COMETS: A MATLAB toolbox for simulating local electric fields generated by transcranial direct current stimulation (tDCS)

Transcranial electrical stimulation modulates emotional experience and metabolites in the prefrontal cortex in a donation task

Understanding the neural, metabolic, and psychological mechanisms underlying human altruism and decision-making is a complex and important topic both for science and society. Here, we investigated whether transcranial Direct Current Stimulation (tDCS) applied to two prefrontal cortex regions, the ventromedial prefrontal cortex (vmPFC, anode) and the right dorsolateral prefrontal cortex (DLPFC, cathode) can induce changes in self-reported emotions and to modulate local metabolite concentrations. We employed in vivo quantitative MR Spectroscopy in healthy adult participants and quantified changes in GABA and Glx (glutamate + glutamine) before and after five sessions of tDCS delivered at 2 mA for 20 min (active group) and 1 min (sham group) while participants were engaged in a charitable donation task. In the active group, we observed increased levels of GABA in vmPFC. Glx levels decreased in both prefrontal regions and self-reported happiness increased significantly over time in the active group. Self-reported guiltiness in both active and sham groups tended to decrease. The results indicate that self-reported happiness can be modulated, possibly due to changes in Glx concentrations following repeated stimulation. Therefore, local changes may induce remote changes in the reward network through interactions with other metabolites, previously thought to be unreachable with noninvasive stimulation techniques.

TDCS over PPC or DLPFC does not improve visual working memory capacity

Non-invasive brain stimulation has been highlighted as a possible intervention to induce cognitive benefits, including on visual working memory (VWM). However, findings are inconsistent, possibly due to methodological issues. A recent high-profile study by Wang et al.1 reported that anodal transcranial direct current stimulation (tDCS) over posterior parietal cortex (PPC), but not over dorsolateral prefrontal cortex (DLPFC), selectively improved VWM capacity but not precision, especially at a high VWM load. Thus, in the current pre-registered conceptual replication study, we accounted for the key potential methodological issues in the original study and tested an adequate number of participants required to demonstrate the previously reported effects (n = 48 compared to n = 20). Participants underwent counterbalanced PPC, DLPFC and sham stimulation before completing 360 trials of a continuous orientation-reproduction task with a slight variation of task stimuli and setup. We found no evidence for the selective effect of PPC stimulation. Instead, our results showed that tDCS effects were absent regardless of stimulation region and VWM load, which was largely supported by substantial to strong Bayesian evidence. Therefore, our results challenge previously reported benefits of single-session anodal PPC-tDCS on VWM.

Optimizing electrode placement for transcranial direct current stimulation in nonsuperficial cortical regions: a computational modeling study

Transcranial direct current stimulation (tDCS) is a noninvasive brain stimulation technique for modulating neuronal excitability by sending a weak current through electrodes attached to the scalp. For decades, the conventional tDCS electrode for stimulating the superficial cortex has been widely reported. However, the investigation of the optimal electrode to effectively stimulate the nonsuperficial cortex is still lacking. In the current study, the optimal tDCS electrode montage that can deliver the maximum electric field to nonsuperficial cortical regions is investigated. Two finite element head models were used for computational simulation to determine the optimal montage for four different nonsuperficial regions: the left foot motor cortex, the left dorsomedial prefrontal cortex (dmPFC), the left medial orbitofrontal cortex (mOFC), and the primary visual cortex (V1). Our findings showed a good consistency in the optimal montage between two models, which led to the anode and cathode being attached to C4-C3 for the foot motor, F4-F3 for the dmPFC, Fp2-F7 for the mOFC, and Oz-Cz for V1. Our suggested montages are expected to enhance the overall effectiveness of stimulation of nonsuperficial cortical areas.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13534-023-00335-2.

Determination of optimal injection current pattern for multichannel transcranial electrical stimulation without individual MRI using multiple head models

BACKGROUND AND OBJECTIVE

Multichannel transcranial electrical stimulation (tES) is widely used to achieve improved stimulation focality. In the multichannel tES, the injection current pattern is generally determined through an optimization process with a finite element (FE) head model extracted from individual magnetic resonance images (MRIs). Although using an individual head model ensures the best outcome, acquiring MRIs of individual subjects in many practical applications is often difficult. Alternatively, a standard head model can be used to determine the optimal injection current pattern to stimulate a specific target; however, this may result in a relatively inaccurate delivery of stimulation current owing to the difference in individual anatomical structures. To address this issue, we propose a new approach for determining the injection current pattern using multiple head models, which can improve the stimulation focality compared to that achieved with a single standard head model.

METHODS

Twenty FE head models were used to optimize the injection current patterns to stimulate three cortical regions that are widely considered targets for tES. The individual injection current patterns were then averaged to obtain each target's mean injection current pattern. The stimulation focality for each target was then calculated by applying different current patterns (the mean current, individual current, and current from a standard model).

RESULTS

Our results showed that the stimulation focality obtained using the mean injection current pattern was significantly higher than that obtained using the injection current pattern from a standard head model. Additionally, our results demonstrated that a minimum of 13 head models are required to determine mean current pattern, allowing for a higher stimulation focality than when using the current from a standard head model.

CONCLUSIONS

Hence, using multiple head models can provide a viable solution for improving the stimulation efficacy of multichannel tES when individual MRIs are not available.

Alpha transcranial alternating current stimulation modulates auditory perception

BACKGROUND

Studies using transcranial alternating current stimulation (tACS), a type of non-invasive brain stimulation, have demonstrated a relationship between the positive versus negative phase of both alpha and delta/theta oscillations with variable near-threshold auditory perception. These findings have not been directly compared before. Furthermore, as perception was better in the positive versus negative phase of two different frequencies, it is unclear whether changes in polarity (independent of a specific frequency) could also modulate auditory perception.

OBJECTIVE

We investigated whether auditory perception depends on the phase of alpha, delta/theta, or polarity alone.

METHODS

We stimulated participants with alpha, delta, and positive and negative direct current (DC) over temporal and central scalp sites while they identified near-threshold tones-in-noise. A Sham condition without tACS served as a control condition. A repeated-measures analysis of variance was used to assess differences in proportions of hits between conditions and polarities. Permutation-based circular-logistic regressions were used to assess the relationship between circular-predictors and single-trial behavioral responses. An exploratory analysis compared the full circular-logistic regression model to the intercept-only model.

RESULTS

Overall, there were a greater proportion of hits in the Alpha condition in comparison to Delta, DC, and Sham conditions. We also found an interaction between polarity and stimulation condition; post-hoc analyses revealed a greater proportion of hits in the positive versus negative phase of Alpha tACS. In contrast, no significant differences were found in the Delta, DC, or Sham conditions. The permutation-based circular-logistic regressions did not reveal a statistically significant difference between the obtained RMS of the sine and cosine coefficients and the mean of the surrogate distribution for any of the conditions. However, our exploratory analysis revealed that circular-predictors explained the behavioral data significantly better than an intercept-only model for the Alpha condition, and not the other three conditions.

CONCLUSION

These findings suggest that alpha tACS, and not delta nor polarity alone, modulates auditory perception.

Single Session Anodal Transcranial Direct Current Stimulation on Different Cortical Areas

Abstract. Transcranial direct current stimulation (tDCS) studies in healthy volunteers have shown conflicting results in terms of modulation in pain thresholds. The aim of this study was to investigate how single session anodal tDCS and modulated tDCS (mtDCS) of distinct cortical areas affected pain and perception thresholds in healthy participants. Five different stimulation conditions were applied at different cortical sites to 20 healthy volunteers to investigate the effects of tDCS and mtDCS (20 Hz) on pain and perception thresholds. TDCS over the motor cortex (M1), mtDCS over the motor cortex, tDCS over the dorsolateral prefrontal cortex (DLPFC), mtDCS of the DLPFC, and mtDCS over the occipital cortex were the stimulation conditions. All of the stimulations were anodal. The stimulations were given in a randomized order at 20-minute intervals. For comparison, electrical pain and perception thresholds were obtained from the right middle finger before and during the tDCS. After each measurement, participants were asked to give a score to their pain. In repeated measures analysis of variance (RM-ANOVA) test, the Condition × Time interaction showed no significant influence on changes in pain, perception thresholds, and pain scores ( p = .48, p = .89, and p = .50, respectively). However, regardless of the condition types, there was a significant difference in pain and perceptual thresholds during tDCS ( p = .01, p = .025, respectively). Our findings did not support difference in pain and perception modulation by a single session anodal tDCS over M1 and DLPFC compared to the occipital cortex in healthy volunteers. The increase in all thresholds during tDCS, irrespective of conditions, and peripheral sensations, including an active control group, taken together, suggest a placebo effect of active tDCS. Future studies about pain and perception in healthy subjects should consider the level of experimental pain and a strong placebo effect.

The impact of cathodal tDCS on the GABAergic system in the epileptogenic zone: A multimodal imaging study

Objectives

We aimed to investigate the antiepileptic effects of cathodal transcranial direct current stimulation (c-tDCS) and mechanisms of action based on its effects on the neurotransmitters responsible for the abnormal synchrony patterns seen in pharmacoresistant epilepsy. This is the first study to test the impact of neurostimulation on epileptiform interictal discharges (IEDs) and to measure brain metabolites in the epileptogenic zone (EZ) and control regions simultaneously in patients with pharmacoresistant epilepsy.

Methods

This is a hypothesis-driven pilot prospective single-blinded repeated measure design study in patients diagnosed with pharmacoresistant epilepsy of temporal lobe onset. We included seven patients who underwent two sessions of c-tDCS (sham followed by real). The real tDCS session was 20 min in duration and had a current intensity of 1.5 mA delivered via two surface electrodes that had dimensions of 3 × 4 cm. The cathode electrode was placed at FT7 in the center whereas the anode at Oz in the center. After each session, we performed electroencephalographic recording to count epileptiform IEDs over 30 min. We also performed magnetic resonance spectroscopy (MRS) to measure brain metabolite concentrations in the two areas of interest (EZ and occipital region), namely, gamma-aminobutyric acid (GABA), glutamate (Glx), and glutathione. We focused on a homogenous sample where the EZ and antiepileptic medications are shared among patients.

Results

Real tDCS decreased the number of epileptiform IEDs per min (from 9.46 ± 2.68 after sham tDCS to 5.37 ± 3.38 after real tDCS), p = 0.018, as compared to sham tDCS. GABA was decreased in the EZ after real c-tDCS stimulation as compared to sham tDCS (from 0.129 ± 0.019 to 0.096 ± 0.018, p = 0.02). The reduction in EZ GABA correlated with the reduction in the frequency of epileptiform IED per min (rho: 0.9, p = 0.003).

Conclusion

These results provide a window into the antiepileptic mechanisms of action of tDCS, based on local and remote changes in GABA and neural oscillatory patterning responsible for the generation of interictal epileptiform discharges.

Inter-Individual Variability in tDCS Effects: A Narrative Review on the Contribution of Stable, Variable, and Contextual Factors

Due to its safety, portability, and cheapness, transcranial direct current stimulation (tDCS) use largely increased in research and clinical settings. Despite tDCS's wide application, previous works pointed out inconsistent and low replicable results, sometimes leading to extreme conclusions about tDCS's ineffectiveness in modulating behavioral performance across cognitive domains. Traditionally, this variability has been linked to significant differences in the stimulation protocols across studies, including stimulation parameters, target regions, and electrodes montage. Here, we reviewed and discussed evidence of heterogeneity emerging at the intra-study level, namely inter-individual differences that may influence the response to tDCS within each study. This source of variability has been largely neglected by literature, being results mainly analyzed at the group level. Previous research, however, highlighted that only a half-or less-of studies' participants could be classified as responders, being affected by tDCS in the expected direction. Stable and variable inter-individual differences, such as morphological and genetic features vs. hormonal/exogenous substance consumption, partially account for this heterogeneity. Moreover, variability comes from experiments' contextual elements, such as participants' engagement/baseline capacity and individual task difficulty. We concluded that increasing knowledge on inter-dividual differences rather than undermining tDCS effectiveness could enhance protocols' efficiency and reproducibility.

Reconciling psychological and neuroscientific accounts of reduced motivation in aging

Motivation is a hallmark of healthy aging, but the motivation to engage in effortful behavior diminishes with increasing age. Most neurobiological accounts of altered motivation in older adults assume that these deficits are caused by a gradual decline in brain tissue, while some psychological theories posit a switch from gain orientation to loss avoidance in motivational goals. Here, we contribute to reconcile the psychological and neural perspectives by providing evidence that the frontopolar cortex (FPC), a brain region involved in cost-benefit weighting, increasingly underpins effort avoidance rather than engagement with age. Using anodal transcranial direct current stimulation together with effort-reward trade-offs, we find that the FPC's function in effort-based decisions remains focused on cost-benefit calculations but appears to switch from reward-seeking to cost avoidance with increasing age. This is further evidenced by the exploratory, independent analysis of structural brain changes, showing that the relationship between the density of the frontopolar neural tissue and the willingness to exert effort differs in young vs older adults. Our results inform aging-related models of decision-making by providing preliminary evidence that, in addition to cortical thinning, changes in goal orientation need to be considered in order to understand alterations in decision-making over the life span.

No effects of transcranial direct current stimulation on visual evoked potential and peak gamma frequency

Evidence suggests that the visual evoked potential (VEP) and gamma oscillations elicited by visual stimuli reflect the balance of excitatory and inhibitory (E-I) cortical processes. As tDCS has been shown to modulate E-I balance, the current study investigated whether amplitudes of VEP components (N1 and P2) and peak gamma frequency are modulated by transcranial direct current stimulation (tDCS). Healthy adults underwent two electroencephalography (EEG) recordings while viewing stimuli designed to elicit a robust visual response. Between the two recordings, participants were randomly assigned to three tDCS conditions (anodal-, cathodal-, and sham-tDCS) or received no-tDCS. tDCS electrodes were placed over the occipital cortex (Oz) and the left cheek with an intensity of 2 mA for 10 min. Data of 39 participants were analysed for VEP amplitudes and peak gamma frequency using mixed-model ANOVAs. The results showed no main effects of tDCS in any metric. Possible explanations for the absence of tDCS effects are discussed.

Limb apparent motion perception: Modification by tDCS, and clinically or experimentally altered bodily states

Limb apparent motion perception (LAMP) refers to the illusory visual perception of a moving limb upon observing two rapidly alternating photographs depicting the same limb in two different postures. Fast stimulus onset asynchronies (SOAs) induce the more visually guided perception of physically impossible movements. Slow SOAs induce the perception of physically possible movements. According to the motor theory of LAMP, the latter perception depends upon the observer's sensorimotor representations. Here, we tested this theory in two independent studies by performing a central (study 1) and peripheral (study 2) manipulation of the body's sensorimotor states during two LAMP tasks. In the first sham-controlled transcranial direct current stimulation between-subject designed study, we observed that the dampening of left sensorimotor cortex activity through cathodal stimulation biased LAMP towards the more visually guided perception of physically impossible movements for stimulus pairs at slow SOAs. In the second, online within-subject designed study, we tested three participant groups twice: (1) individuals with an acquired lower limb amputation, either while wearing or not wearing their prosthesis (2) individuals with body integrity dysphoria (i.e., with a desire for amputation of a healthy leg) while sitting in a regular position or binding up the undesired leg (to simulate the desired amputation); (3) able-bodied individuals while sitting in a normal position or sitting on one of their legs. We found that the momentary sensorimotor state crucially impacted LAMP in individuals with an amputation and able-bodied participants, but not in BID individuals. Taken together, the results of these two studies substantiate the motor theory of LAMP.

The residential stability mindset increases racial in-group bias in empathy

With the deepening of internationalization, the population's mobility has greatly increased, which can impact people's intergroup relationships. The current research examined the hypothesis that residential mobility plays a crucial role in racial in-group bias in empathy (RIBE) with three studies. By manipulating the residential mobility/stability mindset and measuring subjective pain intensity ratings (Study 1) and event-related potentials (ERPs, Study 2) of Chinese adults on painful and neutral expressions of Asian and Caucasian faces, we found that the RIBE in subjective ratings and N1 amplitudes increased and P3 amplitudes decreased in the stability group. Transcranial direct current stimulation (tDCS) manipulation in Study 3 further found that anodal stimulation of the left dorsolateral prefrontal cortex (DLPFC) increased the RIBE of participants with residential stability experience but had no effect on those with residential mobility experience. As residential mobility continues to increase worldwide, we may observe concomitant changes in racial intergroup relationships.

Within-session repeated transcranial direct current stimulation of the posterior parietal cortex enhances spatial working memory

Spatial working memory (SWM) is an essential cognitive ability that supports complex tasks, but its capacity is limited. Studies using transcranial direct current stimulation (tDCS) have shown potential benefits for SWM performance. Recent studies have shown that repeated short applications of tDCS affected corticospinal excitability. Moreover, neuroimaging studies have indicated that the pattern of neural activity measured in the posterior parietal cortex (PPC) tracks SWM ability. It is unknown whether repeated tDCS can enhance SWM and whether varied tDCS protocols (single 10 min tDCS, 10 min tDCS-5 min break-10 min tDCS, 10 min tDCS-20 min break-10 min tDCS) over the right PPC have different effects on SWM. The current study investigated whether offline single-session and repeated tDCS over the right PPC affects SWM updating, as measured by spatial 2-back and 3-back tasks. The results showed that stimulating the right PPC with repeated 10 min anodal tDCS significantly improved the response speed of the spatial 2-back task relative to single-session tDCS. Repeated 10 min tDCS with a longer interval (i.e. inter-stimulation interval of 20 min) enhanced the response speed of the spatial 3-back task. Altogether these findings provide causal evidence that suggests that the right PPC plays an important role in SWM. Furthermore, repeated tDCS with longer intervals may be a promising intervention for improving SWM-related function.

Anodal transcranial direct current stimulation over the posterior parietal cortex enhances three-dimensional mental rotation ability

Prior neuroimaging and neurophysiological studies have found that the right posterior parietal cortex (PPC) plays an important role in mental rotation ability. Transcranial direct-current stimulation (tDCS) has been shown the potential to enhance cognitive ability by delivering a low current to the brain cortex of interest, via electrodes on the scalp. Here, we tested whether stimulating the PPC with tDCS can improve three-dimensional mental rotation performance and narrow gender difference. The classic three-dimensional Shepard-Metzler task was measured after three stimulation conditions (right PPC, left PPC, sham stimulation). The results indicated that stimulating the right PPC induced an improvement in accuracy and response time of mental rotation relative to sham stimulation. Stimulating the left PPC caused an enhancement in the accuracy but not in the response time. Gender difference during mental rotation was diminished after stimulation. These findings indicated that the PPC regions played a causal role in mental rotation ability. tDCS could be used as a promising non-invasive method to improve mental rotation skills in individuals with lower ability and to provide an effective therapeutic tool for neurological disorder rehabilitation.

Reading proficiency influences the effects of transcranial direct current stimulation: Evidence from selective modulation of dorsal and ventral pathways of reading in bilinguals

INTRODUCTION

tDCS can modulate reading which is processed by lexical (ventral) and sub-lexical (dorsal) pathways. Previous research indicates that pathway recruitment in bilinguals depends on a script's orthographic depth and a reader's proficiency with it. The effect of tDCS on each reading pathway has not been investigated in bilinguals. We stimulated the left dorsal and ventral pathways separately in Chinese-English (C-E) bilinguals to understand whether pathway-specific modulation by tDCS is possible and, if so, how it is influenced by orthographic depth and script proficiency.

METHODS

A double-blind, sham-controlled, within-subject experiment was designed wherein 16 balanced bilinguals received anodal tDCS in dorsal, ventral and sham sessions. Two tDCS montages of electrode sizes 5 × 5 cm² with 1) anode at CP5 and cathode at CZ, and 2) anode at TP7 and cathode at nape of the neck, were applied for stimulating the dorsal and ventral pathways respectively. Bilinguals were asked to read word lists for each language before and after stimulation. RTs for accurate trials were analysed using linear mixed-effect modelling that included proficiency scores for reading English pseudo-words (PW) and Chinese pinyin.

RESULTS

For both languages, word reading RTs were faster following dorsal pathway stimulation. The dorsal stimulation effect (change in RT) was negatively correlated with pseudoword reading and pinyin proficiency. Stimulation of the ventral pathway decreased RTs only for Chinese reading.

CONCLUSION

Dorsal and ventral reading pathways can be selectively modulated by tDCS in bilingual readers with dorsal (sub-lexical) pathway stimulation affecting reading in both scripts and ventral (lexical) pathway stimulation selectively affecting Chinese reading. Dorsal pathway tDCS effects are modulated by sub-lexical reading proficiency.

Modality effects in verbal working memory updating: Transcranial direct current stimulation over human inferior frontal gyrus and posterior parietal cortex

Verbal working memory (VWM) involves visual and auditory verbal information. Neuroimaging studies have shown significant modality effects for VWM in the left posterior parietal cortex (PPC). The left inferior frontal gyrus (IFG) is more sensitive to auditory and phonological information. However, much less is known about the effects of transcranial direct current stimulation (tDCS) over the left PPC and IFG on different sensory modalities of VWM (auditory vs. visual). Therefore, the present study aimed to examine whether tDCS over the left PPC and IFG affects visual and auditory VWM updating performance using a single-blind design. Fifty-one healthy participants were randomly assigned to three tDCS groups (left PPC/left IFG/sham) and were asked to complete both the visual and auditory letter 3-back tasks. Results showed that stimulating the left PPC enhanced the response efficiency of visual, but not auditory, VWM compared with the sham condition. Anodal stimulation to the left IFG improved the response efficiency of both tasks. The present study revealed a modality effect of VWM in the left PPC, while the left IFG had a causal role in VWM updating of different sensory modalities.

Converging Evidence That Neural Plasticity Underlies Transcranial Direct-Current Stimulation

It is not definitely known how direct-current stimulation causes its long-lasting effects. Here, we tested the hypothesis that the long time course of transcranial direct-current stimulation (tDCS) is because of the electrical field increasing the plasticity of the brain tissue. If this is the case, then we should see tDCS effects when humans need to encode information into long-term memory, but not at other times. We tested this hypothesis by delivering tDCS to the ventral visual stream of human participants during different tasks (i.e., recognition memory vs. visual search) and at different times during a memory task. We found that tDCS improved memory encoding, and the neural correlates thereof, but not retrieval. We also found that tDCS did not change the efficiency of information processing during visual search for a certain target object, a task that does not require the formation of new connections in the brain but instead relies on attention and object recognition mechanisms. Thus, our findings support the hypothesis that direct-current stimulation modulates brain activity by changing the underlying plasticity of the tissue.

The Effects of Acute Transcranial Direct Current Stimulation on Attentional Bias in Pedophilic Disorder: A Preregistered Pilot Study

OBJECTIVES

Individuals with pedophilic disorder (PD) experience personal and interpersonal difficulties and are at risk of sexually offending against children. As such, innovative and empirically validated treatments are needed. Recent studies have indicated that men who have sexually offended against children (SOC) with PD display an automatic attention bias for child-related stimuli as well as reduced activity in the dorsolateral prefrontal cortex (dlPFC), a brain area involved in cognitive control, including control over sexual arousal. In this preregistered pilot study, we are the first to investigate whether acutely increasing prefrontal activity could reduce the putative pedophilic attention bias.

MATERIALS AND METHODS

We delivered a single 20-min session of active anodal versus sham transcranial direct current stimulation (tDCS) over the left dlPFC to 16 SOC with PD and 16 matched healthy controls, while they performed a task requiring controlled attention to computer-generated images of clothed and nude children and adults. We collected responses unobtrusively by recording eye movements.

RESULTS

Our results did not support the presence of the expected automatic attention bias across outcome measures. Nonetheless, we found a response facilitation with child targets in patients and, unexpectedly, in controls, likely due to unwanted salience effects. Active versus sham tDCS reduced this bias across groups, as indicated by a significant group*condition interaction (p = 0.04). However, no attentional bias and no tDCS effects on attentional responses to child and adult images emerged following tDCS.

CONCLUSIONS

These results suggest enhanced cognitive control in response to salient stimuli during active tDCS. Thus, to assist future studies on neuromodulation in PD, we provide suggestions for design improvement.

Therapeutic effects of anodal transcranial direct current stimulation in a rat model of ADHD

Most therapeutic candidates for treating attention-deficit hyperactivity disorder (ADHD) have focused on modulating the dopaminergic neurotransmission system with neurotrophic factors. Regulation of this system by transcranial direct current stimulation (tDCS) could contribute to the recovery of cognitive symptoms observed in patients with ADHD. Here, male spontaneously hypertensive rats (SHR) were subjected to consecutive high-definition tDCS (HD-tDCS) (20 min, 50 μA, current density 63.7 A/m², charge density 76.4 kC/m²) over the prefrontal cortex. This treatment alleviated cognitive deficits, with an increase in tyrosine hydroxylase and vesicular monoamine transporter two and significantly decreased plasma membrane reuptake transporter (DAT). HD-tDCS application increased the expression of several neurotrophic factors, particularly brain-derived neurotrophic factor (BDNF), and activated hippocampal neurogenesis. Our results suggest that anodal HD-tDCS over the prefrontal cortex may ameliorate cognitive dysfunction via regulation of DAT and BDNF in the mesocorticolimbic dopaminergic pathways, and therefore represents a potential adjuvant therapy for ADHD.

Enhancing Memory for Relationship Actions by Transcranial Direct Current Stimulation of the Superior Temporal Sulcus

We examine the effect of transcranial direct current stimulation (tDCS) of right superior temporal sulcus (rSTS) in memorization of approach/avoidance relationship-action sentences; for example, “Alejandro accepted/rejected Marta in his group.” Sixty-five university students participated in a tDCS study, in which a between-subjects design was adopted. Sixty-four participants were also given the behavioral approach system (BAS) and behavioral inhibition system (BIS) scales. Participants were subjected to 20 min of stimulation: anodal (N = 24), cathodal (N = 21), or sham (N = 20); subsequently, they were given a list of 40 sentences (half approach and half avoidance) and told to try to memorize them. Finally, they performed a changed/same memory task (half the sentences were the “same” and half were “changed”). Previously, we had examined performance in the memory task without tDCS with another group of participants (N = 20). We found that anodal stimulation improved d’ index of discriminability (hits-false alarms) compared to sham and cathodal conditions for both approach and avoidance sentences. Moreover, the comparison between anodal and task-alone performance showed that stimulation improved d’ index of approach sentences more, as task-alone performance showed better discrimination for avoidance than for approach. Likewise, we explored a potential modulation of tDCS effect by (BAS) and (BIS) traits. We found that d’ index improvement in anodal stimulation condition only benefited low BAS and low BIS participants. Implications of these results are discussed in the context of rSTS function in encoding and memorizing verbally described intentional relationship-actions and the role of individual differences on modulating tDCS effect.

Individually customized transcranial temporal interference stimulation for focused modulation of deep brain structures: a simulation study with different head models

Temporal interference (TI) stimulation was recently proposed that allows for the stimulation of deep brain structures with neocortical regions being minimally stimulated. For human brain modulation, TI current patterns are known to be considerably affected by the complex structures of the human head, and thus, it is hard to deliver TI current to a specific deep brain region. In this study, we optimized scalp electrode configurations and injection currents that can deliver maximum TI stimulation currents to a specific deep brain region, the head of the right hippocampus in this study, considering the real anatomical head structures of each individual. Three realistic finite element (FE) head models were employed for the optimization of TI stimulation. To generate TI current patterns, two pairs of scalp electrodes were selected, which carry two sinusoidally alternating currents with a small frequency difference. For every possible combination of electrode pairs, optimal injection currents delivering the maximal TI currents to the head of the right hippocampus were determined. The distribution of the optimized TI currents was then compared with that of the unoptimized TI currents and the conventional single frequency alternating current stimulation. Optimization of TI stimulation parameters allows for the delivery of the desired amount of TI current to the target region while effectively reducing the TI currents delivered to cortical regions compared to the other stimulation approaches. Inconsistency of the optimal stimulation conditions suggest that customized stimulation, considering the individual anatomical differences, is necessary for more effective transcranial TI stimulation. Customized transcranial TI stimulation based on the numerical field analysis is expected to enhance the overall effectiveness of noninvasive stimulation of the human deep brain structures.

Anodal tDCS of right temporo-parietal junction promotes threat detection in low-spatial-frequency channels

Fast detection of threat is crucial for survival. Previous studies have suggested the involvement of low- and high-spatial-frequency (LSF and HSF) information in the process of threat detection. However, the specific contributions of LSF and HSF information to it are still controversial. Here we probed this issue by utilizing a prime procedure coupled with the transcranial direct current stimulation (tDCS) technique. Specifically, in a single-blind design, participants were exposed to LSF or HSF faces prior to the presentation of looming or receding spheres. Meanwhile, tDCS was applied over the right or left temporo-parietal junction (TPJ), which has been found to be preferentially associated with the processing of LSF and HSF information, respectively. The results showed significant LSF-related facilitation of threat detection when anodal tDCS was applied to the right TPJ. However, HSF-related facilitation of threat detection was not observed when anodal tDCS was applied to the left TPJ. The findings support the idea that LSF information can facilitate threat detection, and provide direct evidence that the right TPJ is vital for LSF-related facilitation of threat detection.

Development of a transcranial direct current stimulation (tDCS) device based on polarity interchangeable electrodes

BACKGROUND:

Transcranial direct current stimulation (tDCS) is a noninvasive brain stimulation technique that can help modulate cortical excitability by transmitting direct current (DC) between a pair of scalp electrodes. To date, most studies on tDCS have been relatively short-lived, and the DC stimulations only lasted a few minutes. Conventional tDCS devices usually have some problems such as needing a lot of patches and lead lines.

OBJECTIVE:

Since conventional tDCS devices are unsuitable for use in long-term stimulations, we developed a new tDCS which can easily be used by unskilled persons.

METHODS:

We developed a new tDCS device that does not have lead lines for tDCS electrodes and has a simple structure.

RESULT:

This device can achieve stimulation with polarity interchangeable DC without physically swapping the anode and cathode. The performance of the proposed device was verified through an experiment.

CONCLUSION:

The developed tDCS device can contribute to long-term research as it uses neuroelectric stimulation.

Mind the stimulation site: Enhancing and diminishing sentence comprehension with anodal tDCS

In a previous sham-controlled study, we showed the feasibility of increasing language comprehension in healthy participants by applying anodal transcranial direct current stimulation (atDCS) over the left inferior frontal gyrus (LIFG). In the present work, we present a follow-up experiment targeting with atDCS the left inferior parietal cortex (LIPC) while participants performed the same auditory comprehension task used in our previous experiment. Both neural sites (LIFG and LIPC) are crucial hubs of Baddeley's model of verbal short-term memory (vSTM). AtDCS over LIPC decreased accuracy as compared to sham and LIFG stimulation, suggesting the involvement of this area in sentence comprehension. Crucially, our results highlighted that applying tDCS over different hubs of the same neural network can lead to opposite behavioural results, with relevant implications from a clinical perspective.

Encoding strategy mediates the effect of electrical stimulation over posterior parietal cortex on visual short-term memory

Over past decades, converging neuroimaging and electrophysiological findings have suggested a crucial role of posterior parietal cortex (PPC) in supporting the storage capacity of visual short-term memory (VSTM). Moreover, a few recent studies have shown that electrical stimulation over PPC can enhance VSTM capacity, making it a promising method for improving VSTM function. However, the reliability of these results is still in question because null findings have also been observed. Among studies that reported significant effects, some found increased VSTM capacity only in people with low capacity. Here, we hypothesized that subjects' encoding strategy might be a key source of these variable results. To directly test this hypothesis, we stimulated PPC using transcranial direct-current stimulation (tDCS) in male and female human subjects instructed to employ different encoding strategies during a VSTM recall task. We found that VSTM capacity was higher in subjects who were instructed to remember all items in the supra-capacity array of visual stimuli (i.e., the remember-all group), compared to subjects who were told to focus on a subset of these stimuli (i.e., the remember-subset group). As predicted, anodal tDCS over PPC significantly enhanced VSTM capacity only in the remember-subset group, but not in the remember-all group. Additionally, no effect of encoding strategy or its interaction with electrical stimulation was found on VSTM precision. Together, these results suggest that encoding strategy has a selective influence on VSTM capacity and this influence of encoding strategy mediates the effect of electrical stimulation over PPC on VSTM function.

Enhanced tES and tDCS computational models by meninges emulation

OBJECTIVE

Understanding how current reaches the brain during transcranial electrical stimulation (tES) underpins efforts to rationalize outcomes and optimize interventions. To this end, computational models of current flow relate applied dose to brain electric field. Conventional tES modeling considers distinct tissues like scalp, skull, cerebrospinal fluid (CSF), gray matter and white matter. The properties of highly conductive CSF are especially important. However, modeling the space between skull and brain as entirely CSF is not an accurate representation of anatomy. The space conventionally modeled as CSF is approximately half meninges (dura, arachnoid, and pia) with lower conductivity. However, the resolution required to describe individual meningeal layers is computationally restrictive in an MRI-derived head model. Emulating the effect of meninges through CSF conductivity modification could improve accuracy with minimal cost.

APPROACH

Models with meningeal layers were developed in a concentric sphere head model. Then, in a model with only CSF between skull and brain, CSF conductivity was optimized to emulate the effect of meningeal layers on cortical electric field for multiple electrode positions. This emulated conductivity was applied to MRI-derived models.

MAIN RESULTS

Compared to a model with conventional CSF conductivity (1.65 S m-1), emulated CSF conductivity (0.85 S m-1) produced voltage fields better correlated with intracranial recordings from epilepsy patients.

SIGNIFICANCE

Conventional tES models have been validated using intracranial recording. Residual errors may nonetheless impact model utility. Because CSF is so conductive to current flow, misrepresentation of the skull-brain interface as entirely CSF is not realistic for tES modeling. Updating the conventional model with a CSF conductivity emulating the effect of the meninges enhances modeling accuracy without increasing model complexity. This allows existing modeling pipelines to be leveraged with a simple conductivity change. Using 0.85 S m-1 emulated CSF conductivity is recommended as the new standard in non-invasive brain stimulation modeling.

Can Anodal Transcranial Direct Current Stimulation Increase Steady-State Visual Evoked Potential Responses?

BACKGROUND

It has been frequently reported that non-negligible numbers of individuals have steady-state visual evoked potential (SSVEP) responses of low signal-to-noise-ratio (SNR) to specific stimulation frequencies, which makes detection of the SSVEP difficult especially in brain-computer interface applications. We investigated whether SSVEP can be modulated by anodal transcranial direct-current stimulation (tDCS) of the visual cortex.

METHODS

Each participant participated in two 20-min experiments-an actual tDCS experiment and a sham tDCS experiment-that were conducted on different days. Two representative electroencephalogram (EEG) features used for the SSVEP detection, SNR and amplitude, were tested for pre- and post-tDCS conditions to observe the effect of the anodal tDCS.

RESULTS

The EEG features were significantly enhanced by the anodal tDCS for the electrodes with low pre-tDCS SNR values, whereas the effect was not significant for electrodes with relatively higher SNR values.

CONCLUSION

Anodal tDCS of the visual cortex may be effective in enhancing the SNR and amplitude of the SSVEP response especially for individuals with low-SNR SSVEP.

Electric field simulations for transcranial brain stimulation using FEM: an efficient implementation and error analysis

OBJECTIVE

Transcranial magnetic stimulation (TMS) and transcranial electric stimulation (TES) modulate brain activity non-invasively by generating electric fields either by electromagnetic induction or by injecting currents via skin electrodes. Numerical simulations based on anatomically detailed head models of the TMS and TES electric fields can help us to understand and optimize the spatial stimulation pattern in the brain. However, most realistic simulations are still slow, and the role of anatomical fidelity on simulation accuracy has not been evaluated in detail so far.

APPROACH

We present and validate a new implementation of the finite element method (FEM) for TMS and TES that is based on modern algorithms and libraries. We also evaluate the convergence of the simulations and estimate errors stemming from numerical and modelling aspects.

MAIN RESULTS

Comparisons with analytical solutions for spherical phantoms validate our new FEM implementation, which is three to six times faster than previous implementations. The convergence results suggest that accurately capturing the tissue geometry in addition to choosing a sufficiently accurate numerical method is of fundamental importance for accurate simulations.

SIGNIFICANCE

The new implementation allows for a substantial increase in computational efficiency of FEM TMS and TES simulations. This is especially relevant for applications such as the systematic assessment of model uncertainty and the optimization of multi-electrode TES montages. The results of our systematic error analysis allow the user to select the best tradeoff between model resolution and simulation speed for a specific application. The new FEM code is openly available as a part of our open-source software SimNIBS 3.0.

Stimulating the ventrolateral prefrontal cortex (VLPFC) modulates frustration-induced aggression: A tDCS experiment

BACKGROUND

The prefrontal cortex is crucial for top-down regulation of aggression, but the neural underpinnings of aggression are still poorly understood. Past research showed the transcranial direct current stimulation (tDCS) over the ventrolateral prefrontal cortex (VLPFC) modulates aggression following exposure to risk factors for aggression (e.g., social exclusion, violent media). Although frustration is a key risk factor for aggression, no study to date has examined the modulatory role of tDCS on frustration-induced aggression.

OBJECTIVES

By exploring the VLPFC involvement in frustration-aggression link, we tested the hypothesis that the anodal tDCS over right and left VLPFC modulates frustration-induced aggression. We also explored whether tDCS interacts with gender to influence frustration-induced aggression.

METHODS

90 healthy participants (45 men) were randomly assigned to receive anodal or sham tDCS over the right or left VLPFC before being frustrated by an accomplice. To increase reliability, several tasks were used to measure aggression.

RESULTS

We found that anodal tDCS over the left VLPFC, compared to sham stimulation, increased aggression. Unexpectedly, no main effect was found following tDCS of right VLPFC. However, we also found a significant interaction between gender and tDCS, showing that males were more aggressive than females following sham stimulation, but females became as aggressive as males following active tDCS.

CONCLUSION

Overall, these results shed light on the neural basis of frustration-induced aggression, providing further evidence for the involvement of VLPFC in modulating aggressive responses, and on gender differences in aggression. Future research should further investigate the role of stimulating the VLPFC on frustration-induced aggression.

Modulation of Emotional Conflict Processing by High-Definition Transcranial Direct Current Stimulation (HD-TDCS)

Cognitive control is characterized by selective attention to relevant stimuli while irrelevant, distracting stimuli are inhibited. While the classical color-word Stroop task was implemented to investigate the processes of cognitive control, a variant of it—the face-word Stroop task—allows for directly investigating processes of emotional conflict control. It is thought that the prefrontal cortex (PFC) is especially involved in processes of cognitive control, while the rostral cingulate is mainly associated with the resolution of emotional conflict. In recent years, the role of the dorsolateral PFC (DLPFC) during the performance of the classical Stroop was investigated by means of transcranial direct current stimulation (tDCS) with divergent results. However, investigations to the causal role of the DLPFC during emotional conflict processing are rare. For this purpose, we used a combined high-definition tDCS (HD-tDCS)/electroencephalogram (EEG) setting to investigate the impact of anodal stimulation of the left DLPFC on behavioral and electrophysiological responses during an emotional face-word Stroop task. In two separate sessions, participants (n = 18) received either sham or anodal HD-tdc stimulation while responding to the emotional expression of the face and ignoring the word. Our results show that anodal stimulation of the left DLPFC increases the behavioral interference effect, that is, the already decelerated reaction times (RTs) to incongruent trials further increase while RTs to congruent trials remain largely unaffected. Furthermore, the stimulation modulates brain response to emotional facial expressions during the face-word Stroop generally—independent of the valence of the emotional expression and the congruency of the combined face-word presentation, the N170 decreases during anodal stimulation. These results reveal that the left DLPFC has a causal role in emotional conflict processing during a face-word Stroop.

Simulation Analyses of tDCS Montages for the Investigation of Dorsal and Ventral Pathways

Modulating higher cognitive functions like reading with transcranial direct current stimulation (tDCS) can be challenging as reading involves regions in the dorsal and ventral cortical areas that lie in close proximity. If the two pathways are stimulated simultaneously, the function of dorsal pathway (predominantly used for graphophonological conversion) might interfere with the function of the ventral pathway (used for semantics), and vice-versa. To achieve functional specificity in tDCS for investigating the two pathways of reading, it is important to stimulate each pathway per session such that the spread of current across the cortical areas due to the two montages has minimal overlap. The present study intends to achieve this by introducing a systematic approach for tDCS analysis. We employed the COMETS2 software to simulate 10 montage configurations (5 for each pathway) for three electrode sizes: 5 × 5, 3 × 3, and 5 × 7 cm2. This diversity in montage configuration is chosen since previous studies found the position and the size of anode and cathode to play an important role. The values of the magnitude of current density (MCD) obtained from the configuration were used to calculate: (i) average MCD in each cortical lobe, (ii) number of overlapping coordinates, and (iii) cortical areas with high MCD. The measures (i) and (iii) ascertained the current spread by each montage within a cortical lobe, and (ii) verified the overlap of the spread of current between a pair of montages. The analyses show that a montage using the electrode size of 5 × 5 cm2 with the anode at CP5 and cathode at CZ, and another with anode at TP7 and cathode at nape of the neck are optimal choices for dorsal and ventral pathways, respectively. To verify, we cross-validated the results with ROAST. This systematic approach was helpful in reducing the ambiguity of montage selection prior to conducting a tDCS study.

Comparative performance of the finite element method and the boundary element fast multipole method for problems mimicking transcranial magnetic stimulation (TMS)

OBJECTIVE

A study pertinent to the numerical modeling of cortical neurostimulation is conducted in an effort to compare the performance of the finite element method (FEM) and an original formulation of the boundary element fast multipole method (BEM-FMM) at matched computational performance metrics.

APPROACH

We consider two problems: (i) a canonic multi-sphere geometry and an external magnetic-dipole excitation where the analytical solution is available and; (ii) a problem with realistic head models excited by a realistic coil geometry. In the first case, the FEM algorithm tested is a fast open-source getDP solver running within the SimNIBS 2.1.1 environment. In the second case, a high-end commercial FEM software package ANSYS Maxwell 3D is used. The BEM-FMM method runs in the MATLAB^® 2018a environment.

MAIN RESULTS

In the first case, we observe that the BEM-FMM algorithm gives a smaller solution error for all mesh resolutions and runs significantly faster for high-resolution meshes when the number of triangular facets exceeds approximately 0.25 M. We present other relevant simulation results such as volumetric mesh generation times for the FEM, time necessary to compute the potential integrals for the BEM-FMM, and solution performance metrics for different hardware/operating system combinations. In the second case, we observe an excellent agreement for electric field distribution across different cranium compartments and, at the same time, a speed improvement of three orders of magnitude when the BEM-FMM algorithm used.

SIGNIFICANCE

This study may provide a justification for anticipated use of the BEM-FMM algorithm for high-resolution realistic transcranial magnetic stimulation scenarios.

Selective changes in moral judgment by noninvasive brain stimulation of the medial prefrontal cortex

Multiple cortical networks intervene in moral judgment, among which the dorsolateral prefrontal cortex (DLPFC) and the medial prefrontal structures (medial PFC) emerged as two major territories, which have been traditionally attributed, respectively, to cognitive control and affective reactions. However, some recent theoretical and empirical accounts disputed this dualistic approach to moral evaluation. In the present study, to further assess the functional contribution of the medial PFC in moral judgment, we modulated its cortical excitability by means of transcranial direct current stimulation (tDCS) and tracked the change in response to different types of moral dilemmas, including switch-like and footbridge-like moral dilemmas, with and without personal involvement. One hundred participants (50 males) completed a questionnaire to assess the baseline levels of deontology. Next, participants were randomly assigned to receive anodal, sham, or cathodal tDCS over the medial prefrontal structures and then were asked to address a series of dilemmas. The results showed that participants who received anodal stimulation over the medial PFC provided more utilitarian responses to switch-like (but not footbridge-like) dilemmas than those who received cathodal tDCS. We also found that neurostimulation modulated the influence that deontology has on moral choices. Specifically, in the anodal tDCS group, participants' decisions were less likely to be influenced by their baseline levels of deontology compared with the sham or cathodal groups. Overall, our results seem to refute a functional role of the medial prefrontal structures purely restricted to affective reactions for moral dilemmas, providing new insights on the functional contribution of the medial PFC in moral judgment.

Neuromodulation of Right Auditory Cortex Selectively Increases Activation in Speech-Related Brain Areas in Brainstem Auditory Agnosia

Auditory agnosia is an inability to make sense of sound that cannot be explained by deficits in low-level hearing. In view of recent promising results in the area of neurorehabilitation of language disorders after stroke, we examined the effect of transcranial direct current stimulation (tDCS) in a young woman with general auditory agnosia caused by traumatic injury to the left inferior colliculus. Specifically, we studied activations to sound embedded in a block design using functional magnetic resonance imaging before and after application of anodal tDCS to the right auditory cortex. Before tDCS, auditory discrimination deficits were associated with abnormally reduced activations of the auditory cortex and bilateral unresponsiveness of the anterior superior temporal sulci and gyri. This session replicated a previous functional scan with the same paradigm a year before the current experiment. We then applied anodal tDCS over right auditory cortex for 20 min-utes and immediately re-scanned the patient. We found increased activation of bilateral auditory cortices and, for speech sounds, selectively increased activation in Broca's and Wernicke's areas. Future research might consider the long-term behavioral effects after neurostimulation in auditory agnosia and its potential use in the neurorehabilitation of more general auditory disorders.

Seeing the World as it is: Mimicking Veridical Motion Perception in Schizophrenia Using Non-invasive Brain Stimulation in Healthy Participants

Schizophrenia (Sz) is a mental health disorder characterized by severe cognitive, emotional, social, and perceptual deficits. Visual deficits are found in tasks relying on the magnocellular/dorsal stream. In our first experiment we established deficits in global motion processing in Sz patients compared to healthy controls. We used a novel task in which background optic flow produces a distortion of the apparent trajectory of a moving stimulus, leading control participants to provide biased estimates of the true motion trajectory under conditions of global stimulation. Sz patients were significantly less affected by the global background motion, and reported trajectories that were more veridically accurate than those of controls. In order to study the mechanism of this effect, we performed a second experiment where we applied transcranial electrical stimulation over area MT+ to selectively modify global motion processing of optic flow displays in healthy participants. Cathodal and high frequency random noise stimulation had opposite effects on trajectory perception in optic flow. The brain stimulation over a control site and in a control task revealed that the effect of stimulation was specific for global motion processing in area MT+. These findings both support prior studies of impaired early visual processing in Sz and provide novel approaches for measurement and manipulation of the underlying circuits.

Anodal frontal tDCS for chronic cluster headache treatment: a proof-of-concept trial targeting the anterior cingulate cortex and searching for nociceptive correlates

BACKGROUND

Percutaneous occipital nerve stimulation (ONS) is effective in refractory chronic cluster headache (rCCH) patients. Responders to ONS differ from non-responders by greater glucose metabolism in subgenual anterior cingulate cortex (sgACC). We reasoned that transcranial direct current stimulation (tDCS), a non-invasive approach, might be able to activate this area and thus improve rCCH patients. Our objective was to explore in a pilot trial the therapeutic potential of tDCS (anode at Fz, cathode over C7) and its possible effects on pain perception, frontal executive functions and mood in rCCH patients.

METHODS

Thirty-one patients were asked to apply daily 20-min sessions of 2 mA tDCS for 4 or 8 weeks after a 1-month baseline. CH attacks were monitored with paper diaries. The primary outcome measure was change in weekly attacks between baseline and the last week of tDCS. Twenty-three patients were available for a modified ITT analysis, 21 for per-protocol analysis. We also explored treatment-related changes in thermal pain thresholds and nociceptive blink reflexes (nBR), frontal lobe function and mood scales.

RESULTS

In the per-protocol analysis there was a mean 35% decrease of attack frequency (p = 0.0001) with 41% of patients having a ≥ 50% decrease. Attack duration and intensity were also significantly reduced. After 8 weeks (n = 10), the 50% responder rate was 45%, but at follow-up 2 weeks after tDCS (n = 16) mean attack frequency had returned to baseline levels. The treatment effect was significant in patients with high baseline thermal pain thresholds in the forehead (n = 12), but not in those with low thresholds (n = 9). The Frontal Assessment Battery score increased after tDCS (p = 0.01), while there was no change in depression scores or nBR.

CONCLUSION

tDCS with a Fz-C7 montage may have a preventive effect in rCCH patients, especially those with low pain sensitivity, suggesting that a sham-controlled trial in cluster headache is worthwhile. Whether the therapeutic effect is due to activation of the sgACC that can in theory be reached by the electrical field, or of other prefrontal cortical areas remains to be determined.

Modulation of negative emotions through anodal tDCS over the right ventrolateral prefrontal cortex

Increasing evidence suggests that the right ventrolateral prefrontal cortex (rVLPFC) plays a critical role in emotion regulation, in particular concerning negative feelings. In the present research, we applied anodal transcranial direct current stimulation (tDCS) over the rVLPFC with a twofold purpose. First, we aimed at exploring the feasibility of modulating the subjective experience of emotions through tDCS in healthy participants. Second, we wanted to assess which specific emotion can be regulated (and which cannot) with this brain stimulation approach. We designed a double-blind, between-subjects, sham-controlled study in which 96 participants watched short video clips eliciting different emotions during anodal or sham tDCS over the rVLPFC. Emotional reactions to each video clip were assessed with self-report scales measuring eight basic emotions. Results showed that, in contrast to the sham condition, tDCS over the rVLPFC reduced the perceived extent of specific negative emotions, namely, fear, anxiety, and sadness, compared to other negative or positive feelings. Overall, these results support the role of rVLPFC in regulating negative emotions, mostly associated with the prevention of dangerous situations (i.e., fear, anxiety, and sadness).

Brain Stimulation Over the Frontopolar Cortex Enhances Motivation to Exert Effort for Reward

BACKGROUND

Loss of motivation is a characteristic feature of several psychiatric and neurological disorders. However, the neural mechanisms underlying human motivation are far from being understood. Here, we investigate the role that the frontopolar cortex (FPC) plays in motivating cognitive and physical effort exertion by computing subjective effort equivalents.

METHODS

We manipulated neural processing with transcranial direct current stimulation targeting the FPC while 141 healthy participants decided whether or not to engage in cognitive or physical effort to obtain rewards.

RESULTS

We found that brain stimulation targeting the FPC increased the amount of both types of effort participants were willing to exert for rewards.

CONCLUSIONS

Our findings provide important insights into the neural mechanisms involved in motivating effortful behavior. Moreover, they suggest that considering the motivation-related activity of the FPC could facilitate the development of treatments for the loss of motivation commonly seen in psychiatric and other neurological disorders.

The causal role of the somatosensory cortex in prosocial behaviour

Witnessing another person’s suffering elicits vicarious brain activity in areas that are active when we ourselves are in pain. Whether this activity influences prosocial behavior remains the subject of debate. Here participants witnessed a confederate express pain through a reaction of the swatted hand or through a facial expression, and could decide to reduce that pain by donating money. Participants donate more money on trials in which the confederate expressed more pain. Electroencephalography shows that activity of the somatosensory cortex I (SI) hand region explains variance in donation. Transcranial magnetic stimulation (TMS) shows that altering this activity interferes with the pain–donation coupling only when pain is expressed by the hand. High-definition transcranial direct current stimulation (HD-tDCS) shows that altering SI activity also interferes with pain perception. These experiments show that vicarious somatosensory activations contribute to prosocial decision-making and suggest that they do so by helping to transform observed reactions of affected body-parts into accurate perceptions of pain that are necessary for decision-making.

When we experience physical pain, certain areas in our brain that process bodily sensation and emotions switch on. If we see someone else in pain, many of the same regions also get activated. In contrast, convicted criminals with psychopathic traits have less activation in these areas of the brain when witnessing someone’s pain; they also show less empathy and disregard the needs of others. This suggests that a lack of this ‘shared activations’ may lead to problems in empathy. In fact, many scientists believe that shared activations are why we feel empathy for people in pain, and why we are driven to help them. Yet, there is little direct evidence about how the activity in the pain processing parts of the brain actually influences helpful behavior. As a result, some scientists now argue that empathy-related processes may actually contribute very little to helping behavior.

Gallo et al. designed an experiment where participants watched videos of someone having their hand swatted with a belt, and showing different levels of pain as a result. The volunteers could decide to reduce the amount of pain the person received by donating money they could have taken home. The more pain the participants thought the victim was in, the more money they gave up to lessen it. During the study, the activity in the brain region that processes pain in the hand was also measured in the participants. The more active this region was, the more money people donated to help.

Then, Gallo et al. used techniques that interfered with the activity of the brain area involved in perceiving sensations from the hand. This interference changed how accurately participants assessed the victim's pain. It also disrupted the link between donations and the victim's perceived pain: the amount of money people gave no longer matched the level of pain they had witnessed. This suggests that the brain areas that perceive sensations of pain in the self, which evolved primarily to experience our own sensations, also have a social function. They transform the sight of bodily harm into an accurate feeling for how much pain the victim experiences. The findings also show that we need this feeling so we can adapt our help to the needs of others. In the current debate about the role of empathy in helping behaviors, this study demonstrates that empathy-related brain activity indeed promotes helping by allowing us to detect those that need our assistance.

Understanding the relationship between helping behavior and the activity of the brain may further lead to treatments for individuals with antisocial behavior and for children with callous and unemotional traits, a disorder that is associated with a lack of empathy and a general disregard for others.

Binding oneself to the mast: stimulating frontopolar cortex enhances precommitment

Humans often give in to temptations that are in conflict with valuable long-term goals like health or saving for the future. Such willpower failures represent a prevalent problem in everyday life and in many psychiatric disorders. Strategies that increase resistance to temptations could therefore improve overall societal well-being. One important strategy is to voluntarily precommit, i.e. to restrict one’s future action space by removing the tempting short-term option from the choice set, thereby leaving only the long-term option for implementation. The neural mechanisms necessary to implement precommitment have remained unknown. Here, we test whether anodal transcranial direct current stimulation (tDCS) over the frontopolar cortex (FPC) can improve precommitment. Participants performed a self-control task in which they could precommit to obtain a delayed larger reward by removing an immediately available smaller reward from the future choice options. We found that anodal stimulation over FPC selectively increased the propensity to precommit. In contrast, tDCS had no effects on non-binding decisions, impulse control or reward preference. Our data establish a causal role for the FPC in the implementation of precommitment, revealing a novel route to improving resistance against temptations.

Facilitation of Function and Manipulation Knowledge of Tools Using Transcranial Direct Current Stimulation (tDCS)

Using a variety of tools is a common and essential component of modern human life. Patients with brain damage or neurological disorders frequently have cognitive deficits in their recognition and manipulation of tools. In this study, we focused on improving tool-related cognition using transcranial direct current stimulation (tDCS). Converging evidence from neuropsychology, neuroimaging and non- invasive brain stimulation has identified the anterior temporal lobe (ATL) and inferior parietal lobule (IPL) as brain regions supporting action semantics. We observed enhanced performance in tool cognition with anodal tDCS over ATL and IPL in two cognitive tasks that require rapid access to semantic knowledge about the function or manipulation of common tools. ATL stimulation improved access to both function and manipulation knowledge of tools. The effect of IPL stimulation showed a trend toward better manipulation judgments. Our findings support previous studies of tool semantics and provide a novel approach for manipulation of underlying circuits.

Transcranial Direct Current Stimulation (tDCS): A Beginner's Guide for Design and Implementation

Transcranial direct current stimulation (tDCS) is a popular brain stimulation method that is used to modulate cortical excitability, producing facilitatory or inhibitory effects upon a variety of behaviors. There is, however, a current lack of consensus between studies, with many results suggesting that polarity-specific effects are difficult to obtain. This article explores some of these differences and highlights the experimental parameters that may underlie their occurrence. We provide a general, practical snapshot of tDCS methodology, including what it is used for, how to use it, and considerations for designing an effective and safe experiment. Our aim is to equip researchers who are new to tDCS with the essential knowledge so that they can make informed and well-rounded decisions when designing and running successful experiments. By summarizing the varied approaches, stimulation parameters, and outcomes, this article should help inform future tDCS research in a variety of fields.