Sex Mediates the Effects of High-Definition Transcranial Direct Current Stimulation on “Mind-Reading”

Effects of tDCS on emotion recognition and brain oscillations

INTRODUCTION

Emotion recognition, the ability to interpret the emotional state of individuals by looking at their facial expressions, is essential for healthy social interactions and communication. There is limited research on the effects of tDCS on emotion recognition in the literature. This study aimed to investigate the effects of anodal stimulation of the ventromedial prefrontal cortex (vmPFC), a key region for emotion recognition from facial expressions, on emotion recognition and brain oscillations.

METHOD

A single-blind randomized-controlled study was conducted with 54 healthy participants. Before and after brain stimulation emotion recognition tasks were administered and resting-state EEG were recorded. The changes in task performances and brain oscillations were analyzed using repeated-measures two-way ANOVA analysis.

RESULTS

There was no significant difference in the emotion recognition tasks between groups in pre-post measurements. The changes in delta, theta, alpha, beta and gamma frequency bands in the frontal, temporal, and posterio-occipital regions, which were determined as regions of interest in resting state EEG data before and after tDCS, were compared between groups. The results showed that there was a significant difference between groups only in delta frequency before and after tDCS in the frontal and temporal regions. While an increase in delta activity was observed in the experimental group in the frontal and temporal regions, a decrease was observed in the control group.

CONCLUSIONS

The tDCS may not have improved emotion recognition because it may not have had the desired effect on the vmPFC, which is in the lower part of the prefrontal lobe. The changes in EEG frequencies observed section tDCS may be similar to those seen in some pathological processes, which could explain the lack of improvement in emotion recognition. Future studies to be carried out for better understand this effect are important.

Multi-polygenic prediction of frailty highlights chronic pain and educational attainment as key risk and protective factors

Frailty is a complex trait. Twin studies and recent Genome-Wide Association Studies have demonstrated a strong genetic basis of frailty but there remains a lack of genetic studies exploring genetic prediction of Frailty. Previous work has shown that a single polygenic predictor - represented by a Frailty polygenic score - predicts Frailty, measured via the frailty index, in independent samples within the United Kingdom. We extended this work, using a multi-polygenic score (MPS) approach to increase predictive power. Predictor variables - twenty-six polygenic scores (PGS) were modelled in regularised Elastic net regression models, with repeated cross-validation, to estimate joint prediction of the polygenic scores and order the predictions by their contributing strength to Frailty in two independent cohorts aged 65+ - the English Longitudinal Study of Ageing (ELSA) and Lothian Birth Cohort 1936 (LBC1936). Results showed that the MPS explained 3.6% and 4.7% of variance compared to the best single-score prediction of 2.6% and 2.2% of variance in ELSA and LBC1936 respectively. The strongest polygenic predictors of worsening frailty came from PGS for Chronic pain, Frailty and Waist circumference; whilst PGS for Parental Death, Educational attainment, and Rheumatoid Arthritis were found to be protective to frailty. Results from the predictors remaining in the final model were then validated using the longitudinal LBC1936, with equivalent PGS scores from the same GWAS summary statistics. Thus, this MPS approach provides new evidence for the genetic contributions to frailty in later life and sheds light on the complex structure of the Frailty Index measurement.

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.

The influence of the COMT Val158Met polymorphism on prefrontal TDCS effects on aggression

Increasing dorsolateral prefrontal cortex (DLPFC) activity by anodal transcranial direct current stimulation (tDCS) enhances cognitive control and might reduce aggression. The Val158Met polymorphism within the catechol-O-methyltransferase gene (rs4680) plays a pivotal role in prefrontal dopamine signaling, displaying associations with aggressive behavior, and potentially influencing the effects of tDCS. In a double-blind, sham-controlled study, we investigated the influence of rs4680 on tDCS effects on aggression. While undergoing functional magnetic resonance imaging, 89 healthy male participants performed the Taylor aggression paradigm before and immediately after tDCS. Actively stimulated participants (n = 45) received anodal tDCS (1.5 mA) for 20 min targeting the right DLPFC. Carriers of the val-allele (val+; n = 46; active tDCS n = 23) were compared to met-allele homozygotes (val-; n = 43; active tDCS n = 22). Analysis revealed decreased aggressive behavior in the val- group following active tDCS (p < 0.001). The val+ group showed increased aggression during the second session (p < 0.001) with an even higher increase following active as compared to sham tDCS (p < 0.001). No effects of stimulation or rs4680 on brain activation were found. Our study provides evidence for opposite tDCS effects on aggressive behavior in val-carriers and val-noncarriers. By shedding light on genetic factors predicting tDCS responsivity, the study will help to pave the way toward individualized-and thus more effective-tDCS treatment options.

A Causal Role of Right Temporoparietal Junction in Prosocial Learning: A Transcranial Direct Current Stimulation Study

Prosocial behavior is a common and important aspect of everyday social life. To behave prosocially, we need to learn the consequences of our actions for other people, known as prosocial learning. Previous studies have identified the right temporoparietal junction (rTPJ) as the critical neurological substrate for prosocial behavior. However, little is known about the causal role of the rTPJ in prosocial learning. To clarify the role of the rTPJ in prosocial learning, we used a reinforcement learning paradigm and transcranial direct current stimulation (tDCS). A total of 75 participants were recruited and randomly assigned to the anodal or sham tDCS group. While receiving tDCS stimulation over the rTPJ, participants were instructed to choose between different stimuli that were probabilistically associated with rewards for themselves in the self-learning condition or for another person in the prosocial-learning condition. Participants were able to learn to obtain rewards for themselves or others, and learning performance in the self-learning condition was better than that in the prosocial-learning condition. However, anodal tDCS over the rTPJ significantly improved learning performance in the prosocial-learning condition. These results indicate that the rTPJ plays a causal role in prosocial learning.

The role of prefrontal cortex and temporoparietal junction in interpersonal comfort and emotional approach

Our perception of physical distance to individuals and stimuli is influenced by our mental distance and relatedness. The present study aimed to investigate the role of the dorsolateral prefrontal cortex (dlPFC), ventromedial prefrontal cortex (vmPFC), and right temporoparietal junction (rTPJ) in interpersonal comfortable distance and approach behaviors towards emotional stimuli. Twenty healthy volunteers received brain stimulation in four separate sessions with a one-week interval, including anodal left dlPFC, anodal right vmPFC, anodal rTPJ, and sham condition, with an extracranial return electrode. Our results revealed an increase in interpersonal distance during anodal rTPJ stimulation and a decrease in distance to positive pictures during anodal vmPFC stimulation. These findings suggest that the rTPJ plays a role in the perceptual component of self-other distancing, while the vmPFC is involved in approaching positive emotions.

The Effects of Transcranial Direct Current Stimulation (tDCS) on the Cognitive Functions: A Systematic Review and Meta-analysis

Previous studies have investigated the effect of transcranial direct current stimulation (tDCS) on cognitive functions. However, these studies reported inconsistent results due to differences in experiment design, measurements, and stimulation parameters. Nonetheless, there is a lack of meta-analyses and review studies on tDCS and its impact on cognitive functions, including working memory, inhibition, flexibility, and theory of mind. We performed a systematic review and meta-analysis of tDCS studies published from the earliest available data up to October 2021, including studies reporting the effects of tDCS on cognitive functions in human populations. Therefore, these systematic review and meta-analysis aim to comprehensively analyze the effects of anodal and cathodal tDCS on cognitive functions by investigating 69 articles with a total of 5545 participants. Our study reveals significant anodal tDCS effects on various cognitive functions. Specifically, we observed improvements in working memory reaction time (RT), inhibition RT, flexibility RT, theory of mind RT, working memory accuracy, theory of mind accuracy and flexibility accuracy. Furthermore, our findings demonstrate noteworthy cathodal tDCS effects, enhancing working memory accuracy, inhibition accuracy, flexibility RT, flexibility accuracy, theory of mind RT, and theory of mind accuracy. Notably, regarding the influence of stimulation parameters of tDCS on cognitive functions, the results indicated significant differences across various aspects, including the timing of stimulation (online vs. offline studies), population type (clinical vs. healthy studies), stimulation duration (< 15 min vs. > 15 min), electrical current intensities (1-1.5 m.A vs. > 1.5 m.A), stimulation sites (right frontal vs. left frontal studies), age groups (young vs. older studies), and different cognitive tasks in each cognitive functioning aspect. In conclusion, our results demonstrate that tDCS can effectively enhance cognitive task performance, offering valuable insights into the potential benefits of this method for cognitive improvement.

Sex matters for the enhancement of cognitive training with transcranial direct current stimulation (tDCS)

BACKGROUND

Transcranial direct current stimulation (tDCS) can influence brain network activity and associated cognitive and behavioural functions. In addition to the extensive variety in stimulation parameters, numerous biological factors drive these effects, however these are yet poorly understood. Here, we investigate one of the major biological factors by focusing on sex-dependent effects of tDCS on a challenging cognitive control task (adaptive paced auditory serial addition task [PASAT]) in healthy humans.

METHODS

This sex-specific re-analysis was performed on data of 163 subjects who underwent a 2-week cognitive control training (6 sessions in total). Subjects received either verum (anodal/cathodal) or sham tDCS. Electrodes were placed over the left or right dorsolateral prefrontal cortex and the respective contralateral deltoid muscle. Cognitive control was measured as performance in the PASAT and was analysed in respect to stimulation conditions (sham, anodal, cathodal) and sex.

RESULTS

Regardless of stimulation condition, performance gains between the sexes were higher in females compared to males (p = 0.0038). Female's performance during anodal tDCS exceeded male's (p = 0.0070), yet no effects were found for cathodal or sham tDCS. Moreover, in females we found a superior effect for anodal tDCS over sham stimulation (fanodal: p = 0.0354; fcathodal: p = 0.6181), but no such effect in males (manodal: p = 0.6882; mcathodal: p = 0.4822).

CONCLUSIONS

This study highlights the relevance of biological sex for the effects of tDCS on cognitive training. Thus, an increased attention to biological sex is advisable in future brain stimulation research to highlight and in consequence better understand potentially underlying sex-specific mechanisms. Considering biological sex will further advance customisation and individualisation of tDCS interventions. Trial registration ClinicalTrials.gov, NCT04108663.

Increased neural activity of right temporo-parietal junction causes different effect on altruism in situations of advantageous and disadvantageous inequity

Altruism is defined as the performance of "costly acts that confer economic benefits on other individuals", which is one of the major puzzles in the behavioural sciences today. Altruistic behaviour not only facilitates interpersonal adaptation and harmony but also enhances social welfare and social responsibility. The right temporo-parietal junction (rTPJ) has been proposed as playing a key role in guiding human altruistic behaviour, but its precise functional contribution to altruistic behaviour in situations of advantageous and disadvantageous inequity remains unclear. The purpose of this study was to modulate the activation of the rTPJ through transcranial direct current stimulation (tDCS) in order to clarify the causal role of the rTPJ in altruistic behaviour in situations of advantageous and disadvantageous inequity. A total of 106 participants were randomly assigned to one of three stimulation conditions: anodal tDCS stimulation on the rTPJ; sham tDCS stimulation on the rTPJ and anodal tDCS stimulation on the primary visual cortex (VC)as the control group, and. After 20 min of stimulation, participants undertook a modified dictator game that measured altruistic behaviour. Mixed-effect logistic regressions were applied to statistical analyses in this study. The results indicated that anodal tDCS over the rTPJ increased participants' altruistic tendency by increasing their tendency to choose altruistic options in trials with higher cost, as well as their tendency to behave altruistically in situations of advantageous but not disadvantageous inequity. These results suggested that increased neural activity of the rTPJ leads to different impacts on altruism in these two different inequity situations.

Distinct role of the right temporoparietal junction in advantageous and disadvantageous inequity: A tDCS study

Fairness is a hallmark of humans' ability to maintain cooperative relationships with large numbers of unrelated others. It influences many aspects of daily life, from how people share their resources with partners to how policymakers shape income distribution policy. The right temporoparietal junction (rTPJ) is a hub of the mentalizing network and it has been proposed to play a key role in guiding human reciprocal behavior; however, its precise functional contribution to fair behavior in situations of advantageous and disadvantageous inequity remains unclear. The purpose of this study was to clarify the role of the rTPJ in relation to fair behavior in situations of advantageous and disadvantageous inequity by modulating the activation of the rTPJ through transcranial direct current stimulation (tDCS). Anodal tDCS at 1.5 mA over the primary visual cortex (VC) or rTPJ was performed and participants subsequently played a binary-choice version of the Dictator Game. We found that anodal tDCS over the rTPJ increased the participants' equity choices in the disadvantageous inequity situation but not in the advantageous inequity situation. The tDCS effect is moderated by sex and, in particular, the tDCS effect increases female equity choices. The results suggest that the rTPJ plays a distinct role in inequity aversion in these two types of inequity situations.

Modulating the Activity of the Right Dorsolateral Prefrontal Cortex Alters Altruism in Situations of Advantageous Inequity

Altruism is highly valued and cherished by human society. However, human preferences and behavior are sensitive to inequality considerations. Currently, remarkably little is known about the neurobiological mechanisms underlying the process of altruistic acts in inequity situations. Therefore, to clarify the causal role of the right dorsolateral prefrontal cortex (rDLPFC) in altruism during situations of advantageous and disadvantageous inequity, we applied transcranial direct current stimulation (tDCS) to demonstrate the involvement of the rDLPFC in altruism in situations of inequity. A total of 71 participants (38 female and 33 male) received anodal tDCS at 1.5 mA over the rDLPFC (n = 38) or the primary visual cortex (n = 33) and subsequently participated in a modified dictator game that measures altruism. We found that anodal tDCS over the rDLPFC decreased subjects' sensitivity to altruistic efficiency and cost in situations of advantageous inequity. Our results suggested that the rDLPFC plays an important role in overriding self-interest to enforce altruism in situations of advantageous inequity.

¬Transcranial direct current stimulation improves sleep quality in patients with insomnia after traumatic brain injury

INTRODUCTION

Insomnia is a serious problem after traumatic brain injury (TBI) and partially improves via sleeping pills. We investigated the efficacy of transcranial direct current stimulation (tDCS) with a focus on the role of age and gender.

MATERIALS AND METHODS

In a randomized double-blind clinical trial, 60 eligible TBI-induced insomnia patients were assigned to real and sham tDCS groups and were treated for three weeks. Sham but not real tDCS took sleeping pills for the first three weeks of the study and then used the placebo until the end of the study. The placebo was used by the real-tDCS group throughout the study. Sleep quality and insomnia severity were respectively evaluated by Pittsburg Sleep Quality Index (PSQI) and Insomnia Severity Index (ISI) at three time points.

RESULTS

Real tDCS group reported lower mean ISI and PSQI scores at 3 weeks post treatment onset and maintained this decline for six weeks post treatment onset (P < 0.001). In younger participants and those identified as men, the treatment-induced attenuation of the mean PSQI score was reported higher and more lasting in real than sham tDCS groups.

CONCLUSION

Gender and age-specific tDCS protocols may be warranted to optimize the therapeutic effect of tDCS.

The effect of home-based transcranial direct current stimulation in cognitive performance in fibromyalgia: A randomized, double-blind sham-controlled trial

Background

Transcranial Direct Current Stimulation (tDCS) is a promising approach to improving fibromyalgia (FM) symptoms, including cognitive impairment. So, we evaluated the efficacy and safety of home-based tDCS in treating cognitive impairment. Besides, we explored if the severity of dysfunction of the Descendant Pain Modulation System (DPMS) predicts the tDCS effect and if its effect is linked to changes in neuroplasticity as measured by the brain-derived neurotrophic factor (BDNF).

Methods

This randomized, double-blind, parallel, sham-controlled clinical trial, single-center, included 36 women with FM, aged from 30 to 65 years old, assigned 2:1 to receive a-tDCS (n = 24) and s-tDCS (n = 12). The primary outcome was the Trail Making Test's assessment of executive attention, divided attention, working memory (WM), and cognitive flexibility (TMT-B-A). The secondary outcomes were the Controlled Oral Word Association Test (COWAT), the WM by Digits subtest from the Wechsler Adult Intelligence Scale (WAIS-III), and quality of life. Twenty-minute daily sessions of home-based tDCS for 4 weeks (total of 20 sessions), 2 mA anodal-left (F3) and cathodal-right (F4) prefrontal stimulation with 35 cm² carbon electrodes.

Results

GLM showed a main effect for treatment in the TMT-B-A [Wald χ2 = 6.176; Df = 1; P = 0.03]. The a-tDCS improved cognitive performance. The effect size estimated by Cohen's d at treatment end in the TMT-B-A scores was large [-1.48, confidence interval (CI) 95% = -2.07 to-0.90]. Likewise, the a-tDCS effects compared to s-tDCS improved performance in the WM, verbal and phonemic fluency, and quality-of-life scale. The impact of a-tDCS on the cognitive tests was positively correlated with the reduction in serum BDNF from baseline to treatment end. Besides, the decrease in the serum BDNF was positively associated with improving the quality of life due to FM symptoms.

Conclusion

These findings revealed that daily treatment with a home-based tDCS device over l-DLPFC compared to sham stimulation over 4 weeks improved the cognitive impairment in FM. The a-tDCS at home was well-tolerated, underlining its potential as an alternative treatment for cognitive dysfunction. Besides, the a-tDCS effect is related to the severity of DPMS dysfunction and changes in neuroplasticity state.

Clinical trial registration

[www.ClinicalTrials.gov], identifier [NCT03843203].

Sensorimotor performance after high-definition transcranial direct current stimulation over the primary somatosensory or motor cortices in men versus women

The primary somatosensory (S1) cortex is a central structure in motor performance. However, transcranial direct current stimulation (tDCS) research aimed at improving motor performance usually targets the primary motor cortex (M1). Recently, sex was found to mediate tDCS response. Thus, we investigated whether tDCS with an anodal electrode placed over S1 improves motor performance and sensation perception in men versus women. Forty-five participants randomly received 15-min high-definition tDCS (HD-tDCS) at 1 mA to S1, M1, or sham stimulation. Reaching performance was tested before and immediately following stimulation. Two-point orientation discrimination (TPOD) of fingers and proprioception of a reaching movement were also tested. Although motor performance did not differ between groups, reaching reaction time improved in the M1 group men. Reaching movement time and endpoint error improved in women and men, respectively. Correct trials percentage for TPOD task was higher in the S1 compared to the M1 group in the posttest and improved only in the S1 group. Reaching movement time for the proprioception task improved, overall, and endpoint error did not change. Despite the reciprocal connections between S1 and M1, effects of active tDCS over S1 and M1 may specifically influence sensation perception and motor performance, respectively. Also, sex may mediate effects of HD-tDCS on motor performance.

Reading language of the eyes

The need for assessment of social skills in clinical and neurotypical populations has led to the widespread, and still increasing use of the 'Reading the Mind in the Eyes Test' (RMET) developed more than two decades ago by Simon Baron-Cohen and colleagues for evaluation of social cognition in autism. By analyzing most recent clinical and brain imaging data, we illuminate a set of factors decisive for using the RMET. Converging evidence indicates: (i) In neurotypical individuals, RMET scores are tightly correlated with other social skills (empathy, emotional intelligence, and body language reading); (ii) The RMET assesses recognition of facial affect, but also heavily relies on receptive language skills, semantic knowledge, and memory; (iii) RMET performance is underwritten by the large-scale ensembles of neural networks well-outside the social brain; (iv) The RMET is limited in its capacity to differentiate between neuropsychiatric conditions as well as between stages and severity of a single disorder, though it reliably distinguishes individuals with altered social cognition or elevated pathological traits from neurotypical persons; (v) Merely gender (as a social construct) rather than neurobiological sex influences performance on the RMET; (vi) RMET scores do not substantially decline in healthy aging, and they are higher with higher education level, cognitive abilities, literacy, and mental well-being; (vii) Accuracy on the RMET, and engagement of the social brain, are greater when emotions are expressed and recognized by individuals with similar cultural/ethnic background. Further research is required to better inform usage of the RMET as a tool for swift and reliable examination of social cognition. In light of comparable visual input from the RMET images and faces covered by masks due to COVID-19 regulations, the analysis is of value for keeping efficient social interaction during the current pandemic, in particular, in professional settings related to social communication.

More focal, less heterogeneous? Multi-level meta-analysis of cathodal high-definition transcranial direct current stimulation effects on language and cognition

High-definition transcranial direct current stimulation (HD-tDCS) is a relatively focal, novel non-invasive brain stimulation method with the potential to investigate the causal contributions of specific cortical brain regions to language and cognition. Studies with HD-tDCS typically employ a 4 × 1 electrode design with a single central target electrode surrounded by four return electrodes, among which return current intensity is evenly distributed. With cathodal HD-tDCS, neural excitability in the target region is assumed to be reduced, which offers interesting perspectives for neuropsychological research and interventions. This multi-level meta-analysis compiles published studies using cathodal HD-tDCS in 4 × 1 configuration to modulate cognition and behavior. Regarding HD-tDCS, 77 effect sizes were gathered from 11 eligible reports. We extended this database with 52 effect sizes from 11 comparable reports using conventional tDCS with cathodal polarity. We observed no significant overall effect and no moderation by within-study and between-study variables in HD. In the extended analysis, results suggested a non-linear moderation of cathodal tDCS effects by intensity, driven by negative effect sizes at 1.5 mA. However, studies varied tremendously in task parameters, outcomes, and even technical parameters. Interestingly, within-study heterogeneity exceeded between-study heterogeneity in the present sample, and moderators hardly reduced the residual heterogeneity. Across domains and configurations, both positive and negative effect sizes are possible. We discuss the findings in relation to conventional cathodal tDCS and the framework of polarity specificity. Fundamental aspects of cathodal HD-tDCS are still to be addressed in future research.

Distinguishing the Roles of the Dorsomedial Prefrontal Cortex and Right Temporoparietal Junction in Altruism in Situations of Inequality: A Transcranial Direct Current Stimulation Study

The right temporoparietal junction (rTPJ) and dorsomedial prefrontal cortex (dmPFC), which are involved in social cognition, have been proposed to play key roles in guiding human altruistic behavior. However, no study has provided empirical evidence that the rTPJ and dmPFC play distinct roles in altruism under situations of inequality. A total of 107 healthy young adults were randomly assigned to receive anodal or sham transcranial direct current stimulation (tDCS) to either the dmPFC or rTPJ, and they participated in a modified dictator game. The stimulation of the dmPFC increased the level of altruistic behavior, while the stimulation of the rTPJ did not. Furthermore, we determined that the increase in altruism induced by tDCS of the dmPFC could be modulated by perspective taking. These results demonstrate that the dmPFC and rTPJ play distinct roles in the enhancement of altruism in situations of inequality; this finding is consistent with theories proposing that the dmPFC has evolved mechanisms dedicated to perspective taking.

Transcranial direct current stimulation (tDCS) reduces motivation to drink ethanol and reacquisition of ethanol self-administration in female mice

Transcranial direct current stimulation (tDCS) is an emerging noninvasive brain neuromodulation technique aimed at relieving symptoms associated with psychiatric disorders, including addiction. The goal of the present study was to better identify which phase of alcohol-related behavior (hedonic effect, behavioral sensitization, self-administration, or motivation to obtain the drug) might be modulated by repeated anodal tDCS over the frontal cortex (0.2 mA, 20 min, twice a day for 5 consecutive days), using female mice as a model. Our data showed that tDCS did not modulate the hedonic effects of ethanol as assessed by a conditioned place preference test (CPP) or the expression of ethanol-induced behavioral sensitization. Interestingly, tDCS robustly reduced reacquisition of ethanol consumption (50% decrease) following extinction of self-administration in an operant paradigm. Furthermore, tDCS significantly decreased motivation to drink ethanol on a progressive ratio schedule (30% decrease). Taken together, our results show a dissociation between the effects of tDCS on “liking” (hedonic aspect; no effect in the CPP) and “wanting” (motivation; decreased consumption on a progressive ratio schedule). Our tDCS procedure in rodents will allow us to better understand its mechanisms of action in order to accelerate its use as a complementary and innovative tool to help alcohol-dependent patients maintain abstinence or reduce ethanol intake.

Sex difference in tDCS current mediated by changes in cortical anatomy: A study across young, middle and older adults

INTRODUCTION

The observed variability in the effects of transcranial direct current stimulation (tDCS) is influenced by the amount of current reaching the targeted region-of-interest (ROI). Age and sex might affect current density at target ROI due to their impact on cortical anatomy. The present tDCS simulation study investigates the effects of cortical anatomical parameters (volumes, dimension, and torque) on simulated tDCS current density in healthy young, middle-aged, and older males and females.

METHODOLOGY

Individualized head models from 240 subjects (120 males, 18-87 years of age) were used to identify the estimated current density (2 mA current intensity, 25 cm² electrode) from two simulated tDCS montages (CP5_CZ and F3_FP2) targeting the inferior parietal lobule (IPL) and middle frontal gyrus (MFG), respectively. Cortical parameters including segmented brain volumes (cerebrospinal fluid [CSF], grey and white matter), cerebral-dimensions (length/width &length/height) and brain-torque (front and back shift, petalia, and bending) were measured using the magnetic resonance images (MRIs) from each subject. The present study estimated sex differences in current density at these target ROIs mediated by these cortical parameters within each age group.

RESULTS

For both tDCS montages, females in the older age group received higher current density than their male counterparts at the target ROIs. No sex differences were observed in the middle-aged group. Males in the younger age group had a higher current density than females, only for the parietal montage. Across all age groups, CSF, and grey matter volumes significantly predicted the current intensity estimated at the target sites. In the older age group only, brain-torque was a significant mediator of the sex difference.

CONCLUSIONS

Our findings demonstrate the presence of sex differences in the simulated tDCS current density, however this pattern differed across age groups and stimulation locations. Future studies should consider influence of age and sex on individual cortical anatomy and tailor tDCS stimulation parameters accordingly.

Effect of High-definition Transcranial Direct Current Stimulation on Conditioned Pain Modulation in Healthy Adults: A Crossover Randomized Controlled Trial

The disorder of the conditioned pain modulation (CPM) system is one of the main causes of pain perception in individuals. High-definition transcranial direct current stimulation (HD-tDCS) targeting specific brain areas was indicated to have an analgesic effect possibly by activating the endogenous pain inhibition pathway evident in CPM. However, discrepancies were found in previous limited studies of varied homogeneity and quality. Therefore, the present study applied 2 mA HD-tDCS (20 min) in the left primary motor cortex (M1) among 35 healthy adults with a blinded crossover study design, to investigate its effectiveness on optimizing the analgesic effect in healthy individuals through assessing changes of the CPM. The univariate and multivariate general linear models were used to evaluate the intervention effect between-group on the Δ-value (after-intervention minus before-intervention) during CPM (primary outcome), pressure pain threshold (PPT), and cold pressure threshold (CPT) (secondary outcome), respectively. A significant between-group difference in Δ-CPM was found for active stimulation. HD-tDCS significantly improved the analgesic efficiency of Δ-CPM, compared with the sham control, after adjusting the confounding factors including age, gender, psychological status, as well as the sequence effect. The changes of CPM were positively correlated with the total physical activity volume. In conclusion, our findings provide evidence support to the effectiveness of HD-tDCS on endogenous pain modulation among healthy adults. Further studies are required to explore the analgesic effect of tDCS among patients with chronic pain, thereby facilitating optimal chronic pain management.

Investigating the sex-dependent effects of prefrontal cortex stimulation on response execution and inhibition

Context-dependent execution or inhibition of a response is an important aspect of executive control, which is impaired in neuropsychological and addiction disorders. Transcranial direct current stimulation (tDCS) of the dorsolateral prefrontal cortex (DLPFC) has been considered a remedial approach to address deficits in response control; however, considerable variability has been observed in tDCS effects. These variabilities might be related to contextual differences such as background visual-auditory stimuli or subjects' sex. In this study, we examined the interaction of two contextual factors, participants' sex and background acoustic stimuli, in modulating the effects of tDCS on response inhibition and execution. In a sham-controlled and cross-over (repeated-measure) design, 73 participants (37 females) performed a Stop-Signal Task in different background acoustic conditions before and after tDCS (anodal or sham) was applied over the DLPFC. Participants had to execute a speeded response in Go trials but inhibit their response in Stop trials. Participants' sex was fully counterbalanced across all experimental conditions (acoustic and tDCS). We found significant practice-related learning that appeared as changes in indices of response inhibition (stop-signal reaction time and percentage of successful inhibition) and action execution (response time and percentage correct). The tDCS and acoustic stimuli interactively influenced practice-related changes in response inhibition and these effects were uniformly seen in both males and females. However, the effects of tDCS on response execution (percentage of correct responses) were sex-dependent in that practice-related changes diminished in females but heightened in males. Our findings indicate that participants' sex influenced the effects of tDCS on the execution, but not inhibition, of responses.

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.

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.

The Hypnotic Analgesia Suggestion Mitigated the Effect of the Transcranial Direct Current Stimulation on the Descending Pain Modulatory System: A Proof of Concept Study

Objective

We evaluated whether active(a)-tDCS combined with hypnotic analgesia suggestion (HS) would be more effective than a single active(a)-tDCS, and/or sham-(s)-tDCS and s-tDCS/HS on the following outcomes: function of descending pain modulatory system (DPMS) during the conditioned pain modulation test (CPM-test) (primary outcome), heat pain threshold (HPT), heat pain tolerance (HPTo) and cold pressor test (CPT) (secondary outcomes). We also examined whether their effects are related to neuroplasticity state evaluated by serum brain-derived-neurotropic factor (BDNF).

Materials and Methods

Forty-eight females received one session of one of the four interventions (a-tDCS/HS, s-tDCS/HS, a-tDCS, and s-tDCS) in an incomplete randomized crossover sequence. The a-tDCS or s-tDCS was applied over the left dorsolateral prefrontal cortex (DLPFC) for 30 minutes at 2mA.

Results

A generalized linear model revealed a significant main effect for the intervention group (P <0.032). The delta-(Δ) pain score on the Numerical Pain Scale (NPS0-10) during CPM-test in the a-tDCS/HS group was -0.25 (0.43). The (Δ) pain score on NPS (0-10) during CPM-test in the other three groups was a-tDCS=-0.54 (0.41), HS -0.01 (0.41) and s-tDCS/HS=-0.19 (0.43). A-tDCS/HS intervention increased the CPT substantially compared to all other interventions. Also, higher baseline levels of BDNF were associated with a larger change in CPT and HPTo.

Conclusion

These findings indicate that the HS combined with a-tDCS mitigated the effect of the a-tDCS on the DPMS. The a-tDCS up-regulates the inhibition on DPMS, and the HS improved pain tolerance. And, together they enhanced the reaction time substantially upon the CPT.

Clinical Trial Registration

www.ClinicalTrials.gov, identifier NCT03744897.

Transcranial electric stimulation as a neural interface to gain insight on human brain functions: current knowledge and future perspective

The use of brain-stimulation approaches in social and affective science has greatly increased over the last two decades. The interest in social factors has grown along with technological advances in brain research. Transcranial electric stimulation (tES) is a research tool that allows scientists to establish contributory causality between brain functioning and social behaviour, therefore deepening our understanding of the social mind. Preliminary evidence is also starting to demonstrate that tES, either alone or in combination with pharmacological or behavioural interventions, can alleviate the symptomatology of individuals with affective or social cognition disorders. This review offers an overview of the application of tES in the field of social and affective neuroscience. We discuss issues and challenges related to this application and suggest avenue for future basic and translational research.

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.

Influence of gender-related differences in transcranial direct current stimulation: A Computational Study

Gender-related anatomical differences have been reported with respect to brain and cerebrospinal fluid. The cortical induced electric field (EF) in transcranial direct current stimulation (tDCS) depends both on the dose (electrode montage and applied current) and the individual anatomical details. Therefore in spite of fixed dose, one can expect gender-related differences to impact induced EF which in turn would influence tDCS outcome. The abundance of promise with tDCS in both research and clinical domains is also accompanied with variability in response. The in-constant induced cortical EF is one of the main contributors of this variability.The aim of this study is to quantify the effects of the gender-related morphological changes on tDCS induced cortical EF. MRI data were obtained for 10 healthy individuals (5 males: M1-M5 and 5 females: F1-F5) spanning ages 27-47 years. Finite element models derived from the individual MRI and simulating the classic left motor cortex-contralateral supraorbital (C3-SO) montage were used to predict the cortical EF. The percentage tissue volume were also determined to illustrate anatomical differences in the dataset considered.Findings indicate that induced EF is higher in female head models on an average than male head models across several metrics. While the average peak EF value in female head models was comparable to that of male head models, the mean and median values were 11.6% and 10% higher. On an individual basis, the highest peak value was observed in a female subject F3 (0.83 V/m) while the lowest peak value was observed in male subject M2 (0.34 V/m) -indicating a variation of ~2.4-fold across the dataset considered. The average gray matter percentage volume in females was 11.6% higher than in males. The average white matter percentage volume was 8.7% higher in females while negligible CSF percentage volume difference was noted across gender. The results of our study indicate gender-related differences in tDCS induced current flow and quantify the extent of this variation.

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.

Transforming treatments for schizophrenia: Virtual reality, brain stimulation and social cognition

Schizophrenia is characterised by delusions, hallucinations, anhedonia and apathy; while impairments in social cognition are often less recognised. Poor social cognition can lead to difficulties in obtaining and maintaining employment, academic progression, interpersonal relationships, and community functioning. Current interventions are highly intensive, require significant resources and have only modest effects on functional outcomes. Virtual reality (VR) and non-invasive brain stimulation (NIBS) may have a role in addressing these limitations. VR allows treatments that are potentially more accessible, less delivery intensive, and have higher ecological validity. While NIBS is able to directly modulate activity in social brain areas in order to promote neuroplasticity, strengthen neural connections and enhance brain function related to social cognitive behaviours. Therefore, the combination of VR and NIBS may allow for more efficient and transferrable interventions than those currently available. This review will explore the potential role of these technologies in the treatment of social cognitive impairment.

Theory of Mind Performance Predicts tDCS-Mediated Effects on the Medial Prefrontal Cortex: A Pilot Study to Investigate the Role of Sex and Age

Transcranial Direct Current Stimulation (tDCS) has become an increasingly promising tool for understanding the relationship between brain and behavior. The purpose of this study was to investigate whether the magnitude of sex- and age-related tDCS effects previously found in the medial prefrontal cortex (mPFC) during a Theory of Mind (ToM) task correlates with social cognition performance; in particular, we explored whether different patterns of activity would be detected in high- and low-performing participants. For this, young and elderly, male and female participants were categorized as a low- or high-performer according to their score on the Reading the Mind in the Eyes task. Furthermore, we explored whether sex- and age-related effects associated with active tDCS on the mPFC were related to cognitive functioning. We observed the following results: (i) elderly participants experience a significant decline in ToM performance compared to young participants; (ii) low-performing elderly females report slowing of reaction time when anodal tDCS is applied over the mPFC during a ToM task; and (iii) low-performing elderly females are characterized by lower scores in executive control functions, verbal fluency and verbal short-term memory. The relationship between tDCS results and cognitive functioning is discussed in light of the neuroscientific literature on sex- and age-related differences.

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.

Aging, sex and cognitive Theory of Mind: a transcranial direct current stimulation study

Aging is accompanied by changes in cognitive abilities and a great interest is spreading among researchers about aging impact on social cognition skills, such as the Theory of Mind (ToM). Transcranial direct current stimulation (tDCS) has been used in social cognition studies founding evidence of sex-related different effects on cognitive ToM task in a young people sample. In this randomized, double-blind, sham-controlled study, we applied one active and one sham tDCS session on the medial prefrontal cortex (mPFC) during a cognitive ToM task, including both social (i.e., communicative) and nonsocial (i.e., private) intention attribution conditions, in sixty healthy aging individuals (30 males and 30 females). In half of the participants the anode was positioned over the mPFC, whereas in the other half the cathode was positioned over the mPFC. The results showed that: (i) anodal tDCS over the mPFC led to significant slower reaction times (vs. sham) for social intention attribution task only in female participants; (ii) No effects were found in both females and males during cathodal stimulation. We show for the first time sex-related differences in cognitive ToM abilities in healthy aging, extending previous findings concerning young participants.

Enhancing theory of mind in behavioural variant frontotemporal dementia with transcranial direct current stimulation

Behavioural variant frontotemporal dementia (bvFTD) is a form of frontotemporal degeneration characterized by early changes in personality, emotional blunting, and/or loss of empathy. Recent research has highlighted that these features may be at least partially explained by impairments in the theory of mind (ToM; i.e., the ability to understand and predict other people's behaviour by attributing independent mental states to them). The aim of this randomized, double-blind, placebo-controlled study was to test the hypothesis that transcranial direct current stimulation (tDCS) over the medial frontal cortex (MFC) selectively enhances communicative intention processing, a specific ToM ability. Using a single-session online design, we administered a ToM task that measures the ability to represent other people's private and communicative intentions during active or sham tDCS to 16 bvFTD patients. To assess the impact of dementia on performance on the ToM task, we included 16 age-matched healthy volunteers who were asked to perform the entire experimental ToM task. BvFTD is characterized by an impairment in the comprehension of both communicative and private intentions relative to a healthy control group and by a disproportional impairment in communicative intention compared with private intention processing. Significant and selective accuracy improvement in the comprehension of communicative intentions after active stimulation was observed in patients with bvFTD. This is the first study that analyses ToM ability in patients with bvFTD using tDCS stimulation. Our findings could potentially contribute to the development of an effective, noninvasive brain stimulation treatment of ToM impairments in patients with bvFTD.

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.

What Do You Have in Mind? Measures to Assess Mental State Reasoning in Neuropsychiatric Populations

Social interaction is closely associated with both functional capacity and well-being. Previous research has not only revealed evidence of social dysfunction in individuals with a wide range of psychiatric and neurological disorders but also generated an abundance of potential measures for assessing social cognition. This review explores the most popular measures used within neuropsychiatric populations to investigate the ability to recognize or reason about the mental states of others. Measures are also critically analyzed in terms of strengths and limitations to aid task selection in future clinical studies. The most frequently applied assessment tools use verbal, visual or audiovisual forms of presentation and assess recognition of mental states from facial features, self-rated empathy, the understanding of other's cognitive mental states such as beliefs and intentions, or the ability to combine knowledge of other's thoughts and emotions in order to understand subtle communications or socially inappropriate behavior. Key weaknesses of previous research include limited investigation of relationships with clinical symptoms, and underutilization of measures of everyday social functioning that offer a useful counterpart to traditional "lab" tasks. Future studies should aim to carefully select measures not only based on the range of skills to be assessed but also taking into account potential difficulties with interpretation and the need to gain insight into the application of social cognitive skills as well as ability per se. Some of the best measures include those with well-matched control trials (e.g., Yoni Task) or those that restrict the influence of verbal deficits (e.g., intentions comic strip task), elicit spontaneous mentalizing (e.g., Animations Task), and possess greater ecological validity (e.g., Movie for the Assessment of Social Cognition). Social cognitive research within psychiatric populations will be further enhanced through the development of more closely matched control tasks, and the exploration of relationships between task performance, medication, strategy use, and broader emotional and motor functions.

Transcranial direct current stimulation enhances theory of mind in Parkinson's disease patients with mild cognitive impairment: a randomized, double-blind, sham-controlled study

Background

Parkinson’s Disease (PD) with mild cognitive impairment (MCI) (PD-MCI) represents one of the most dreaded complications for patients with PD and is associated with a higher risk of developing dementia. Although transcranial direct current stimulation (tDCS) has been demonstrated to improve motor and non-motor symptoms in PD, to date, no study has investigated the effects of tDCS on Theory of Mind (ToM), i.e., the ability to understand and predict other people’s behaviours, in PD-MCI.

Methods

In this randomized, double-blind, sham-controlled study, we applied active tDCS over the medial frontal cortex (MFC) to modulate ToM performance in twenty patients with PD-MCI. Twenty matched healthy controls (HC) were also enrolled and were asked to perform the ToM task without receiving tDCS.

Results

In the patients with PD-MCI, i) ToM performance was worse than that in the HC, ii) ToM abilities were poorer in those with fronto-executive difficulties, and iii) tDCS over the MFC led to significant shortening of latency for ToM tasks.

Conclusions

We show for the first time that active tDCS over the MFC enhances ToM in patients with PD-MCI, and suggest that non-invasive brain stimulation could be used to ameliorate ToM deficits observed in these patients.

Acute effects of single dose transcranial direct current stimulation on muscle strength: A systematic review and meta-analysis

Previous studies investigating the effects of transcranial direct current stimulation (tDCS) on muscle strength showed no consensus. Therefore, the purpose of this article was to systematically review the literature on the effects of single dose tDCS to improve muscle strength. A systematic literature search was conducted on PubMeb, ISI Web of Science, SciELO, and Scopus using search terms regarding tDCS and muscle strength. Studies were included in accordance with Population, Intervention, Comparison, Outcomes, and Setting (PICOS) including criteria. Healthy men and women, strength training practitioners or sedentary were selected. The acute effects of single dose anode stimulus of tDCS (a-tDCS) and the placebo stimulus of tDCS (sham) or no interventions were considered as an intervention and comparators, respectively. Measures related to muscle strength were analyzed. To conduct the analyses a weighted mean difference (WMD) and the standardized mean difference (SMD) were applied as appropriate. A total of 15 studies were included in this systematic review and 14 in meta-analysis. Regarding the maximal isometric voluntary contraction (MIVC), a small effect was seen between tDCS and Sham with significant difference between the conditions (SMD = 0.29; CI95% = 0.05 to 0.54; Z = 2.36; p = 0.02). The muscular endurance measured by the seconds sustaining a percentage of MIVC demonstrated a large effect between tDCS and Sham (WMD = 43.66; CI95% = 29.76 to 57.55; Z = 6.16; p < 0.001), showing an improvement in muscular endurance after exposure to tDCS. However, muscular endurance based on total work showed a trivial effect between tDCS and Sham with no significant difference (SMD = 0.22; CI95% = -0.11 to 0.54; Z = 1.32, p = 0.19). This study suggests that the use of tDCS may promote increase in maximal voluntary contraction and muscular endurance through isometric contractions.

The Effect of High-Definition Transcranial Direct Current Stimulation of the Right Inferior Frontal Gyrus on Empathy in Healthy Individuals

Empathy, including cognitive and emotional empathy, refers to the ability to infer the mental states of others and to the capacity to share emotions. The neural mechanisms involved in empathy are complex and not yet fully understood, and previous studies have shown that both cognitive and emotional empathy are closely associated with the inferior frontal gyrus (IFG). In this study, we examined whether empathy can be modulated by high-definition transcranial direct current stimulation (HD-tDCS) of the right IFG. Twenty-three healthy participants took part in all three experimental conditions (i.e., anodal, cathodal and sham stimulation) in a randomized order. Participants then completed the Chinese version of the Multifaceted Empathy Test (MET), which assesses both cognitive and emotional empathy. The results show that scores obtained for cognitive empathy following cathodal stimulation are significantly lower than those obtained following sham stimulation. In addition, scores obtained for cognitive empathy following anodal stimulation are higher than those obtained following sham stimulation, though the difference is only marginally significant. However, the results fail to show whether the stimulation of the right IFG via HD-tDCS plays a role in emotional empathy. Our results suggest that the right IFG plays a key role in cognitive empathy and indicate that HD-tDCS can regulate cognitive empathy by inducing excitability changes in the right IFG.

Effects of neuromodulation on cognitive performance in individuals exhibiting addictive behaviors: A systematic review

BACKGROUND

There is growing interest in non-invasive brain stimulation techniques as treatments for addictive disorders. While multiple reviews have examined the effects of neuromodulation on craving and consumption, there has been no review of how neuromodulation affects cognitive functioning in addiction. This systematic review examined studies of the cognitive effects of transcranial direct current stimulation (tDCS) and transcranial magnetic stimulation (TMS) in individuals exhibiting addictive behavior.

METHODS

Articles were identified through searches in PubMed and PsycINFO conducted in October 2017. Eligible studies investigated the effects of tDCS or TMS on cognitive task performance in participants reporting substance use (e.g., alcohol, tobacco, or drugs) or addictive behaviors (e.g., gambling). Tasks were organized into five domains: (1) Inhibitory control, (2) Risk-taking, (3) Impulsive choice (delay discounting), (4) Executive function, and (5) Implicit biases.

RESULTS

Twenty-four articles met the inclusion criteria. Fifty-seven percent of studies used tDCS and 43% used TMS, with nearly all studies (96%) targeting the dorsolateral prefrontal cortex. Ten studies reported significant within-subject modulation of cognitive functioning associated with active TMS or tDCS, with the same number reporting no change in cognitive performance. Of four studies that included both an experimental and control participant group, three showed between-group differences in the effects of neuromodulation.

CONCLUSIONS

While positive effects in several studies suggest that tDCS and TMS improve cognitive functioning in addiction, there is substantial heterogeneity across studies. We discuss person-related and methodological factors that could explain inconsistencies, and propose individualized stimulation protocols may sharpen the cognitive effects of neuromodulation in addiction.

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.

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.