Safety, Tolerability, Blinding Efficacy and Behavioural Effects of a Novel MRI-Compatible, High-Definition tDCS Set-Up

Improving mentalizing deficits in older age with region-specific transcranial direct current stimulation

Older adults have difficulties to detect the intentions, thoughts, and feelings of others, indicating an age-associated decline of socio-cognitive abilities that are known as "mentalizing". These deficits in mental state recognition are driven by neurofunctional alterations in brain regions that are implicated in mentalizing, such as the right temporo-parietal junction (rTPJ) and the dorso-medial prefrontal cortex (dmPFC). We tested whether focal transcranial current stimulation (tDCS) of the rTPJ and dmPFC has the potential to eliminate mentalizing deficits in older adults. Mentalizing deficits were assessed with a novel mindreading task that required the recognition of mental states in child faces. Older adults (n = 60) performed worse than younger adults (n = 30) on the mindreading task, indicating age-dependent deficits in mental state recognition. These mentalizing deficits were ameliorated in older adults who received sham-controlled andodal tDCS over the rTPJ (n = 30) but remained unchanged in older adults who received sham-controlled andodal tDCS over the dmPFC (n = 30). We, thus, showed for the first time that anodal tDCS over the rTPJ has the potential to remediate age-dependent mentalizing deficits in a region-specific way. This provides a rationale for exploring stimulation-based interventions targeting mentalizing deficits in older age.

The role of frontal cortex in novel-word learning and consolidation: Evidence from focal transcranial direct current stimulation

Previous studies have demonstrated that conventional transcranial direct current stimulation (tDCS) can enhance novel-word learning. However, because of the widespread current that is induced by these setups and lack of appropriate control conditions, little is known about the underlying neural mechanisms. In the present double-blinded and sham-tDCS controlled study, we investigated for the first time if regionally precise focal tDCS targeting two key nodes of the novel-word learning network at different time points would result in regionally and temporally distinct effects. 156 participants completed a contextual novel-word-learning paradigm and learning success was probed immediately after the acquisition period and 30-min later. Participants were randomly assigned to six stimulation conditions: Active tDCS (1.5 mA) was administered to left inferior frontal (IFG) or middle temporal gyrus (MTG), either during acquisition or delayed recall. Control groups received sham-tDCS either during acquisition or delayed recall (50% IFG/MTG). Data were analyzed with a generalized linear mixed model with a binomial link function in a Bayesian framework. Our results showed that frontal tDCS selectively increased accuracy gains from immediate to delayed recall, irrespective of timing of the stimulation. There was no evidence for beneficial effects of middle temporal gyrus tDCS. Our findings confirm that IFG tDCS can enhance novel-word learning in a regionally, but not timing specific way. Tentatively, this may be explained by enhancement of semantic selection processes resulting in more effective consolidation and/or retrieval. Future studies using longer time intervals between assessments are required to clarify the potential contribution of neurophysiological after-effects of IFG tDCS administered during acquisition to enhanced consolidation.

Investigating the neural mechanisms of transcranial direct current stimulation effects on human cognition: current issues and potential solutions

Transcranial direct current stimulation (tDCS) has been studied extensively for its potential to enhance human cognitive functions in healthy individuals and to treat cognitive impairment in various clinical populations. However, little is known about how tDCS modulates the neural networks supporting cognition and the complex interplay with mediating factors that may explain the frequently observed variability of stimulation effects within and between studies. Moreover, research in this field has been characterized by substantial methodological variability, frequent lack of rigorous experimental control and small sample sizes, thereby limiting the generalizability of findings and translational potential of tDCS. The present manuscript aims to delineate how these important issues can be addressed within a neuroimaging context, to reveal the neural underpinnings, predictors and mediators of tDCS-induced behavioral modulation. We will focus on functional magnetic resonance imaging (fMRI), because it allows the investigation of tDCS effects with excellent spatial precision and sufficient temporal resolution across the entire brain. Moreover, high resolution structural imaging data can be acquired for precise localization of stimulation effects, verification of electrode positions on the scalp and realistic current modeling based on individual head and brain anatomy. However, the general principles outlined in this review will also be applicable to other imaging modalities. Following an introduction to the overall state-of-the-art in this field, we will discuss in more detail the underlying causes of variability in previous tDCS studies. Moreover, we will elaborate on design considerations for tDCS-fMRI studies, optimization of tDCS and imaging protocols and how to assure high-level experimental control. Two additional sections address the pressing need for more systematic investigation of tDCS effects across the healthy human lifespan and implications for tDCS studies in age-associated disease, and potential benefits of establishing large-scale, multidisciplinary consortia for more coordinated tDCS research in the future. We hope that this review will contribute to more coordinated, methodologically sound, transparent and reproducible research in this field. Ultimately, our aim is to facilitate a better understanding of the underlying mechanisms by which tDCS modulates human cognitive functions and more effective and individually tailored translational and clinical applications of this technique in the future.

Electrode positioning errors reduce current dose for focal tDCS set-ups: Evidence from individualized electric field mapping

OBJECTIVE

Electrode positioning errors contribute to variability of transcranial direct current stimulation (tDCS) effects. We investigated the impact of electrode positioning errors on current flow for tDCS set-ups with different focality.

METHODS

Deviations from planned electrode positions were determined using data acquired in an experimental study (N = 240 datasets) that administered conventional and focal tDCS during magnetic resonance imaging (MRI). Comparison of individualized electric field modeling for planned and empirically derived "actual" electrode positions was conducted to quantify the impact of positioning errors on the electric field dose in target regions for tDCS.

RESULTS

Planned electrode positions resulted in higher current dose in the target regions for focal compared to conventional montages (7-12%). Deviations from planned positions significantly reduced current flow in the target regions, selectively for focal set-ups (26-30%). Dose reductions were significantly larger for focal compared to conventional set-ups (29-43%).

CONCLUSIONS

Precise positioning is crucial when using focal tDCS set-ups to avoid significant reductions of current dose in the intended target regions.

SIGNIFICANCE

Our results highlight the urgent need to routinely implement methods for improving electrode positioning, minimization of electrode drift, verification of electrode positions before and/or after tDCS and also to consider positioning errors when investigating dose-response relationships, especially for focal set-ups.

Small effects of electric field on motor cortical excitability following anodal tDCS

The dose-response characteristics of transcranial direct current stimulation (tDCS) remain uncertain but may be related to variability in brain electric fields due to individual anatomical factors. Here, we investigated whether the electric fields influence the responses to motor cortical tDCS. In a randomized cross-over design, 21 participants underwent 10 min of anodal tDCS with 0.5, 1.0, 1.5, or 2.0 mA or sham. Compared to sham, all active conditions increased the size of motor evoked potentials (MEP) normalized to the pre-tDCS baseline, irrespective of anterior or posterior magnetic test stimuli. The electric field calculated in the motor cortex of each participant had a nonlinear effect on the normalized MEP size, but its effects were small compared to those of other participant-specific factors. The findings support the efficacy of anodal tDCS in enhancing the MEP size but do not demonstrate any benefits of personalized electric field modeling in explaining tDCS response variability.

Determining the effects of targeted high-definition transcranial direct current stimulation on reducing post-stroke upper limb motor impairments-a randomized cross-over study

BACKGROUND

Stroke is one of the leading causes of death in the USA and is a major cause of serious disability for adults. This randomized crossover study examines the effect of targeted high-definition transcranial direct current transcranial brain stimulation (tDCS) on upper extremity motor recovery in patients in the post-acute phase of stroke recovery.

METHODS

This randomized double-blinded cross-over study includes four intervention arms: anodal, cathodal, and bilateral brain stimulation, as well as a placebo stimulation. Participants receive each intervention in a randomized order, with a 2-week washout period between each intervention. The primary outcome measure is change in Motor Evoked Potential. Secondary outcome measures include the Fugl-Meyer Upper Extremity (FM-UE) score, a subset of FM-UE (A), related to the muscle synergies, and the Modified Ashworth Scale.

DISCUSSION

We hypothesize that anodal stimulation to the ipsilesional primary motor cortex will increase the excitability of the damaged cortico-spinal tract, reducing the UE flexion synergy and enhancing UE motor function. We further hypothesize that targeted cathodal stimulation to the contralesional premotor cortex will decrease activation of the cortico-reticulospinal tract (CRST) and the expression of the upper extremity (UE) flexion synergy and spasticity. Finally, we hypothesize bilateral stimulation will achieve both results simultaneously. Results from this study could improve understanding of the mechanism behind motor impairment and recovery in stroke and perfect the targeting of tDCS as a potential stroke intervention. With the use of appropriate screening, we anticipate no ethical or safety concerns. We plan to disseminate these research results to journals related to stroke recovery, engineering, and medicine.

TRIAL REGISTRATION

ClinicalTrials.gov NCT05479006 . Registered on 26 July 2022.

Lasting enhancements in neural efficiency by multi-session transcranial direct current stimulation during working memory training

The neural basis for long-term behavioral improvements resulting from multi-session transcranial direct current stimulation (tDCS) combined with working memory training (WMT) remains unclear. In this study, we used task-related electroencephalography (EEG) measures to investigate the lasting neurophysiological effects of anodal high-definition (HD)-tDCS applied over the left dorsolateral prefrontal cortex (dlPFC) during a challenging WMT. Thirty-four healthy young adults were randomized to sham or active tDCS groups and underwent ten 30-minute training sessions over ten consecutive days, preceded by a pre-test and followed by post-tests performed one day and three weeks after the last session, respectively, by performing high-load WM tasks along with EEG recording. Multi-session HD-tDCS significantly enhanced the behavioral benefits of WMT. Compared to the sham group, the active group showed facilitated increases in theta, alpha, beta, and gamma task-related oscillations at the end of training and significantly increased P300 response 3 weeks post-training. Our findings suggest that applying anodal tDCS over the left dlPFC during multi-session WMT can enhance the behavioral benefits of WMT and facilitate sustained improvements in WM-related neural efficiency.

High-definition transcranial direct current stimulation for upper extremity rehabilitation in moderate-to-severe ischemic stroke: a pilot study

Introduction

Previous studies found that post-stroke motor impairments are associated with damage to the lesioned corticospinal tract (CST) and hyperexcitability of the contralesional cortico-reticulospinal tract (CRST). This proof-of-concept study aims to develop a non-invasive brain stimulation protocol that facilitates the lesioned CST and inhibits the contralesional CRST to improve upper extremity rehabilitation in individuals with moderate-to-severe motor impairments post-stroke.

Methods

Fourteen individuals (minimum 3 months post ischemic stroke) consented. Physician decision of the participants baseline assessment qualified eight to continue in a randomized, double-blind cross-over pilot trial (ClinicalTrials.gov Identifier: NCT05174949) with: (1) anodal high-definition transcranial direct stimulation (HD-tDCS) over the ipsilesional primary motor cortex (M1), (2) cathodal HD-tDCS over contralesional dorsal premotor cortex (PMd), (3) sham stimulation, with a two-week washout period in-between. Subject-specific MR images and computer simulation were used to guide HD-tDCS and verified by Transcranial Magnetic Stimulation (TMS) induced Motor Evoked Potential (MEP). The motor behavior outcome was evaluated by an Fugl-Meyer Upper Extremity score (primary outcome measure) and the excitability of the ipslesoinal CST and contralesional CRST was determined by the change of MEP latencies and amplitude (secondary outcome measures).

Results

The baseline ipsilesional M1 MEP latency and amplitude were correlated with FM-UE. FM-UE scores were improved post HD-tDCS, in comparison to sham stimulation. Both anodal and cathodal HD-tDCS reduced the latency of the ipsilesional M1 MEP. The contralesional PMd MEP disappeared/delayed after HD-tDCS.

Discussion

These results suggest that HD-tDCS could improve the function of the lesioned corticospinal tract and reduce the excitability of the contralesional cortico-reticulospinal tract, thus, improving motor function of the upper extremity in more severely impaired individuals.

Modulation of the executive control network by anodal tDCS over the left dorsolateral prefrontal cortex improves task shielding in dual tasking

Task shielding is an important executive control demand in dual-task performance enabling the segregation of stimulus-response translation processes in each task to minimize between-task interference. Although neuroimaging studies have shown activity in left dorsolateral prefrontal cortex (dlPFC) during various multitasking performances, the specific role of dlPFC in task shielding, and whether non-invasive brain stimulation (NIBS) may facilitate task shielding remains unclear. We therefore applied a single-blind, crossover sham-controlled design in which 34 participants performed a dual-task experiment with either anodal transcranial direct current stimulation (atDCS, 1 mA, 20 min) or sham tDCS (1 mA, 30 s) over left dlPFC. Task shielding was assessed by the backward-crosstalk effect, indicating the extent of between-task interference in dual tasks. Between-task interference was largest at high temporal overlap between tasks, i.e., at short stimulus onset asynchrony (SOA). Most importantly, in these conditions of highest multitasking demands, atDCS compared to sham stimulation significantly reduced between-task interference in error rates. These findings extend previous neuroimaging evidence and support modulation of successful task shielding through a conventional tDCS setup with anodal electrode over the left dlPFC. Moreover, our results demonstrate that NIBS can improve shielding of the prioritized task processing, especially in conditions of highest vulnerability to between-task interference.

Robust enhancement of motor sequence learning with 4 mA transcranial electric stimulation

BACKGROUND AND OBJECTIVES

Motor learning experiments with transcranial direct current stimulation (tDCS) at 2 mA have produced mixed results. We hypothesize that tDCS boosts motor learning provided sufficiently high field intensity on the motor cortex.

METHODS

In a single-blinded design, 108 healthy participants received either anodal (N = 36) or cathodal (N = 36) tDCS at 4 mA total, or no stimulation (N = 36) while they practiced a 12-min sequence learning task. Anodal stimulation was delivered across four electrode pairs (1 mA each), with anodes above the right parietal lobe and cathodes above the right frontal lobe. Cathodal stimulation, with reversed polarities, served as an active control for sensation, while the no-stimulation condition established baseline performance. fMRI-localized targets on the primary motor cortex in 10 subjects were used in current flow models to optimize electrode placement for maximal field intensity. A single electrode montage was then selected for all participants.

RESULTS

We found a significant difference in performance with anodal vs. cathodal stimulation (Cohen's d = 0.71) and vs. no stimulation (d = 0.56). This effect persisted for at least 1 h, and subsequent learning for a new sequence and the opposite hand also improved. Sensation ratings were comparable in the active groups and did not exceed moderate levels. Current flow models suggest the new electrode montage can achieve stronger motor cortex polarization than alternative montages.

CONCLUSION

The present paradigm shows a medium to large effect size and is well-tolerated. It may serve as a go-to experiment for future studies on motor learning and tDCS.

High-definition transcranial direct current stimulation (HD-tDCS) as augmentation therapy in late-life depression (LLD) with suboptimal response to treatment-a study protocol for a double-blinded randomized sham-controlled trial

BACKGROUND

Late-life depression (LLD) has a poorer prognosis and higher relapse rate than younger adults, with up to one third of patients with LLD showing suboptimal response to antidepressant therapy. LLD has been associated with significant impairment in cognition and daily functioning. Few studies have evaluated the therapeutic effects of high-definition transcranial direct current stimulation (HD-tDCS) on depressive and cognitive symptoms of LLD. The current randomized controlled trial assesses the efficacy of HD-tDCS as an augmentation therapy with antidepressants compared to sham-control in subjects with LLD.

METHODS

Fifty-eight patients with LLD will be recruited and randomly assigned to the active HD-tDCS or sham HD-tDCS group. In both groups, patients will receive the active or sham intervention in addition to their pre-existing antidepressant therapy, for 2 weeks with 5 sessions per week, each lasting 30 min. The primary outcome measures will be the change of depressive symptoms, clinical response and the remission rate as measured with the Hamilton Depression Rating scale (HAMD-17) before and after the intervention, and at the 4th and 12th week after the completed intervention. Secondary outcome measures include cognitive symptoms, anxiety symptoms, daily functioning and adverse effects.  DISCUSSION: Older adults with depression are associated with poorer outcomes or unsatisfactory responses to antidepressant therapy, and significant cognitive decline. Therefore, a new effective treatment option is needed. This randomized control trial aims at assessing the efficacy of HD-tDCS on ameliorating the depressive, cognitive and anxiety symptoms, and improving the daily functioning of subjects with LLD.

TRIAL REGISTRATION

ClinicalTrials.gov NCT05322863. Registered on 11 April 2022.

Online closed-loop real-time tES-fMRI for brain modulation: A technical report

Recent studies suggest that transcranial electrical stimulation (tES) can be performed during functional magnetic resonance imaging (fMRI). The novel approach of using concurrent tES-fMRI to modulate and measure targeted brain activity/connectivity may provide unique insights into the causal interactions between the brain neural responses and psychiatric/neurologic signs and symptoms, and importantly, guide the development of new treatments. However, tES stimulation parameters to optimally influence the underlying brain activity may vary with respect to phase difference, frequency, intensity, and electrode's montage among individuals. Here, we propose a protocol for closed-loop tES-fMRI to optimize the frequency and phase difference of alternating current stimulation (tACS) for two nodes (frontal and parietal regions) in individual participants. We carefully considered the challenges in an online optimization of tES parameters with concurrent fMRI, specifically in its safety, artifact in fMRI image quality, online evaluation of the tES effect, and parameter optimization method, and we designed the protocol to run an effective study to enhance frontoparietal connectivity and working memory performance with the optimized tACS using closed-loop tES-fMRI. We provide technical details of the protocol, including electrode types, electrolytes, electrode montages, concurrent tES-fMRI hardware, online fMRI processing pipelines, and the optimization algorithm. We confirmed the implementation of this protocol worked successfully with a pilot experiment.

Repeated High-Definition Transcranial Direct Current Stimulation Modulated Temporal Variability of Brain Regions in Core Neurocognitive Networks Over the Left Dorsolateral Prefrontal Cortex in Mild Cognitive Impairment Patients

Background: Early intervention of amnestic mild cognitive impairment (aMCI) may be the most promising way for delaying or even preventing the progression to Alzheimer’s disease. Transcranial direct current stimulation (tDCS) is a noninvasive brain stimulation technique that has been recognized as a promising approach for the treatment of aMCI. Objective: In this paper, we aimed to investigate the modulating mechanism of tDCS on the core neurocognitive networks of brain. Methods: We used repeated anodal high-definition transcranial direct current stimulation (HD-tDCS) over the left dorsolateral prefrontal cortex and assessed the effect on cognition and dynamic functional brain network in aMCI patients. We used a novel method called temporal variability to depict the characteristics of the dynamic brain functional networks. Results: We found that true anodal stimulation significantly improved cognitive performance as measured by the Montreal Cognitive Assessment after simulation. Meanwhile, the Mini-Mental State Examination scores showed a clear upward trend. More importantly, we found significantly altered temporal variability of dynamic functional connectivity of regions belonging to the default mode network, central executive network, and the salience network after true anodal stimulation, indicating anodal HD-tDCS may enhance brain function by modulating the temporal variability of the brain regions. Conclusion: These results imply that ten days of anodal repeated HD-tDCS over the LDLPFC exerts beneficial effects on the temporal variability of the functional architecture of the brain, which may be a potential neural mechanism by which HD-tDCS enhances brain functions. Repeated HD-tDCS may have clinical uses for the intervention of brain function decline in aMCI patients.

Safety of Special Waveform of Transcranial Electrical Stimulation (TES): In Vivo Assessment

Intermittent theta burst (iTBS) powered by direct current stimulation (DCS) can safely be applied transcranially to induce neuroplasticity in the human and animal brain cortex. tDCS-iTBS is a special waveform that is used by very few studies, and its safety needs to be confirmed. Therefore, we aimed to evaluate the safety of tDCS-iTBS in an animal model after brain stimulations for 1 h and 4 weeks. Thirty-one Sprague Dawley rats were divided into two groups: (1) short-term stimulation for 1 h/session (sham, low, and high) and (2) long-term for 30 min, 3 sessions/week for 4 weeks (sham and high). The anodal stimulation applied over the primary motor cortex ranged from 2.5 to 4.5 mA/cm². The brain biomarkers and scalp tissues were assessed using ELISA and histological analysis (H&E staining) after stimulations. The caspase-3 activity, cortical myelin basic protein (MBP) expression, and cortical interleukin (IL-6) levels increased slightly in both groups compared to sham. The serum MBP, cortical neuron-specific enolase (NSE), and serum IL-6 slightly changed from sham after stimulations. There was no obvious edema or cell necrosis seen in cortical histology after the intervention. The short- and long-term stimulations did not induce significant adverse effects on brain and scalp tissues upon assessing biomarkers and conducting histological analysis.

LIVE-streaming 3D images: A neuroscience approach to full-body illusions

Inspired by recent technological advances in the gaming industry, we used capture cards to create and LIVE-stream high quality 3D-images. With this novel technique, we developed a real-life stereoscopic 3D full-body illusion paradigm (3D projection). Unlike previous versions of the full-body illusion that rely upon unwieldy head-mounted displays, this paradigm enables the unobstructed investigation of such illusions with neuroscience methods (e.g., transcranial direct current stimulation, transcranial magnetic stimulation, electroencephalography, and near-infrared spectroscopy) and examination of their neural underpinnings. This paper has three aims: (i) to provide a step-by-step guide on how to implement 3D LIVE-streaming, (ii) to explain how this can be used to create a full-body illusion paradigm; and (iii) to present evidence that documents the effectiveness of our methods (de Boer et al., 2020), including suggestions for potential applications. Particularly significant is the fact that 3D LIVE-streaming is not GPU-intensive and can easily be applied to any device or screen that can display 3D images (e.g., TV, tablet, mobile phone). Therefore, these methods also have potential future clinical and commercial benefits. 3D LIVE-streaming could be used to enhance future clinical observations or educational tools, or potentially guide medical interventions with real-time high-quality 3D images. Alternatively, our methods can be used in future rehabilitation programs to aid recovery from nervous system injury (e.g., spinal cord injury, brain damage, limb loss) or in therapies aimed at alleviating psychosis symptoms. Finally, 3D LIVE-streaming could set a new standard for immersive online gaming as well as augmenting online and mobile experiences (e.g., video chat, social sharing/events).

A checklist for assessing the methodological quality of concurrent tES-fMRI studies (ContES checklist): a consensus study and statement

Low-intensity transcranial electrical stimulation (tES), including alternating or direct current stimulation, applies weak electrical stimulation to modulate the activity of brain circuits. Integration of tES with concurrent functional MRI (fMRI) allows for the mapping of neural activity during neuromodulation, supporting causal studies of both brain function and tES effects. Methodological aspects of tES-fMRI studies underpin the results, and reporting them in appropriate detail is required for reproducibility and interpretability. Despite the growing number of published reports, there are no consensus-based checklists for disclosing methodological details of concurrent tES-fMRI studies. The objective of this work was to develop a consensus-based checklist of reporting standards for concurrent tES-fMRI studies to support methodological rigor, transparency and reproducibility (ContES checklist). A two-phase Delphi consensus process was conducted by a steering committee (SC) of 13 members and 49 expert panelists through the International Network of the tES-fMRI Consortium. The process began with a circulation of a preliminary checklist of essential items and additional recommendations, developed by the SC on the basis of a systematic review of 57 concurrent tES-fMRI studies. Contributors were then invited to suggest revisions or additions to the initial checklist. After the revision phase, contributors rated the importance of the 17 essential items and 42 additional recommendations in the final checklist. The state of methodological transparency within the 57 reviewed concurrent tES-fMRI studies was then assessed by using the checklist. Experts refined the checklist through the revision and rating phases, leading to a checklist with three categories of essential items and additional recommendations: (i) technological factors, (ii) safety and noise tests and (iii) methodological factors. The level of reporting of checklist items varied among the 57 concurrent tES-fMRI papers, ranging from 24% to 76%. On average, 53% of checklist items were reported in a given article. In conclusion, use of the ContES checklist is expected to enhance the methodological reporting quality of future concurrent tES-fMRI studies and increase methodological transparency and reproducibility.

A New Approach to Assess Blinding for Transcranial Direct Current Stimulation Treatment in Patients with Fibromyalgia. A Randomized Clinical Trial

Correct blinding is essential for preventing potential biases. The aim of this study was to assess the blinding of participants and a therapist following treatment with transcranial direct current stimulation in subjects with fibromyalgia using James’ and Bang’s blinding indexes. Eighty subjects were randomly allocated either active or sham stimulation groups in an intervention of five sessions lasting 20 min each. A questionnaire was delivered to both the therapist and patients after the last session to record their guess of which treatment had been applied. No differences between the groups were noted at baseline in terms of demographic or clinical data. James’ BI was 0.83 (CI 95%: 0.76–0.90) for the patients and 0.55 (CI 95%: 0.45–0.64) for the therapist. Bang’s BI for subjects was −0.08 (CI 95%: −0.24–0.09) and −0.8 (CI 95%: −0.26–0.1) for the active and sham transcranial direct current stimulation groups, respectively. Bang’s BI for the therapist was 0.21 (CI 95%: −0.02–0.43) and 0.13 (CI 95%: −0.09–0.35) for the active and sham transcranial direct current stimulation groups, respectively. Protocols of active and sham transcranial direct current stimulation applied in this study have shown satisfactory blinding of the therapist and subjects with fibromyalgia.

Stimulation of the Social Brain Improves Perspective Selection in Older Adults: A HD-tDCS Study

There is evidence for dissociable, causal roles for two key social brain regions in young adults. Specifically, the right temporoparietal junction (rTPJ) is associated with embodied perspective taking, whereas the dorsomedial prefrontal cortex (dmPFC) is associated with the integration of social information. However, it is unknown whether these causal brain-behaviour associations are evident in older adults. Fifty-two healthy older adults were stratified to receive either rTPJ or dmPFC anodal high-definition transcranial direct current stimulation in a sham-controlled, double-blinded, repeated-measures design. Self-other processing was assessed across implicit and explicit level one (line-of-sight) and level two (embodied rotation) visual perspective taking (VPT) tasks, and self-other encoding effects on episodic memory. Both rTPJ and dmPFC stimulation reduced the influence of the alternate perspective during level one VPT, indexed by a reduced congruency effect (difference between congruent and incongruent perspectives). There were no stimulation effects on level two perspective taking nor self-other encoding effects on episodic memory. Stimulation to the rTPJ and dmPFC improved perspective selection during level one perspective taking. However, dissociable effects on self-other processing, previously observed in young adults, were not identified in older adults. The results provide causal evidence for age-related changes in social brain function that requires further scrutinization.

Transcranial alternating current stimulation at theta frequency to left parietal cortex impairs associative, but not perceptual, memory encoding

Neural oscillations in the theta range (4-8 Hz) are thought to underlie associative memory function in the hippocampal-cortical network. While there is ample evidence supporting a role of theta oscillations in animal and human memory, most evidence is correlational. Non-invasive brain stimulation (NIBS) can be employed to modulate cortical oscillatory activity to influence brain activity, and possibly modulate deeper brain regions, such as hippocampus, through strong and reliable cortico-hippocampal functional connections. We applied focal transcranial alternating current stimulation (tACS) at 6 Hz over left parietal cortex to modulate brain activity in the putative cortico-hippocampal network to influence associative memory encoding. After encoding and brain stimulation, participants completed an associative memory and a perceptual recognition task. Results showed that theta tACS significantly decreased associative memory performance but did not affect perceptual memory performance. These results show that parietal theta tACS modulates associative processing separately from perceptual processing, and further substantiate the hypothesis that theta oscillations are implicated in the cortico-hippocampal network and associative encoding.

Prefrontal high definition cathodal tDCS modulates executive functions only when coupled with moderate aerobic exercise in healthy persons

Transcranial direct current stimulation (tDCS) is a promising tool to enhance cognitive performance. However, its effectiveness has not yet been unequivocally shown. Thus, here we tested whether coupling tDCS with a bout of aerobic exercise (AE) is more effective in modulating cognitive functions than tDCS or AE alone. One hundred twenty-two healthy participants were assigned to five randomized controlled crossover experiments. Two multimodal target experiments (EXP-4: anodal vs. sham tDCS during AE; EXP-5: cathodal vs. sham tDCS during AE) investigated whether anodal (a-tDCS) or cathodal tDCS (c-tDCS) applied during AE over the left dorsolateral prefrontal cortex (left DLPFC) affects executive functioning (inhibition ability). In three unimodal control experiments, the participants were either stimulated (EXP-1: anodal vs. sham tDCS, EXP-2: cathodal vs. sham tDCS) or did AE (EXP-3: AE vs. active control). Participants performed an Eriksen flanker task during ergometer cycling at moderate intensity (in EXP. 3-5). Only c-tDCS during AE had a significant adverse effect on the inhibition task, with decreased accuracy. This outcome provides preliminary evidence that c-tDCS during AE over the left DLPFC might effectively modulate inhibition performance compared to c-tDCS alone. However, more systematic research is needed in the future.

Safety evaluation of a new setup for transcranial electric stimulation during magnetic resonance imaging

BACKGROUND

Transcranial electric stimulation during MR imaging can introduce safety issues due to coupling of the RF field with the stimulation electrodes and leads.

OBJECTIVE

To optimize the stimulation setup for MR current density imaging (MRCDI) and increase maximum stimulation current, a new low-conductivity (σ = 29.4 S/m) lead wire is designed and tested.

METHOD

The antenna effect was simulated to investigate the effect of lead conductivity. Subsequently, specific absorption rate (SAR) simulations for realistic lead configurations with low-conductivity leads and two electrode types were performed at 128 MHz and 298 MHz being the Larmor frequencies of protons at 3T and 7T. Temperature measurements were performed during MRI using high power deposition sequences to ensure that the electrodes comply with MRI temperature regulations.

RESULTS

The antenna effect was found for copper leads at ¼ RF wavelength and could be reliably eliminated using low-conductivity leads. Realistic lead configurations increased the head SAR and the local head SAR at the electrodes only minimally. The highest temperatures were measured on the rings of center-surround electrodes, while circular electrodes showed little heating. No temperature increase above the safety limit of 39 °C was observed.

CONCLUSION

Coupling to the RF field can be reliably prevented by low-conductivity leads, enabling cable paths optimal for MRCDI. Compared to commercial copper leads with safety resistors, the low-conductivity leads had lower total impedance, enabling the application of higher currents without changing stimulator design. Attention must be paid to electrode pads.

High-definition transcranial direct current stimulation of the lateral occipital cortex influences figure-ground perception

Prior work has shown that the lateral occipital cortex (LO) is involved in recognition of objects and their parts, as well as segregation of that object (or "figure") from its background. No studies, though, have examined how LO's functioning is influenced by non-invasive brain stimulation, particularly during a figure-ground perception task. The present study tested whether high-definition transcranial direct current stimulation (HD-tDCS) to right LO influences the effects of familiarity on figure-ground perception. Following 20 min of offline anodal stimulation (or sham), participants viewed masked stimuli consisting of two regions separated by a vertical border and were asked to report which region they perceived as figure. One region was the "critical" region, which either depicted a portion of a familiar object ("Familiar" stimuli), or a familiar object with its parts rearranged into a novel configuration ("Part-rearranged" stimuli). Previous research using these stimuli has found higher reports of the critical region as figure for Familiar vs. Part-rearranged displays, demonstrating the effect of familiarity on figure assignment. The results of the current study showed that HD-tDCS to right LO significantly influenced this typical behavioral pattern. Specifically, stimulation (vs. sham) increased reports of the critical region as figure for Part-rearranged stimuli, bringing perception of these displays up to the level of the Familiar stimuli. We interpret this finding as evidence that stimulation of right LO increased participants' reliance on the familiarity of the parts in their figure-ground judgements-a finding consistent with and extending previous research showing that LO is indeed sensitive to object parts. This is the first study showing that HD-tDCS to LO can influence the effects of familiarity on figure-ground perception.

High definition transcranial direct current stimulation (HD-tDCS): A systematic review on the treatment of neuropsychiatric disorders

HD-tDCS (High-definition transcranial direct current stimulation) is a novel non-invasive brain stimulation (NIBS) technique based on the principle that when weak intensity electric currents are targeted on specific areas of the scalp, they cause underlying cortical stimulation. HD-tDCS shares its technical methodology with conventional tDCS (montage comprising of one anode and one cathode) except for a few modifications that are believed to have focal and longer-lasting neuromodulation effects. Although HD-tDCS is a recently available NIBS technique, impactful studies, case reports, and few controlled trials have been conducted in this context, facilitating an understanding of its neurobiological effects and the clinical translation of the same in health care set-up. The current article narratively reviews the mechanism of action of HD-tDCS, and it systematically examines the cognitive, clinical, and neurobiological effects of HD-tDCS in healthy volunteers as well as patients with neuropsychiatric conditions. Thus, this review attempts to explore the role of HD-tDCS in present-day practice and the future in the context of various neurological and psychiatric disorders.

Improving Cross-cultural "Mind-reading" with Electrical Brain Stimulation

A cross-cultural disadvantage exists when inferring the mental state of others, which may be detrimental for individuals acting in an increasingly globalized world. The dorsomedial prefrontal cortex (dmPFC) is a key hub of the social brain involved in ToM. We explored whether facilitation of dmPFC function by focal high-definition tDCS can improve cross-cultural mind-reading. 52 (26 F/M) Singaporeans performed the Caucasian version of the Reading the Mind in the Eyes Test (RMET) and received HD-tDCS to either the dmPFC or a control site (right temporoparietal junction, rTPJ) in sham-controlled, double-blinded, crossover studies. Contact with Caucasians was determined for the Singaporean cohort as a potential mediator of RMET performance and HD-tDCS response. 52 Caucasians completed the RMET during sham-tDCS and served as a comparison group. A cross-cultural disadvantage on the RMET was confirmed in the Singaporean cohort and this disadvantage was more pronounced in those participants who had less contact with Caucasians. Importantly, HD-tDCS to the dmPFC improved RMET performance in those with less contact. No effect was identified for rTPJ HD-tDCS or for the age/sex control task demonstrating task and site specificity of the stimulation effects. Electrical stimulation of the dmPFC selectively improves the rate of cross-cultural ToM inference from facial cues, effectively removing cross-cultural disadvantage that was found in individuals with lower cross-cultural exposure.

A causal role for the right angular gyrus in self-location mediated perspective taking

Recent theories suggest that self-consciousness, in its most elementary form, is functionally disconnected from the phenomenal body. Patients with psychosis frequently misattribute their thoughts and actions to external sources; and in certain out-of-body experiences, lucid states, and dreams body-ownership is absent but self-identification is preserved. To explain these unusual experiences, we hypothesized that self-identification depends on inferring self-location at the right angular gyrus (i.e., perspective-taking). This process relates to the discrimination of self-produced signals (endogenous attention) from environmental stimulation (exogenous attention). Therefore, when this mechanism fails, this causes altered sensations and perceptions. We combined a Full-body Illusion paradigm with brain stimulation (HD-tDCS) and found a clear causal association between right angular gyrus activation and alterations in self-location (perspective-taking). Anodal versus sham HD-tDCS resulted in: a more profound out-of-body shift (with reduced sense of agency); and a weakened ability to discriminate self from other perspectives. We conclude that self-identification is mediated in the brain by inferring self-location (i.e., perspective-taking). Self-identification can be decoupled from the bodily self, explaining phenomena associated with disembodiment. These findings present novel insights into the relationship between mind and body, and may offer important future directions for treating psychosis symptoms and rehabilitation programs to aid in the recovery from a nervous system injury. The brain's ability to locate itself might be the key mechanism for self-identification and distinguishing self from other signals (i.e., perspective-taking).

Effects of Electrode Configurations and Injected Current Intensity on the Electrical Field of Transcranial Direct Current Stimulation: A Simulation Study

Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation technology that modulates the excitability of the brain by delivering weak electric currents to the brain via scalp electrodes. Electrode configuration and injected current intensity are two important parameters in the tDCS design. This simulation study examined three commercially available electrode configurations, i.e. conventional low definition rectangular pad, high-definition Disc, and high-definition 4 x 1 with different electrode distances and different injected current intensity. Simulation results show that increasing the injected current intensity of HD-tDCS mainly increases the electrical field strength for all configurations. Both Disc and 4 x 1 high definition tDCS (HD-tDCS) have better focality than the conventional low-definition rectangular pad. Increasing the inter-electrode distance in HD-tDCS enlarges the electrical field strength and the depth of stimulation but reduces the focality. In motor rehabilitation, a trade-off needs to be made in the tDCS design to allow the electrical field reaching the white matter to facilitate the usage of the cortico-spinal tract without influencing other undesirable regions in the brain.

Modulation of Adaptive Cognitive Control by Prefrontal High-Definition Transcranial Direct Current Stimulation in Older Adults

OBJECTIVE

Adaptive cognitive control frequently declines in advanced age. Because high-definition transcranial direct current stimulation (HD-tDCS) of the right dorsolateral prefrontal cortex (DLPFC) improved cognitive control in young adults, we investigated if this montage can also improve cognitive control in older individuals.

METHOD

In a double-blind, sham HD-tDCS controlled, cross-over design, 36 older participants received right DLPFC HD-tDCS during a visual flanker task. Conflict adaptation (CA) effects on response time (RT) and error rates (ER) assessed adaptive cognitive control. Biophysical modeling assessed the magnitude and distribution of induced current in older adults.

RESULTS

Active HD-tDCS enhanced CA in older adults. However, this positive behavioral effect was limited to CA in ER. Similar to results obtained in healthy young adults, current modeling analysis demonstrated focal current delivery to the DLPFC with sufficient magnitude of the induced current to modulate neural function in older adults.

DISCUSSION

This study confirms the effectiveness of HD-tDCS to modulate adaptive cognitive control in advanced age.

Multisession transcranial direct current stimulation facilitates verbal learning and memory consolidation in young and older adults

This study investigated effects of multisession transcranial direct-current stimulation on learning and maintenance of novel memory content and scrutinised effects of baseline cognitive status and the role of multi-session tDCS on overnight memory consolidation. In a prospective, randomized, double-blind, parallel-group, sham-tDCS controlled design, 101 healthy young and older adults completed a five-day verbal associative learning paradigm while receiving multisession tDCS to the task-relevant left prefrontal cortex. In older adults, active multisession tDCS enhanced recall performance after each daily training session. Effects were maintained the next morning and during follow-up assessments (one week; three months). In young adults, multisession tDCS significantly increased long-term recall. Unlike previous findings in the motor domain, beneficial effects of multisession tDCS on cognitive learning and memory were notexclusively due to enhanced memory consolidation. Positive stimulation effects were primarily found in participants with lower baseline learning ability, suggesting that multisession tDCS may counteract memory impairment in health and disease.

Modeling radio-frequency energy-induced heating due to the presence of transcranial electric stimulation setup at 3T

PURPOSE

The purpose of the present study was to develop a numerical workflow for simulating temperature increase in a high-resolution human head and torso model positioned in a whole-body magnetic resonance imaging (MRI) radio-frequency (RF) coil in the presence of a transcranial electric stimulation (tES) setup.

METHODS

A customized human head and torso model was developed from medical image data. Power deposition and temperature rise (ΔT) were evaluated with the model positioned in a whole-body birdcage RF coil in the presence of a tES setup. Multiphysics modeling at 3T (123.2 MHz) on unstructured meshes was based on RF circuit, 3D electromagnetic, and thermal co-simulations. ΔT was obtained for (1) a set of electrical and thermal properties assigned to the scalp region, (2) a set of electrical properties of the gel used to ensure proper electrical contact between the tES electrodes and the scalp, (3) a set of electrical conductivity values of skin tissue, (4) four gel patch shapes, and (5) three electrode shapes.

RESULTS

Significant dependence of power deposition and ΔT on the skin's electrical properties and electrode and gel patch geometries was observed. Differences in maximum ΔT (> 100%) and its location were observed when comparing the results from a model using realistic human tissue properties and one with an external container made of acrylic material. The electrical and thermal properties of the phantom container material also significantly (> 250%) impacted the ΔT results.

CONCLUSION

Simulation results predicted that the electrode and gel geometries, skin electrical conductivity, and position of the temperature sensors have a significant impact on the estimated temperature rise. Therefore, these factors must be considered for reliable assessment of ΔT in subjects undergoing an MRI examination in the presence of a tES setup.

Methodology for tDCS integration with fMRI

Understanding and reducing variability of response to transcranial direct current stimulation (tDCS) requires measuring what factors predetermine sensitivity to tDCS and tracking individual response to tDCS. Human trials, animal models, and computational models suggest structural traits and functional states of neural systems are the major sources of this variance. There are 118 published tDCS studies (up to October 1, 2018) that used fMRI as a proxy measure of neural activation to answer mechanistic, predictive, and localization questions about how brain activity is modulated by tDCS. FMRI can potentially contribute as: a measure of cognitive state-level variance in baseline brain activation before tDCS; inform the design of stimulation montages that aim to target functional networks during specific tasks; and act as an outcome measure of functional response to tDCS. In this systematic review, we explore methodological parameter space of tDCS integration with fMRI spanning: (a) fMRI timing relative to tDCS (pre, post, concurrent); (b) study design (parallel, crossover); (c) control condition (sham, active control); (d) number of tDCS sessions; (e) number of follow up scans; (f) stimulation dose and combination with task; (g) functional imaging sequence (BOLD, ASL, resting); and (h) additional behavioral (cognitive, clinical) or quantitative (neurophysiological, biomarker) measurements. Existing tDCS-fMRI literature shows little replication across these permutations; few studies used comparable study designs. Here, we use a representative sample study with both task and resting state fMRI before and after tDCS in a crossover design to discuss methodological confounds. We further outline how computational models of current flow should be combined with imaging data to understand sources of variability. Through the representative sample study, we demonstrate how modeling and imaging methodology can be integrated for individualized analysis. Finally, we discuss the importance of conducting tDCS-fMRI with stimulation equipment certified as safe to use inside the MR scanner, and of correcting for image artifacts caused by tDCS. tDCS-fMRI can address important questions on the functional mechanisms of tDCS action (e.g., target engagement) and has the potential to support enhancement of behavioral interventions, provided studies are designed rationally.

The Right Temporoparietal Junction Is Causally Associated with Embodied Perspective-taking

A prominent theory claims that the right temporoparietal junction (rTPJ) is especially associated with embodied processes relevant to perspective-taking. In the present study, we use high-definition transcranial direct current stimulation to provide evidence that the rTPJ is causally associated with the embodied processes underpinning perspective-taking. Eighty-eight young human adults were stratified to receive either rTPJ or dorsomedial PFC anodal high-definition transcranial direct current stimulation in a sham-controlled, double-blind, repeated-measures design. Perspective-tracking (line-of-sight) and perspective-taking (embodied rotation) were assessed using a visuo-spatial perspective-taking task that required understanding what another person could see or how they see it, respectively. Embodied processing was manipulated by positioning the participant in a manner congruent or incongruent with the orientation of an avatar on the screen. As perspective-taking, but not perspective-tracking, is influenced by bodily position, this allows the investigation of the specific causal role for the rTPJ in embodied processing. Crucially, anodal stimulation to the rTPJ increased the effect of bodily position during perspective-taking, whereas no such effects were identified during perspective-tracking, thereby providing evidence for a causal role for the rTPJ in the embodied component of perspective-taking. Stimulation to the dorsomedial PFC had no effect on perspective-tracking or taking. Therefore, the present study provides support for theories postulating that the rTPJ is causally involved in embodied cognitive processing relevant to social functioning.SIGNIFICANCE STATEMENT The ability to understand another's perspective is a fundamental component of social functioning. Adopting another perspective is thought to involve both embodied and nonembodied processes. The present study used high-definition transcranial direct current stimulation (HD-tDCS) and provided causal evidence that the right temporoparietal junction is involved specifically in the embodied component of perspective-taking. Specifically, HD-tDCS to the right temporoparietal junction, but not another hub of the social brain (dorsomedial PFC), increased the effect of body position during perspective-taking, but not tracking. This is the first causal evidence that HD-tDCS can modulate social embodied processing in a site-specific and task-specific manner.

Differences in high-definition transcranial direct current stimulation over the motor hotspot versus the premotor cortex on motor network excitability

The effectiveness of transcranial direct current stimulation (tDCS) placed over the motor hotspot (thought to represent the primary motor cortex (M1)) to modulate motor network excitability is highly variable. The premotor cortex-particularly the dorsal premotor cortex (PMd)-may be a promising alternative target to reliably modulate motor excitability, as it influences motor control across multiple pathways, one independent of M1 and one with direct connections to M1. This double-blind, placebo-controlled preliminary study aimed to differentially excite motor and premotor regions using high-definition tDCS (HD-tDCS) with concurrent functional magnetic resonance imaging (fMRI). HD-tDCS applied over either the motor hotspot or the premotor cortex demonstrated high inter-individual variability in changes on cortical motor excitability. However, HD-tDCS over the premotor cortex led to a higher number of responders and greater changes in local fMRI-based complexity than HD-tDCS over the motor hotspot. Furthermore, an analysis of individual motor hotspot anatomical locations revealed that, in more than half of the participants, the motor hotspot is not located over anatomical M1 boundaries, despite using a canonical definition of the motor hotspot. This heterogeneity in stimulation site may contribute to the variability of tDCS results. Altogether, these preliminary findings provide new considerations to enhance tDCS reliability.

Transcranial direct current stimulation of default mode network parietal nodes decreases negative mind-wandering about the past

Mind-wandering is a cognitive process in which people spontaneously have thoughts that are unrelated to their current activities. The types of mind-wandering thoughts that people have when affected by a negative mood resemble thoughts associated with mood disorders (e.g., negative thoughts about the past). Transcranial direct current stimulation (tDCS) is a form of noninvasive brain stimulation that can modulate cognition and affect in healthy and clinical populations. Ninety participants received either excitatory, inhibitory, or sham tDCS to bilateral inferior parietal lobe (IPL) nodes of the default mode network (DMN) to assess changes in maladaptive mind-wandering following criticism. tDCS did not change mind-wandering frequency after hearing criticism, but it did change what people mind-wandered about. Specifically, cathodal stimulation decreased the frequency of negative mind-wandering thoughts about the past. Future studies could investigate tDCS of DMN regions as an intervention for patients with mood disorders who suffer from negative, past-oriented cognitions.

Common and unique effects of HD-tDCS to the social brain across cultural groups

Cultural background influences social cognition, however no study has examined brain stimulation differences attributable to cultural background. 104 young adults [52 South-East Asian Singaporeans (SEA); 52 Caucasian Australians (CA)] received anodal high-definition transcranial direct current stimulation (HD-tDCS) to the dorsomedial prefrontal cortex (dmPFC) or the right temporoparietal junction (rTPJ). Participants completed tasks with varying demands on self-other processing including visual perspective taking (VPT)and episodic memory with self and other encoding. At baseline, SEA showed greater self-other integration than CA in the level one (line-of-sight) VPT task as indexed by greater interference from the alternate perspective. Anodal HD-tDCS to the dmPFC resulted in the CA performing closer to the SEA during egocentric perspective judgements. Baseline performance on level two (embodied rotation) VPT task and the self-reference effect (SRE) in episodic memory was comparable between the two groups. In the combined sample, HD-tDCS to the rTPJ decreased the interference from the egocentric perspective during level two VPT and dmPFC HD-tDCS removed the SRE in episodic memory. Stimulation effects were comparable when baseline performance was comparable. When baseline performance differed, stimulation differences were identified. Therefore, social cognitive differences due to cultural background are an important consideration in social brain stimulation studies.

High-Definition Transcranial Direct Current Stimulation Improves Verb Recovery in Aphasic Patients Depending on Current Intensity

High-definition transcranial direct current stimulation (HD-tDCS) is a variant of tDCS, which produces more focal stimulation, delimiting brain current flow to a defined region compared to conventional tDCS. To date, only one study has been conducted to investigate HD-tDCS effects on language recovery in aphasia. Here, we aimed to assess the effects of cathodal HD-tDCS on verb naming by comparing two current intensities: 1 vs 2 mA. In a double-blinded cross over study, two groups of 10 aphasic individuals were submitted to active cathodal HD-tDCS and sham stimulation over the right homolog of Broca's area, while performing a verb naming task. Indeed, we reasoned that, by applying inhibitory current over the right Broca's area, we would decrease the inhibitory impact from the right hemisphere to the left perilesional cortex, thus boosting language recovery. The groups differed in the intensity of the active stimulation (1 mA or 2 mA). In both groups, each condition was carried out in five consecutive daily sessions with one week of interval between the two experimental conditions. A significant improvement in verb naming was found only after cathodal HD-tDCS at 2 mA, which endured one week after the end of treatment. The improvement was not observed on the group receiving cathodal HD-tDCS at 1 mA. Our findings showed that HD-tDCS applied to the right intact hemisphere are efficacious for language recovery. These results indicate that HD-tDCS represents a promising new technique for language rehabilitation. However, systematic determination of stimulation intensity appears to be crucial for obtaining relevant effects.

Long-Term Transcranial Direct Current Stimulation Does Not Improve Executive Function in Healthy Older Adults

Background: Executive function tends to decline as people age. Transcranial direct current stimulation (tDCS) is assumed to have beneficial effects on various cognitive functions. Some prior investigations have shown that repeated sessions of tDCS enhance the executive function performance of healthy elderly people by mediating cognitive training gains. However, studies of the effect of long-term stimulation on executive function without cognitive training are absent. Objective: The purpose of this study was to explore whether the executive function of healthy older adults could be enhanced with long-term tDCS alone applied on the prefrontal cortex. Methods: Sixty-five cognitively normal older adults were enrolled and randomly assigned to two groups: an anodal tDCS group and a sham tDCS group. The participants in the two groups received anodal stimulation or sham stimulation over the left dorsolateral prefrontal lobe, for 30 min per day for 10 consecutive days. Executive function was tested before stimulation, immediately after stimulation and 3 months after stimulation. Three core components of executive function were tested using a two-back task for updating, a flanker task for inhibition, and a switching task for shifting. Results: Across the three tasks, we failed to discover any differences between the anodal and sham stimulation. Moreover, we found no statistically significant stimulation effect in the follow-up session. Conclusion: Our study does not support the assumption that multiple sessions of tDCS that are independent of cognitive training have a beneficial effect on executive function in healthy older adults, presumably because the effect of the stimulation lies in its amplification of training gains. It indicates that combining traditional cognitive training methods with brain stimulation may be a better approach to improve older adults' executive function.

Dissociable Roles Within the Social Brain for Self–Other Processing: A HD-tDCS Study

Theories of right temporoparietal junction (rTPJ) function in social cognition include self–other distinction, self-inhibition, or embodied rotation, whereas the dorsomedial prefrontal cortex (dmPFC) is associated with integrating social information. However, no study has provided causal evidence for dissociable roles of the rTPJ and dmPFC in social cognition. A total of 52 healthy young adults were stratified to receive either dmPFC or rTPJ anodal high-definition transcranial direct current stimulation (HD-tDCS) in a sham-controlled, double-blinded, repeated measures design. Self–other processing was assessed across implicit and explicit level 1 (line-of-sight) and level 2 (mental rotation) visual perspective taking (VPT) tasks, and self–other effects on memory. DmPFC stimulation selectively increased the influence of the allocentric perspective during egocentric perspective taking, indexed by an increase in congruency effect across explicit VPT tasks. Moreover, dmPFC stimulation removed the self-reference effect in episodic memory by increasing the recognition of other and decreasing the recognition of self-encoded words. Stimulation of the rTPJ resulted in improved inhibition of the egocentric-perspective during level 2 VPT only, indexed by a reduction of the congruency effect when taking the allocentric perspective. This research supports theories suggesting that the rTPJ facilitates embodied mental rotation of the self into an alternate perspective, whereas the dmPFC integrates social information relevant to self-directed processes.

Tolerability and blinding of 4x1 high-definition transcranial direct current stimulation (HD-tDCS) at two and three milliamps

BACKGROUND

Transcranial direct current stimulation (tDCS) is an in-demand form of neuromodulation generally regarded as safe and well tolerated. However, few studies have examined the safety, tolerability, or blinding of High Definition (HD-) tDCS, especially in older adults and at stimulation intensities of 2 milliamps (mA) or greater.

OBJECTIVE

We examined the rates of serious adverse events and common side effects to establish safety and tolerability, respectively, in HD-tDCS. Blinding was evaluated using participants' accuracy in correctly stating their condition (i.e., active or sham).

METHODS

The sample included 101 older adults (M_age = 69.69, SD = 8.33; M_educ = 16.27, SD = 2.42) who participated in our double blind randomized controlled studies or in case studies that used HD-tDCS for 20-30 min at 2 mA (n = 66, 31 active) or 3 mA (n = 35, 20 active). Participants completed a standardized side effect questionnaire and were asked whether they received active or sham stimulation at the end of each session.

RESULTS

There were no serious adverse events and no participants withdrew, suggesting that HD-tDCS meets basic safety parameters. Tolerability was comparable between active and sham HD-tDCS regardless of intensity (2 mA and 3 mA) in first session (allp > .09). Tingling was the most commonly endorsed item (59% active; 56% sham) followed by burning sensation (51% active; 50% sham), the majority of which were mild in nature. "Severe" ratings were reported in fewer than 4% of sessions. Blinding appeared adequate since there were no significant group differences between individuals correctly stating their stimulation condition (χ2 = 0.689, p = .679). The above tolerability and blinding findings generally persisted when multiple session data (i.e., 186 total sessions) were considered.

CONCLUSIONS

HD-tDCS appears well-tolerated and safe with effective sham-control in older adults, even at 3 mA. These data support the use of HD-tDCS in randomized controlled trials and clinical translation efforts.

Causal evidence for task-specific involvement of the dorsomedial prefrontal cortex in human social cognition

The dorsomedial prefrontal cortex (dmPFC) is a key hub of the 'social brain', but little is known about specific processes supported by this region. Using focal high-definition transcranial direct current stimulation (HD-tDCS) and a social cognitive battery with differing demands on self-other processing, we demonstrate specific involvement of the dmPFC in tasks placing high demands on self-other processing. Specifically, excitatory (anodal) HD-tDCS enhanced the integration of external information into the self for explicit higher-order socio-cognitive tasks across cognitive domains; i.e. visual perspective taking (VPT) and episodic memory. These effects were task specific, as no stimulation effects were found for attributing mental states from the eyes or implicit VPT. Inhibitory (cathodal) HD-tDCS had weaker effects in the opposite direction towards reduced integration of external information into the self. We thus demonstrate for the first time a specific and causal role of the dmPFC in integrating higher-order information from others/external source into that of the self across cognitive domains.

Recent Trends in the Use of Electrical Neuromodulation in Parkinson's Disease

PURPOSE OF REVIEW

This review aims to survey recent trends in electrical forms of neuromodulation, with a specific application to Parkinson's disease (PD). Emerging trends are identified, highlighting synergies in state-of-the-art neuromodulation strategies, with directions for future improvements in stimulation efficacy suggested.

RECENT FINDINGS

Deep brain stimulation remains the most common and effective form of electrical stimulation for the treatment of PD. Evidence suggests that transcranial direct current stimulation (tDCS) most likely impacts the motor symptoms of the disease, with the most prominent results relating to rehabilitation. However, utility is limited due to its weak effects and high variability, with medication state a key confound for efficacy level. Recent innovations in transcranial alternating current stimulation (tACS) offer new areas for investigation.

SUMMARY

Our understanding of the mechanistic foundations of electrical current stimulation is advancing and as it does so, trends emerge which steer future clinical trials towards greater efficacy.

Incomplete evidence that increasing current intensity of tDCS boosts outcomes

BACKGROUND

Transcranial direct current stimulation (tDCS) is investigated to modulate neuronal function by applying a fixed low-intensity direct current to scalp.

OBJECTIVES

We critically discuss evidence for a monotonic response in effect size with increasing current intensity, with a specific focus on a question if increasing applied current enhance the efficacy of tDCS.

METHODS

We analyzed tDCS intensity does-response from different perspectives including biophysical modeling, animal modeling, human neurophysiology, neuroimaging and behavioral/clinical measures. Further, we discuss approaches to design dose-response trials.

RESULTS

Physical models predict electric field in the brain increases with applied tDCS intensity. Data from animal studies are lacking since a range of relevant low-intensities is rarely tested. Results from imaging studies are ambiguous while human neurophysiology, including using transcranial magnetic stimulation (TMS) as a probe, suggests a complex state-dependent non-monotonic dose response. The diffusivity of brain current flow produced by conventional tDCS montages complicates this analysis, with relatively few studies on focal High Definition (HD)-tDCS. In behavioral and clinical trials, only a limited range of intensities (1-2 mA), and typically just one intensity, are conventionally tested; moreover, outcomes are subject brain-state dependent. Measurements and models of current flow show that for the same applied current, substantial differences in brain current occur across individuals. Trials are thus subject to inter-individual differences that complicate consideration of population-level dose response.

CONCLUSION

The presence or absence of simple dose response does not impact how efficacious a given tDCS dose is for a given indication. Understanding dose-response in human applications of tDCS is needed for protocol optimization including individualized dose to reduce outcome variability, which requires intelligent design of dose-response studies.

High-definition tDCS of the temporo-parietal cortex enhances access to newly learned words

Learning associations between words and their referents is crucial for language learning in the developing and adult brain and for language re-learning after neurological injury. Non-invasive transcranial direct current stimulation (tDCS) to the posterior temporo-parietal cortex has been suggested to enhance this process. However, previous studies employed standard tDCS set-ups that induce diffuse current flow in the brain, preventing the attribution of stimulation effects to the target region. This study employed high-definition tDCS (HD-tDCS) that allowed the current flow to be constrained to the temporo-parietal cortex, to clarify its role in novel word learning. In a sham-controlled, double-blind, between-subjects design, 50 healthy adults learned associations between legal non-words and unfamiliar object pictures. Participants were stratified by baseline learning ability on a short version of the learning paradigm and pairwise randomized to active (20 mins; N = 25) or sham (40 seconds; N = 25) HD-tDCS. Accuracy was comparable during the baseline and experimental phases in both HD-tDCS conditions. However, active HD-tDCS resulted in faster retrieval of correct word-picture pairs. Our findings corroborate the critical role of the temporo-parietal cortex in novel word learning, which has implications for current theories of language acquisition.

Low intensity transcranial electric stimulation: Safety, ethical, legal regulatory and application guidelines

Low intensity transcranial electrical stimulation (TES) in humans, encompassing transcranial direct current (tDCS), transcutaneous spinal Direct Current Stimulation (tsDCS), transcranial alternating current (tACS), and transcranial random noise (tRNS) stimulation or their combinations, appears to be safe. No serious adverse events (SAEs) have been reported so far in over 18,000 sessions administered to healthy subjects, neurological and psychiatric patients, as summarized here. Moderate adverse events (AEs), as defined by the necessity to intervene, are rare, and include skin burns with tDCS due to suboptimal electrode-skin contact. Very rarely mania or hypomania was induced in patients with depression (11 documented cases), yet a causal relationship is difficult to prove because of the low incidence rate and limited numbers of subjects in controlled trials. Mild AEs (MAEs) include headache and fatigue following stimulation as well as prickling and burning sensations occurring during tDCS at peak-to-baseline intensities of 1-2mA and during tACS at higher peak-to-peak intensities above 2mA. The prevalence of published AEs is different in studies specifically assessing AEs vs. those not assessing them, being higher in the former. AEs are frequently reported by individuals receiving placebo stimulation. The profile of AEs in terms of frequency, magnitude and type is comparable in healthy and clinical populations, and this is also the case for more vulnerable populations, such as children, elderly persons, or pregnant women. Combined interventions (e.g., co-application of drugs, electrophysiological measurements, neuroimaging) were not associated with further safety issues. Safety is established for low-intensity 'conventional' TES defined as <4mA, up to 60min duration per day. Animal studies and modeling evidence indicate that brain injury could occur at predicted current densities in the brain of 6.3-13A/m2 that are over an order of magnitude above those produced by tDCS in humans. Using AC stimulation fewer AEs were reported compared to DC. In specific paradigms with amplitudes of up to 10mA, frequencies in the kHz range appear to be safe. In this paper we provide structured interviews and recommend their use in future controlled studies, in particular when trying to extend the parameters applied. We also discuss recent regulatory issues, reporting practices and ethical issues. These recommendations achieved consensus in a meeting, which took place in Göttingen, Germany, on September 6-7, 2016 and were refined thereafter by email correspondence.

Safety parameter considerations of anodal transcranial Direct Current Stimulation in rats

A commonly referenced transcranial Direct Current Stimulation (tDCS) safety threshold derives from tDCS lesion studies in the rat and relies on electrode current density (and related electrode charge density) to support clinical guidelines. Concerns about the role of polarity (e.g. anodal tDCS), sub-lesion threshold injury (e.g. neuroinflammatory processes), and role of electrode montage across rodent and human studies support further investigation into animal models of tDCS safety. Thirty-two anesthetized rats received anodal tDCS between 0 and 5mA for 60min through one of three epicranial electrode montages. Tissue damage was evaluated using hemotoxylin and eosin (H&E) staining, Iba-1 immunohistochemistry, and computational brain current density modeling. Brain lesion occurred after anodal tDCS at and above 0.5mA using a 25.0mm² electrode (electrode current density: 20.0A/m²). Lesion initially occurred using smaller 10.6mm² or 5.3mm² electrodes at 0.25mA (23.5A/m²) and 0.5mA (94.2A/m²), respectively. Histological damage was correlated with computational brain current density predictions. Changes in microglial phenotype occurred in higher stimulation groups. Lesions were observed using anodal tDCS at an electrode current density of 20.0A/m², which is below the previously reported safety threshold of 142.9A/m² using cathodal tDCS. The lesion area is not simply predicted by electrode current density (and so not by charge density as duration was fixed); rather computational modeling suggests average brain current density as a better predictor for anodal tDCS. Nonetheless, under the assumption that rodent epicranial stimulation is a hypersensitive model, an electrode current density of 20.0A/m² represents a conservative threshold for clinical tDCS, which typically uses an electrode current density of 2A/m² when electrodes are placed on the skin (resulting in a lower brain current density).

Stimulation of Dorsolateral Prefrontal Cortex Enhances Adaptive Cognitive Control: A High-Definition Transcranial Direct Current Stimulation Study

Conflict adaptation is a hallmark effect of adaptive cognitive control and refers to the adjustment of control to the level of previously experienced conflict. Conflict monitoring theory assumes that the dorsolateral prefrontal cortex (DLPFC) is causally involved in this adjustment. However, to date, evidence in humans is predominantly correlational, and heterogeneous with respect to the lateralization of control in the DLPFC. We used high-definition transcranial direct current stimulation (HD-tDCS), which allows for more focal current delivery than conventional tDCS, to clarify the causal involvement of the DLPFC in conflict adaptation. Specifically, we investigated the regional specificity and lateralization of potential beneficial stimulation effects on conflict adaptation during a visual flanker task. One hundred twenty healthy participants were assigned to four HD-tDCS conditions: left or right DLPFC or left or right primary motor cortex (M1). Each group underwent both active and sham HD-tDCS in crossover, double-blind designs. We obtained a sizeable conflict adaptation effect (measured as the modulation of the flanker effect as a function of previous response conflict) in all groups and conditions. However, this effect was larger under active HD-tDCS than under sham stimulation in both DLPFC groups. In contrast, active stimulation had no effect on conflict adaptation in the M1 groups. In sum, the present results indicate that the DLPFC plays a causal role in adaptive cognitive control, but that the involvement of DLPFC in control is not restricted to the left or right hemisphere. Moreover, our study confirms the potential of HD-tDCS to modulate cognition in a regionally specific manner.

SIGNIFICANCE STATEMENT

Conflict adaptation is a hallmark effect of adaptive cognitive control. While animal studies have suggested causal involvement of the DLPFC in this phenomenon, such evidence is currently lacking in humans. The present study used high-definition transcranial direct current stimulation (HD-tDCS) to demonstrate that the DLPFC is causally involved in conflict adaptation in humans. Our study confirms a central claim of conflict monitoring theory, which up to now has predominantly relied on correlational studies. Our results further indicate an equal involvement of the left and right DLPFC in adaptive control, whereas stimulation of a control region-the primary motor cortex-had no effect on adaptive control. The study thus confirms the potential of HD-tDCS to modulate cognition in a regionally specific manner.