Unilateral tDCS Research Articles

One Form of tDCS Does Not Fit All: The intersectionality of severity level and expected tDCS benefit

AbstractBackgroundWe investigated if a mixed group of people living with dementia (Alzheimer’s Disease (n = 16) or Frontotemporal dementia (n = 8)) would have faster response times on an N‐Back task (1‐back), a measure of working memory, after practicing the task with 20 minutes of tDCS, versus an analogous placebo condition. Crucially, half of these participants were slower than normal on the task at baseline, while the other half has normal response times. Our primary outcome measure was improvement on the MMSE task.MethodEach participant practiced the n‐back in three separate nine‐session rounds, two months apart, with a different form of stimulation in each round: (1) 4mA unilateral stimulation, with one anode electrode over the left dorsolateral prefrontal cortex area and a cathode electrode over the occipital lobe area; (2) 4mA bilateral stimulation, using the same configuration, but with an extra active anode electrode over the right dorsolateral prefrontal cortex, and (3) SHAM stimulation.ResultWe found the condition x time x severity variable was significant (F (12, 264) = 2.17, p < .05), which suggests the results founds were dependent both of the tDCS montage received and the initial severity level of the participants. More specifically, as supported by post‐hoc tests, participants initially impaired on the N‐Back task improved on the MMSE when they received the bilateral tDCS montage, but those participants with normal response times improved on the MMSE when they received the unilateral tDCS montage.ConclusionConsistent with past studies, results suggest tDCS can be used to improve cognition in people living with dementia. Optimization, however, may depend both on the montage applied and the indiosyncratic nature of the participant. A bilateral montage, for example, that theoretically impacts a wider range of brain areas, may be superior to a unilateral one when people with dementia exhibit a greater degree of impairment.

One Form of tDCS Does Not Fit All: The intersectionality of severity level and expected tDCS benefit

AbstractBackgroundWe investigated if a mixed group of people living with dementia (Alzheimer’s Disease (n = 16) or Frontotemporal dementia (n = 8)) would have faster response times on an N‐Back task (1‐back), a measure of working memory, after practicing the task with 20 minutes of tDCS, versus an analogous placebo condition. Crucially, half of these participants were slower than normal on the task at baseline, while the other half has normal response times. Our primary outcome measure was improvement on the MMSE task.MethodEach participant practiced the n‐back in three separate nine‐session rounds, two months apart, with a different form of stimulation in each round: (1) 4mA unilateral stimulation, with one anode electrode over the left dorsolateral prefrontal cortex area and a cathode electrode over the occipital lobe area; (2) 4mA bilateral stimulation, using the same configuration, but with an extra active anode electrode over the right dorsolateral prefrontal cortex, and (3) SHAM stimulation.ResultWe found the condition x time x severity variable was significant (F (12, 264) = 2.17, p < .05), which suggests the results founds were dependent both of the tDCS montage received and the initial severity level of the participants. More specifically, as supported by post‐hoc tests, participants initially impaired on the N‐Back task improved on the MMSE when they received the bilateral tDCS montage, but those participants with normal response times improved on the MMSE when they received the unilateral tDCS montage.ConclusionConsistent with past studies, results suggest tDCS can be used to improve cognition in people living with dementia. Optimization, however, may depend both on the montage applied and the indiosyncratic nature of the participant. A bilateral montage, for example, that theoretically impacts a wider range of brain areas, may be superior to a unilateral one when people with dementia exhibit a greater degree of impairment.

Limited Add-On Effects of Unilateral and Bilateral Transcranial Direct Current Stimulation on Visuo-Motor Grip Force Tracking Task Training Outcome in Chronic Stroke. A Randomized Controlled Trial

Background: This randomized controlled trial investigated if uni- and bihemispheric transcranial direct current stimulation (tDCS) of the motor cortex can enhance the effects of visuo-motor grip force tracking task training and transfer to clinical assessments of upper extremity motor function.Methods: In a randomized, double-blind, sham-controlled trial, 40 chronic stroke patients underwent 5 days of visuo-motor grip force tracking task training of the paretic hand with either unilateral or bilateral (N = 15/group) or placebo tDCS (N = 10). Immediate and long-term (3 months) effects on training outcome and motor recovery (Upper Extremity Fugl-Meyer, UE-FM, Wolf Motor Function Test, and WMFT) were investigated.Results: Trained task performance significantly improved independently of tDCS in a curvilinear fashion. In the anodal stimulation group UE-FM scores were higher than in the sham group at day 5 (adjusted mean difference: 2.6, 95%CI: 0.6–4.5, p = 0.010) and at 3 months follow up (adjusted mean difference: 2.8, 95%CI: 0.8–4.7, p = 0.006). Neither training alone, nor the combination of training and tDCS improved WMFT performance.Conclusions: Visuo-motor grip force tracking task training can facilitate recovery of upper extremity function. Only minimal add-on effects of anodal but not dual tDCS were observed.Clinical Trial Registration: https://clinicaltrials.gov/ct2/results?recrs=&cond=&term=NCT01969097&cntry=&state=&city=&dist=, identifier: NCT01969097, retrospectively registered on 25/10/2013.

Alcohol Abuse and Brain Excitability as Revealed by EEG and Noninvasive Brain Stimulation

Alcohol use disorder is one of the common substance use disorders leading to mental and health problems. Despite the potential positive effects of transcranial direct current stimulation (tDCS) on symptoms of various substance use disorder, how specific tDCS protocols effectively influence on individuals with alcohol use disorder is still controversial. This systematic review and meta-analysis investigated beneficial effects of tDCS on symptoms of alcohol use disorder.Eighteen total studies met our inclusion criteria, and we used 25 total comparisons from the qualified studies for the data synthesis. We estimated effect sizes by quantifying changes in alcohol craving and consumption between active tDCS protocol and sham groups. In addition, three moderator variable analyses determined whether tDCS effects on symptoms of alcohol use disorder were different based on (a) bilateral versus unilateral tDCS protocols, (b) specific targeted regions, and (c) multiple sessions versus single session of tDCS protocols.Random-effects model meta-analysis revealed small positive tDCS effects on alcohol craving and consumption. Specifically, bilateral tDCS protocols significantly reduced alcohol craving, and further anodal tDCS on right dorsolateral prefrontal cortex (DLPFC) and cathodal tDCS on left DLPFC revealed significant positive effects. The multiple sessions of tDCS protocols showed better effects on reducing alcohol craving.The current findings suggested that bilateral tDCS protocols including anodal tDCS on right DLPFC and cathodal tDCS on left DLPFC with multiple sessions may effectively improve tDCS effects on symptoms of alcohol use disorder.

The Effect of Bilateral Anodal Transcranial Direct Current Stimulation versus Treadmill Training on Brain Activities, Gait Functions, Level of Participation and Enjoyment of Children with Cerebral Palsy: A Randomized Controlled Trial Protocol

ABSTRACT Background Cerebral palsy (CP) possesses bilateral sensory-motor cortical excitability alterations. In past studies, researchers have applied unilateral anodal transcranial direct stimulation (tDCS) with inconsistent findings. Objective Examining the effects of treadmill training with either unilateral or bilateral anodal tDCS stimulation on brain activities, gait, and body functions of children with CP. Design Randomized-clinical-trial. Participants Eighty children with CP will be allocated into: treadmill/bilateral-tDCS, treadmill/unilateral-tDCS, treadmill/sham-tDCS, and treadmill groups. Additional 20 neurotypical children will be recruited for comparison. Intervention Planned intervention will implement ten-sessions of treadmill training (50% of maximum-ground speed) either with unilateral-tDCS over left sensory-motor, or bilateral-tDCS over left/right sensory-motor areas. Brain activities, gait, body functions, and participation will be assessed at pre-intervention, post-intervention, and at one-month follow-up. Result and Discussion This study would contribute to extant findings on the use of tDCS stimulation in children with CP and to our understanding of the appropriateness of the planned outcome measures.

Effects of unilateral tDCS over left prefrontal cortex on emotion regulation in depression: Evidence from concurrent functional magnetic resonance imaging.

Major depressive disorder (MDD) is the most common mood disorder and a primary cause of disability worldwide. MDD symptomatology entails disturbances in emotion regulation, namely one's ability to modify the intensity and duration of emotional reactions towards affective events. Transcranial direct current stimulation (tDCS) has emerged as a promising treatment for MDD. Yet, positive tDCS outcomes vary across studies, while the precise effects of the procedure for cortical excitability in MDD during emotion regulation remain largely unexplored. Here, we leveraged functional magnetic resonance imaging (fMRI)-compatible tDCS technology to examine the functional consequences of a unilateral anodal tDCS montage at 1.5 mA over left PFC (area F3; with the reference electrode over an extracephalic location) for brain activity during an emotion-regulation task in MDD patients and age-matched healthy control subjects. Our results revealed down-regulation of negative emotions in the right amygdala and visual cortex of healthy controls but not MDD patients prior to stimulation, the degree of which correlated with the magnitude of the participants' reappraisal scores. TDCS did not elicit significant changes in neural activation patterns for either group. These findings contribute to the literature on the pathophysiology of MDD by showing that a key disturbance in the disorder entails the ineffective down-regulation of activity not only within the amygdala, but also within visual cortical areas in response to negative information. Further, these results suggest that relative to bifrontal tDCS montages, unilateral stimulation of moderate intensity over left PFC may not be sufficient to elicit therapeutic effects for MDD.

Effects of Transcranial Direct Current Stimulation on Different Electrode Sites on the Activity of the Upper Extremity in Chronic Stroke Patients: a Randomized, Double-Blind, Sham-Controlled Trial

This study examined the effects of transcranial direct current stimulation (tDCS) on different electrode sites on the activity of the upper extremity in chronic stroke patients. Forty-five subjects were randomly assigned to unilateral tDCS, dual tDCS, and control groups, and tDCS was applied to the unilateral tDCS and dual tDCS groups for 20 minutes per day, 5 times a week for a total of 4 weeks. The activity of the subject’s upper extremity was evaluated by a hand grip strength test, an elbow flexor and wrist flexor muscle tone test, and a Jebsen Taylor hand function test. In the unilateral tDCS group, dual tDCS group, and control group, significant improvement was observed in the hand grip strength test and the Jebsen Taylor hand function test after intervention (p.05), and there was significant improvement in the hand grip strength test and the Jebsen Taylor hand function test compared to the control group (p.05). The results of this study suggest that tDCS applied to the primary motor cortex (M1) positively affects the activity of the upper extremity in stroke patients.

The effect of transcranial direct current stimulation of pharyngeal motor cortex on swallowing function in patients with chronic dysphagia after stroke: A retrospective cohort study.

About 11% to 13% of patients with acute dysphagia induced by stroke remain chronic dysphagia 6 months after stroke which usually leads to many severe complications and poor quality of life.To investigate the effect of transcranial direct current stimulation (tDCS) on swallowing function in the patients with chronic dysphagia after stroke.26 post-stroke patients with chronic dysphagia who received tDCS were identified by electronic medical records between July 2016 and April 2018. Of which, 13 were treated by unilateral hemispheric anodal tDCS at affected pharyngeal motor cortex. 13 eligible patients only treated by conventional therapies but without tDCS were randomly selected by matching on date of admission (±2 weeks) of the patients receiving unilateral tDCS. The swallowing function and quality of life were evaluated before and 2 weeks after treatment.The patients in three groups were comparable. The swallowing function and quality of life of the patients in all the 3 groups had been improved over time. Comparing to the group without tDCS, both the groups with unilateral or bilateral tDCS had shorter oral transit time (1.69 ± 0.95, 0.97 ± 0.71 seconds, respectively) and higher scores of quality of life (159.76 ± 12.59, 179.69 ± 11.81, respectively) after treatment.Both unilateral and bilateral hemispheric anodal tDCS combined with conventional therapies are helpful for recovery of swallowing function in patients with chronic dysphagia induced by stroke, but bilateral anodal tDCS substantially improve more.

Effects of prefrontal tDCS on executive function: Methodological considerations revealed by meta-analysis

A meta-analysis of studies using single-session transcranial direct current stimulation (tDCS) to target the dorsolateral prefrontal cortex (DLPFC) was undertaken to examine the effect of stimulation on executive function (EF) in healthy samples. 27 studies were included in analyses, yielding 71 effect sizes. The most relevant measure for each task was determined a priori and used to calculate Hedge's g. Methodological characteristics of each study were examined individually as potential moderators of effect size. Stimulation effects on three domains of EF (inhibition of prepotent responses, mental set shifting, and information updating and monitoring) were analyzed separately. In line with previous work, the current study found no significant effect of anodal unilateral tDCS, cathodal unilateral tDCS, or bilateral tDCS on EF. Further moderator and subgroup analyses were only carried out for anodal unilateral montages due to the small number of studies using other montages. Subgroup analyses revealed a significant effect of anodal unilateral tDCS on updating tasks, but not on inhibition or set-shifting tasks. Cathode location significantly moderated the effect of anodal unilateral tDCS. Extracranial cathodes yielded a significant effect on EF while cranial cathodes yielded no effect. Anode size also significantly moderated effect of anodal unilateral tDCS, with smaller anodes being more effective than larger anodes. In summary, anodal DLPFC stimulation is more effective at improving updating ability than inhibition and set-shifting ability, but anodal stimulation can significantly improve general executive function when extracranial cathodes or small anodes are used. Future meta-analyses may examine how stimulation's effects on specific behavioral tasks, rather than broader domains, might be affected by methodological moderators.

Frontoparietal tDCS Benefits Visual Working Memory in Older Adults With Low Working Memory Capacity.

Working memory (WM) permits maintenance of information over brief delays and is an essential executive function. Unfortunately, WM is subject to age-related decline. Some evidence supports the use of transcranial direct current stimulation (tDCS) to improve visual WM. A gap in knowledge is an understanding of the mechanism characterizing these tDCS linked effects. To address this gap, we compared the effects of two tDCS montages designed on visual working memory (VWM) performance. The bifrontal montage was designed to stimulate the heightened bilateral frontal activity observed in aging adults. The unilateral frontoparietal montage was designed to stimulate activation patterns observed in young adults. Participants completed three sessions (bilateral frontal, right frontoparietal, sham) of anodal tDCS (20 min, 2 mA). During stimulation, participants performed a visual long-term memory (LTM) control task and a visual WM task. There was no effect of tDCS on the LTM task. Participants receiving right unilateral tDCS showed a WM benefit. This pattern was most robust in older adults with low WM capacity. To address the concern that the key difference between the two tDCS montages could be tDCS over the posterior parietal cortex (PPC), we included new analyses from a previous study applying tDCS targeting the PPC paired with a recognition VWM task. No significant main effects were found. A subsequent experiment in young adults found no significant effect of either tDCS montage on either task. These data indicate that tDCS montage, age and WM capacity should be considered when designing tDCS protocols. We interpret these findings as suggestive that protocols designed to restore more youthful patterns of brain activity are superior to those that compensate for age-related changes.

The effect of transcranial direct current stimulation on motor sequence learning and upper limb function after stroke.

To assess the impact of electrode arrangement on the efficacy of tDCS in stroke survivors and determine whether changes in transcallosal inhibition (TCI) underlie improvements. 24 stroke survivors (3-124months post-stroke) with upper limb impairment participated. They received blinded tDCS during a motor sequence learning task, requiring the paretic arm to direct a cursor to illuminating targets on a monitor. Four tDCS conditions were studied (crossover); anodal to ipsilesional M1, cathodal to contralesional M1, bihemispheric, sham. The Jebsen Taylor hand function test (JTT) was assessed pre- and post-stimulation and TCI assessed as the ipsilateral silent period (iSP) duration using transcranial magnetic stimulation. The time to react to target illumination reduced with learning of the movement sequence, irrespective of tDCS condition (p>0.1). JTT performance improved after unilateral tDCS (anodal or cathodal) compared with sham (p<0.05), but not after bihemispheric (p>0.1). There was no effect of tDCS on change in iSP duration (p>0.1). Unilateral tDCS is effective for improving JTT performance, but not motor sequence learning. This has implications for the design of future clinical trials.

P244 The effect of transcranial direct current stimulation on motor sequence learning and upper limb function after stroke

Transcranial direct current stimulation (tDCS) is a safe and non-invasive brain stimulation technique with the potential to improve upper limb function after stroke. Ipsilesional primary motor cortex (M1) excitability can be increased with anodal tDCS, contralesional M1 excitability can be decreased with cathodal tDCS or both anodal and cathodal tDCS can be used simultaneously on both cortices (bihemispheric). The impact of these different electrode arrangements on the efficacy of tDCS, and whether any of the changes are due to callosal connections between cortices, is unclear. This study aimed to investigate the effect of tDCS electrode arrangement on motor sequence learning and upper limb function in chronic stroke survivors. 21 stroke survivors (range 3–124 months post-stroke, 34–81 years of age) with upper limb impairment received 20 min of 1 mA tDCS (0.04 mA·cm−2) during performance of a motor sequence learning task which involved movement of a computer mouse with the paretic arm to circular targets on a monitor in a repeating pattern. Four tDCS conditions were studied in a repeated-measures design; (i) anodal to the ipsilesional M1, (ii) cathodal to the contralesional M1, (iii) bihemispheric and (iv) sham. Upper limb function was assessed before and after tDCS, using the Jebsen–Taylor hand function test (JTT). Changes in transcallosal inhibition (TCI) were assessed using transcranial magnetic stimulation (ipsilateral silent period duration). There was no effect of tDCS condition on performance of the motor sequence learning task. Performance on the JTT improved significantly after unilateral tDCS (anodal or cathodal) compared to sham (p < 0.05), but not after bihemispheric (Fig. 1). There was no effect on TCI (p > 0.5), and no relationship between changes in TCI and upper limb function. Unilateral, but not bihemispheric, tDCS improves upper limb function. The response to tDCS does not appear to be driven by changes in TCI. These results have implications for the use of tDCS for upper limb rehabilitation.Download : Download high-res image (77KB)Download : Download full-size image

P284 Bilateral effects of unilateral anodal tDCS on motor cortex plasticity and the cross-transfer of strength

Introduction Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation technique that utilizes weak direct currents to induce polarity-dependent modulation of corticospinal excitability. The cross-transfer of strength following unilateral training may be modulated by increased corticospinal excitability of the ipsilateral primary motor cortex (iM1). Although tDCS exerts a modulatory effect over the stimulation region, several studies have also demonstrated that distal areas of the brain connected to the region of stimulation may also be affected. However, no studies have investigated the effect of unilateral anodal-tDCS (a-tDCS) on the contralateral unstimulated hemisphere and whether this increases or decreases bilateral M1 activity and its subsequent effects on motor performance. Objectives : We examined the bilateral effect of unilateral a-tDCS applied to either the iM1 or contralateral M1 prior to a single bout of unilateral strength training on corticospinal excitability, inhibition, and the cross-transfer of strength. Materials and methods In a randomized cross-over design, changes in strength, corticospinal excitability and inhibition of the untrained arm, were analysed in 13 participants in both left and right M1. Participants were exposed to 20 min of anodal and sham tDCS in a contralateral (anode over the left M1) or ipsilateral (anode over right M1) arrangement, followed by an acute bout of strength training of the right biceps. Results Both contralateral and ipsilateral a-tDCS and strength training of the right biceps, increased strength of left untrained biceps by 10% and 12% compared to 3% following sham tDCS. TMS revealed that both arrangements of a-tDCS in combination with strength training, increased corticospinal excitability and decreased silent period duration in both hemispheres, however these changes were greater in the left hemisphere compared to sham tDCS and training ( P Conclusion The findings show that a-tDCS in both contralateral and ipsilateral arrangements in combination with an acute bout of strength training induces bilateral corticospinal plasticity and increases the cross-transfer of strength.

Effects of Unilateral Transcranial Direct Current Stimulation of Left Prefrontal Cortex on Processing and Memory of Emotional Visual Stimuli.

The dorsolateral prefrontal cortex (DLPFC) is generally thought to be involved in affect and emotional processing; however, the specific contribution of each hemisphere continues to be debated. In the present study, we employed unilateral tDCS to test the unique contribution of left DLPFC in the encoding and retrieval of emotional stimuli in healthy subjects. Forty-two right handed undergraduate students received either anodal, cathodal or sham stimulation of left DLPFC while viewing neutral, pleasant, and unpleasant pictures. After completing a filler task, participants were asked to remember as many pictures as possible. Results showed that participants were able to remember a larger amount of emotional (both pleasant and unpleasant) pictures than of neutral ones, regardless of the type of tDCS condition. Participants who received anodal stimulation recalled a significantly higher number of pleasant images than participants in the sham and cathodal conditions, while no differences emerged in the recall of neutral and unpleasant pictures. We conclude that our results provide some support to the role of left prefrontal cortex in the encoding and retrieval of pleasant stimuli.

Sensorimotor Rhythm BCI with Simultaneous High Definition-Transcranial Direct Current Stimulation Alters Task Performance.

Transcranial direct current stimulation (tDCS) has been used to alter the excitability of neurons within the cerebral cortex. Improvements in motor learning have been found in multiple studies when tDCS was applied to the motor cortex before or during task learning. The motor cortex is also active during the performance of motor imagination, a cognitive task during which a person imagines, but does not execute, a movement. Motor imagery can be used with noninvasive brain computer interfaces (BCIs) to control virtual objects in up to three dimensions, but to master control of such devices requires long training times. To evaluate the effect of high-definition tDCS on the performance and underlying electrophysiology of motor imagery based BCI. We utilize high-definition tDCS to investigate the effect of stimulation on motor imagery-based BCI performance across and within sessions over multiple training days. We report a decreased time-to-hit with anodal stimulation both within and across sessions. We also found differing electrophysiological changes of the stimulated sensorimotor cortex during online BCI task performance for left vs. right trials. Cathodal stimulation led to a decrease in alpha and beta band power during task performance compared to sham stimulation for right hand imagination trials. These results suggest that unilateral tDCS over the sensorimotor motor cortex differentially affects cortical areas based on task specific neural activation.

Abstract WP154: Bihemispheric Modulation of the Motor Cortex by Single-session Transcranial Direct Current Stimulation During Training in Subacute Stroke Patients

Introduction: Non-invasive transcranial direct current stimulation (tDCS) can induce polarity-specific changes in cortical excitability. Compared to unilateral anodal tDCS over the non-dominant motor cortex (M1), dual-hemispheric tDCS to the M1 may further accelerate motor reaction of the target hand and alter excitability of the corticospinal tract (CST). However, the effects and individual variability of bi-hemispheric tDCS to the M1 during motor training remain largely unclear in healthy and stroke subjects. Purpose: We assessed the hypothesis that bi-hemispheric tDCS during single-session motor training alter inter-hemispheric inhibition, CST excitability and bilateral cortical oscillations. Methods: We enrolled first-time, unilateral ischemic stroke patients between two and four weeks after stroke and matched healthy controls. They were subjected to two 20 min-sessions of dual-hemispheric tDCS (anode over non-dominant or ipsilesional M1, cathode over dominant or contralesional M1; 2mA for 20 mins) and sham tDCS (2mA for 2 mins) in a randomized crossover design during repetitive extension of the non-dominant or paretic extensor carpi radialis muscle. We compared the post-stimulation changes of motor evoked potentials (MEPs), ipsilateral silent period (iSP), short interval intracoritcal inhibition (SICI), as well as resting and unilateral finger lifting-related cortical oscillations by magnetoencephalography (MEG). Results: Compared to the sham tDCS, the dual-hemispheric tDCS significantly increased MEP amplitudes and reduced SICI at the anodal-stimulated M1, as well as decreased inter-hemispheric inhibition from the cathodal-stimulated M1 with shortened iSP for about 30 mins in healthy controls (n=8). In contrast, reduced MEP amplitudes were observed at the cathodal-stimulated M1. The tDCS effects on cortical oscillations and in stroke patients are currently under investigation. Conclusions: Task-concurrent dual tDCS may enhance activity-dependent motor plasticity in subacute stroke.

Transcranial direct current stimulation (tDCS) of frontal cortex decreases performance on the WAIS-IV intelligence test

Transcranial direct current stimulation (tDCS) modulates excitability of motor cortex. However, there is conflicting evidence about the efficacy of this non-invasive brain stimulation modality to modulate performance on cognitive tasks. Previous work has tested the effect of tDCS on specific facets of cognition and executive processing. However, no randomized, double-blind, sham-controlled study has looked at the effects of tDCS on a comprehensive battery of cognitive processes. The objective of this study was to test if tDCS had an effect on performance on a comprehensive assay of cognitive processes, a standardized intelligence quotient (IQ) test. The study consisted of two substudies and followed a double-blind, between-subjects, sham-controlled design. In total, 41 healthy adult participants were included in the final analysis. These participants completed the Wechsler Adult Intelligence Scale, Fourth Edition (WAIS-IV) as a baseline measure. At least one week later, participants in substudy 1 received either bilateral tDCS (anodes over both F4 and F3, cathode over Cz, 2mA at each anode for 20min) or active sham tDCS (2mA for 40s), and participants in substudy 2 received either right or left tDCS (anode over either F4 or F3, cathode over Cz, 2mA for 20min). In both studies, the WAIS-IV was immediately administered following stimulation to assess for performance differences induced by bilateral and unilateral tDCS. Compared to sham stimulation, right, left, and bilateral tDCS reduced improvement between sessions on Full Scale IQ and the Perceptual Reasoning Index. This demonstration that frontal tDCS selectively degraded improvement on specific metrics of the WAIS-IV raises important questions about the often proposed role of tDCS in cognitive enhancement.

Differential influences of unilateral tDCS over the intraparietal cortex on numerical cognition.

Recent neuro-imaging research identified the bilateral intraparietal sulcus (IPS) to be a key area associated with number processing. However, causal structure-function relationships are hard to evaluate from neuro-imaging techniques such as fMRI. Nevertheless, brain stimulation methods like transcranial direct current stimulation (tDCS) allow for investigating the functional relevance of the IPS for number processing. Following up on a study using bilateral bi-cephalic tDCS over the IPS, the current study aimed at evaluating the differential lateralized functional contributions of the left and right IPS to number processing using unilateral bi-cephalic tDCS over either the left or right IPS. Results indicated a right lateralization for the processing of the place-value structure of the Arabic number system. Importantly, the processing of number magnitude information was not affected by unilateral IPS corroborating the assumption that number magnitude is processed in the bilateral IPS. Taken together, these data suggest that even though number magnitude is represented bilaterally, the left and right IPS seem to contribute differentially to numerical cognition with respect to the processing of specific other aspects of numerical information.

Polarity specific effects of transcranial direct current stimulation on interhemispheric inhibition.

Transcranial direct current stimulation (tDCS) has been used as a useful interventional brain stimulation technique to improve unilateral upper-limb motor function in healthy humans, as well as in stroke patients. Although tDCS applications are supposed to modify the interhemispheric balance between the motor cortices, the tDCS after-effects on interhemispheric interactions are still poorly understood. To address this issue, we investigated the tDCS after-effects on interhemispheric inhibition (IHI) between the primary motor cortices (M1) in healthy humans. Three types of tDCS electrode montage were tested on separate days; anodal tDCS over the right M1, cathodal tDCS over the left M1, bilateral tDCS with anode over the right M1 and cathode over the left M1. Single-pulse and paired-pulse transcranial magnetic stimulations were given to the left M1 and right M1 before and after tDCS to assess the bilateral corticospinal excitabilities and mutual direction of IHI. Regardless of the electrode montages, corticospinal excitability was increased on the same side of anodal stimulation and decreased on the same side of cathodal stimulation. However, neither unilateral tDCS changed the corticospinal excitability at the unstimulated side. Unilateral anodal tDCS increased IHI from the facilitated side M1 to the unchanged side M1, but it did not change IHI in the other direction. Unilateral cathodal tDCS suppressed IHI both from the inhibited side M1 to the unchanged side M1 and from the unchanged side M1 to the inhibited side M1. Bilateral tDCS increased IHI from the facilitated side M1 to the inhibited side M1 and attenuated IHI in the opposite direction. Sham-tDCS affected neither corticospinal excitability nor IHI. These findings indicate that tDCS produced polarity-specific after-effects on the interhemispheric interactions between M1 and that those after-effects on interhemispheric interactions were mainly dependent on whether tDCS resulted in the facilitation or inhibition of the M1 sending interhemispheric volleys.

Effects of different electrical brain stimulation protocols on subcomponents of motor skill learning.

Noninvasive electrical brain stimulation (NEBS) with transcranial direct current (tDCS) or random noise stimulation (tRNS) applied to the primary motor cortex (M1) can augment motor learning. We tested whether different types of stimulation alter particular aspects of learning a tracing task over three consecutive days, namely skill acquisition (online/within session effects) or consolidation (offline/between session effects). Motor training on a tracing task over three consecutive days was combined with different types and montages of stimulation (tDCS, tRNS). Unilateral M1 stimulation using tRNS as well as unilateral and bilateral M1 tDCS all enhanced motor skill learning compared to sham stimulation. In all groups, this appeared to be driven by online effects without an additional offline effect. Unilateral tDCS resulted in large skill gains immediately following the onset of stimulation, while tRNS exerted more gradual effects. Control stimulation of the right temporal lobe did not enhance skill learning relative to sham. The mechanisms of action of tDCS and tRNS are likely different. Hence, the time course of skill improvement within sessions could point to specific and temporally distinct interactions with the physiological process of motor skill learning. Exploring the parameters of NEBS on different tasks and in patients with brain injury will allow us to maximize the benefits of NEBS for neurorehabilitation.