d = 0.55). The effects of frontopolar tDCS were greatest during the first two extinction trials, suggesting that tDCS may have promoted fear inhibition prior to safety learning. Return of fear to the CS + during tests were comparable across conditions (ps > 0.50). These findings suggest that frontopolar tDCS may modulate the processing of threat cues and associated circuitry or promote the inhibition of fear. This has clear implications for the treatment of anxiety and related disorders with therapeutic exposure.

Transcranial magnetic stimulation and transcranial direct current stimulation affect explicit but not implicit emotion regulation: a meta-analysis

Emotion regulation (ER) refers to the process through which people influence the occurrence, experience, and expression of emotions. It can be established in an explicit (voluntary) or implicit (automatic) way, both of which are essential for mental and physical well-being. Recent evidence has highlighted the potential of transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS) targeting the prefrontal cortex (PFC) to modulate ER. However, previous studies have only evaluated the effects of TMS and tDCS on explicit ER, leaving implicit ER relatively unexplored. In this review and meta-analysis, we systematically evaluated the effects of TMS and tDCS over the PFC on the two forms of ER, using both subjective and physiological response as outcome indicators. Twenty-seven studies were included in our study. Both subjective (Hedges' g = - 0.20) and physiological (Hedges' g = - 0.65) results indicated a significant effect of TMS and tDCS targeting PFC on down-regulation of explicit ER, but not implicit ER (Hedges' g = - 0.04). Moreover, moderation analysis indicated that the effect of TMS and tDCS on the down-regulating of subjective experience was moderated by several factors, including stimulation method, target area, target hemisphere, and stimulation timing. Specifically, our results showed that applying TMS or targeting the right PFC, particularly the right ventrolateral prefrontal cortex, or using offline TMS and tDCS produced a larger stimulation effect on ER. In summary, these findings suggest that TMS and tDCS has a positive effect on explicit, but not implicit ER. The distinct TMS and tDCS effect on the two forms of ER help deepen our understanding of TMS and tDCS use and provide valuable insights for the development of tailored TMS and tDCS protocols for explicit and implicit regulation.

The Role of Serotonin in Fear Learning and Memory: A Systematic Review of Human Studies

Fear is characterized by distinct behavioral and physiological responses that are essential for the survival of the human species. Fear conditioning (FC) serves as a valuable model for studying the acquisition, extinction, and expression of fear. The serotonin (5-hydroxytryptamine, 5-HT) system is known to play a significant role in emotional and motivational aspects of human behavior, including fear learning and expression. Accumulating evidence from both animal and human studies suggests that brain regions involved in FC, such as the amygdala, hippocampus, and prefrontal cortex, possess a high density of 5-HT receptors, implicating the crucial involvement of serotonin in aversive learning. Additionally, studies exploring serotonin gene polymorphisms have indicated their potential influence on FC. Therefore, the objective of this work was to review the existing evidence linking 5-HT with fear learning and memory in humans. Through a comprehensive screening of the PubMed and Web of Science databases, 29 relevant studies were included in the final review. These studies investigated the relationship between serotonin and fear learning using drug manipulations or by studying 5-HT-related gene polymorphisms. The results suggest that elevated levels of 5-HT enhance aversive learning, indicating that the modulation of serotonin 5-HT2A receptors regulates the expression of fear responses in humans. Understanding the role of this neurochemical messenger in associative aversive learning can provide insights into psychiatric disorders such as anxiety and post-traumatic stress disorder (PTSD), among others.

Facilitated extinction but impaired extinction recall by eye movement manipulation in humans - Indications for action mechanisms and the applicability of eye movement desensitization

Eye movement desensitization and reprocessing (EMDR) therapy utilizes the manipulation of eye movements to reduce affective distress during fear-exposure. Animal research recently suggested a potential neural mechanism underlying these effects, by which increased activity of the superior colliculus (SC), mediating visual attention, increases the inhibition of the basolateral amygdala (BLA), mediating defensive plasticity. We tested such mechanism in forty healthy humans using a multiple-day single-cue fear conditioning and extinction paradigm. The activity of the SC during extinction was experimentally manipulated by eye movements, as half of the participants executed saccadic eye movements (n = 20; major SC involvement), while the other half executed smooth eye pursuits (n = 20; minor SC involvement). Amygdala-mediated fear-potentiated startle responses and fear bradycardia, as well as threat expectancy was analyzed. Saccadic eye movements facilitated the extinction of fear bradycardia and fear-potentiated startle responses. Higher saccadic accuracy and range correlated with reduced fear-potentiated startle. However, during extinction recall, fear-potentiated startle and fear bradycardia resurged and partly reached levels obtained after fear acquisition. Threat expectancy was not affected by different eye movements and was not elevated during extinction recall. Within limitations, results support an inhibitory SC-BLA pathway in humans by which eye movements may reduce low-level defensive responding, but not threat expectancy. Yet, manipulating eye movements during extinction learning seems to impair extinction recall for behavioral and physiological defensive response indices. Thus, increasing SC activity might enhance initial efficacy of exposure treatment, but additional strategies seem necessary for sustained fear attenuation.

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.

Stimulation of the ventromedial prefrontal cortex blocks the return of subcortically mediated fear responses

The ventromedial prefrontal cortex (vmPFC) mediates the inhibition of defensive responses upon encounters of cues, that had lost their attribute as a threat signal via previous extinction learning. Here, we investigated whether such fear extinction recall can be facilitated by anodal transcranial direct current stimulation (tDCS). Extinction recall was tested twenty-four hours after previously acquired fear was extinguished. Either anodal tDCS or sham stimulation targeting the vmPFC was applied during this test. After stimulation ceased, we examined return of fear after subjects had been re-exposed to aversive events. Fear was assessed by reports of threat expectancy and modulations of autonomic (skin conductance, heart rate) and protective reflex (startle potentiation) measures, the latter of which are mediated by subcortical defense circuits. While tDCS did not affect initial extinction recall, it abolished the return of startle potentiation and autonomic components of the fear response. Results suggest hierarchical multi-level vmPFC functions in human fear inhibition and indicate, that its stimulation might immunize against relapses into pathological subcortically mediated defensive activation.

Enhanced fear acquisition in individuals with evening chronotype. A virtual reality fear conditioning/extinction study

Circadian rhythms have received increasing attention within the context of mental disorders. Evening chronotype has been associated with enhanced risk to develop anxiety and post-traumatic stress disorder (PTSD). The classical fear conditioning paradigm is a powerful tool to reveal key mechanisms of anxiety and PTSD. We used this paradigm to study the neurocognitive basis of the association between chronotype and fear responses in healthy humans. 20 participants with evening chronotype and 20 controls (i.e., intermediate chronotype) completed a 2-day Pavlovian fear learning and extinction virtual reality task. Participants received fear conditioning, and extinction learning on day 1. Extinction memory recall was tested on day 2. To address interactions between chronotype and time of day of the fear conditioning, and extinction performance, half of the participants were tested in the morning, and the other half in the evening. Skin conductance response (SCR) and subjective fear ratings were measured as primary outcomes. Chronotype was established via the morningness-eveningness questionnaire (MEQ). We found an overall higher SCR for fear acquisition in participants with the evening chronotype profile, compared to controls. Moreover, the higher the MEQ scores -indicative of less eveningness - the lower the SCR was. No effects of chronotype were found for extinction and extinction recall. The higher vulnerability of the evening chronotype for anxiety and related disorders may thus be explained by enhanced fear acquisition of this group.

Prefrontal cortex, amygdala, and threat processing: implications for PTSD

Posttraumatic stress disorder can be viewed as a disorder of fear dysregulation. An abundance of research suggests that the prefrontal cortex is central to fear processing-that is, how fears are acquired and strategies to regulate or diminish fear responses. The current review covers foundational research on threat or fear acquisition and extinction in nonhuman animals, healthy humans, and patients with posttraumatic stress disorder, through the lens of the involvement of the prefrontal cortex in these processes. Research harnessing advances in technology to further probe the role of the prefrontal cortex in these processes, such as the use of optogenetics in rodents and brain stimulation in humans, will be highlighted, as well other fear regulation approaches that are relevant to the treatment of posttraumatic stress disorder and involve the prefrontal cortex, namely cognitive regulation and avoidance/active coping. Despite the large body of translational research, many questions remain unanswered and posttraumatic stress disorder remains difficult to treat. We conclude by outlining future research directions related to the role of the prefrontal cortex in fear processing and implications for the treatment of posttraumatic stress disorder.

Transcranial direct current stimulation targeting the medial prefrontal cortex modulates functional connectivity and enhances safety learning in obsessive-compulsive disorder: Results from two pilot studies

BACKGROUND

Exposed-based psychotherapy is a mainstay of treatment for obsessive-compulsive disorder (OCD) and anxious psychopathology. The medial prefrontal cortex (mPFC) and the default mode network (DMN), which is anchored by the mPFC, promote safety learning. Neuromodulation targeting the mPFC might augment therapeutic safety learning and enhance response to exposure-based therapies.

METHODS

To characterize the effects of mPFC neuromodulation on functional connectivity, 17 community volunteers completed resting-state functional magnetic resonance imaging scans before and after 20 min of frontopolar anodal multifocal transcranial direct current stimulation (tDCS). To examine the effects of tDCS on therapeutic safety learning, 24 patients with OCD completed a pilot randomized clinical trial; they were randomly assigned (double-blind, 50:50) to receive active or sham frontopolar tDCS before completing an in vivo exposure and response prevention (ERP) challenge. Changes in subjective emotional distress during the ERP challenge were used to index therapeutic safety learning.

RESULTS

In community volunteers, frontal pole functional connectivity with the middle and superior frontal gyri increased, while connectivity with the anterior insula and basal ganglia decreased (ps < .001, corrected) after tDCS; functional connectivity between DMN and salience network also decreased after tDCS (ps < .001, corrected). OCD patients who received active tDCS exhibited more rapid therapeutic safety learning (ps < .05) during the ERP challenge than patients who received sham tDCS.

CONCLUSIONS

Frontopolar tDCS may modulate mPFC and DMN functional connectivity and can accelerate therapeutic safety learning. Though limited by small samples, these findings motivate further exploration of the effects of frontopolar tDCS on neural and behavioral targets associated with exposure-based psychotherapies.

Is there a neuroscience-based, mechanistic rationale for transcranial direct current stimulation as an adjunct treatment for posttraumatic stress disorder?

It is well-known that there is considerable variation in the effectiveness of evidence-based treatments for psychiatric disorders, and a continued need to improve the real-world effectiveness of these treatments. In the last 20+ years the examination of noninvasive brain stimulation techniques for psychiatric treatment has increased dramatically. However, in order to test these techniques for effective therapeutic use, it is critical to understand (a) (what are) the key neural circuits to engage for specific disorders or clusters of symptoms, and (b) (how) can these circuits be reached effectively using neurostimulation? Here we focus on the research toward the application of transcranial direct current stimulation (tDCS) for posttraumatic stress disorder (PTSD). tDCS is a portable and inexpensive technique that lends itself well to be combined with, and thus potentially augment, exposure-based treatment for PTSD. In this review, we discuss the behavioral model of threat and safety learning and memory as it relates to PTSD, the underlying neurobiology of PTSD, as well as the current understandings of tDCS action, including its limitations and opportunities. Through this lens, we summarize the research on the application of tDCS to modulated threat and safety learning and memory to date, and propose new directions for its future research. (PsycInfo Database Record (c) 2021 APA, all rights reserved).

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.

Transcranial Direct Current Stimulation of the Ventromedial Prefrontal Cortex Modulates Perceptual and Neural Patterns of Fear Generalization

BACKGROUND

Overgeneralization of fear is a pathogenic marker of anxiety and stress-related disorders and has been linked with perceptual discrimination deficits, reduced fear inhibition, and prefrontal hyporeactivity to safety-signaling stimuli. We aimed to examine whether behavioral and neural patterns of fear generalization are influenced by the fear-inhibiting ventromedial prefrontal cortex (vmPFC).

METHODS

Three groups of healthy participants received excitatory (n = 27), inhibitory (n = 26), or sham (n = 26) transcranial direct current stimulation of the vmPFC after a fear conditioning phase and before a fear generalization phase. We obtained, as dependent variables, fear ratings and unconditioned stimulus-expectancy ratings, perceptual aspects of fear generalization (perceptual discrimination), pupil dilations, and source estimations of event-related fields elicited by conditioned and generalization stimuli.

RESULTS

After inhibitory (compared with excitatory and sham) vmPFC stimulation, we observed reduced performance in perceptual discrimination and less negative inhibitory gradients in frontal structures at midlatency and late time intervals. Fear and unconditioned stimulus-expectancy ratings as well as pupil dilation remained unaffected by stimulation.

CONCLUSIONS

These findings reveal a causal contribution of vmPFC reactivity to generalization patterns and suggest that vmPFC hyporeactivity consequent on inhibitory vmPFC stimulation may serve as a model for pathological processes of fear generalization (reduced discrimination, impaired fear inhibition via frontal brain structures). This encourages further basic and clinical research on the potential of targeted brain stimulation to modulate fear generalization and overgeneralization.

Does non-invasive brain stimulation modulate emotional stress reactivity?

Excessive emotional responses to stressful events can detrimentally affect psychological functioning and mental health. Recent studies have provided evidence that non-invasive brain stimulation (NBS) targeting the prefrontal cortex (PFC) can affect the regulation of stress-related emotional responses. However, the reliability and effect sizes have not been systematically analyzed. In the present study, we reviewed and meta-analyzed the effects of repetitive transcranial magnetic (rTMS) and transcranial direct current stimulation (tDCS) over the PFC on acute emotional stress reactivity in healthy individuals. Forty sham-controlled single-session rTMS and tDCS studies were included. Separate random effects models were performed to estimate the mean effect sizes of emotional reactivity. Twelve rTMS studies together showed no evidence that rTMS over the PFC influenced emotional reactivity. Twenty-six anodal tDCS studies yielded a weak beneficial effect on stress-related emotional reactivity (Hedges’ g = −0.16, CI_95% = [−0.33, 0.00]). These findings suggest that a single session of NBS is insufficient to induce reliable, clinically significant effects but also provide preliminary evidence that specific NBS methods can affect emotional reactivity. This may motivate further research into augmenting the efficacy of NBS protocols on stress-related processes.

Memories are not written in stone: Re-writing fear memories by means of non-invasive brain stimulation and optogenetic manipulations

The acquisition of fear associative memory requires brain processes of coordinated neural activity within the amygdala, prefrontal cortex (PFC), hippocampus, thalamus and brainstem. After fear consolidation, a suppression of fear memory in the absence of danger is crucial to permit adaptive coping behavior. Acquisition and maintenance of fear extinction critically depend on amygdala-PFC projections. The robust correspondence between the brain networks encompassed cortical and subcortical hubs involved into fear processing in humans and in other species underscores the potential utility of comparing the modulation of brain circuitry in humans and animals, as a crucial step to inform the comprehension of fear mechanisms and the development of treatments for fear-related disorders. The present review is aimed at providing a comprehensive description of the literature on recent clinical and experimental researches regarding the noninvasive brain stimulation and optogenetics. These innovative manipulations applied over specific hubs of fear matrix during fear acquisition, consolidation, reconsolidation and extinction allow an accurate characterization of specific brain circuits and their peculiar interaction within the specific fear processing.

Defining focal brain stimulation targets for PTSD using neuroimaging

INTRODUCTION

Focal brain stimulation has potential as a treatment for posttraumatic stress disorder (PTSD). In this review, we aim to inform selection of focal brain stimulation targets for treating PTSD by examining studies of the functional neuroanatomy of PTSD and treatment response. We first briefly review data on brain stimulation interventions for PTSD. Although published data suggest good efficacy overall, the neurobiological rationale for each stimulation target is not always clear.

METHODS

Therefore, we assess pre- and post-treatment (predominantly psychotherapy) functional neuroimaging studies in PTSD to determine which brain changes seem critical to treatment response. Results of these studies are presented within a previously proposed functional neural systems model of PTSD.

RESULTS

While not completely consistent, research suggests that downregulating the fear learning and threat and salience detection circuits (i.e., amygdala, dorsal anterior cingulate cortex and insula) and upregulating the emotion regulation and executive function and contextual processing circuits (i.e., prefrontal cortical regions and hippocampus) may mediate PTSD treatment response.

CONCLUSION

This literature review provides some justification for current focal brain stimulation targets. However, the examination of treatment effects on neural networks is limited, and studies that include the stimulation targets are lacking. Further, additional targets, such as the cingulate, medial prefrontal cortex, and inferior parietal lobe, may also be worth investigation, especially when considering how to achieve network level changes. Additional research combining PTSD treatment with functional neuroimaging will help move the field forward by identifying and validating novel targets, providing better rationale for specific treatment parameters and personalizing treatment for PTSD.

A Systematic Review on the Effect of Transcranial Direct Current and Magnetic Stimulation on Fear Memory and Extinction

Anxiety disorders are among the most prevalent mental disorders. Present treatments such as cognitive behavior therapy and pharmacological treatments show only moderate success, which emphasizes the importance for the development of new treatment protocols. Non-invasive brain stimulation methods such as repetitive transcranial magnetic stimulation (rTMS) and transcranial direct current stimulation (tDCS) have been probed as therapeutic option for anxiety disorders in recent years. Mechanistic information about their mode of action, and most efficient protocols is however limited. Here the fear extinction model can serve as a model of exposure therapies for studying therapeutic mechanisms, and development of appropriate intervention protocols. We systematically reviewed 30 research articles that investigated the impact of rTMS and tDCS on fear memory and extinction in animal models and humans, in clinical and healthy populations. The results of these studies suggest that tDCS and rTMS can be efficient methods to modulate fear memory and extinction. Furthermore, excitability-enhancing stimulation applied over the vmPFC showed the strongest potential to enhance fear extinction. We further discuss factors that determine the efficacy of rTMS and tDCS in the context of the fear extinction model and provide future directions to optimize parameters and protocols of stimulation for research and treatment.

Augmentation of fear extinction by theta-burst transcranial magnetic stimulation of the prefrontal cortex in humans

BACKGROUND

Fear extinction alone does not erase the original fear memory. Interventions that enhance extinction can be beneficial for the treatment of fear-related disorders. Repetitive transcranial magnetic stimulation has been shown to improve memory performance. The present study examined the effects of intermittent theta-burst stimulation (iTBS) on fear extinction and the return of fear memory in humans.

METHODS

Ninety-one young healthy volunteers underwent 3 experiments using a randomized controlled experimental design. Participants first acquired fear conditioning, after which they received 30 Hz iTBS before and after extinction training. The iTBS was applied to 1 of 2 targets: the left dorsolateral prefrontal cortex (dlPFC) and the vertex (control). Fear responses were measured 24 hours later and 1 month later.

RESULTS

During the spontaneous recovery and reinstatement tests, iTBS of the left dlPFC before and after extinction significantly reduced fear response, whereas iTBS of the vertex had no effect on fear memory performance. This combined approach had a relatively long-lasting effect (i.e., at least 1 month).

LIMITATIONS

We did not explore the effect of iTBS of the dlPFC on the expression of fear without extinction training. The neural mechanisms of iTBS with fear extinction to inhibit the fear response are unclear. Our results are preliminary and should be interpreted with caution.

CONCLUSION

`The present results showed that 30 Hz iTBS of the left dlPFC enhanced retention of fear extinction. Our study introduces a new intervention for fear memory and suggests that the left dlPFC may be a treatment target for fear-related disorders.

Simultaneous Application of Transcranial Direct Current Stimulation during Virtual Reality Exposure

Transcranial direct current stimulation (tDCS) is a form of non-invasive brain stimulation that changes the likelihood of neuronal firing through modulation of neural resting membranes. Compared to other techniques, tDCS is relatively safe, cost-effective, and can be administered while individuals are engaged in controlled, specific cognitive processes. This latter point is important as tDCS may predominantly affect intrinsically active neural regions. In an effort to test tDCS as a potential treatment for psychiatric illness, the protocol described here outlines a novel procedure that allows the simultaneous application of tDCS during exposure to trauma-related cues using virtual reality (tDCS+VR) for veterans with posttraumatic stress disorder (NCT03372460). In this double-blind protocol, participants are assigned to either receive 2 mA tDCS, or sham stimulation, for 25 minutes while passively watching three 8-minute standardized virtual reality drives through Iraq or Afghanistan, with virtual reality events increasing in intensity during each drive. Participants undergo six sessions of tDCS+VR over the course of 2-3 weeks, and psychophysiology (skin conductance reactivity) is measured throughout each session. This allows testing for within and between session changes in hyperarousal to virtual reality events and adjunctive effects of tDCS. Stimulation is delivered through a built-in rechargeable battery-driven tDCS device using a 1 (anode) x 1 (cathode) unilateral electrode set-up. Each electrode is placed in a 3 x 3 cm (current density 2.22 A/m²) reusable sponge pocket saturated with 0.9% normal saline. Sponges with electrodes are attached to the participant's skull using a rubber headband with the electrodes placed such that they target regions within the ventromedial prefrontal cortex. The virtual reality headset is placed over the tDCS montage in such a way as to avoid electrode interference.

Effects of repeated anodal transcranial direct current stimulation on auditory fear extinction in C57BL/6J mice

BACKGROUND

Trauma-based psychotherapy is a first line treatment for post-traumatic stress disorder (PTSD) but not all patients achieve long-term remission. Transcranial direct current stimulation (tDCS) received considerable attention as a neuromodulation method that may improve trauma-based psychotherapy.

OBJECTIVE

We explored the effects of repeated anodal tDCS over the prefrontal cortex (PFC) on fear extinction in mice as a preclinical model for trauma-based psychotherapy.

METHODS

We performed auditory fear conditioning with moderate or high shock intensity on C57BL6/J mice. Next, mice received anodal tDCS (0.2 mA, 20 min) or sham stimulation over the PFC twice daily for five consecutive days. Extinction training was performed by repeatedly exposing mice to the auditory cue the day after the last stimulation session. Early and late retention of extinction were evaluated one day and three weeks after extinction training respectively.

RESULTS

We observed no significant effect of tDCS on the acquisition or retention of fear extinction in mice subjected to fear conditioning with moderate intensity. However, when the intensity of fear conditioning was high, tDCS significantly lowered freezing during the acquisition of extinction, regardless of the extinction protocol. Moreover, when tDCS was combined with a strong extinction protocol, we also observed a significant improvement of early extinction recall. Finally, we found that tDCS reduced generalized fear induced by contextual cues when the intensity of conditioning is high and extinction training limited.

CONCLUSIONS

Our data provide a rationale to further explore anodal tDCS over the PFC as potential support for trauma-based psychotherapy for PTSD.

Timing matters: Transcranial direct current stimulation after extinction learning impairs subsequent fear extinction retention

BACKGROUND

Transcranial direct current stimulation (tDCS) has previously been shown to improve fear extinction learning and retention when administered prior to or during extinction learning. This study investigates whether tDCS immediately following extinction learning improves efficacy of extinction memory retention.

METHODS

30 participants completed a 2-day fear learning and extinction paradigm, where they acquired fear of a stimulus conditioned to an aversive electric shock on day 1. Extinction learning occurred on day 1, with tDCS or sham tDCS administered immediately following the learning phase. Participants returned for a second day test of extinction memory recall. Skin conductance was measured as the primary outcome.

RESULTS/CONCLUSIONS

Participants in the tDCS group showed impaired fear extinction retention on day 2, marked by significant generalisation of fear to the safety stimulus. This contrasts with earlier studies showing improved extinction retention when stimulation occurred during encoding of extinction learning, compared to immediate consolidation as in our study. These findings may have important implications for the use of tDCS during exposure therapy for anxiety and trauma disorders.

Repeated noninvasive stimulation of the ventromedial prefrontal cortex reveals cumulative amplification of pleasant compared to unpleasant scene processing: A single subject pilot study

The ventromedial prefrontal cortex (vmPFC) is a major hub of the reward system and has been shown to activate specifically in response to pleasant / rewarding stimuli. Previous studies demonstrate enhanced pleasant cue reactivity after single applications of transcranial direct current stimulation (tDCS) to the vmPFC. Here we present a pilot case study in which we assess the cumulative impact of multiple consecutive vmPFC-tDCS sessions on the processing of visual emotional stimuli in an event-related MEG recording design. The results point to stable modulation of increased positivity biases (pleasant > unpleasant stimulus signal strength) after excitatory vmPFC stimulation and a reversed pattern (pleasant < unpleasant) after inhibitory stimulation across five consecutive tDCS sessions. Moreover, cumulative effects of these emotional bias modulations were observable for several source-localized spatio-temporal clusters, suggesting an increase in modulatory efficiency by repeated tDCS sessions. This pilot study provides evidence for improvements in the effectiveness and utility of a novel tDCS paradigm in the context of emotional processing.

Prefrontal but not cerebellar tDCS attenuates renewal of extinguished conditioned eyeblink responses

An extended neural network is known to underlie extinction learning. As yet, comparatively little is known about the possible contribution of the cerebellum and the dorsolateral prefrontal cortex (dlPFC). In the present study, transcranial direct current stimulation (tDCS) was used to provide further evidence that the dlPFC and the cerebellum are involved in extinction-related processes. A total of 100 young and healthy human participants were randomly assigned to one of five stimulation groups: (1) anodal tDCS of the cerebellum, (2) cathodal tDCS of the cerebellum, (3) anodal tDCS of the dlPFC, (4) cathodal tDCS of the dlPFC, and (5) sham stimulation. Participants underwent delay eyeblink conditioning using an A-B-A/B renewal paradigm. Two different colors of background light (orange and blue) were used as contexts. On day 1, acquisition of conditioned eyeblink responses was performed in context A, followed by extinction in context B. tDCS was applied during extinction. On day 2, extinction recall was tested in contexts A and B with higher incidence of conditioned responses in acquisition context A compared to extinction context B indicating renewal effects. All groups showed significant effects of acquisition of conditioned eyeblink responses and significant effects of extinction. There was no significant difference in extinction between stimulation groups. During extinction recall, renewal effects were present in all groups, except the group which had received anodal tDCS of the dlPFC during extinction. In the present study, no direct effects of dlPFC or cerebellar tDCS were demonstrated on extinction. Anodal tDCS of the dlPFC, but not the cerebellum, resulted in delayed effects on context-related processes of extinction, possibly explained by shifting attention away from the context and towards the conditioned stimulus during extinction learning. Anodal tDCS of the dlPFC attenuated context-related recall of learned aversive responses. Effects of tDCS, however, were weak and need to be confirmed in future studies. Lack of cerebellar tDCS effects do not exclude a possible role of the cerebellum in extinction-related processes, and are likely explained by methodological limitations of cerebellar tDCS.

The effect of cathodal tDCS on fear extinction: A cross-measures study

BACKGROUND

Extinction-based procedures are often used to inhibit maladaptive fear responses. However, because extinction procedures show efficacy limitations, transcranial direct current stimulation (tDCS) has been suggested as a promising add-on enhancer.

OBJECTIVE

In this study, we tested how cathodal tDCS over the right dorsolateral prefrontal cortex affects extinction and tried to unveil the processes at play that boost the effectiveness of extinction procedures and its translational potential to the treatment of anxiety disorders.

METHODS

We implemented a fear conditioning paradigm whereby 41 healthy women (mean age = 20.51 ± 5.0) were assigned to either cathodal tDCS (n = 27) or sham tDCS (n = 16). Fear responses were measured with self-reports, autonomic responses, and implicit avoidance tendencies.

RESULTS

Cathodal tDCS shows no statistically significant effect in extinction, according to self-reports, and seems to even negatively affect fear conditioned skin conductance responses. However, one to three months after the tDCS session and extinction, we found a group difference in the action tendencies towards the neutral stimuli (F (1, 41) = 12.04, p = .001, ηp2 = .227), with the cathodal tDCS group (as opposed to the sham group) showing a safety learning (a positive bias towards the CS-), with a moderate effect size. This suggests that cathodal tDCS may foster stimuli discrimination, leading to a decreased generalization effect.

DISCUSSION

Cathodal tDCS may have enhanced long-term distinctiveness between threatening cues and perceptively similar neutral cues through a disambiguation process of the value of the neutral stimuli-a therapeutic target in anxiety disorders. Future studies should confirm these results and extend the study of cathodal tDCS effect on short term avoidance tendencies.

Neuromodulatory treatments for post-traumatic stress disorder (PTSD)

Electroconvulsive therapy has been used successfully in some individuals with posttraumatic stress disorder (PTSD) whose symptoms have not improved with other treatments. But there are only a few reports. Meanwhile, an array of new neuromodulation strategies, including repetitive transcranial magnetic stimulation, transcranial direct current stimulation, vagus nerve stimulation, trigeminal nerve stimulation, and deep brain stimulation have been developed and applied experimentally in the treatment of other psychiatric disorders. This article will review the clinical evidence and mechanistic basis for their use in PTSD.

Noninvasive Stimulation of the Ventromedial Prefrontal Cortex Indicates Valence Ambiguity in Sad Compared to Happy and Fearful Face Processing

The ventromedial prefrontal cortex (vmPFC) is known to be specifically involved in the processing of stimuli with pleasant, rewarding meaning to the observer. By the use of non-invasive transcranial direct current stimulation (tDCS), it was previously possible to show evidence for this valence specificity and to modulate the impact of the vmPFC on emotional network processing. Prior results showed increased neural activation during pleasant relative to unpleasant stimulus processing after excitatory compared to inhibitory vmPFC-tDCS. As dysfunctional vmPFC activation patterns are associated with major depressive disorder (MDD), tDCS of this region could render an attractive application in future therapy. Here, we investigated vmPFC-tDCS effects on sad compared to happy face processing, as sad faces are often used in the study of mood disorders. After counterbalanced inhibitory or excitatory tDCS, respectively, healthy participants viewed happy and sad faces during magnetoencephalography (MEG) recording. In addition, tDCS effects on an interpretational bias of ambiguous happy-sad face morphs and an attentional bias of a dot-probe task with happy and sad faces as emotional primes were investigated. Finally, in conjoint analyses with data from a previous sibling study (happy and fearful faces) we examined whether excitatory vmPFC-tDCS would reveal a general increase in processing of pleasant stimuli independent of the type of unpleasant stimuli applied (sad vs. fearful faces). MEG and behavioral results showed that happy faces promoted a relative positivity bias after excitatory compared to inhibitory tDCS, visible in left orbitofrontal cortex and in the emotion-primed dot-probe task. A converse pattern in the MEG data during sad face processing suggests the possible involvement of an empathy network and thus significantly differed from neuronal processing of fearful face processing. Implications for the bearing of vmPFC modulation on emotional face processing and the impact of specific unpleasant face expressions are discussed.

Efficacy and mechanisms of non-invasive brain stimulation to enhance exposure therapy: A review

Though cognitive behavioral techniques are generally effective in the treatment of anxiety disorders, some people fail to benefit from exposure therapy or experience a return of fear after terminating exposure therapy. The burgeoning field of non-invasive brain stimulation provides a potential method of augmenting exposure therapy so that it is more effective. Successful exposure therapy is hypothesized to occur due to inhibition, and research suggests that brain stimulation can alter inhibitory learning and related processes. As such, one can reasonably posit that brain stimulation could be used to test the inhibitory learning theory of exposure therapy and to increase the efficacy of exposure therapy by inducing stronger inhibitory learning during exposures. Four known studies that pair brain stimulation with exposure therapy have yielded promising preliminary evidence in support of the therapeutic use of brain stimulation. In this review we describe research illustrating the mechanisms and efficacy of non-invasive brain stimulation to enhance the understanding of and outcomes produced by exposure therapy.

Augmentation of Extinction and Inhibitory Learning in Anxiety and Trauma-Related Disorders

Although the fear response is an adaptive response to threatening situations, a number of psychiatric disorders feature prominent fear-related symptoms caused, in part, by failures of extinction and inhibitory learning. The translational nature of fear conditioning paradigms has enabled us to develop a nuanced understanding of extinction and inhibitory learning based on the molecular substrates to systems neural circuitry and psychological mechanisms. This knowledge has facilitated the development of novel interventions that may augment extinction and inhibitory learning. These interventions include nonpharmacological techniques, such as behavioral methods to implement during psychotherapy, as well as device-based stimulation techniques that enhance or reduce activity in different regions of the brain. There is also emerging support for a number of psychopharmacological interventions that may augment extinction and inhibitory learning specifically if administered in conjunction with exposure-based psychotherapy. This growing body of research may offer promising novel techniques to address debilitating transdiagnostic fear-related symptoms.

Eternal sunshine of the neuromodulated mind: Altering fear memories through neuromodulation

Anxiety disorders pose one of the greatest threats to mental health. Modern treatment methods exist but are hindered by relapse, toxicity, and low efficacy. The use of neuromodulation to treat anxiety disorders has shown promising results, yet its underpinning mechanisms remain poorly understood. In this review, we make the case for further development of neuromodulation techniques to alter fear memories, with particular regard to future clinical applications in treating anxiety disorders. We start by briefly summarizing the neural circuitry of fear while identifying the pros and cons of possible neuromodulation targets. We then highlight recent advances in neuromodulation techniques that have been used to alter fear memories. Next, we apply a novel network-based approach to elucidate possible mechanisms of neuromodulation which may disrupt the consolidation of fear memory. Finally, we emphasize the need for more systematic neuromodulation studies on animal models and the developing brain. Overall, we aim to provide an integrated framework for future action, identifying key research priorities that must be addressed before effective neuromodulation-based treatments can be developed for practical use.

Fasting enhances extinction retention and prevents the return of fear in humans

Fear is prone to return following extinction that is the basis of exposure therapy for fear-related disorders. Manipulations that enhance the extinction process can be beneficial for treatment. Animal studies have shown that fasting or caloric restriction can enhance extinction and inhibit the return of fear. The present study examined the effects of fasting on fear acquisition, extinction, and the return of fear in humans. One hundred and twenty-five male participants were randomized into a fasting group and food group and exposed to a Pavlovian fear conditioning paradigm. Changes in plasma cortisol and ghrelin levels were examined using enzyme-linked immunosorbent assays. One-night fasting had no effect on fear acquisition but enhanced fear extinction retention and prevented the return of fear, and this effect persisted for at least 6 months. This procedure was also effective for remote fear memory. Plasma ghrelin levels were elevated after fasting and had a negative relationship with the fear response in spontaneous recovery test. However, overnight fasting did not affect cortisol levels. These findings indicate that fasting enhances extinction retention and prevents the return of fear, without influencing fear memory formation. We propose that this novel procedure may open new avenues for promoting extinction-based therapies for fear-related disorders.

Combined transcranial direct current stimulation with virtual reality exposure for posttraumatic stress disorder: Feasibility and pilot results

BACKGROUND

Facilitating neural activity using non-invasive brain stimulation may improve extinction-based treatments for posttraumatic stress disorder (PTSD).

OBJECTIVE/HYPOTHESIS

Here, we examined the feasibility of simultaneous transcranial direct current stimulation (tDCS) application during virtual reality (VR) to reduce psychophysiological arousal and symptoms in Veterans with PTSD.

METHODS

Twelve Veterans with PTSD received six combat-related VR exposure sessions during sham-controlled tDCS targeting ventromedial prefrontal cortex. Primary outcome measures were changes in skin conductance-based arousal and self-reported PTSD symptom severity.

RESULTS

tDCS + VR components were combined without technical difficulty. We observed a significant interaction between reduction in arousal across sessions and tDCS group (p = .03), indicating that the decrease in physiological arousal was greater in the tDCS + VR versus sham group. We additionally observed a clinically meaningful reduction in PTSD symptom severity.

CONCLUSIONS

This study demonstrates feasibility of applying tDCS during VR. Preliminary data suggest a reduction in psychophysiological arousal and PTSD symptomatology, supporting future studies.

Prefrontal Cortex Stimulation Enhances Fear Extinction Memory in Humans

BACKGROUND

Animal fear conditioning studies have illuminated neuronal mechanisms of learned associations between sensory stimuli and fear responses. In rats, brief electrical stimulation of the infralimbic cortex has been shown to reduce conditioned freezing during recall of extinction memory. Here, we translated this finding to humans with magnetic resonance imaging-navigated transcranial magnetic stimulation (TMS).

METHODS

Subjects (N = 28) were aversively conditioned to two different cues (day 1). During extinction learning (day 2), TMS was paired with one of the conditioned cues but not the other. TMS parameters were similar to those used in rat infralimbic cortex: brief pulse trains (300 ms at 20 Hz) starting 100 ms after cue onset, total of four trains (28 TMS pulses). TMS was applied to one of two targets in the left frontal cortex, one functionally connected (target 1) and the other unconnected (target 2, control) with a human homologue of infralimbic cortex in the ventromedial prefrontal cortex. Skin conductance responses were used as an index of conditioned fear.

RESULTS

During extinction recall (day 3), the cue paired with TMS to target 1 showed significantly reduced skin conductance responses, whereas TMS to target 2 had no effect. Further, we built group-level maps that weighted TMS-induced electric fields and diffusion magnetic resonance imaging connectivity estimates with fear level. These maps revealed distinct cortical regions and large-scale networks associated with reduced versus increased fear.

CONCLUSIONS

The results showed that spatiotemporally focused TMS may enhance extinction learning and/or consolidation of extinction memory and suggested novel cortical areas and large-scale networks for targeting in future studies.

Modulating what is and what could have been: The effect of transcranial direct current stimulation on the evaluation of attained and unattained decision outcomes

The affective evaluation of decision outcomes, whether attained (e.g., disappointment) or based on the conscious realization that a decision made differently would have led to a better or worse outcome (e.g., regret), greatly influence future decisions. Prior research has demonstrated a role of the medial and orbitofrontal cortex (M/OFC) in decision valuation and the experience of regret and relief. Here we examined whether inhibitory transcranial direct current stimulation (tDCS) could dampen the experience of decision-induced affect, with a focus on regret and relief. Thirty-eight participants completed a previously used gambling task and were asked to rate their happiness with attained outcomes of a chosen gamble before and after being shown unattained, counterfactual outcomes (i.e., what would have happened had they selected the other gamble). The difference in happiness rating before and after revealing these unattained counterfactual outcomes was taken as a measure of regret (negative shift) or relief (positive shift). During this task, 20 participants received 2 mA cathodal tDCS over EEG coordinate Fp1 for 20 minutes, and 18 participants received sham stimulation over the same location. Linear mixed-model results showed that, compared to sham, participants who received cathodal tDCS reported less intense emotions in response to attained as well as counterfactual outcomes. These findings were not due to the groups differing in the gambles they selected or attained monetary outcomes, demonstrating that tDCS can modulate decision-induced (counterfactual) affect. This may have implications for the ability to modulate value-based decision-making using brain stimulation techniques more broadly.

Bilateral anodal transcranial direct current stimulation effect on balance and fearing of fall in patient with Parkinson's disease

BACKGROUND

A number of studies have examined the therapeutic effects of transcranial direct current stimulation (tDCS) stimulation in patients with Parkinson's disease (PD) using unilateral anodal stimulation applied either on the left or right brain hemisphere. However, PD involves the dysfunctions of both brain hemispheres.

OBJECTIVES

This study investigates the therapeutic effects of bilateral anodal tDCS stimulation on balance and fear of fall outcomes in patient with PD.

METHODS

Eighteen patients with idiopathic PD completed the study. Ten sessions of bilateral anodal tDCS stimulation were applied over the FC1 and FC2 targeting both pre-frontal and motor areas for each patient, 5 sessions per week for 2 weeks. Berg Balance Scale (BBS), Falls Efficacy Scale-International (FES-I), and 10 meters walk test (10mwt) were applied before and after the stimulation therapy.

RESULTS

Paired t-test showed a significant increase in the BBS scores and decrease in the FES-I scores after the bilateral tDCS compared with those scores before tDCS therapy (P < 0.05), as well improvement in the 10mwt scores.

CONCLUSION

Our data showed that bilateral anodal tDCS serves as an effective, safe and feasible approach for rehabilitation of patients with PD with the issues related to balance and fear of fall.

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.

Neural Oscillatory Correlates for Conditioning and Extinction of Fear

The extinction of conditioned-fear represents a hallmark of current exposure therapies as it has been found to be impaired in people suffering from post-traumatic stress disorder (PTSD) and anxiety. A large body of knowledge focusing on psychophysiological animal and human studies suggests the involvement of key brain structures that interact via neural oscillations during the acquisition and extinction of fear. Consequently, neural oscillatory correlates of such mechanisms appear relevant regarding the development of novel therapeutic approaches to counterbalance abnormal activity in fear-related brain circuits, which, in turn, could alleviate fear and anxiety symptoms. Here, we provide an account of state-of-the-art neural oscillatory correlates for the conditioning and extinction of fear, and also deal with recent translational efforts aimed at fear extinction by neural oscillatory modulation.

Augmentation of Fear Extinction by Transcranial Direct Current Stimulation (tDCS)

Although posttraumatic stress disorder (PTSD; DSM-V 309.82) and anxiety disorders (DSM-V 300.xx) are widely spread mental disorders, the effectiveness of their therapy is still unsatisfying. Non-invasive brain-stimulation techniques like transcranial direct current stimulation (tDCS) might be an option to improve extinction learning, which is a main functional factor of exposure-based therapy for anxiety disorders. To examine this hypothesis, we used a fear conditioning paradigm with female faces as conditioned stimuli (CS) and a 95-dB female scream as unconditioned stimulus (UCS). We aimed to perform a tDCS of the ventromedial prefrontal cortex (vmPFC), which is mainly involved in the control of extinction-processes. Therefore, we applied two 4 × 4 cm electrodes approximately at the EEG-positions F7 and F8 and used a direct current of 1.5 mA. The 20-min stimulation was started during a 10-min break between acquisition and extinction and went on overall extinction-trials. The healthy participants were randomly assigned in two double-blinded process into two sham stimulation and two verum stimulation groups with opposite current flow directions. To measure the fear reactions, we used skin conductance responses (SCR) and subjective ratings. We performed a generalized estimating equations model for the SCR to assess the impact of tDCS and current flow direction on extinction processes for all subjects that showed a successful conditioning (N = 84). The results indicate that tDCS accelerates early extinction processes with a significantly faster loss of CS+/CS– discrimination. The discrimination loss was driven by a significant decrease in reaction toward the CS+ as well as an increase in reaction toward the CS– in the tDCS verum groups, whereas the sham groups showed no significant reaction changes during this period. Therefore, we assume that tDCS of the vmPFC can be used to enhance early extinction processes successfully. But before it should be tested in a clinical context further investigation is needed to assess the reason for the reaction increase on CS–. If this negative side effect can be avoided, tDCS may be a tool to improve exposure-based anxiety therapies.

Memory creation and modification: Enhancing the treatment of psychological disorders

Modification of the ongoing influence of maladaptive cognitive, emotional, and behavioral patterns is a fundamental feature of many psychological treatments. Accordingly, a clear understanding of the nature of memory adaptation and accommodation to therapeutic learning becomes an important issue for (1) understanding the impact of clinical interventions, and (2) considering innovations in treatment strategies. In this article, we consider advances in the conceptualization of memory processes and memory modification research relative to clinical treatment. We review basic research on the formation of memories, the way in which new learning is integrated within memory structures, and strategies to influence the nature and degree to which new learning is integrated. We then discuss cognitive/behavioral and pharmacological strategies for influencing memory formation in relation to disorder prevention or treatment. Our goal is to foster awareness of current strategies for enhancing therapeutic learning and to encourage research on potential new avenues for memory enhancement in service of the treatment of mental health disorders. (PsycINFO Database Record

Noninvasive stimulation of the ventromedial prefrontal cortex modulates emotional face processing

The ventromedial prefrontal cortex (vmPFC) is associated with emotional states that can be characterized as positive affect. Moreover, a variety of psychiatric disorders that are associated with disturbed reactions toward reward- or safety-signaling stimuli reveal functional or structural anomalies within this area. Thus, neuromodulation of this region via transcranial direct current stimulation (tDCS) offers an attractive opportunity to noninvasively influence pleasant emotional and reward processing. Recent experiments revealed hemodynamic and electrophysiological evidence for valence specific modulations of emotional scene processing after excitatory and inhibitory tDCS of the vmPFC. Here, we identified that tDCS modulation of vmPFC during emotional face processing results in effects convergent with scene processing, in that excitatory tDCS increased neural reactivity during happy compared to fearful face perception, whereas inhibitory stimulation led to a converse effect. In addition, behavioral data (affect identification of ambiguous expressive faces) revealed a bias toward preferential processing of happy compared to fearful faces after excitatory compared to after inhibitory stimulation. These results further support the vmPFC as an appropriate target for noninvasive neuromodulation of an appetitive processing network in patients suffering from disturbed cognition of reward- and safety-signaling stimuli. It should however be noted that electrophysiological pre-tDCS differences at earlier time intervals of emotional face and scene processing appeared amplified by tDCS, which remains to be investigated.

Interference effects of transcranial direct current stimulation over the right frontal cortex and adrenergic system on conditioned fear

RATIONALE

The effects of pharmacological interventions on fear memory have widely been studied, but there are very few studies about the effects of brain electrical stimulation on fear memory function.

OBJECTIVE

Therefore, our aim was to determine whether anodal/cathodal transcranial direct current stimulation (tDCS) over the right frontal cortex would modify propranolol-induced contextual and auditory fear memory deficits, before or after training.

METHODS

The adult NMRI male mice were randomly assigned into three groups: the sham group, the anodal tDCS group, and the cathodal tDCS group. Fear memories were evaluated using a classical fear conditioning apparatus.

RESULTS

While the anodal stimulation did not affect fear retrieval, post-training cathodal stimulation improved fear memory retrieval. Regardless of when propranolol (0.1 mg/kg) was administered, it impaired fear memory retrieval. However, when anodal stimulation and propranolol were applied prior to the training, contextual fear memory retrieval was increased and auditory fear memory was reversed. An enhanced contextual retrieval was also observed when propranolol was administered prior to the training and stimulation occurred after the training. Only when the stimulation occurred prior to the training and propranolol was administered after the training was there a selective improvement in contextual fear memory retrieval, leaving the auditory fear memory retrieval impaired. Interestingly, cathodal stimulation improved the effects of propranolol on auditory fear memory only when it occurred prior to the training.

CONCLUSION

The results highlight possible improving effects for anodal/cathodal tDCS on propranolol-induced deficits on fear memories. The timing of the interventions related to the specific phases of memory formation is important in modulating fear behaviors.