Introduction
Knowledge of the neural circuits underlying emotion dysregulation in anxiety and trauma-related disorders can be leveraged to identify cortical targets for non-invasive brain stimulation (NIBS), either as an adjunct to behavioral interventions or as a standalone treatment (Ressler & Mayberg, Reference Ressler and Mayberg2007; Vicario, Salehinejad, Felmingham, Martino, & Nitsche, Reference Vicario, Salehinejad, Felmingham, Martino and Nitsche2019). Extinction is the emotional learning process underlying exposure therapy (Graham & Milad, Reference Graham and Milad2011). The ventromedial prefrontal cortex (vmPFC) has been implicated in neural circuits for consolidation and retrieval of fear extinction memory in animal and human studies by consistent evidence, such as vmPFC lesions impairing extinction retrieval, vmPFC stimulation strengthening extinction memory, vmPFC metabolic and electrophysiological responses being potentiated by extinction, and vmPFC activation being correlated with extinction behavior (Barrett & Gonzalez-Lima, Reference Barrett and Gonzalez-Lima2018; Gilmartin, Balderston, & Helmstetter, Reference Gilmartin, Balderston and Helmstetter2014; Quirk, Garcia, & Gonzalez-Lima, Reference Quirk, Garcia and González-Lima2006; Quirk & Mueller, Reference Quirk and Mueller2008). Preliminary transcranial direct current stimulation (tDCS) studies with humans show that stimulating the vmPFC improves fear extinction (Nuñez, Zinbarg, & Mittal, Reference Nuñez, Zinbarg and Mittal2019). Additionally, targeting the dorsolateral prefrontal cortex (dlPFC) with either standalone tDCS or transcranial magnetic stimulation (TMS) has been shown to reduce anxiety disorder symptom severity (Vicario et al., Reference Vicario, Salehinejad, Felmingham, Martino and Nitsche2019). Importantly, meta-analyses of TMS for anxiety and trauma-related disorders have shown medium to large treatment effects (Cirillo et al., Reference Cirillo, Gold, Nardi, Ornelas, Nierenberg, Camprodon and Kinrys2019; Cui et al., Reference Cui, Jiang, Wei, Li, Li, Zhu and Li2019) and tDCS has shown large effect sizes for reducing core symptoms of post-traumatic stress disorder (PTSD), supporting the efficacy of NIBS (Kan, Zhang, Zhang, & Kranz, Reference Kan, Zhang, Zhang and Kranz2020).
Here we present the first clinical trial treating pathological fear with transcranial infrared laser stimulation (TILS), a mechanistically distinct form of NIBS that improves neuronal bioenergetics by stimulating cytochrome-c-oxidase (CCO) activity (Gonzalez-Lima & Barrett, Reference Gonzalez-Lima and Barrett2014; Hamblin, Reference Hamblin2016; Rojas & Gonzalez-Lima, Reference Rojas and Gonzalez-Lima2013). By targeting CCO, a fundamental enzyme for oxidative energy production, TILS may provide some advantages over other forms of NIBS due to its more well-established mechanistic specificity (Gonzalez-Lima & Barrett, Reference Gonzalez-Lima and Barrett2014). As brain physiology is critically dependent on oxygenation for energy production, the mechanistic details of TILS on CCO are tied to cognitive processing by the brain: (a) near-infrared photons penetrate into the cerebral cortex (Salehpour et al., Reference Salehpour, Cassano, Rouhi, Hamblin, Taboada, Farajdokht and Mahmoudi2019) and oxidize CCO because CCO is the major intracellular acceptor of photons from red-to-near-infrared light (Rojas & Gonzalez-Lima, Reference Rojas and Gonzalez-Lima2013); (b) CCO catalyzes the reduction of oxygen to water that allows mitochondria to produce adenosine triphosphate (ATP), the energy source in cell metabolism (Gonzalez-Lima, Barksdale, & Rojas, Reference Gonzalez-Lima, Barksdale and Rojas2014); and (c) CCO also catalyzes the synthesis of the vasodilator nitric oxide (NO) under low oxygen conditions (Poyton & Hendrickson, Reference Poyton, Hendrickson, Hamblin, Agrawal and de Sousa2016). Therefore, TILS of the PFC augments CCO-catalyzed oxygen consumption, ATP production, and NO-mediated vasodilation, which constitute its downstream mechanism for augmenting brain activity. Direct measurement evidence for this mechanism is provided by 10 randomized sham-controlled brain activity studies showing that TILS influences human PFC activity by photo-oxidation of CCO and causes hemodynamic, electrophysiological, and functional connectivity effects in the default mode network that augment PFC-based cognitive brain functions (Holmes et al., Reference Holmes, Barrett, Saucedo, O'Connor, Liu and Gonzalez-Lima2019; Pruitt et al., Reference Pruitt, Wang, Wu, Kallioniemi, Husain and Liu2020; Saucedo et al., Reference Saucedo, Courtois, Wade, Kelley, Kheradbin, Barrett and Gonzalez-Lima2021; Tian, Hase, Gonzalez-Lima, & Liu, Reference Tian, Hase, Gonzalez-Lima and Liu2016; Urquhart et al., Reference Urquhart, Wanniarachchi, Wang, Gonzalez-Lima, Alexandrakis and Liu2020; Vargas et al., Reference Vargas, Barrett, Saucedo, Huang, Abraham, Tanaka and Gonzalez-Lima2017; Wang, Dmochowski, Husain, Gonzalez-Lima, & Liu, Reference Wang, Dmochowski, Husain, Gonzalez-Lima and Liu2017a; Wang et al., Reference Wang, Tian, Reddy, Nalawade, Barrett, Gonzalez-Lima and Liu2017b, Reference Wang, Reddy, Nalawade, Pal, Gonzalez-Lima and Liu2018, Reference Wang, Dmochowski, Zeng, Kallioniemi, Husain, Gonzalez-Lima and Liu2019). Five randomized sham-controlled human cognitive studies also provide measurement evidence of TILS-induced beneficial effects on PFC-modulated attention, memory, and executive functions (Barrett & Gonzalez-Lima, Reference Barrett and Gonzalez-Lima2013; Blanco, Maddox, & Gonzalez-Lima, Reference Blanco, Maddox and Gonzalez-Lima2017a; Blanco, Saucedo, & Gonzalez-Lima, Reference Blanco, Saucedo and Gonzalez-Lima2017b; Disner, Beevers, & Gonzalez-Lima, Reference Disner, Beevers and Gonzalez-Lima2016; Hwang, Castelli, & Gonzalez-Lima, Reference Hwang, Castelli and Gonzalez-Lima2016). Together these 15 human studies with over 500 subjects demonstrate that TILS to the forehead at 1064 nm wavelength engages PFC oxygenation that is tied to PFC-based cognitive functions by promoting PFC activity that modulates network electrophysiology. Our aim was to examine the potential of TILS as (a) an exposure therapy enhancer by targeting the vmPFC, which plays a critical role in fear extinction (Sevenster, Visser, & D'Hooge, Reference Sevenster, Visser and D'Hooge2018) and (b) a monotherapy by targeting the dlPFC, which has been implicated in emotion regulation (Buhle et al., Reference Buhle, Silvers, Wager, Lopez, Onyemekwu, Kober and Ochsner2013).
The application of fear extinction principles through repeated exposure to fear-provoking targets is a powerful intervention for reducing pathological fear (Milad & Quirk, Reference Milad and Quirk2012; Telch, Cobb, & Lancaster, Reference Telch, Cobb, Lancaster, Emmelkamp and Ehring2014b). However, a substantial minority of patients experience a return of fear in new contexts or after the passage of time, highlighting the need for exposure-augmentation strategies (Vervliet, Craske, & Hermans, Reference Vervliet, Craske and Hermans2013). Although pharmacologic approaches have predominated the exposure-augmentation research, NIBS is a compelling alternative that can selectively target cortical regions with putative roles in extinction learning (Rojas & Gonzalez-Lima, Reference Rojas and Gonzalez-Lima2013; Sathappan, Luber, & Lisanby, Reference Sathappan, Luber and Lisanby2019). Extinction enhancement with methylene blue (MB), a pharmacological agent sharing the same mechanistic CCO target as TILS, has been successfully translated from rodent models to exposure therapy for claustrophobia (Telch et al., Reference Telch, Bruchey, Rosenfield, Cobb, Smits, Pahl and Gonzalez-Lima2014a) and PTSD (Zoellner et al., Reference Zoellner, Telch, Foa, Farach, McLean, Gallop and Gonzalez-Lima2017). However, prior work with TILS and extinction is limited to rodents, where post-extinction TILS improved fear extinction memory retention by boosting CCO in the PFC (Rojas, Bruchey, & Gonzalez-Lima, Reference Rojas, Bruchey and Gonzalez-Lima2012). Therefore, clinical trials of the effect of vmPFC-TILS on exposure therapy are warranted.
Another promising NIBS target is the dlPFC, which is implicated in the cognitive control of emotion (Buhle et al., Reference Buhle, Silvers, Wager, Lopez, Onyemekwu, Kober and Ochsner2013) and is deficiently recruited in pathological fear (Zilverstand, Parvaz, & Goldstein, Reference Zilverstand, Parvaz and Goldstein2017). While dlPFC-TILS facilitated sustained attention (Barrett & Gonzalez-Lima, Reference Barrett and Gonzalez-Lima2013), executive function (Barrett & Gonzalez-Lima, Reference Barrett and Gonzalez-Lima2013; Blanco et al., Reference Blanco, Saucedo and Gonzalez-Lima2017b), and mood (Barrett & Gonzalez-Lima, Reference Barrett and Gonzalez-Lima2013) in healthy subjects and enhanced attention bias modification treatment for sub-clinically depressed individuals (Disner et al., Reference Disner, Beevers and Gonzalez-Lima2016), evidence for the therapeutic effect of dlPFC-TILS for anxiety is limited to one uncontrolled study (Schiffer et al., Reference Schiffer, Johnston, Ravichandran, Polcari, Teicher, Webb and Hamblin2009). This warrants further testing of the therapeutic potential of dlPFC-TILS as a monotherapy for pathological fear.
Drawing from these findings, we conducted a four-arm randomized placebo-controlled trial aimed at addressing two primary questions: (a) Does post-session administration of TILS targeting the vmPFC – a cortical region strongly implicated in fear extinction, enhance exposure therapy outcomes relative to sham TILS? (b) Does TILS monotherapy targeting the dlPFC – a cortical region implicated in emotion regulation, outperform sham stimulation in reducing naturally acquired pathological fear? Lastly, to test whether subjects in the higher clinical range benefit more from either exposure therapy, TILS, or their combination, we also examined baseline symptom severity as a moderator of treatment outcome.
Methods
Participants
Treatment-seeking volunteers (N = 112) with pathological fear of enclosed spaces, contamination, public speaking, or bodily sensations were recruited from the university's student population and greater Austin community. Inclusion criteria were (1) aged 18–65 years; and (2) peak-fear rating of 50 or higher on two behavioral assessments (see measures). The minimum peak-fear criterion was selected to ensure that only participants displaying sufficient fear during the behavioral assessments were included in the trial. Exclusion criteria were (1) suicide risk; (2) concurrent exposure-based treatment; (3) medication change within 6 weeks; and (4) contraindicated medical or neurological condition. Written informed consent was collected prior to procedures, which were approved by the Institutional Review Board of The University of Texas at Austin. The complete protocol is provided in online Supplementary Material 1 and is described in our methods publication (Zaizar, Gonzalez-Lima, & Telch, Reference Zaizar, Gonzalez-Lima and Telch2018).
Procedures
Recruitment
Figure 1 summarizes participant flow through the study from October 2014 to March 2020. An online prescreen was used to assess initial eligibility and recruit individuals who showed elevated scores (2 s.d.s above mean) on domain-specific validated symptom inventories: Claustrophobia Questionnaire (Radomsky, Rachman, Thordarson, McIsaac, & Teachman, Reference Radomsky, Rachman, Thordarson, McIsaac and Teachman2001), Obsessive-Compulsive Inventory-Revised (Foa et al., Reference Foa, Huppert, Leiberg, Langner, Kichic, Hajcak and Salkovskis2002), Liebowitz Social Anxiety Scale Self-Report (Heimberg et al., Reference Heimberg, Horner, Juster, Safren, Brown, Schneier and Liebowitz1999), or Anxiety Sensitivity Index-3 (Taylor et al., Reference Taylor, Zvolensky, Cox, Deacon, Heimberg, Ledley and Cardenas2007). Participants were enrolled by research assistants who conducted the baseline assessment.
CONSORT flow diagram.
Randomization
The randomization sequence was generated by the first author (EDZ). Eligible participants were stratified on fear domain, sex, and pretreatment peak-fear and block randomized to one of four treatment arms: (a) Exposure to feared targets + TILS of the vmPFC (EX + TILS_vmPFC); (b) Exposure to feared targets + sham TILS of the vmPFC (EX + SHAM_vmPFC); (c) TILS of the dlPFC (TILS_dlPFC); and (d) Sham TILS of the dlPFC (SHAM_dlPFC). Participants were blind as to whether they received active or sham TILS in the exposure therapy arms and the monotherapy arms.
Interventions
Transcranial infrared laser stimulation (TILS)
Prior to laser stimulation, all participants viewed a video demonstration of the brain stimulation procedures which included treatment rationales highlighting improved emotion regulation in the two monotherapy conditions (TILS_dlPFC/SHAM_dlPFC) or fear extinction enhancement in the two stimulation plus exposure therapy conditions (EX + TILS_vmPFC/EX + SHAM_vmPFC).
In the two monotherapy conditions, TILS was administered bilaterally to the dlPFC (F3 and F4 on the electroencephalographic International 10–20 System) for 60 s in four alternating cycles for 8 min total with a 1064 nm wavelength Model CG-5000 laser diode supplied by Cell Gen Therapeutics, LLC (HD Laser Center, Dallas, TX, USA). We have shown in a published video our laser device on the head with stimulation targets and laser stimulation procedure and setup (Zaizar et al., Reference Zaizar, Gonzalez-Lima and Telch2018). We used laser parameters and standard operating procedures reviewed and approved by the Laser Safety Office of the University of Texas at Austin and published previously (Zaizar et al., Reference Zaizar, Gonzalez-Lima and Telch2018). The laser had a collimated beam circular area of 13.6 cm2. The measured laser power output was 3.4 W. Each forehead area treated received a power density (irradiance) of 0.25 W/cm2 (3.4 W/13.6 cm2). For the 8 min stimulation, the total laser energy delivered was 1632 J (3.4W × 480 s). The laser energy density (fluence) was 120 J/cm2 (0.25 W/cm2 × 480 s). SHAM administration followed the same 8 min procedure but delivered only 5 s of laser stimulation per cycle. Thus, the SHAM condition received 3.4 W × 5 s equal to 17 J for a total energy density of 0.25 W/cm2 × 5 s equal to 1.25 J/cm2 (i.e. less than 1% of the energy density used in the active laser groups). This 5 s stimulation is sufficient to provide a brief sensation of slight heat (as active placebo) at the onset of each 1 min cycle, using a small fraction of the energy received by the experimental groups, but insufficient to influence behavioral or brain function (Barrett & Gonzalez-Lima, Reference Barrett and Gonzalez-Lima2013; Saucedo et al., Reference Saucedo, Courtois, Wade, Kelley, Kheradbin, Barrett and Gonzalez-Lima2021). Parameters set for our TILS and SHAM stimulation procedures were identical to our previously published studies (Barrett & Gonzalez-Lima, Reference Barrett and Gonzalez-Lima2013; Blanco et al., Reference Blanco, Saucedo and Gonzalez-Lima2017b; Disner et al., Reference Disner, Beevers and Gonzalez-Lima2016; Hwang et al., Reference Hwang, Castelli and Gonzalez-Lima2016). We elected to use an active placebo comparator based on their enhanced utility to reduce the risk of participant unblinding relative to traditional placebos (Jensen, Bielefeldt, & Hróbjartsson, Reference Jensen, Bielefeldt and Hróbjartsson2017).
In the two exposure therapy arms, TILS or Sham stimulation was administered immediately following the completion of exposure therapy and consisted of bilateral stimulation to the vmPFC (FP1 and FP2, International 10–20 System for EEG) for 60 s in four alternating cycles for 8 min total (EX + TILS_vmPFC) or the same procedure (EX + SHAM_vmPFC) for 5 s of laser stimulation per cycle.
Participants were fitted with protective eyewear and were asked to close their eyes for the duration of this procedure. This ensured eye safety and aided in keeping participants blind as to whether they received TILS or SHAM. Furthermore, as a manipulation check, we measured whether participants believed they received active or sham stimulation as well as their confidence in this belief.
Exposure therapy
Participants randomized to EX + TILS_vmPFC and EX + SHAM_vmPFC completed single-session manualized exposure therapy protocols consisting of repeated trials confronting one of four phobic targets: (a) claustrophobia (lying inside a tightly enclosed chamber) (Telch et al., Reference Telch, Bruchey, Rosenfield, Cobb, Smits, Pahl and Gonzalez-Lima2014a); (b) contamination fear (touching a mixture of dirt, dead insects, and hair) (Cougle, Wolitzky-Taylor, Lee, & Telch, Reference Cougle, Wolitzky-Taylor, Lee and Telch2007); (c) public speaking fear (giving a speech in front of a live audience) (Smits, Powers, Buxkamper, & Telch, Reference Smits, Powers, Buxkamper and Telch2006); and (d) fear of benign somatic sensations (repeated inhalations of a gas mixture of 35% CO2/65% O2) (Telch, Rosenfield, Lee, & Pai, Reference Telch, Rosenfield, Lee and Pai2012).
Measures
Primary outcome
The primary outcome was self-reported peak-fear (range: 0–100) measured at the termination of two behavioral approach tests (BATs; Context A; Context B) each conducted at pre-randomization, post-TILS, and at 2-week follow-up (Fig. 2). Assessing the outcome in two contexts allowed us to examine the generalization of treatment effects across two distinct contexts. Note that across groups, the context effect may also involve an order effect because Context A always preceded Context B. In the exposure groups, this effect also captured differences between testing in treatment and non-treatment contexts because Context A used the exposure therapy stimuli and Context B used novel stimuli. Stimuli used for the BAT contexts were specific to each fear domain and are outlined in Table 1. Immediately following each BAT context, subjects rated the maximum level of fear (peak-fear: 0–100) they experienced during the BAT. All BATs lasted up to 15 s (except for the 10 s 35% CO2 challenge and 1 min respiratory task, see Table 1). This method for indexing fear responses is based on prior research (Cougle et al., Reference Cougle, Wolitzky-Taylor, Lee and Telch2007; Smits et al., Reference Smits, Powers, Buxkamper and Telch2006; Telch et al., Reference Telch, Rosenfield, Lee and Pai2012, Reference Telch, Bruchey, Rosenfield, Cobb, Smits, Pahl and Gonzalez-Lima2014a).
Boxplot of peak-fear levels in the behavioral approach tasks.
Note: Boxplots illustrate median and distribution (interquartile range) of raw data values.
Summary of behavioral approach test contexts for each fear domain
Note: Context A stimulus was also encountered during exposure therapy.
Treatment moderator
Pre-randomization peak-fear ratings served as treatment moderators (see analyses section).
Statistical analysis
Sample size calculation
Prior clinical trials from our group have examined the impact of MB, a pharmacologic intervention that shares the same CCO mechanism as TILS, on the treatment of claustrophobia (Telch et al., Reference Telch, Bruchey, Rosenfield, Cobb, Smits, Pahl and Gonzalez-Lima2014a) and PTSD (Zoellner et al., Reference Zoellner, Telch, Foa, Farach, McLean, Gallop and Gonzalez-Lima2017). These studies found evidence of enhancing effects in samples of 19–23 subjects per group. For this pilot trial of TILS as a singular or combined treatment for pathological fear, our aim was to recruit 30 participants per group.
Primary outcome
Analyses were conducted in R (version 3.6.1) and applied a two-tailed significance threshold of α = 0.05. The primary outcome of peak-fear in the BATs was analyzed in a mixed model with Group (EX + TILS_vmPFC, EX + SHAM_vmPFC, TILS_dlPFC, SHAM_dlPFC), Time (Post-treatment, Follow-up), Context (A, B), their interactions, and a random subject effect. The effect of Baseline-Fear was included to adjust for pre-randomization levels of the outcome and its interaction with Group was included to test its moderating impact on treatment effects. Pairwise comparisons of significant interaction effects were used to test differential treatment effects on retention (i.e. Group × Time) and contextual generalization (i.e. Group × Context). Here the retention interaction answers the question ‘Are there group differences in the retention of treatment effects from post-treatment to follow-up?’ and the context interaction answers the question ‘Are there group differences in how treatment effects generalize across distinct contexts?’.
The trial protocol paper (Zaizar et al., Reference Zaizar, Gonzalez-Lima and Telch2018) originally proposed measuring the primary outcome in a 2 × 2 ANCOVA of Exposure Therapy (yes or no) and TILS (yes or no). However, while data collection was ongoing and prior to analyses being conducted, the decision was made to analyze the four groups separately because vmPFC-TILS and dlPFC-TILS represent two distinct treatment approaches given the difference in stimulation site. In order to include all participants, consistent with the principles of intent-to-treat analyses, we used mixed models (which applies robust maximum likelihood estimation to estimate the effect of missing data) in place of ANCOVA (which applies listwise deletion of any participants of missing data).
Treatment moderator
Preliminary analyses indicated the primary outcome model described above fits the data better than a less complex model without Group × Baseline-Fear, χ2(3) = 16.87, p < 0.001. Therefore, the moderating impact of baseline severity was tested within the primary outcome model. The protocol paper (Zaizar et al., Reference Zaizar, Gonzalez-Lima and Telch2018) proposed additional potential moderation analyses (e.g. moderation by fear domain type). However, given the sample size for this pilot project, we decided to limit analyses to the primary outcome and moderation by baseline fear.
Results
Sample characteristics
Table 2 presents baseline demographic and clinical characteristics of the sample. Fear domains were distributed across enclosed spaces [n = 36 (32.14%)), bodily sensations (n = 32, (28.57%)], contamination [n = 22, (19.64%)], or public speaking [n = 22, (19.64%)]. Domain-specific validated measures were in the pathological range based on cutoffs and normative data (Foa et al., Reference Foa, Huppert, Leiberg, Langner, Kichic, Hajcak and Salkovskis2002; Heimberg et al., Reference Heimberg, Horner, Juster, Safren, Brown, Schneier and Liebowitz1999; Radomsky et al., Reference Radomsky, Rachman, Thordarson, McIsaac and Teachman2001; Taylor et al., Reference Taylor, Zvolensky, Cox, Deacon, Heimberg, Ledley and Cardenas2007).
Demographic and clinical characteristics of treatment groups
ASI-3, Anxiety Sensitivity Index 3; CLQ, Claustrophobia Questionnaire; OCI-R, Obsessive-Compulsive Inventory-Revised; LSAS, Liebowitz Social Anxiety Scale; CEQ, Credibility/Expectancy Questionnaire.
Treatment integrity, credibility, safety
Dropout proportions were not significantly different across groups, [EX + TILS_vmPFC 1 (3.45%); EX + SHAM_vmPFC 2 (6.90%); TILS_dlPFC 0; SHAM_dlPFC 0; Fisher's exact test, p = 0.62], and no adverse events were reported. Group differences in the proportion of participants who believed they received active TILS were non-significant [EX + TILS_vmPFC: 20 (71.43%); EX + SHAM_vmPFC: 20 (74.07%); TILS_dlPFC: 19 (73.08%); SHAM_dlPFC: 19 (67.86%); p = 0.96]. Unexpectedly, SHAM_dlPFC had higher mean treatment credibility and expectancy ratings than EX + SHAM_vmPFC and TILS_dlPFC (Table 2). Together, this suggests SHAM stimulation functioned as a rigorous active placebo control condition: across both SHAM groups, most participants believed they received TILS, and treatment expectancy and credibility was higher than or comparable to the groups that included active TILS.
Primary outcome
Omnibus test
Figure 2 shows raw peak-fear values across time-points and contexts; full model statistics are provided in the online Supplementary Materials. Although the Group × Time × Context interaction was non-significant (p = 0.16), significant interactions of Group × Time (p = 0.02) and Group × Context (p = 0.02) indicated differential treatment effects on retention (i.e. between time-points, averaged across contexts) and on generalization (i.e. between contexts, averaged across time).
Retention effects
Across the two post-treatment time-points, EX + TILS_vmPFC did not outperform EX + SHAM_vmPFC (both ps > 0.36), and TILS_dlPFC did not outperform SHAM_dlPFC (both ps > 0.09). However, both exposure groups had lower fear levels relative to the non-exposure groups [EX + TILS_vmPFC v. TILS_dlPFC: b = −13.30, 95% CI (−25.45 to −1.15), p = 0.03; EX + TILS_vmPFC v. SHAM_dlPFC: b = −23.87, 95% CI (−35.7 to −11.95), p < 0.001; EX + SHAM_vmPFC v. TILS_dlPFC: b = −15.88, 95% CI (−28.13 to −3.63), p = 0.01; EX + SHAM_vmPFC v. SHAM_dlPFC: b = −26.44, 95% CI (−38.47 to −14.41), p < 0.001]. But, at follow-up, the two exposure groups significantly outperformed SHAM_dlPFC only [EX + TILS_vmPFC v. SHAM_dlPFC: b = −13.87, 95% CI (−25.98 to −1.76), p = 0.03; EX + SHAM_vmPFC v. SHAM_dlPFC: b = −19.55, 95% CI (−31.78 to −7.32), p = 0.002]. All other comparisons were non-significant.
Contextual generalization effects
Differences between EX + TILS_vmPFC and EX + SHAM_vmPFC were non-significant in each context (both ps > 0.31), suggesting that the addition of TILS_vmPFC did not enhance contextual generalization of exposure effects. However, TILS_dlPFC outperformed SHAM_dlPFC in assessment context A [TILS_dlPFC v. SHAM_dlPFC: b = −13.44, 95% CI (−25.73 to −1.15), p = 0.03], providing some evidence of the impact of dlPFC-TILS on fear reduction in the initial testing context. Additionally, exposure groups had lower fear relative to SHAM_dlPFC in assessment context A [EX + TILS_vmPFC v. SHAM_dlPFC: b = −24.91, 95% CI (−36.92 to −12.9), p < 0.001; EX + SHAM_vmPFC v. SHAM_dlPFC: b = −26.87, 95% CI (−38.96 to −14.78), p < 0.001] and in assessment context B [EX + TILS_vmPFC v. SHAM_dlPFC: b = −12.83, 95% CI (−24.88 to −0.78), p = 0.04; EX + SHAM_vmPFC v. SHAM_dlPFC: b = −19.12, 95% CI (−31.33 to −6.91), p = 0.003]. But only EX + SHAM_vmPFC outperformed TILS_dlPFC (Context A: EX + SHAM_vmPFC v. TILS_dlPFC: b = −13.42, 95% CI (−25.77 to −1.07), p = 0.04; Context B: EX + SHAM_vmPFC v. TILS_dlPFC: b = −12.68, 95% CI (−24.9 to −0.37), p = 0.046]. All other comparisons were non-significant.
Moderation by baseline-fear
Figure 3 shows the significant Group × Baseline-Fear interaction (averaged across time-points and contexts), p < 0.001. At low levels of baseline-fear, the only significant difference was between EX + SHAM_vmPFC and SHAM_dlPFC [b = −16.63, 95% CI (−29.14 to −4.13), p = 0.01], whereas at high levels of baseline-fear, the two exposure conditions outperformed each of the monotherapy conditions [EX + TILS_vmpFC v. TILS_dlPFC: b = −18.31, 95% CI (−30.95 to −5.67), p = 0.01; EX + TILS_vmPFC v. SHAM_dlPFC: b = −26.19, 95% CI (−39.08 to −13.29), p < 0.001; EX + SHAM_vmPFC v. TILS_dlPFC: b = −21.58, 95% CI (−34.42 to −8.75), p < 0.001; EX + SHAM_vmPFC v. SHAM_dlPFC: b = −29.46, 95% CI (−42.53 to −16.39), p < 0.001].
Model-based estimates of moderation effects of the primary outcome by pretreatment peak-fear levels.
Note: The Group × Baseline-Fear interaction (averaged across time-points and contexts) was significant, p < 0.001.
Discussion
This was the first rigorous test of the singular and combined effects of a single administration of TILS and single-session exposure therapy for reducing naturally acquired pathological fear. Internal validity was maximized by utilizing active sham stimulation as a comparator and by measuring the success of this manipulation. Most subjects (69.64%) reported believing that they received real TILS and there were no significant group differences with respect to this belief. External validity was strengthened by including participants with a diverse range of pathological fears.
We did not find evidence that vmPFC-TILS enhanced the retention or contextual generalization of fear extinction learning. Given that systemic engagement of the same mechanistic target (i.e. neuronal bioenergetics) with oral administration of MB (Gonzalez-Lima & Bruchey, Reference Gonzalez-Lima and Bruchey2004; Wrubel, Barrett, Shumake, Johnson, & Gonzalez-Lima, Reference Wrubel, Barrett, Shumake, Johnson and Gonzalez-Lima2007) effectively augmented exposure therapy (Telch et al., Reference Telch, Bruchey, Rosenfield, Cobb, Smits, Pahl and Gonzalez-Lima2014a; Zoellner et al., Reference Zoellner, Telch, Foa, Farach, McLean, Gallop and Gonzalez-Lima2017), targeting the vmPFC with TILS may have failed to influence the extensive network of brain regions involved in post-extinction memory consolidation (Barrett, Shumake, Jones, & Gonzalez-Lima, Reference Barrett, Shumake, Jones and Gonzalez-Lima2003; Sevenster et al., Reference Sevenster, Visser and D'Hooge2018). Though photobiomodulation improved fear extinction in rodents (Rojas et al., Reference Rojas, Bruchey and Gonzalez-Lima2012), the stimulation included all the rodent brain relative to only the vmPFC in humans (Carlén, Reference Carlén2017; Fullana et al., Reference Fullana, Albajes-Eizagirre, Soriano-Mas, Vervliet, Cardoner, Benet and Harrison2018), that may have made it more feasible to engage the fear extinction neuronal network more completely. Further research is necessary to identify optimal targets for enhancing exposure therapy with TILS using brain imaging to assess the specificity of target engagement (Wang et al., Reference Wang, Tian, Reddy, Nalawade, Barrett, Gonzalez-Lima and Liu2017b). Whether the posterior part of the vmPFC was sufficiently engaged by TILS is unclear because the existing human studies documenting the effects of TILS on CCO and/or hemodynamics have measured from the anterior vmPFC at FP1/FP2 points or the anterior dlPFC at F3/F4 points (Holmes et al., Reference Holmes, Barrett, Saucedo, O'Connor, Liu and Gonzalez-Lima2019; Pruitt et al., Reference Pruitt, Wang, Wu, Kallioniemi, Husain and Liu2020; Saucedo et al., Reference Saucedo, Courtois, Wade, Kelley, Kheradbin, Barrett and Gonzalez-Lima2021; Tian et al., Reference Tian, Hase, Gonzalez-Lima and Liu2016; Wang et al., Reference Wang, Tian, Reddy, Nalawade, Barrett, Gonzalez-Lima and Liu2017b, Reference Wang, Reddy, Nalawade, Pal, Gonzalez-Lima and Liu2018). However, recent simulations of TILS penetration into the human head have shown that 1064 nm photons reach deep into the white matter posterior to the vmPFC, although with much reduced power levels than to anterior PFC regions (Huang, Kao, Sung, & Abraham, Reference Huang, Kao, Sung and Abraham2020; Tian, Varghese, Tran, Fang, & Gonzalez-Lima, Reference Tian, Varghese, Tran, Fang and Gonzalez-Lima2020). An intranasal stimulation approach will not likely improve penetration into the mPFC because intranasal lasers have a very small beam area, which results in significantly less penetration than the larger laser beam disk areas that can be used in the forehead (Huang et al., Reference Huang, Kao, Sung and Abraham2020; Tian et al., Reference Tian, Varghese, Tran, Fang and Gonzalez-Lima2020). Additionally, extensive evidence supports the role of the vmPFC in both retrieval and consolidation of extinction (Quirk & Mueller, Reference Quirk and Mueller2008). Therefore, an alternate timing of TILS to the vmPFC could be before the retrieval test, but this timing would have more limited potential in the clinical practice of exposure therapy.
In contrast, we found weak-to-moderate support for the use of bilateral dlPFC-TILS monotherapy for reducing fear. Specifically, dlPFC-TILS attenuated fear in the first two assessment contexts, even though participants did not receive exposure therapy. It is important to note that mean peak-fear levels were lower in the second assessment context relative to the first across both monotherapy conditions (Fig. 2). Given that Context A was more fear provoking, it is plausible that Context A may have provided a more sensitive test of treatment effects than Context B. Our suggestive finding is consistent with research identifying the dlPFC as a stimulation target for improving emotion regulation (Buhle et al., Reference Buhle, Silvers, Wager, Lopez, Onyemekwu, Kober and Ochsner2013; Zilverstand et al., Reference Zilverstand, Parvaz and Goldstein2017). However, assessment of the therapeutic potential of dlPFC-TILS should take into account that EX + SHAM_vmPFC was associated with even greater fear reduction, highlighting the clinically meaningful potency of a single session of exposure therapy. Additionally, the exposure groups were more effective for individuals with high baseline-fear, on average across time-points and contexts. This is consistent with the already well-established efficacy of exposure therapy for clinical fears (Foa & McLean, Reference Foa and McLean2015; Powers, Halpern, Ferenschak, Gillihan, & Foa, Reference Powers, Halpern, Ferenschak, Gillihan and Foa2010) and underscores the importance of comparing novel interventions to this goal standard approach while considering pretreatment severity. Indeed, exposure therapy may be indicated for those in the higher phobic range. Nevertheless, our tentative finding can guide future work with dlPFC-TILS for pathological fear, which should examine increased doses (i.e. repeated stimulation) and lateralizing stimulation to the right dlPFC based on other NIBS studies (Vicario et al., Reference Vicario, Salehinejad, Felmingham, Martino and Nitsche2019) and recent imaging work demonstrating that the right dlPFC is a major hub for outgoing connections to other brain regions (Li et al., Reference Li, Zhang, Yin, Ren, He and Li2018).
Given that we did not find support for vmPFC-TILS as an exposure therapy enhancer but did observe some therapeutic benefit of dlPFC-TILS as a monotherapy, this begs the question as to whether combining exposure therapy with dlPFC-TILS may serve as a promising augmentation treatment approach for future work. Relatedly, a recent meta-analysis highlighted the involvement of the dlPFC in human fear extinction learning (Fullana et al., Reference Fullana, Albajes-Eizagirre, Soriano-Mas, Vervliet, Cardoner, Benet and Harrison2018), consistent with neural models emphasizing higher cognitive circuits in human fear processes (LeDoux & Pine, Reference LeDoux and Pine2016). Of note, one fMRI study observed increased right dlPFC activity and decreased amygdala activity immediately after a single session of exposure therapy (Hauner, Mineka, Voss, & Paller, Reference Hauner, Mineka, Voss and Paller2012). However, at 6-month follow-up, only attenuated amygdala activity was observed. These findings suggest that the right dlPFC may have a ‘time-limited’ role in successful therapeutic learning and provide guidance for timing cortical stimulation to potentially enhance exposure therapy. Therefore, future studies should also test the effects of dlPFC-TILS on exposure therapy by specifically targeting the right dlPFC immediately after exposure.
Limitations
Our design limits inferences (a) beyond a single administration of both interventions either alone or in combination; (b) regarding the effects of vmPFC stimulation outside of the context of exposure; or (c) regarding the effects of dlPFC stimulation within the context of exposure. This would have required a fully crossed six-arm design which was not feasible given our resources for this pilot. Second, our sample size precluded investigation of additional treatment moderation effects (e.g. fear domain type). Third, the relatively young age of the subjects may play a role in the TILS effects, as a new study comparing young and older subjects showed that there was a greater TILS effect on CCO with increasing age, while TILS effects on hemodynamics decreased with increasing age (Saucedo et al., Reference Saucedo, Courtois, Wade, Kelley, Kheradbin, Barrett and Gonzalez-Lima2021).
Conclusions
The field of NIBS for pathological fear is still in its infancy, especially with respect to combining this approach with exposure therapy. This pilot trial represents the first attempt to characterize the clinical benefits of TILS alone and in concert with extinction-based therapy for pathological fear. Although we did not find evidence of improved exposure outcomes after vmPFC-TILS, our trial partially demonstrated that anxiolytic effects may be achieved through dlPFC-TILS. Given that this research is in an early phase and that we found only limited support for dlPFC-TILS as a monotherapy, it is important to interpret these initial findings cautiously. Subsequent investigations with this approach are needed to: (a) optimize dosing strategies for vmPFC-TILS or test alternative cortical targets for augmenting exposure therapy with TILS, and (b) evaluate the benefit of repeated and/or lateralized dlPFC-TILS as a monotherapy for pathological fear, similarly to TILS studies done for other purposes that used five sessions (O'Donnell, Barrett, Fink, Garcia-Pittman, & Gonzalez-Lima, Reference O'Donnell, Barrett, Fink, Garcia-Pittman and Gonzalez-Lima2021; Vargas et al., Reference Vargas, Barrett, Saucedo, Huang, Abraham, Tanaka and Gonzalez-Lima2017).
Supplementary material
The supplementary material for this article can be found at https://doi.org/10.1017/S0033291721002270.
Financial support
FGL reported receiving support from the NIH and the Oskar Fischer Project Fund during the conduct of the study. SP reported receiving support from the NIMH and the Donald D. Harrington Foundation during the conduct of the study. All of these sources of funding were unrelated to the current project. This research received no specific grant from any funding agency, commercial or not-for-profit sectors.
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Ethical standards
The authors assert that all procedures contributing to this work comply with the ethical standards of the relevant national and institutional committees on human experimentation and with the Helsinki Declaration of 1975, as revised in 2008.
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