Evaluation of local electric fields generated by transcranial direct current stimulation with an extracephalic reference electrode based on realistic 3D body modeling

Direct Current Stimulation over the Primary Motor Cortex, Cerebellum, and Spinal Cord to Modulate Balance Performance: A Randomized Placebo-Controlled Trial

OBJECTIVES

Existing applications of non-invasive brain stimulation in the modulation of balance ability are focused on the primary motor cortex (M1). It is conceivable that other brain and spinal cord areas may be comparable or more promising targets in this regard. This study compares transcranial direct current stimulation (tDCS) over (i) the M1, (ii) the cerebellum, and (iii) trans-spinal direct current stimulation (tsDCS) in the modulation of balance ability.

METHODS

Forty-two sports students were randomized in this placebo-controlled study. Twenty minutes of anodal 1.5 mA t/tsDCS over (i) the M1, (ii) the cerebellum, and (iii) the spinal cord, as well as (iv) sham tDCS were applied to each subject. The Y Balance Test, Single Leg Landing Test, and Single Leg Squat Test were performed prior to and after each intervention.

RESULTS

The Y Balance Test showed significant improvement after real stimulation of each region compared to sham stimulation. While tsDCS supported the balance ability of both legs, M1 and cerebellar tDCS supported right leg stand only. No significant differences were found in the Single Leg Landing Test and the Single Leg Squat Test.

CONCLUSIONS

Our data encourage the application of DCS over the cerebellum and spinal cord (in addition to the M1 region) in supporting balance control. Future research should investigate and compare the effects of different stimulation protocols (anodal or cathodal direct current stimulation (DCS), alternating current stimulation (ACS), high-definition DCS/ACS, closed-loop ACS) over these regions in healthy people and examine the potential of these approaches in the neurorehabilitation.

Transcranial alternating current stimulation does not affect microscale learning

A theory has been posited that microscale learning, which involves short intervals of a few seconds during explicit motor skill learning, considerably enhances performance. This phenomenon correlates with diminished beta-band activity in the frontal and parietal regions. However, there is a lack of neurophysiological studies regarding the relationship between microscale learning and implicit motor skill learning. In the present study, we aimed to determine the effects of transcranial alternating current stimulation (tACS) during short rest periods on microscale learning in an implicit motor task. We investigated the effects of 20-Hz β-tACS delivered during short rest periods while participants performed an implicit motor task. In Experiments 1 and 2, β-tACS targeted the right dorsolateral prefrontal cortex and the right frontoparietal network, respectively. The participants performed a finger-tapping task using their nondominant left hand, and microscale learning was separately analyzed for micro-online gains (MOnGs) and micro-offline gains (MOffGs). Contrary to our expectations, β-tACS exhibited no statistically significant effects on MOnGs or MOffGs in either Experiment 1 or Experiment 2. In addition, microscale learning during the performance of the implicit motor task was improved by MOffGs in the early learning phase and by MOnGs in the late learning phase. These results revealed that the stimulation protocol employed in this study did not affect microscale learning, indicating a novel aspect of microscale learning in implicit motor tasks. This is the first study to examine microscale learning in implicit motor tasks and may provide baseline information that will be useful in future studies.

Transcranial direct current stimulation combined with speech therapy in Fragile X syndrome patients: a pilot study

Background

Fragile X syndrome (FXS) is the leading cause of genetic intellectual disability. Among the neurobehavioral dysfunctions in FXS individuals, language development and literacy are compromised. Recent evidence hypothesized that the disruption of excitatory glutamatergic and GABAergic inhibitory neurotransmission balance might be responsible for impairment in cognitive function. In this study, we evaluated for the first time, the safety, tolerability, and efficacy of anodal prefrontal transcranial direct current stimulation (tDCS) combined with standard speech therapy to enhance language function in FXS patients.

Methods

In total, 16 adult FXS patients were enrolled. Participants underwent 45 min of anodic tDCS combined with speech therapy for 5 weeks (3 times per week). Language function was evaluated using the Test for Reception of Grammar-Version 2 (TROG-2) and subtests of the Italian Language Examination (Esame del Linguaggio - II, EDL-II). Right and left dorsolateral prefrontal cortex transcranial magnetic stimulation and concurrent electroencephalography (TMS-EEG) recordings were collected at baseline and after the treatment to evaluate cortical reactivity and connectivity changes.

Results

After 5 weeks of combined therapy, we observed a significant improvement in the writing (7.5%), reading (20.3%), repetition (13.3%), and TROG-2 (10.2%) tests. Parallelly with clinical change, TMS-EEG results showed a significant difference in TMS-evoked potential amplitude over the left frontal cortex after treatment (-0.73 ± 0.87 μV) compared to baseline (0.18 ± 0.84 μV).

Conclusion

Our study provides novel evidence that left anodal prefrontal tDCS combined with standard speech therapy could be effective in enhancing language function in FXS patients, mainly by inducing a rebalance of the dysfunctional prefrontal cortical excitability.

Are we really targeting and stimulating DLPFC by placing transcranial electrical stimulation (tES) electrodes over F3/F4?

In many clinical trials involving transcranial electrical stimulation (tES), target electrodes are typically placed over DLPFC with the assumption that this will primarily stimulate the underlying brain region. However, our study aimed to evaluate the electric fields (EF) that are actually delivered and identify prefrontal regions that may be inadvertently targeted in DLPFC tES. Head models were generated from the Human Connectome Project database's T1 + T2-weighted MRIs of 80 healthy adults. Two common DLPFC montages were simulated; symmetric-F4/F3, and asymmetric-F4/Fp1. Averaged EF was extracted from (1) the center of the target electrode (F4), and (2) the top 1% of voxels showing the strongest EF in individualized EF maps. Interindividual variabilities were quantified with the standard deviation of EF peak location/value. Similar steps were repeated with 66 participants with methamphetamine use disorder (MUDs) as an independent clinical population. In healthy adults, the group-level location of EF peaks was situated in the medial-frontopolar, and the individualized EF peaks were positioned in a cube with a volume of 29 cm3 /46 cm3 (symmetric/asymmetric montages). EFs in the frontopolar area were significantly higher than EF "under" the target electrode in both symmetric (peak: 0.41 ± 0.06, F4:0.22 ± 0.04) and asymmetric (peak: 0.38 ± 0.04, F4:0.2 ± 0.04) montages (Heges'g > 0.7). Similar results with slight between-group differences were found in MUDs. We highlighted that in common DLPFC tES montages, in addition to interindividual/intergroup variability, the frontopolar received the highest EFs rather than DLPFC as the main target. We specifically recommended considering the potential involvement of the frontopolar area as a mechanism underlying the effectiveness of DLPFC tES protocols.

Monopolar tDCS might affect brainstem reflexes: A computational and neurophysiological study

OBJECTIVE

To assess whether monopolar multi-electrode transcranial direct current stimulation (tDCS) montages might selectively affect deep brain structures through computational predictions and neurophysiological assessment.

METHODS

Electric field distribution in deep brain structures (i.e., thalamus and midbrain) were estimated through computational models simulating tDCS with two monopolar and two monopolar multi-electrode montages. Monopolar multi-electrode tDCS was then applied to healthy subject, and effects on pontine and medullary circuitries was evaluated studying changes in blink reflex (BR) and masseter inhibitory reflex (MIR).

RESULTS

Computational results suggest that tDCS with monopolar multi-electrode montages might induce electric field intensities in deep brain structure comparable to those in grey matter, while neurophysiological results disclosed that BR and MIR were selectively modulated by tDCS only when cathode was placed over the right deltoid.

CONCLUSIONS

Multi-electrode tDCS (anodes over motor cortices, cathode over right deltoid) could induce significant electric fields in the thalamus and midbrain, and selectively affect brainstem neural circuits.

SIGNIFICANCE

Multi-electrode tDCS (anodes over motor cortices, cathode over right deltoid) might be further explored to affect brainstem activity, also in the context of non-invasive deep brain stimulation.

The role of posterior parietal cortex and medial prefrontal cortex in distraction and mind-wandering

Distraction reflects a drift of attention away from the task at hand towards task-irrelevant external or internal information (mind-wandering). The right posterior parietal cortex (PPC) and the medial prefrontal cortex (mPFC) are known to mediate attention to external information and mind-wandering, respectively, but it is not clear whether they support each process selectively or rather they play similar roles in supporting both. In this study, participants performed a visual search task including salient color singleton distractors before and after receiving cathodal (inhibitory) transcranial direct current stimulation (tDCS) to the right PPC, the mPFC, or sham tDCS. Thought probes assessed the intensity and contents of mind-wandering during visual search. The results show that tDCS to the right PPC but not mPFC reduced the attentional capture by the singleton distractor during visual search. tDCS to both mPFC and PPC reduced mind-wandering, but only tDCS to the mPFC specifically reduced future-oriented mind-wandering. These results suggest that the right PPC and mPFC play a different role in directing attention towards task-irrelevant information. The PPC is involved in both external and internal distraction, possibly by mediating the disengagement of attention from the current task and its reorienting to salient information, be this a percept or a mental content (mind-wandering). By contrast, the mPFC uniquely supports mind-wandering, possibly by mediating the endogenous generation of future-oriented thoughts capable to draw attention inward, away from ongoing activities.

Causal role of the dorsolateral prefrontal cortex in modulating the balance between Pavlovian and instrumental systems in the punishment domain

Previous literature suggests that a balance between Pavlovian and instrumental decision-making systems is critical for optimal decision-making. Pavlovian bias (i.e., approach toward reward-predictive stimuli and avoid punishment-predictive stimuli) often contrasts with the instrumental response. Although recent neuroimaging studies have identified brain regions that may be related to Pavlovian bias, including the dorsolateral prefrontal cortex (dlPFC), it is unclear whether a causal relationship exists. Therefore, we investigated whether upregulation of the dlPFC using transcranial current direct stimulation (tDCS) would reduce Pavlovian bias. In this double-blind study, participants were assigned to the anodal or the sham group; they received stimulation over the right dlPFC for 3 successive days. On the last day, participants performed a reinforcement learning task known as the orthogonalized go/no-go task; this was used to assess each participant's degree of Pavlovian bias in reward and punishment domains. We used computational modeling and hierarchical Bayesian analysis to estimate model parameters reflecting latent cognitive processes, including Pavlovian bias, go bias, and choice randomness. Several computational models were compared; the model with separate Pavlovian bias parameters for reward and punishment domains demonstrated the best model fit. When using a behavioral index of Pavlovian bias, the anodal group showed significantly lower Pavlovian bias in the punishment domain, but not in the reward domain, compared with the sham group. In addition, computational modeling showed that Pavlovian bias parameter in the punishment domain was lower in the anodal group than in the sham group, which is consistent with the behavioral findings. The anodal group also showed a lower go bias and choice randomness, compared with the sham group. These findings suggest that anodal tDCS may lead to behavioral suppression or change in Pavlovian bias in the punishment domain, which will help to improve comprehension of the causal neural mechanism.

State of the art: non-invasive electrical stimulation for the treatment of chronic tinnitus

Subjective tinnitus is the perception of sound in the absence of external stimulation. Neuromodulation is a novel method with promising properties for application in tinnitus management. This study sought to review the types of non-invasive electrical stimulation in tinnitus to provide the foothold for further research. PubMed, EMBASE, and Cochrane databases were searched for studies on the modulation of tinnitus by non-invasive electrical stimulation. Among the four forms of non-invasive electrical modulation, transcranial direct current stimulation, transcranial random noise stimulation, and transauricular vagus nerve stimulation yielded promising results, whereas the effect of transcranial alternating current stimulation in the treatment of tinnitus has not been confirmed. Non-invasive electrical stimulation can effectively suppress tinnitus perception in some patients. However, the heterogeneity in parameter settings leads to scattered and poorly replicated findings. Further high-quality studies are needed to identify optimal parameters to develop more acceptable protocols for tinnitus modulation.

An in-silico analysis of retinal electric field distribution induced by different electrode design of trans-corneal electrical stimulation

OBJECTIVE

Trans-corneal electrical stimulation (TcES) produces therapeutic effects on many ophthalmic diseases non-invasively. Existing clinical TcES devices use largely variable design of electrode distribution and stimulation parameters. Better understanding of how electrode configuration paradigms and stimulation parameters influence the electric field distribution on the retina, will be beneficial to the design of next-generation TcES devices.

APPROACH

In this study, we constructed a realistic finite element human head model with fine eyeball structure. Commonly used DTL-Plus and ERG-Jet electrodes were simulated. We then conducted in silico investigations of retina observation surface (ROS) electric field distributions induced by different return electrode configuration paradigms and different stimulus intensities.

MAIN RESULTS

Our results suggested that the ROS electric field distribution could be modulated by re-designing TcES electrode settings and stimulus parameters. Under far return location (FRL) paradigms, either DTL-Plus or ERG-Jet approach could induce almost identical ROS electric field distribution regardless where the far return was located. However, compared with the ERG-Jet mode, DTL-Plus stimulation induced stronger nasal lateralization. In contrast, ERG-Jet stimulation induced relatively stronger temporal lateralization. The ROS lateralization can be further tweaked by changing the DTL-Plus electrode length.

SIGNIFICANCE

These results may contribute to the understanding of the characteristics of DTL-Plus and ERG-Jet electrodes based electric field distribution on the retina, providing practical implications for the therapeutic application of TcES.

Effective Transcranial Direct Current Stimulation Parameters for the Modulation of Eating Behavior: A Systematic Literature Review and Meta-Analysis

OBJECTIVE

This study aimed to consider the effect of differing transcranial direct current stimulation (tDCS) parameters on eating-related measures and how issues with experimental design (e.g., inadequate blinding) or parameters variation may drive equivocal effects.

METHODS

Literature searches were conducted across MEDLINE, PsycINFO, Scopus, and Science Direct. Studies using conventional sham-controlled tDCS to modify eating-related measures in adult human participants were included. A total of 1135 articles were identified and screened by two independent authors. Study quality was assessed using the Risk of Bias tool. Random-effects meta-analyses were performed, with subgroup analyses to determine differences between parameter sets.

RESULTS

We identified 28 eligible studies; 7 showed low risk of bias, with the remaining studies showing bias arising from issues implementing or reporting blinding protocols. Large variation in applied parameters was found, including montage, current intensity and density, participant and researcher blinding, and the use of online or offline tasks. The application of differing parameters seemed to alter the effects of tDCS on eating-related measures, particularly for current density ( g = -0.25 to 0.31), and when comparing single-session ( g = -0.08 to 0.01) versus multisession protocols ( g = -0.34 to -0.29). Some parameters result in null effects.

CONCLUSIONS

The absence of tDCS-mediated change in eating-related measures may be driven by variation in applied parameters. Consistent application of parameters that seem to be effective for modulating eating behavior is important for identifying the potential impact of tDCS. Using the findings of this review, we propose a series of parameters that researchers should apply in their work.

Dissociating the role of dACC and dlPFC for emotion appraisal and mood regulation using cathodal tDCS

Several neuroimaging studies have shown that a distributed network of brain regions is involved in our ability to appraise the emotions we experience in daily life. In particular, scholars suggested that the dorsal anterior cingulate cortex (dACC) may play a role in the appraisal of emotional stimuli together with subcortical regions, especially when stimuli are negatively valenced, and the dorsolateral prefrontal cortex (dlPFC) may play a role in regulating emotions. However, proofs of the causal role of these regions are lacking. In the present study, we aim at testing this model by stimulating both the dACC and the left dlPFC via cathodal tDCS. Twenty-four participants were asked to attend and rate the arousal and valence of negative and neutral emotional stimuli (pictures and words) in three different experimental sessions: cathodal stimulation of dACC, left dlPFC, or sham. In addition to the experimental task, the baseline affective state was measured before and after the stimulation to further assess the effect of stimulation over the baseline affective state after the experimental session. Results showed that cathodal stimulation of dACC, but not the left dlPFC, was associated with reduced arousal ratings of emotional stimuli, both compared with the sham condition. Moreover, cathodal stimulation of left dlPFC decreased participant's positive affective state after the session. These findings suggest for the first time, a dissociation between the dACC and dlPFC, with the former more involved in emotion appraisal, and the latter more involved in mood modulation.

Gamma-transcranial alternating current stimulation on the cerebellum and supplementary motor area improves bimanual motor skill

BACKGROUND

Bimanual movements require sophisticated coordination of both hands. For improving bimanual motor skills, previous studies employed non-invasive brain stimulation methods to evaluate their effects on symmetrical and/or gross bimanual motor skills. However, asymmetrical and elaborate movements were not sufficiently improved. Studies using non-invasive brain stimulation have examined the effects of stimulation on the primary and supplementary motor areas (SMA),) but not on the cerebellar regions.

OBJECTIVE

We investigated whether the transcranial alternating current stimulation (tACS), which modulates oscillations in the cerebral cortex, of the cerebellum and SMA improves bimanual movements.

METHODS

Bimanual movements were assessed in 22 healthy young adults (mean age: 21.3 ± 1.5 years) via 13 trials of the Purdue Pegboard Test (PPT). A DC stimulator delivered 70Hz tACS (γ-tACS) at 1mA intensity via electrodes placed over the SMA, cerebellum and left shoulder in 5s fade in/out cycles of 5s for a total stimulus duration of 60s for in each trial. Four stimulation conditions were applied and compared for statistical differences.

RESULTS

The γ-tACS of the cerebellum, γ-tACS of the SMA and simultaneous stimulation of both regions caused significant improvement in PPT performance scores. The γ-tACS of the cerebellum improved PPT performance in all subjects and was more effective than the γ-tACS of the SMA.

CONCLUSION

The γ-tACS of the cerebellum effectively and reliably improves complex bimanual motor skills. Although the neural mechanisms of the stimulation effect remain unclear, these results can guide the future development of new stimulation methods for improving bimanual motor skills.

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.

Direct Current Stimulation in Cell Culture Systems and Brain Slices-New Approaches for Mechanistic Evaluation of Neuronal Plasticity and Neuromodulation: State of the Art

Non-invasive direct current stimulation (DCS) of the human brain induces neuronal plasticity and alters plasticity-related cognition and behavior. Numerous basic animal research studies focusing on molecular and cellular targets of DCS have been published. In vivo, ex vivo, and in vitro models enhanced knowledge about mechanistic foundations of DCS effects. Our review identified 451 papers using a PRISMA-based search strategy. Only a minority of these papers used cell culture or brain slice experiments with DCS paradigms comparable to those applied in humans. Most of the studies were performed in brain slices (9 papers), whereas cell culture experiments (2 papers) were only rarely conducted. These ex vivo and in vitro approaches underline the importance of cell and electric field orientation, cell morphology, cell location within populations, stimulation duration (acute, prolonged, chronic), and molecular changes, such as Ca2+-dependent intracellular signaling pathways, for the effects of DC stimulation. The reviewed studies help to clarify and confirm basic mechanisms of this intervention. However, the potential of in vitro studies has not been fully exploited and a more systematic combination of rodent models, ex vivo, and cellular approaches might provide a better insight into the neurophysiological changes caused by tDCS.

Frontopolar tDCS Induces Frequency-Dependent Changes of Spontaneous Low-Frequency Fluctuations: A Resting-State fMRI Study

Transcranial direct current stimulation (tDCS) is a noninvasive neuromodulation technique that can modulate cortical excitability and behavioral performance. However, its effects on spontaneous low-frequency fluctuations of brain activity are still poorly understood. Here, we systematically investigated the frontopolar tDCS effects on resting-state brain activity and connectivity. Twelve healthy participants were recruited and received anode, cathode, and sham stimulation in a randomized order. Resting-state functional magnetic resonance imaging was performed before and after stimulation. Functional connectivity was calculated to examine tDCS effects within and beyond the frontopolar network. To assess the frequency-dependent changes of brain activity, fractional amplitude of low-frequency fluctuations (fALFF) was computed in the slow-4 (0.027-0.073 Hz) and slow-5 (0.01-0.027 Hz) bands. The results showed anodal tDCS-induced widespread connectivity reduction within and beyond the frontopolar network. Regardless of tDCS polarity, stimulation effect on fALFF was significantly larger in slow-5 band compared with the slow-4. Notably, anodal tDCS-induced connectivity changes were associated with pre-tDCS fALFF in slow-4 band, showing positive correlations in the frontal regions and negative correlations in the temporal regions. Our findings imply that tDCS-induced brain alterations may be frequency-dependent, and pre-tDCS regional brain activity could be used to predict post-tDCS connectivity changes.

Using Transcranial Electrical Stimulation in Audiological Practice: The Gaps to Be Filled

The effects of transcranial electrical stimulation (tES) approaches have been widely studied for many decades in the motor field, and are well known to have a significant and consistent impact on the rehabilitation of people with motor deficits. Consequently, it can be asked whether tES could also be an effective tool for targeting and modulating plasticity in the sensory field for therapeutic purposes. Specifically, could potentiating sensitivity at the central level with tES help to compensate for sensory loss? The present review examines evidence of the impact of tES on cortical auditory excitability and its corresponding influence on auditory processing, and in particular on hearing rehabilitation. Overall, data strongly suggest that tES approaches can be an effective tool for modulating auditory plasticity. However, its specific impact on auditory processing requires further investigation before it can be considered for therapeutic purposes. Indeed, while it is clear that electrical stimulation has an effect on cortical excitability and overall auditory abilities, the directionality of these effects is puzzling. The knowledge gaps that will need to be filled are discussed.

Standard Non-Personalized Electric Field Modeling of Twenty Typical tDCS Electrode Configurations via the Computational Finite Element Method: Contributions and Limitations of Two Different Approaches

Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation procedure to modulate cortical excitability and related brain functions. tDCS can effectively alter multiple brain functions in healthy humans and is suggested as a therapeutic tool in several neurological and psychiatric diseases. However, variability of results is an important limitation of this method. This variability may be due to multiple factors, including age, head and brain anatomy (including skull, skin, CSF and meninges), cognitive reserve and baseline performance level, specific task demands, as well as comorbidities in clinical settings. Different electrode montages are a further source of variability between tDCS studies. A procedure to estimate the electric field generated by specific tDCS electrode configurations, which can be helpful to adapt stimulation protocols, is the computational finite element method. This approach is useful to provide a priori modeling of the current spread and electric field intensity that will be generated according to the implemented electrode montage. Here, we present standard, non-personalized model-based electric field simulations for motor, dorsolateral prefrontal, and posterior parietal cortex stimulation according to twenty typical tDCS electrode configurations using two different current flow modeling software packages. The resulting simulated maximum intensity of the electric field, focality, and current spread were similar, but not identical, between models. The advantages and limitations of both mathematical simulations of the electric field are presented and discussed systematically, including aspects that, at present, prevent more widespread application of respective simulation approaches in the field of non-invasive brain stimulation.

Efficacy of anodal suboccipital direct current stimulation for endogenous pain modulation and tonic thermal pain control in healthy participants: a randomised controlled clinical trial

OBJECTIVE

The aim of this study was to assess whether anodal DCS applied to the suboccipital (SO) target area could potentiate antinociception assessed primarily with conditioned pain modulation of tonic thermal test stimuli.

DESIGN

Randomised double-blinded control trial.

SETTING

Rehabilitation hospital.

SUBJECTS

Healthy participants.

METHODS

Forty healthy participants were randomized to receive either SO-DCS or M1-DCS. The 20-minute 1.5mA anodal or sham DCS intervention were applied to each participant in randomised order during two test sessions. The primary outcome measure included heterotopic cold-pressor conditioned pain modulation (CPM) of tonic heat pain. Secondary measures included pressure pain threshold and tonic thermal pain intensity.

RESULTS

Heterotopic CPM of tonic heat pain intensity was unaffected by either SO-DCS or active M1, including the secondary measures of pressure pain threshold and tonic thermal pain intensity. Although low-power non-significant interactions were identified for DCS intervention (active versus sham) and time (before and after), a significant within-group inhibition of tonic cold pain was identified following SO-DCS (p = 0.011, mean [SD]: -0.76±0.88 points) and M1-DCS (p < 0.002: -0.84±0.82 points), without a significant change following sham DCS.

CONCLUSIONS

Although heterotopic CPM was not facilitated with either SO-DCS or M1-DCS, a general significant inhibition of tonic cold pain intensity was demonstrated following both interventions. The general effects of active DCS compared to sham on tonic cold pain-irrespective of the M1 or SO target-need to be confirmed using standard quantitative sensory testing.

Safety of Transcranial Direct Current Stimulation in Neurorehabilitation

Transcranial direct current stimulation (tDCS) has considerable potential as a useful method in the field of neurorehabilitation. However, the safety of tDCS for the human is primarily based on theoretical evidence related to electricity, and the safety information of applying tDCS to the human is only available from researcher's reporting. Based on tDCS studies with human and animal subjects and simulation-based studies of the safety of current stimulation in the past 20 years, this review investigated the safety of tDCS application to the human body. No severe complications have been reported in either adults or children for tDCS at an intensity of 4 mA or less, within a period of 60 minutes per day, using commonly applied 25 or 35 cm² electrodes. According to animal studies, the amount of electricity used for tDCS is less than 5% of the amount that permanently changes brain tissue, thereby ensuring safety to a certain extent. In order to increase the efficacy of tDCS for neurorehabilitation and to minimize even trivial complications in the human screening of exclusion criteria should be conducted with detailed observations of complications.

Region-Specific Effects of 10-Hz Transcranial Alternate Current Stimulation Over the Left Posterior Parietal Cortex and Primary Somatosensory Area on Tactile Two-Point Discrimination Threshold

Changes in α-band cortical oscillatory activity (8-13 Hz) affect perception; however, how these changes in the left posterior parietal cortex (PPC) and primary somatosensory cortex (S1), which play different roles in determining the two-point discrimination (TPD) threshold, affect TPD threshold remains unelucidated. Therefore, to determine TPD threshold, we aimed to investigate the function of the left PPC and S1 by applying α-band transcranial alternating current stimulation (α-tACS; 10 Hz). TPD threshold was examined at the pad of the right index finger, contralateral to the stimulation site, in 17 healthy adults using a custom-made, computer-controlled, two-point tactile stimulation device, with random application of either active or sham α-tACS over the left PPC (Experiment 1) and left S1 (Experiment 2). Then, 50% TPD threshold was obtained in the active and sham conditions via logistic regression analysis. Afterward, we compared the difference between the active and sham conditions at 50% TPD threshold in each region and found that α-tACS reduced TPD threshold when applied over the left PPC (P = 0.010); however, its effect was insignificant when applied over the left S1 (P = 0.74). Moreover, a comparison of the change in 50% TPD threshold among the regions revealed that α-tACS applied over the left PPC significantly reduced TPD threshold compared with that applied over the left S1 (P = 0.003). Although we did not reveal the actual changes in cortical activity induced by α-tACS, this is the first empirical evidence that α-tACS applied over the left PPC and left S1 exerts region-specific effects on determining TPD threshold assessed in the contralateral index finger pad by stimulation.

Effect of skin conductivity on the electric field induced by transcranial stimulation techniques in different head models

This study aims at quantifying the effect that using different skin conductivity values has on the estimation of the electric (E)-field distribution induced by transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS) in the brain of two anatomical models. The induced E-field was calculated with numerical simulations inside MIDA and Duke models, assigning to the skin a conductivity value estimated from a multi-layered skin model and three values taken from literature. The effect of skin conductivity variations on the local E-field induced by tDCS in the brain was up to 70%. In TMS, minor local differences, in the order of 20%, were obtained in regions of interest for the onset of possible side effects. Results suggested that an accurate model of the skin is necessary in all numerical studies that aim at precisely estimating the E-field induced during TMS and tDCS applications. This also highlights the importance of further experimental studies on human skin characterization, especially at low frequencies.

tDCS over posterior parietal cortex increases cortical excitability but decreases learning: An ERPs and TMS-EEG study

The application of anodal transcranial direct current stimulation (AtDCS) is generally associated with increased neuronal excitability and enhanced cognitive functioning. Nevertheless, previous work showed that applying this straight reasoning does not always lead to the desired results at behavioural level. Here, we investigated electrophysiological markers of AtDCS-mediated effects on visuo-spatial contextual learning (VSCL). In order to assess cortical excitability changes after 3 mA AtDCS applied over posterior parietal cortex, event-related potentials (ERPs) were collected during task performance. Additionally, AtDCS-induced effects on cortical excitability were explored by measuring TMS-evoked potentials (TEPs) collected before AtDCS, after AtDCS and after AtDCS and VSCL interaction. Behavioural results revealed that the application of AtDCS induced a reduction of VSCL. At the electrophysiological level, ERPs showed enhanced cortical response (P2 component) in the group receiving Real-AtDCS as compared to Sham-AtDCS. Cortical responsiveness at rest as measured by TEP, did not indicate any significant difference between Real- and Sham-tDCS groups, albeit a trend was present. Overall, our results suggest that AtDCS increases cortical response to incoming visuo-spatial stimuli, but with no concurrent increase in learning. Detrimental effects on behaviour could result from the interaction between AtDCS- and task-mediated cortical activation. This interaction might enhance cortical excitability and hinder normal task-related neuroplastic phenomena subtending learning.

Implicit visual sensitivity towards slim versus overweight bodies modulates motor resonance in the primary motor cortex: A tDCS study

Motor resonance (MR) can be influenced by individual differences and similarity in the physical appearance between the actor and observer. Recently, we reported that action simulation is modulated by an implicit visual sensitivity towards normal-weight compared with overweight bodies. Furthermore, recent research has suggested the existence of an action observation network responsible for MR, with limited evidence whether the primary motor cortex (M1) is part of this. We expanded our previous findings with regards to the role of an implicit normal-weight-body preference in the MR mechanism. At the same time, we tested the functional relevance of M1 to MR, by using a transcranial direct current stimulation (tDCS) protocol. Seventeen normal-weight and 17 overweight participants were asked to observe normal-weight or overweight actors reaching and grasping a light or heavy cube, and then, at the end of each video-clip to indicate the correct cube weight. Before the task, all participants received 15 min of sham or cathodal tDCS over the left M1. Measures of anti-fat attitudes were also collected. During sham tDCS, all participants were better in simulating the actions performed by normal-weight compared with overweight models. Surprisingly, cathodal tDCS selectively improved the ability in the overweight group to simulate actions performed by the overweight models. This effect was not associated with scores of fat phobic attitudes or implicit anti-fat bias. Our findings are discussed in the context of relevance of M1 to MR and its social modulation by anti-fat attitudes.

Anodal transcranial direct current stimulation enhances the efficiency of functional brain network communication during auditory attentional control

Attentional control is crucial for selectively attending to relevant information when our brain is confronted with a multitude of sensory signals. Graph-theoretical measures provide a powerful tool for investigating the efficiency of brain network communication in separating and integrating information. Albeit, it has been demonstrated that anodal transcranial direct current stimulation (atDCS) can boost auditory attention in situations with high control demands, its effect on neurophysiological mechanisms of functional brain network communication in situations when attentional focus conflicts with perceptual saliency remain unclear. This study investigated the effects of atDCS on network connectivity and θ-oscillatory power under different levels of attentional-perceptual conflict. We hypothesized that the benefit of atDCS on network communication efficiency would be particularly apparent in conditions requiring high attentional control. Thirty young adults participated in a dichotic listening task with intensity manipulation, while EEG activity was recorded. In a cross-over design, participants underwent right frontal atDCS and sham stimulations in two separate sessions. Time-frequency decomposition and graph-theoretical analyses of network efficiency (using "small-world" properties) were used to quantify θ-oscillatory power and brain network efficiency, respectively. The atDCS-induced effect on task efficiency in the most demanding condition was mirrored only by an increase in network efficiency during atDCS compared with the sham stimulation. These findings are corroborated by Bayesian analyses. AtDCS-induced performance enhancement under high levels of attentional-perceptual conflicts is accompanied by an increase in network efficiency. Graph-theoretical measures can serve as a metric to quantify the effects of noninvasive brain stimulation on the separation and integration of information in the brain.NEW & NOTEWORTHY As compared with sham stimulation, application of atDCS enhances θ-oscillation-based network efficiency, but it has no impact on θ-oscillation power. Individual differences in θ-oscillation-based network efficiency correlated with performance efficiency under the sham stimulation.

Electrical stimulation of cranial nerves in cognition and disease

The cranial nerves are the pathways through which environmental information (sensation) is directly communicated to the brain, leading to perception, and giving rise to higher cognition. Because cranial nerves determine and modulate brain function, invasive and non-invasive cranial nerve electrical stimulation methods have applications in the clinical, behavioral, and cognitive domains. Among other neuromodulation approaches such as peripheral, transcranial and deep brain stimulation, cranial nerve stimulation is unique in allowing axon pathway-specific engagement of brain circuits, including thalamo-cortical networks. In this review we amalgamate relevant knowledge of 1) cranial nerve anatomy and biophysics; 2) evidence of the modulatory effects of cranial nerves on cognition; 3) clinical and behavioral outcomes of cranial nerve stimulation; and 4) biomarkers of nerve target engagement including physiology, electroencephalography, neuroimaging, and behavioral metrics. Existing non-invasive stimulation methods cannot feasibly activate the axons of only individual cranial nerves. Even with invasive stimulation methods, selective targeting of one nerve fiber type requires nuance since each nerve is composed of functionally distinct axon-types that differentially branch and can anastomose onto other nerves. None-the-less, precisely controlling stimulation parameters can aid in affecting distinct sets of axons, thus supporting specific actions on cognition and behavior. To this end, a rubric for reproducible dose-response stimulation parameters is defined here. Given that afferent cranial nerve axons project directly to the brain, targeting structures (e.g. thalamus, cortex) that are critical nodes in higher order brain networks, potent effects on cognition are plausible. We propose an intervention design framework based on driving cranial nerve pathways in targeted brain circuits, which are in turn linked to specific higher cognitive processes. State-of-the-art current flow models that are used to explain and design cranial-nerve-activating stimulation technology require multi-scale detail that includes: gross anatomy; skull foramina and superficial tissue layers; and precise nerve morphology. Detailed simulations also predict that some non-invasive electrical or magnetic stimulation approaches that do not intend to modulate cranial nerves per se, such as transcranial direct current stimulation (tDCS) and transcranial magnetic stimulation (TMS), may also modulate activity of specific cranial nerves. Much prior cranial nerve stimulation work was conceptually limited to the production of sensory perception, with individual titration of intensity based on the level of perception and tolerability. However, disregarding sensory emulation allows consideration of temporal stimulation patterns (axon recruitment) that modulate the tone of cortical networks independent of sensory cortices, without necessarily titrating perception. For example, leveraging the role of the thalamus as a gatekeeper for information to the cerebral cortex, preventing or enhancing the passage of specific information depending on the behavioral state. We show that properly parameterized computational models at multiple scales are needed to rationally optimize neuromodulation that target sets of cranial nerves, determining which and how specific brain circuitries are modulated, which can in turn influence cognition in a designed manner.

Feasibility of Transcranial Direct Current Stimulation in Patients with Deep Brain Stimulation: a Case Report

Although deep brain stimulation (DBS) has been reported to be effective to ameliorate motor and non-motor dysfunctions, freezing of gait (FoG) is often resistant to DBS in patients with Parkinson's disease (PD). Transcranial direct current stimulation (tDCS) has been reported as an alternative therapeutic strategy to ameliorate FoG in PD patients. In this case report, we describe the effects of cumulative tDCS over the primary motor cortex of the lower leg to reduce FoG in 2 cases of PD patients with DBS. Two PD patients who had undergone DBS of the subthalamic nucleus visited the rehabilitation medicine department for refractory FoG. Each patient received cumulative tDCS over the primary motor cortex of the lower leg over to reduce FoG. Neither patient required change in dose of dopaminergic medication during the tDCS period nor a significant side effect during and after tDCS. Although the FoG-questionnaire (FoG-Q) in case 1 showed no change after 10 tDCS treatments, the patient in case 2 reported a significant improvement of FoG-Q from 11 to 3 after 5 days of tDCS. We present the safety and feasibility of tDCS in PD patients with DBS who showed refractory FoG.

Examining bi-anodal transcranial direct current stimulation (tDCS) over bilateral dorsolateral prefrontal cortex coupled with bilateral extracephalic references as a treatment for negative symptoms in non-acute schizophrenia patients: A randomized, double-blind, sham-controlled trial

No studies have examined the efficacy of bi-anodal transcranial direct current stimulation (tDCS) over bilateral dorsolateral prefrontal cortex (DLPFC) coupled with bilateral extracephalic references in treating negative symptoms of non-acute schizophrenia patients. This study aimed to investigate the therapeutic effects of the new approach of tDCS on negative symptoms, other schizophrenia symptoms, cognitive deficits and psychosocial functioning in a double-blind, randomized, sham-controlled trial. Patients with non-acute schizophrenia (N = 60) in randomized order received sham treatment or bilaterally provided tDCS (2 mA, twice-daily sessions for five consecutive days) with the anode over the DLPFC and the reference (cathode) over the ipsilateral forearm. The negative symptoms as measured by a dimensional approach of Positive and Negative Syndrome Scale (PANSS) were rapidly reduced by bimodal tDCS relative to sham stimulation (F = 24.86, Cohen's d = 0.661, p = 6.11 × 10^-6). The beneficial effect on negative symptoms lasted for up to 3 months. The authors also observed improvement with tDCS of psychosocial functioning as measured by the global score of Personal and Social Performance scale (PSP) and psychopathological symptoms especially for disorganization and cognitive symptoms as measured by the PANSS. No effects were observed on other schizophrenia symptom dimensions and the performance on a series of neurocognitive tests. Our results show promise for bi-anodal tDCS over bilateral DLPFC using bilateral extracephalic references in treating negative symptoms and other selected manifestations of schizophrenia. Further studies with electrophysiological or imaging evaluation help unravel the exact mechanism of action of this novel stimulation parameter of tDCS in schizophrenia patients. (ClinicalTrials.gov ID:NCT03701100).

A Study of Null Effects for the Use of Transcranial Direct Current Stimulation (tDCS) in Adults With and Without Reading Impairment

There is evidence to support the hypothesis that the delivery of anodal transcranial direct current stimulation (tDCS) to the left temporoparietal junction can enhance performance on reading speed and reading accuracy (Costanzo et al., 2016b; Heth & Lavidor, 2015). Here, we explored whether we could demonstrate similar effects in adults with and without reading impairments.

METHOD

Adults with (N = 33) and without (N = 29) reading impairment were randomly assigned to anodal or sham stimulation conditions. All individuals underwent a battery of reading assessments pre and post stimulation. The stimulation session involved 15 min of anodal/sham stimulation over the left temporoparietal junction while concurrently completing a computerized nonword segmentation task known to activate the temporoparietal junction.

RESULTS

There were no conclusive findings that anodal stimulation impacted reading performance for skilled or impaired readers.

CONCLUSIONS

While tDCS may provide useful gains on reading performance in the paediatric population, much more work is needed to establish the parameters under which such findings would transfer to adult populations. The documentation, reporting, and interpreting of null effects of tDCS are immensely important to a field that is growing exponentially with much uncertainty.

Investigating the effects of tDCS on Visual Orientation Discrimination Task Performance: 'The possible influence of placebo'

The non-invasive neuromodulation technique tDCS offers the promise of a low cost tool for both research and clinical applications in psychology, psychiatry and neuroscience. However, findings regarding its efficacy are often equivocal. A key issue is that the clinical and cognitive applications studied are often complex and thus effects of tDCS are difficult to predict given its known effects on the basic underlying neurophysiology, namely alterations in cortical inhibition-excitation balance. As such, it may be beneficial to assess the effects of tDCS in tasks whose performance has a clear link to cortical inhibition-excitation balance such as the visual orientation discrimination task (ODT). In prior studies in our laboratory no practise effects were found during 2 consecutive runs of the ODT, thus in the current investigation, to examine the effects of tDCS, subjects received 10 minutes of 2mA occipital tDCS (sham, anode, cathode) between a first and second run of ODT. Surprisingly, subjects' performance significantly improved in the second run of ODT compared to the first one regardless of the tDCS stimulation type they received (anodal, cathodal, or sham-tDCS). Possible causes for such an improvement could have been due to either a generic 'placebo' effect of tDCS (as all subjects received some form of tDCS) or an increased delay period between the two runs of ODT of the current study compared to our previous work (10 minutes duration required to administer tDCS as opposed to ~2 minutes in previous studies as a 'break'). As such, we tested these two possibilities with a subsequent experiment in which subjects received 2 minutes or 10 minutes delay between the 2 runs (with no tDCS) or 10 minutes of sham-tDCS. Only sham-tDCS resulted in improved performance thus these data add to a growing literature suggesting that tDCS has powerful placebo effect that may occur even in the absence of active cortical modulation.

Transcending the brain: is there a cost to hacking the nervous system?

Great advancements have recently been made to understand the brain and the potential that we can extract out of it. Much of this has been centred on modifying electrical activity of the nervous system for improved physical and cognitive performance in those with clinical impairment. However, there is a risk of going beyond purely physiological performance improvements and striving for human enhancement beyond traditional human limits. Simple ethical guidelines and legal doctrine must be examined to keep ahead of technological advancement in light of the impending mergence between biology and machine. By understanding the role of modern ethics, this review aims to appreciate the fine boundary between what is considered ethically justified for current neurotechnology.

Investigation into the effect of transcranial direct current stimulation on cardiac pacemakers

BACKGROUND

Studies investigating the therapeutic applications of transcranial direct current stimulation (tDCS) in the treatment of age-related neurodegenerative disease have been promising. However, exclusion criteria for these studies invariably disqualify patients implanted with internal cardiac pacemakers, citing safety concerns. Because the majority of cardiac pacemaker implantees are over 65, this criterion may limit candidacy for tDCS based research and/or treatment of age-related neurodegenerative disease.

OBJECTIVE/HYPOTHESIS

We will test the hypothesis that tDCS impacts pacemaker function. Strong electrical potentials, such as those generated by external defibrillators (∼500 V, ∼10 A), are known to occasionally damage pacemaker circuitry and software, but it seems unlikely tDCS would damage a pacemaker because it involves about 1/200th the energy (∼12 V, ∼2 mA) of an external defibrillator.

METHODS

We delivered tDCS to seven participants (ages 70-92) with bipolar non-dependent pacemakers and subsequently collected data from the internal memory of the pacemakers to assess the tDCS signal detection, as well as alterations in mode switches, impedance levels, and pacing. Subsequently, similar assessments were carried out in participants who were pacemaker-dependent (ages 89-91).

RESULTS

After a review of the recordings, it was found that tDCS had no impact on the non-dependant, as well as the dependent, pacemakers. There were zero mode switches nor any impact on impedance levels.

CONCLUSION

Results in this small series of cases found no evidence that tDCS interferes with the function of the pacemakers and suggests tDCS can be delivered to patients equipped with a cardiac pacemaker. Further studies are needed to generalize these results to other pacemakers.

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.

Comparison of Transcranial Direct Current Stimulation Electrode Montages for the Lower Limb Motor Cortex

Transcranial direct current stimulation (tDCS) has been widely explored as a neuromodulatory adjunct to modulate corticomotor excitability and improve motor behavior. However, issues with the effectiveness of tDCS have led to the exploration of empirical and experimental alternate electrode placements to enhance neuromodulatory effects. Here, we conducted a preliminary study to compare a novel electrode montage (which involved placing 13 cm² electrodes anterior and posterior to the target location) to the traditionally used electrode montage (13 cm² stimulating electrode over the target area and the 35 cm² reference electrode over the contralateral orbit). We examined the effects of tDCS of the lower limb motor area (M1) by measuring the corticomotor excitability (CME) of the tibialis anterior muscle using transcranial magnetic stimulation in twenty healthy participants. We examined behavioral effects using a skilled motor control task performed with the ankle. We did not find one electrode montage to be superior to the other for changes in the CME or motor control. When the group was dichotomized into responders and non-responders (based on upregulation in CME), we found that the responders showed significant upregulation from baseline after tDCS for both montages. However, only the responders in the traditional montage group showed significant changes in motor control after tDCS. These results do not support the superiority of the new anterior–posterior montage over the traditional montage. Further work with a larger cohort and multiple cumulative sessions may be necessary to confirm our results.

10 Hz transcranial alternating current stimulation over posterior parietal cortex facilitates tactile temporal order judgment

The temporal order judgment (TOJ) task has been widely used to investigate spatial attentional bias and the sensitivity of temporal discrimination during the processing of bilateral tactile information. Previous studies have shown that TOJ is impaired in patients who are suffering from chronic pain, stroke, and Parkinson's disease. In addition, studies have indicated that the posterior parietal cortex (PPC) is involved in the TOJ task. However, the neural basis of the TOJ task has not been fully elucidated. To investigate the causal relationship between cortical oscillation and certain behaviors, transcranial alternating current stimulation (tACS) has been used. tACS can entrain an oscillation in the cortex to the applying frequency. In previous studies, increased alpha-band (around 10 Hz) oscillation in the PPC is associated with attentional inhibition of the contralateral side. Therefore, we hypothesized that 10 Hz tACS over PPC would inhibit tactile processing in the contralateral side, leading to ipsilateral spatial attentional bias and impaired temporal discrimination. However, we found that 10 Hz tACS over either side of the PPC facilitated temporal discrimination, with 10 Hz tACS over the right PPC leading to a rightward shift of attentional bias. These findings indicated that 10 Hz tACS over the PPC has a facilitative effect in the processing of bilateral tactile information, and may be useful for modulating or treating spatial bias or temporal discrimination during the integration of bilateral stimulation, at least in the somatosensory domain.

Transcranial Alternating Current Stimulation With Gamma Oscillations Over the Primary Motor Cortex and Cerebellar Hemisphere Improved Visuomotor Performance

Transcranial alternating current stimulation (tACS) can be used to modulate oscillatory brain activity. In this study, we investigated whether tACS applied over the primary motor cortex (M1) and cerebellar cortex region improved motor performance. We applied tACS (1.0 mA) to 20 healthy adults while they performed an isometric force task with some visuomotor control using their right index finger. Gamma (70 Hz) oscillations in the Experiment 1 or beta (20 Hz) oscillations in the Experiment 2 were applied for 30 s over the left M1, right cerebellar hemisphere or both regions ("M1-Cerebellum"), and errors performing the task were compared. Beta-oscillation tACS did not affect motor performance. With the gamma-oscillation tACS, a negative correlation was found between the difference of error in the M1-Cerebellum condition and the number of errors in the sham condition (P = 0.005, Pearson's r = -0.597), indicating that motor performance improved with M1-Cerebellum tACS for subjects with low motor performance in the sham condition. Those who performed poorly in the sham condition made significantly fewer errors with M1-Cerebellum tACS (P = 0.004). Thus, for subjects with poorer motor performance, tACS with gamma oscillations applied over the M1 and contralateral cerebellar hemisphere improved their performance.

Anodal and Cathodal tDCS Over the Right Frontal Eye Fields Impacts Spatial Probability Processing Differently in Pro- and Anti-saccades

Learning regularities that exist in the environment can help the visual system achieve optimal efficiency while reducing computational burden. Using a pro- and anti-saccade task, studies have shown that probabilistic information regarding spatial locations can be a strong modulator of frontal eye fields (FEF) activities and consequently alter saccadic behavior. One recent study has also shown that FEF activities can be modulated by transcranial direct current stimulation, where anodal tDCS facilitated prosaccades but cathodal tDCS prolonged antisaccades. These studies together suggest that location probability and tDCS can both alter FEF activities and oculomotor performance, yet how these two modulators interact with each other remains unclear. In this study, we applied anodal or cathodal tDCS over right FEF, and participants performed an interleaved pro- and anti-saccade task. Location probability was manipulated in prosaccade trials but not antisaccade trials. We observed that anodal tDCS over rFEF facilitated prosaccdes toward low-probability locations but not to high-probability locations; whereas cathodal tDCS facilitated antisaccades away from the high-probability location (i.e., same location as the low-probability locations in prosaccades). These observed effects were specific to rFEF as tDCS over the SEF in a separate control experiment did not yield similar patterns. These effects were also more pronounced in low-performers who had slower saccade reaction time. Together, we conclude that (1) the overlapping spatial endpoint between prosaccades (i.e., toward low-probability location) and antisaccades (i.e., away from high-probability location) possibly suggest an endpoint-selective mechanism within right FEF, (2) anodal tDCS and location probability cannot be combined to produce a bigger facilitative effect, and (3) anodal rFEF tDCS works best on low-performers who had slower saccade reaction time. These observations are consistent with the homeostasis account of tDCS effect and FEF functioning.

Modulation of the frontal-parietal network by low intensity anti-phase 20 Hz transcranial electrical stimulation boosts performance in the attentional blink task

Performance in the attentional blink task has been demonstrated to be directly influenced by alpha and beta neural oscillatory activity. In two experiments we stimulated the right parietal cortex and left frontal cortex with transcranial alternating current stimulation. For the first experiment we targeted only the right parietal cortex and found a non-significant increase in performance from 20 Hz stimulation. In the second experiment we applied two stimulators to the right parietal and left frontal cortex and found a significant increase in performance from 20 Hz tACS with a phase difference of 180°. Since low intensity stimulation has been shown to inhibit cortical excitability, and anti-phasic stimulation has been hypothesized to decrease presynaptic activation in one region and drive postsynaptic spikes in the other, we suggest that low intensity anti-phasic 20 Hz stimulation inhibited the parietal cortex, thereby disinhibiting the frontal cortex. This visual attention mechanism supposedly reduces processing of distractor stimuli and enhances processing of target stimuli. This study reveals that the frontal-parietal visual attention network may be modulated with low intensity 20 Hz anti-phase tACS.

The critical role of phase difference in theta oscillation between bilateral parietal cortices for visuospatial working memory

Visual working memory (VWM) refers to people's ability to maintain and manipulate visual information on line. Its capacity varies between individuals, and neuroimaging studies have suggested a link between one's VWM capacity and theta power in the parietal cortex. However, it is unclear how the parietal cortices communicate with each other in order to support VWM processing. In two experiments we employed transcranial alternate current stimulation (tACS) to use frequency-specific (6 Hz) alternating current to modulate theta oscillation between the left and right parietal cortex with either in-phase (0° difference, Exp 1), anti-phase (180° difference, Exp 2), or sham sinusoidal current stimulation. In Experiment 1, in-phase theta tACS induced an improved VWM performance, but only in low-performers, whereas high-performers suffered a marginally-significant VWM impairment. In Experiment 2, anti-phase theta tACS did not help the low-performers, but significantly impaired high-performers' VWM capacity. These results not only provide causal evidence for theta oscillation in VWM processing, they also highlight the intricate interaction between tACS and individual differences-where the same protocol that enhances low-performers' VWM can backfire for the high-performers. We propose that signal complexity via coherent timing and phase synchronization within the bilateral parietal network is crucial for successful VWM functioning.

Significance of Biological Membranes for Accurate Computational Dosimetry of Low Frequency Electric Fields

Computational dosimetry has become the main tool for estimating induced electric fields within brain tissues in transcranial direct current stimulation (tDCS) which is recently attracting the attention of researches for motor function disturbances such as Parkinson's disease. This paper investigates the effect of including or excluding the very thin meninges in computing tDCS electric fields using CST software. For this purpose, two models of the brain with and without meninges were used to induce electric field with two DC current electrodes (2 mA) in regions of the model referring to M1 and Prefrontal Cortex (FP2) similar to tDCS. Considering meninges, the results have shown differences in the induced field showing that there might be problems with conventional models in which meninges are not taken into account.

Transcranial Alternating Current Stimulation Modulates Risky Decision Making in a Frequency-Controlled Experiment

In this study, we investigated the effect of transcranial alternating current stimulation (tACS) on voluntary risky decision making and executive control in humans. Stimulation was delivered online at 5 Hz (θ), 10 Hz (α), 20 Hz (β), and 40 Hz (γ) on the left and right frontal area while participants performed a modified risky decision-making task. This task allowed participants to voluntarily select between risky and certain decisions associated with potential gains or losses, while simultaneously measuring the cognitive control component (voluntary switching) of decision making. The purpose of this experimental design was to test whether voluntary risky decision making and executive control can be modulated with tACS in a frequency-specific manner. Our results revealed a robust effect of a 20-Hz stimulation over the left prefrontal area that significantly increased voluntary risky decision making, which may suggest a possible link between risky decision making and reward processing, underlined by β-oscillatory activity.

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.

Combination of a short cognitive training and tDCS to enhance visuospatial skills: A comparison between online and offline neuromodulation

Visuospatial skills can be enhanced thanks to specific intervention programs, but the additional benefits of neuromodulation on these skills have not been fully investigated yet, although transcranial direct current stimulation (tDCS) has demonstrated to boost the effects of cognitive trainings. When combining cognitive intervention with neuromodulation, the time-window of tDCS application in relation to task execution has to be taken into account since it has been shown to affect stimulation outcomes. The aim of the present experiment was to investigate the influence of tDCS in enhancing the effects of a training for visuospatial skills. We hypothesized that tDCS applied during training execution (online) would improve the cognitive performance at a larger extent than tDCS applied before training execution (offline). Participants received anodal tDCS over the dorsolateral prefrontal cortex during (online) or before (offline) the completion of the training. A control sham condition was included. Visuospatial abilities were measured 24 h before (day 1, pre-test) and 24 h after (day 3, post-test) the stimulation and training session (day 2). tDCS enhanced gains for mental folding performance when applied during the execution of the training (online). Participants' mental rotation and mental folding performance improved from pre-test to post-test regardless of the stimulation condition. However participants in the online tDCS condition showed the largest improvement in mental folding performance. Findings indicate that tDCS enhanced the effects of the training when applied during its execution, showing cumulative positive aftereffects on visuospatial performance 24 h after the stimulation session. The time-dependent effect points out the importance of the time-window of tDCS application in influencing behavior when combined with cognitive programs.

Combining tDCS with prismatic adaptation for non-invasive neuromodulation of the motor cortex

BACKGROUND

Prismatic adaptation (PA) shifts visual field laterally and induces lateralized deviations of spatial attention. Recently, it has been suggested that prismatic goggles are also able to modulate brain excitability, with cognitive after-effects documented even in tasks not necessarily spatial in nature.

OBJECTIVE

The aim of the present study was to test whether neuromodulatory effects obtained from tDCS and prismatic goggles could interact and induce homeostatic changes in corticospinal excitability.

METHODS

Thirty-four subjects were submitted to single-pulse transcranial magnetic stimulation (TMS) over the right primary motor cortex to measure Input-Output (IO) curve as a measure of corticospinal excitability. Assessment was made in three experimental conditions: before and after rightward PA and anodal tDCS of the right motor cortex; before and after rightward PA; before and after anodal tDCS of the right motor cortex.

RESULTS

A significant decrease of MEPs amplitude and of IO curve slope steepness was found after the combination of rightward PA and anodal tDCS; on the other hand, an increase of MEPs amplitude and of the steepness of IO curve slope on the right motor cortex was found following either rightward PA or anodal tDCS.

CONCLUSION

These findings suggest that priming of motor cortex excitability using PA could be an additional tool to modulate cortical metaplasticity.

Developing clinically practical transcranial direct current stimulation protocols to improve saccadic eye movement control

Recent research indicates that anodal transcranial direct current stimulation (tDCS) applied over the frontal eye field (FEF) can improve saccadic eye movement control in healthy young adults. The current research set out to determine whether similar results can be produced using a clinically practical protocol, whether tDCS applied over the dorsolateral prefrontal cortex (DLPFC) might also afford benefits, and whether benefits extend to older adults. Twenty young and 10 older adults completed two active (FEF and DLPFC) and one sham stimulation session. To aid clinical translation, the method of positioning the electrodes entailed simple measurements only. Saccadic performance following anodal tDCS applied over the FEF or DLPFC did not differ from the sham condition in either age group. Additionally, saccadic performance contralateral to the active electrodes showed no evidence of benefits over ipsilateral performance. These results call into question whether the protocol utilized can be applied effectively using only simple measurements to localize the relevant frontal subregion. Future efforts to develop a clinically practical tDCS protocol to improve saccadic eye movement control should include a sham control condition and consider adjusting the tDCS electrode montage and current strength to optimize the chances of conferring benefits in the population under study.

Modulation of Brain Activity with Noninvasive Transcranial Direct Current Stimulation (tDCS): Clinical Applications and Safety Concerns

Transcranial direct current stimulation (tDCS) is a widely-used tool to induce neuroplasticity and modulate cortical function by applying weak direct current over the scalp. In this review, we first introduce the underlying mechanism of action, the brief history from discovery to clinical scientific research, electrode positioning and montages, and parameter setup of tDCS. Then, we review tDCS application in clinical samples including people with drug addiction, major depression disorder, Alzheimer's disease, as well as in children. This review covers the typical characteristics and the underlying neural mechanisms of tDCS treatment in such studies. This is followed by a discussion of safety, especially when the current intensity is increased or the stimulation duration is prolonged. Given such concerns, we provide detailed suggestions regarding safety procedures for tDCS operation. Lastly, future research directions are discussed. They include foci on the development of multi-tech combination with tDCS such as with TMS and fMRI; long-term behavioral and morphological changes; possible applications in other research domains, and more animal research to deepen the understanding of the biological and physiological mechanisms of tDCS stimulation.

Anatomical Parameters of tDCS to Modulate the Motor System after Stroke: A Review

Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation method to modulate the local field potential in neural tissue and consequently, cortical excitability. As tDCS is relatively portable, affordable, and accessible, the applications of tDCS to probe brain-behavior connections have rapidly increased in the last 10 years. One of the most promising applications is the use of tDCS to modulate excitability in the motor cortex after stroke and promote motor recovery. However, the results of clinical studies implementing tDCS to modulate motor excitability have been highly variable, with some studies demonstrating that as many as 50% or more of patients fail to show a response to stimulation. Much effort has therefore been dedicated to understand the sources of variability affecting tDCS efficacy. Possible suspects include the placement of the electrodes, task parameters during stimulation, dosing (current amplitude, duration of stimulation, frequency of stimulation), individual states (e.g., anxiety, motivation, attention), and more. In this review, we first briefly review potential sources of variability specific to stroke motor recovery following tDCS. We then examine how the anatomical variability in tDCS placement [e.g., neural target(s) and montages employed] may alter the neuromodulatory effects that tDCS exerts on the post-stroke motor system.

COMETS2: An advanced MATLAB toolbox for the numerical analysis of electric fields generated by transcranial direct current stimulation

BACKGROUND

Since there is no way to measure electric current generated by transcranial direct current stimulation (tDCS) inside the human head through in vivo experiments, numerical analysis based on the finite element method has been widely used to estimate the electric field inside the head. In 2013, we released a MATLAB toolbox named COMETS, which has been used by a number of groups and has helped researchers to gain insight into the electric field distribution during stimulation. The aim of this study was to develop an advanced MATLAB toolbox, named COMETS2, for the numerical analysis of the electric field generated by tDCS.

NEW METHOD

COMETS2 can generate any sizes of rectangular pad electrodes on any positions on the scalp surface. To reduce the large computational burden when repeatedly testing multiple electrode locations and sizes, a new technique to decompose the global stiffness matrix was proposed.

RESULTS

As examples of potential applications, we observed the effects of sizes and displacements of electrodes on the results of electric field analysis. The proposed mesh decomposition method significantly enhanced the overall computational efficiency.

COMPARISON WITH EXISTING METHODS

We implemented an automatic electrode modeler for the first time, and proposed a new technique to enhance the computational efficiency.

CONCLUSIONS

In this paper, an efficient toolbox for tDCS analysis is introduced (freely available at http://www.cometstool.com). It is expected that COMETS2 will be a useful toolbox for researchers who want to benefit from the numerical analysis of electric fields generated by tDCS.