Pharmacological modulation of cortical excitability shifts induced by transcranial direct current stimulation in humans

Non-invasive brain stimulation as therapy: systematic review and recommendations with a focus on the treatment of Tourette syndrome

Tourette syndrome (TS) is a neurodevelopmental condition characterised by tics, which are stereotyped movements and/or vocalisations. Tics often cause difficulties in daily life and many with TS express a desire to reduce and/or gain control over them. No singular effective treatment exists for TS, and while pharmacological and behavioural interventions can be effective, the results are variable, and issues relating to access, availability and side effects can be barriers to treatment. Consequently, over the past decade, there has been increasing interest into the potential benefits of non-invasive brain stimulation (NIBS) approaches. This systematic review highlights work exploring NIBS as a potential treatment for TS. On balance, the results tentatively suggest that multiple sessions of stimulation applied over the supplementary motor area (SMA) may help to reduce tics. However, a number of methodological and theoretical issues limit the strength of this conclusion, with the most problematic being the lack of large-scale sham-controlled studies. In this review, methodological and theoretical issues are discussed, unanswered questions highlighted and suggestions for future work put forward.

The role of sodium channels in direct current stimulation-axonal perspective

Transcranial neurostimulation methods are utilized as therapies for various neuropsychiatric disorders. Primarily, they entail the delivery of weak subthreshold currents across the brain, which modulate neuronal excitability. However, it is still a puzzle how such weak electrical fields actuate their effects. Previous studies showed that axons are the most sensitive subcellular compartment for direct current stimulation, and maximal polarization is achieved at their terminals. Nonetheless, polarization of axon terminals according to models was predicted to be weak, and the mechanism for substantial axon terminals polarization was obscure. Here, we show that a weak subthreshold electrical field modifies the conductance of voltage-dependent sodium channels in axon terminals, subsequently amplifying their membrane polarization. Moreover, we show that this amplification has substantial effects on synaptic functioning. Finally, we employ analytical modeling to explain how sodium currents modifications enhance axon terminal polarization. These findings relate to the mechanistic aspects of any neurostimulation technique.

A novel technique for accurate electrode placement over cortical targets for transcranial electrical stimulation (tES) clinical trials

Objective. We present an easy-to-implement technique for accurate electrode placement over repeated transcranial electrical stimulation (tES) sessions across participants and time. tES is an emerging, non-invasive neuromodulation technique that delivers electrical stimulation using scalp electrodes.Approach.The tES electrode placement technique was developed during an exploratory clinical trial aimed at targeting a specific MNI-atlas cortical coordinate inN= 59 depressed participants (32 F, mean age: 31.1 ± 8.3 SD). Each participant completed 12 sessions of active or sham stimulation, administered using high-definition (HD) or conventional sized electrode montages placed according to the proposed technique. Neuronavigation data measuring the distances between the identified and the intended stimulation site, simulations, and cerebral blood flow (CBF) data at baseline and post-treatment were acquired to evaluate the targeting characteristics of the proposed technique.Main results.Neuronavigation measurements indicate accurate electrode placement to within 1 cm of the stimulation target on average across repeated sessions. Simulations predict that these placement characteristics result in minimal electric field differences at the stimulation target (>0.90 correlation, and <10% change in the modal electric field and targeted volume). Additionally, significant changes in %CBF (relative to baseline) under the stimulation target in the active stimulation group relative to sham confirmed that the proposed placement technique introduces minimal bias in the spatial location of the cortical coordinate ultimately targeted. Finally, we show proof of concept that the proposed technique provides similar accuracy of electrode placement at other cortical targets.Significance.For voxel-level cortical targets, existing techniques based on cranial landmarks are suboptimal. Our results show that the proposed electrode placement approach provides high consistency for the accurate targeting of such specific cortical regions. Overall, the proposed technique now enables the accurate targeting of locations not accessible with the existing 10-20 system such as scalp-projections of clinically-relevant cortical coordinates identified by brain mapping studies. Clinical trial ID: NCT03556124.

Current perspectives on the benefits, risks, and limitations of noninvasive brain stimulation (NIBS) for post-stroke dysphagia

INTRODUCTION

Studies have shown that noninvasive brain stimulation (NIBS), including repetitive transcranial magnetic stimulation (rTMS) and transcranial direct current stimulation (tDCS), can promote neuroplasticity, which is considered important for functional recovery of swallowing after stroke. Despite extensive studies on NIBS, there remains a gap between research and clinical practice.

AREAS COVERED

In this article, we update the current knowledge on the benefits and challenges of rTMS and tDCS for post-stroke dysphagia. We identify some key limitations of these techniques that hinder the translation from clinical trials to routine practice. Finally, we discuss the future of NIBS as a treatment for post-stroke dysphagia in real-world settings.

EXPERT OPINION

Current evidence suggests that rTMS and tDCS show promise as a treatment for post-stroke dysphagia. However, these techniques are limited by the response variability, uncertainty on the safety in patients with comorbidities and difficulties in clinical study designs. Such limitations call for further work to enhance their utility through individualized approaches. Despite this, the last decade has seen a growing acceptance toward these techniques among clinical personnel. As such, we advocate caution but support optimism that NIBS will gradually be recognized as a mainstream treatment approach for post-stroke dysphagia in the future.

A Parkinson's disease patient displaying increased neuromelanin-sensitive areas in the substantia nigra after rehabilitation with tDCS: a case report

Previous studies have reported that transcranial direct current stimulation (tDCS) of the frontal polar area (FPA) ameliorated motor disability in patients with Parkinson's disease (PD). Here we report changes in neuromelanin (NM) imaging of dopaminergic neurons before and after rehabilitation combined with anodal tDCS over the FPA for 2 weeks in a PD patient. After the intervention, the patient showed clinically meaningful improvements while the NM-sensitive area in the SN increased by 18.8%. This case study is the first report of NM imaging of the SN in a PD patient who received tDCS.Abbreviations FPA: front polar area; PD: Parkinson's disease; NM: neuromelanin; DCI: DOPA decarboxylase inhibitor; STEF: simple test for evaluating hand function; TUG: timed up and go test; TMT: trail-making test; SN: substantia nigra; NM-MRI: neuromelanin magnetic resonance imaging; MCID: the minimal clinically important difference; SNpc: substantia nigra pars compacta; VTA: ventral tegmental area; LC: locus coeruleus; PFC: prefrontal cortex; M1: primary motor cortex; MDS: Movement Disorder Society; MIBG: ¹²³I-metaiodobenzylguanidine; SBR: specific binding ratio; SPECT: single-photon emission computed tomography; DAT: dopamine transporter; NIBS: noninvasive brain stimulation; tDCS: transcranial direct current stimulation; MAOB: monoamine oxidase B; DCI: decarboxylase inhibitor; repetitive transcranial magnetic stimulation: rTMS; diffusion tensor imaging: DTI; arterial spin labeling: ASL.

Effects of Transcutaneous Spinal Direct Current Stimulation (tsDCS) in Patients With Chronic Pain: A Clinical and Neurophysiological Study

Background and Aims: Chronic pain is a complex clinical condition, often devastating for patients and unmanageable with pharmacological treatments. Converging evidence suggests that transcutaneous spinal Direct Current Stimulation (tsDCS) might represent a complementary therapy in managing chronic pain. In this randomized, double-blind and sham-controlled crossover study, we assessed tsDCS effects in chronic pain patients.

Methods: Sixteen patients (aged 65.06 ± 16.16 years, eight women) with chronic pain of different etiology underwent sham and anodal tsDCS (anode over the tenth thoracic vertebra, cathode over the somatosensory cortical area: 2.5 mA, 20 min, 5 days for 1 week). As outcomes, we considered the Visual Analog Scale (VAS), the Neuropathic Pain Symptom Inventory (NPSI), and the components of the lower limb flexion reflex (LLFR), i.e., RIII threshold, RII latency and area, RIII latency and area, and flexion reflex (FR) total area. Assessments were conducted before (T0), immediately at the end of the treatment (T1), after 1 week (T2) and 1 month (T3).

Results: Compared to sham, anodal tsDCS reduced RIII area at T2 (p = 0.0043) and T3 (p = 0.0012); similarly, FR total area was reduced at T3 (p = 0.03). Clinically, anodal tsDCS dampened VAS at T3 (p = 0.015), and NPSI scores at T1 (p = 0.0012), and T3 (p = 0.0015), whereas sham condition left them unchanged. Changes in VAS and NPSI scores linearly correlated with the reduction in LLFR areas (p = 0.0004).

Conclusions: Our findings suggest that tsDCS could modulate nociceptive processing and pain perception in chronic pain syndromes.

Application of Transcranial Direct Current Stimulation in Psychiatry

Transcranial direct current stimulation (tDCS) is a neuromodulation technique, which noninvasively alters cortical excitability via weak polarizing currents between two electrodes placed on the scalp. Since it is comparably easy to handle, cheap to use and relatively well tolerated, tDCS has gained increasing interest in recent years. Based on well-known behavioral effects, a number of clinical studies have been performed in populations including patients with major depressive disorder followed by schizophrenia and substance use disorders, in sum with heterogeneous results with respect to efficacy. Nevertheless, the potential of tDCS must not be underestimated since it could be further improved by systematically investigating the various stimulation parameters to eventually increase clinical efficacy. The present article briefly explains the underlying physiology of tDCS, summarizes typical stimulation protocols and then reviews clinical efficacy for various psychiatric disorders as well as prevalent adverse effects. Future developments include combined and more complex interactions of tDCS with pharmacological or psychotherapeutic interventions. In particular, using computational models to individualize stimulation protocols, considering state dependency and applying closed-loop technologies will pave the way for tDCS-based personalized interventions as well as the development of home treatment settings promoting the role of tDCS as an effective treatment option for patients with mental health problems.

The Effect of a Single Session of Non-Invasive Brain Stimulation on Balance in Healthy Individuals: A Systematic Review and Best Evidence Synthesis

Aim: To evaluate the effects of a single session of non-invasive brain stimulation (NIBS) on postural balance. Introduction: The NIBS has been used widely in improving balance. However, the effect of a single session of NIBS on balance in healthy individuals has not been systemically reviewed. Methods: A systematic literature review and best evidence synthesis were conducted, according to the Preferred Reporting Items for Systematic Review and Meta-Analysis (PRISMA) guidelines, to determine the effects of different NIBS techniques on balance function in healthy individuals. The methodological quality of included articles was assessed by the risk of bias, and the Downs and Black tool. Data were analyzed by using the best evidence synthesis. Thirty-five articles were included that used the following NIBS techniques: anodal transcranial direct current stimulation (a-tDCS), cathodal transcranial direct current stimulation (c-tDCS), continuous theta burst stimulation (cTBS), and repetitive transcranial magnetic stimulation (rTMS) on primary motor cortex (M1), supplementary motor area (SMA), dorsolateral prefrontal cortex (DLPFC), and cerebellum on balance. Results: Strong evidence showed that a-tDCS of M1, SMA improve balance in healthy participants, and the a-tDCS of DLPFC induces improvement only in dual task balance indices. Also, the findings indicate that cerebellar a-tDCS might significantly improve balance, if at least 10 min cerebellar a-tDCS with an intensity of ≥1 mA, over or maximum 1.5 cm below the inion, is used. Strong evidence showed that c-tDCS, cTBS, and rTMS are not effective on the balance. Conclusion: According to the results, the a-tDCS may be a useful technique to improve balance in healthy adults.

Brain Circuits Involved in the Development of Chronic Musculoskeletal Pain: Evidence From Non-invasive Brain Stimulation

It has been well-documented that the brain changes in states of chronic pain. Less is known about changes in the brain that predict the transition from acute to chronic pain. Evidence from neuroimaging studies suggests a shift from brain regions involved in nociceptive processing to corticostriatal brain regions that are instrumental in the processing of reward and emotional learning in the transition to the chronic state. In addition, dysfunction in descending pain modulatory circuits encompassing the periaqueductal gray and the rostral anterior cingulate cortex may also be a key risk factor for pain chronicity. Although longitudinal imaging studies have revealed potential predictors of pain chronicity, their causal role has not yet been determined. Here we review evidence from studies that involve non-invasive brain stimulation to elucidate to what extent they may help to elucidate the brain circuits involved in pain chronicity. Especially, we focus on studies using non-invasive brain stimulation techniques [e.g., transcranial magnetic stimulation (TMS), particularly its repetitive form (rTMS), transcranial alternating current stimulation (tACS), and transcranial direct current stimulation (tDCS)] in the context of musculoskeletal pain chronicity. We focus on the role of the motor cortex because of its known contribution to sensory components of pain via thalamic inhibition, and the role of the dorsolateral prefrontal cortex because of its role on cognitive and affective processing of pain. We will also discuss findings from studies using experimentally induced prolonged pain and studies implicating the DLPFC, which may shed light on the earliest transition phase to chronicity. We propose that combined brain stimulation and imaging studies might further advance mechanistic models of the chronicity process and involved brain circuits. Implications and challenges for translating the research on mechanistic models of the development of chronic pain to clinical practice will also be addressed.

Prefrontal Transcranial Direct Current Stimulation Globally Improves Learning but Does Not Selectively Potentiate the Benefits of Targeted Memory Reactivation on Awake Memory Consolidation

Targeted memory reactivation (TMR) and transcranial direct current stimulation (tDCS) can enhance memory consolidation. It is currently unknown whether TMR reinforced by simultaneous tDCS has superior efficacy. In this study, we investigated the complementary effect of TMR and bilateral tDCS on the consolidation of emotionally neutral and negative declarative memories. Participants learned neutral and negative word pairs. Each word pair was presented with an emotionally compatible sound. Following learning, participants spent a 20 min retention interval awake under four possible conditions: (1) TMR alone (i.e., replay of 50% of the associated sounds), (2) TMR combined with anodal stimulation of the left DLPFC, (3) TMR combined with anodal stimulation of the right DLPFC and (4) TMR with sham tDCS. Results evidenced selective memory enhancement for the replayed stimuli in the TMR-only and TMR-sham conditions, which confirms a specific effect of TMR on memory. However, memory was enhanced at higher levels for all learned items (irrespective of TMR) in the TMR-anodal right and TMR-anodal left tDCS conditions, suggesting that the beneficial effects of tDCS overshadow the specific effects of TMR. Emotionally negative memories were not modulated by tDCS hemispheric polarity. We conclude that electrical stimulation of the DLPFC during the post-learning period globally benefits memory consolidation but does not potentiate the specific benefits of TMR.

Cognitive functions and underlying parameters of human brain physiology are associated with chronotype

Circadian rhythms have natural relative variations among humans known as chronotype. Chronotype or being a morning or evening person, has a specific physiological, behavioural, and also genetic manifestation. Whether and how chronotype modulates human brain physiology and cognition is, however, not well understood. Here we examine how cortical excitability, neuroplasticity, and cognition are associated with chronotype in early and late chronotype individuals. We monitor motor cortical excitability, brain stimulation-induced neuroplasticity, and examine motor learning and cognitive functions at circadian-preferred and non-preferred times of day in 32 individuals. Motor learning and cognitive performance (working memory, and attention) along with their electrophysiological components are significantly enhanced at the circadian-preferred, compared to the non-preferred time. This outperformance is associated with enhanced cortical excitability (prominent cortical facilitation, diminished cortical inhibition), and long-term potentiation/depression-like plasticity. Our data show convergent findings of how chronotype can modulate human brain functions from basic physiological mechanisms to behaviour and higher-order cognition.

Non-invasive and invasive brain stimulation in alcohol use disorders: A critical review of selected human evidence and methodological considerations to guide future research

INTRODUCTION

Alcohol use disorder (AUD) ranks among the leading causes of decrements in disability-adjusted life-years. Long-term exposure to alcohol leads to an imbalance of activity between frontal cortical systems and the striatum, thereby enhancing impulsive behaviours and weakening inhibitory control. Alternative therapeutic approaches such as non-invasive and invasive brain stimulation have gained some momentum in the field of addictology by capitalizing on their ability to target specific anatomical structures and correct abnormalities in dysfunctional brain circuits.

MATERIALS AND METHODS

The current review, covers original peer-reviewed published research on the use of brain stimulation methods for the rehabilitation of AUD. A broad and systematic search was carried out on four electronic databases: NCBI PubMed, Web of Science, Handbooks and the Cochrane Library. Any original article in English or French language, without restrictions of patient age or gender, article type and publication outlet, were included in the final pool of selected studies.

RESULTS

The outcomes of this systematic review suggest that the dorsolateral prefrontral cortex (DLPFC) is a promising target for treating AUD with high frequency repetitive transcranial magnetic stimulation. Such effect would reduce feelings of craving by enhancing cognitive control and modulating striatal function. Existing literature also supports the notion that changes of DLPFC activity driven by transcranial direct current stimulation, could decrease alcohol craving and consumption. However, to date, no major differences have been found between the efficacy of these two non-invasive brain-stimulation approaches, which require further confirmation. In contrast, beneficial stronger evidence supports an impact of deep brain stimulation reducing craving and improving quality of life in AUD, effects that would be mediated by an impact on the nucleus accumbens, a central structure of the brain's reward circuitry. Overall, neurostimulation shows promise contributing to the treatment of AUD. Nonetheless, progress has been limited by a number of factors such as the low number of controlled randomized trials, small sample sizes, variety of stimulation parameters precluding comparability and incomplete or questionable sham-conditions. Additionally, a lack of data concerning clinical impact on the severity of AUD or craving and the short follow up periods precluding and accurate estimation of effect duration after discontinuing the treatment, has also limited the clinical relevance of final outcomes.

CONCLUSION

Brain stimulation remains a promising approach to contribute to AUD therapy, co-adjuvant of more conventional procedures. However, a stronger therapeutic rational based on solid physio-pathological evidence and accurate estimates of efficacy, are still required to achieve further therapeutic success and expand clinical use.

Personalised, image-guided, noninvasive brain stimulation in gliomas: Rationale, challenges and opportunities

Malignant brain tumours are among the most aggressive human cancers, and despite intensive efforts made over the last decades, patients' survival has scarcely improved. Recently, high-grade gliomas (HGG) have been found to be electrically integrated with healthy brain tissue, a communication that facilitates tumour mitosis and invasion. This link to neuronal activity has provided new insights into HGG pathophysiology and opened prospects for therapeutic interventions based on electrical modulation of neural and synaptic activity in the proximity of tumour cells, which could potentially slow tumour growth. Noninvasive brain stimulation (NiBS), a group of techniques used in research and clinical settings to safely modulate brain activity and plasticity via electromagnetic or electrical stimulation, represents an appealing class of interventions to characterise and target the electrical properties of tumour-neuron interactions. Beyond neuronal activity, NiBS may also modulate function of a range of substrates and dynamics that locally interacts with HGG (e.g., vascular architecture, perfusion and blood-brain barrier permeability). Here we discuss emerging applications of NiBS in patients with brain tumours, covering potential mechanisms of action at both cellular, regional, network and whole-brain levels, also offering a conceptual roadmap for future research to prolong survival or promote wellbeing via personalised NiBS interventions.

The Long-Term Maintenance of Upper Limb Motor Improvements Following Transcranial Direct Current Stimulation Combined with Rehabilitation in People with Stroke: A Systematic Review of Randomized Sham-Controlled Trials

Background: The effectiveness of transcranial direct current stimulation (tDCS) in the upper limb (UL) motor rehabilitation of stroke has been widely studied. However, the long-term maintenance of its improvements has not yet been proven. Methods: A systematic search was conducted in MEDLINE/Pubmed, Web of Science, PEDRo, and Scopus databases from inception to April 2021. Randomized controlled trials were included if they performed a tDCS intervention combined with UL rehabilitation in stroke patients, performed several sessions (five or more), and assessed long-term results (at least three-month follow-up). Risk of bias and methodological quality were evaluated with the Cochrane RoB-2 and the Oxford quality scoring system. Results: Nine studies were included, showing a high methodological quality. Findings regarding UL were categorized into (1) functionality, (2) strength, (3) spasticity. All the studies that showed significant improvements retained them in the long term. Baseline functionality may be a limiting factor in achieving motor improvements, but not in sustaining them over the long term. Conclusion: It seems that the improvements achieved during the application of tDCS combined with UL motor rehabilitation in stroke were preserved until the follow-up time (from 3 months to 1 year). Further studies are needed to clarify the long-term effects of tDCS.

tDCS-Induced Memory Reconsolidation Effects and Its Associations With Structural and Functional MRI Substrates in Subjective Cognitive Decline

Previous evidence suggests that transcranial direct current stimulation (tDCS) to the left dorsolateral prefrontal cortex (l-DLPFC) can enhance episodic memory in subjects with subjective cognitive decline (SCD), known to be at risk of dementia. Our main goal was to replicate such findings in an independent sample and elucidate if baseline magnetic resonance imaging (MRI) characteristics predicted putative memory improvement. Thirty-eight participants with SCD (aged: 60-65 years) were randomly assigned to receive active (N = 19) or sham (N = 19) tDCS in a double-blind design. They underwent a verbal learning task with 15 words (DAY-1), and 24 h later (DAY-2) stimulation was applied for 15 min at 1.5 mA targeting the l-DLPFC after offering a contextual reminder. Delayed recall and recognition were measured 1 day after the stimulation session (DAY-3), and at 1-month follow-up (DAY-30). Before the experimental session, structural and functional MRI were acquired. We identified a group∗time interaction in recognition memory, being the active tDCS group able to maintain stable memory performance between DAY-3 and DAY-30. MRI results revealed that individuals with superior tDCS-induced effects on memory reconsolidation exhibited higher left temporal lobe thickness and greater intrinsic FC within the default-mode network. Present findings confirm that tDCS, through the modulation of memory reconsolidation, is capable of enhancing performance in people with self-perceived cognitive complaints. Results suggest that SCD subjects with more preserved structural and functional integrity might benefit from these interventions, promoting maintenance of cognitive function in a population at risk to develop dementia.

Anodal transcranial direct current stimulation increases corticospinal excitability, while performance is unchanged

Anodal transcranial direct current stimulation (a-tDCS) has been shown to improve bicycle time to fatigue (TTF) tasks at 70–80% of VO_2max and downregulate rate of perceived exertion (RPE). This study aimed to investigate the effect of a-tDCS on a RPE-clamp test, a 250-kJ time trial (TT) and motor evoked potentials (MEP). Twenty participants volunteered for three trials; control, sham stimulation and a-tDCS. Transcranial magnetic stimulation was used to determine the corticospinal excitability for 12 participants pre and post sham stimulation and a-tDCS. The a-tDCS protocol consisted of 13 minutes of stimulation (2 mA) with the anode placed above the Cz. The RPE-clamp test consisted of 5 minutes ergometer bicycling at an RPE of 13 on the Borg scale, and the TT consisted of a 250 kJ (∼10 km) long bicycle ergometer test. During each test, power output, heart rate and oxygen consumption was measured, while RPE was evaluated. MEPs increased significantly by 36% (±36%) post a-tDCS, with 8.8% (±31%) post sham stimulation (p = 0.037). No significant changes were found for any parameter at the RPE-clamp or TT. The lack of improvement may be due to RPE being more controlled by afferent feedback during TT tests than during TTF tests. Based on the results of the present study, it is concluded that a-tDCS applied over Cz, does not enhance self-paced cycling performance.

An Overview of Noninvasive Brain Stimulation: Basic Principles and Clinical Applications

The brain has the innate ability to undergo neuronal plasticity, which refers to changes in its structure and functions in response to continued changes in the environment. Although these concepts are well established in animal slice preparation models, their application to a large number of human subjects could only be achieved using noninvasive brain stimulation (NIBS) techniques. In this review, we discuss the mechanisms of plasticity induction using NIBS techniques including transcranial magnetic stimulation (TMS), transcranial direct current stimulation (tDCS), transcranial alternating current stimulation (tACS), random noise stimulation (RNS), transcranial ultrasound stimulation (TUS), vagus nerve stimulation (VNS), and galvanic vestibular stimulation (GVS). We briefly introduce these techniques, explain the stimulation parameters and potential clinical implications. Although their mechanisms are different, all these NIBS techniques can be used to induce plasticity at the systems level, to examine the neurophysiology of brain circuits and have potential therapeutic use in psychiatric and neurological disorders. TMS is the most established technique for the treatment of brain disorders, and repetitive TMS is an approved treatment for medication-resistant depression. Although the data on the clinical utility of the other modes of stimulation are more limited, the electrical stimulation techniques (tDCS, tACS, RNS, VNS, GVS) have the advantage of lower cost, portability, applicability at home, and can readily be combined with training or rehabilitation. Further research is needed to expand the clinical utility of NIBS and test the combination of different modes of NIBS to optimize neuromodulation induced clinical benefits.

The effects of transcranial direct current stimulation on gait in patients with Parkinson's disease: a systematic review

Background

Gait problems are an important symptom in Parkinson’s disease (PD), a progressive neurodegenerative disease. Transcranial direct current stimulation (tDCS) is a neuromodulatory intervention that can modulate cortical excitability of the gait-related regions. Despite an increasing number of gait-related tDCS studies in PD, the efficacy of this technique for improving gait has not been systematically investigated yet. Here, we aimed to systematically explore the effects of tDCS on gait in PD, based on available experimental studies.

Methods

Using the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) approach, PubMed, Web of Science, Scopus, and PEDro databases were searched for randomized clinical trials assessing the effect of tDCS on gait in patients with PD.

Results

Eighteen studies were included in this systematic review. Overall, tDCS targeting the motor cortex and supplementary motor area bilaterally seems to be promising for gait rehabilitation in PD. Studies of tDCS targeting the dorosolateral prefrontal cortex or cerebellum showed more heterogeneous results. More studies are needed to systematically compare the efficacy of different tDCS protocols, including protocols applying tDCS alone and/or in combination with conventional gait rehabilitation treatment in PD.

Conclusions

tDCS is a promising intervention approach to improving gait in PD. Anodal tDCS over the motor areas has shown a positive effect on gait, but stimulation of other areas is less promising. However, the heterogeneities of methods and results have made it difficult to draw firm conclusions. Therefore, systematic explorations of tDCS protocols are required to optimize the efficacy.

Frontoparietal anodal tDCS reduces ketamine-induced oscillopathies

During the prodromal phase of schizophrenia with its complex and insidious clinical picture, electroencephalographic recordings detect widespread oscillation disturbances (or oscillopathies) during the wake-sleep cycle. Neural oscillations are electrobiomarkers of the connectivity state within systems. A single-systemic administration of ketamine, a non-competitive NMDA glutamate receptor antagonist, transiently reproduces the oscillopathies with a clinical picture reminiscent of the psychosis prodrome. This acute pharmacological model may help the research and development of innovative treatments against psychotic transition. Transcranial electrical stimulation is recognized as an appropriate non-invasive therapeutic modality since it can increase cognitive performance and modulate neural oscillations with little or no side effects. Therefore, our objective was to set up, in the sedated adult rat, a stimulation method that is able to normalize ketamine-induced increase in gamma-frequency (30-80 Hz) oscillations and decrease in sigma-frequency (10-17 Hz) oscillations. Unilateral and bipolar frontoparietal (FP), transcranial anodal stimulation by direct current (<+1 mA) was applied in ketamine-treated rats. A concomitant bilateral electroencephalographic recording of the parietal cortex measured the stimulation effects on its spontaneously occurring oscillations. A 5 min FP anodal tDCS immediately and quickly reduced, significantly with an intensity-effect relationship, the ketamine-induced gamma hyperactivity, and sigma hypoactivity at least in the bilateral parietal cortex. A duration effect was also recorded. The tDCS also tended to diminish the ketamine-induced delta hypoactivity. These preliminary neurophysiological findings are promising for developing a therapeutic proof-of-concept against neuropsychiatric disorders.

Transcranial direct current stimulation over the left anterior temporal lobe during memory retrieval differentially affects true and false recognition in the DRM task

Transcranial direct current stimulation (tDCS) is a form of non-invasive brain stimulation that has been used to modulate human brain activity and cognition. One area which has not yet been extensively explored using tDCS is the generation of false memories. In this study, we combined the Deese-Roediger-McDermott (DRM) task with stimulation of the left anterior temporal lobe (ATL) during retrieval. This area has been shown to be involved in semantic processing in general and retrieval of false memories in the DRM paradigm in particular. During stimulation, 0.7 mA were applied via a 9 cm² electrode over the left ATL, with the 35 cm² return electrode placed over the left deltoid. We contrasted the effects of cathodal, anodal, and sham stimulation, which were applied in the recognition phase of the experiment on a sample of 78 volunteers. Results showed impaired recognition of true memories after both anodal and cathodal stimulation in comparison to sham stimulation, suggesting a reduced signal-to-noise ratio. In addition, the results revealed enhanced false recognition of concept lure items during cathodal stimulation compared to anodal stimulation, indicating a polarity-dependent impact of tDCS on false memories in the DRM task. The pathway by which tDCS modulated false recognition remains unclear: stimulation may have changed the activation of irrelevant lures or affected the weighting and monitoring of lure activations. Nevertheless, these results are a first step towards using brain stimulation to decrease false memories. Practical implications of the findings for real-life settings, for example, in the courtroom, need to be addressed in future work.

A Short-Term Intervention of High-Intensity Exercise and Anodal-tDCS on Motor Learning in Middle-Aged Adults: An RCT

High-intensity exercise has enhanced motor learning in healthy young adults. Anodal-transcranial direct current stimulation (a-tDCS) may optimize these effects. This study aimed to determine the effects of a short-term high-intensity interval exercise intervention either with or without a-tDCS on the learning and retention of a novel motor task in middle-aged adults. Forty-two healthy middle-aged adults (age = 44.6 ± 6.3, female = 76%) were randomized into three groups: exercise and active a-tDCS, exercise and sham a-tDCS, and a non-exercise and sham a-tDCS control. Participants completed a baseline testing session, followed by three intervention sessions 48-h apart. The exercise groups completed 20-min of high-intensity exercise followed by a novel sequential visual isometric pinch task (SVIPT) while receiving 20-min of 1.5 mA a-tDCS, or sham tDCS. The control group completed 20-min of reading before receiving sham a-tDCS during the SVIPT. Learning was assessed by skill change within and between intervention sessions. Participants returned 5–7 days after the final intervention session and performed the SVIPT task to assess retention. All three groups showed evidence of learning on the SVIPT task. Neither group displayed enhanced overall learning or retention when compared to the control group. High-intensity exercise with or without a-tDCS did not improve learning or retention of a novel motor task in middle-aged adults. The methodological framework provides direction for future research to investigate the potential of differing exercise intensity effects on learning and retention.

Comparison of the effects of transcranial direct current stimulation and mindfulness-based stress reduction on mental fatigue, quality of life and aggression in mild traumatic brain injury patients: a randomized clinical trial

BACKGROUND

The rate of traumatic brain injuries (TBIs) due to the accidents is high around the world. Patients with mild TBIs may suffer from some psychological disorders, including aggression, and mental fatigue, and thus their quality of life decreased. Among different treatments for TBI, two treatments, namely transcranial direct current stimulation (tDCS), and mindfulness-based stress reduction (MBSR) have shown to be effective. Therefore, this study aimed to compare the effects of these two treatments on mental fatigue, aggression and quality of life in mTBI patients.

MATERIALS AND METHODS

This randomized controlled trial study was conducted on 48 TBI patients referred to emergency and neurosurgery departments of Shahid Beheshti Hospital, Kashan, Iran. They were selected using the convenience sampling method. Data were collected using the mental fatigue scale, the World Health Organization Quality of Life-BREF (short version), and the Buss-Perry Aggression Questionnaires. Then, the data were analyzed using a Mixed Repeated Measures ANOVAs, and the Levene and Kolmogorov-Smirnov tests by SPSS-23 software.

RESULTS

The mean age of patients in the three groups of MBSR, tDCS and control were 69.38 + 6.11 (25% male), 25.40 + 12.11 (25% male) and 69.37 + 0.2 (18.8% male), respectively. There was no significant difference between the three groups in terms of mental fatigue, quality of life and aggression (P < 0.05). In addition, the results showed that there was a significant difference between the main effect of time and the interaction between time and group (P < 0.001).

CONCLUSIONS

Both MBSR and tDCS methods are effective in reducing the mental fatigue and aggression and increasing quality of life of mTBI patients; MBSR treatment, as indicated in the present study, can be more effective than tDCS in patients with mTBI.

TRIAL REGISTRATION

Thailand Registry of Clinical Trials, TCTR20180827003 Registered on August 24, 2018.

The effects of transcranial direct current stimulation on upper-limb function post-stroke: A meta-analysis of multiple-session studies

OBJECTIVE

To systematically review how patient characteristics and/or transcranial direct current stimulation (tDCS) parameters influence tDCS effectiveness in respect to upper limb function post-stroke.

METHODS

Three electronic databases were searched for sham-controlled randomised trials using the Fugl-Meyer Assessment for upper extremity as outcome measure. A meta-analysis and nine subgroup-analyses were performed to identify which tDCS parameters yielded the greatest impact on upper limb function recovery in stroke patients.

RESULTS

Eighteen high-quality studies (507 patients) were included. tDCS applied in a chronic stage yields greater results than tDCS applied in a (sub)acute stage. Additionally, patients with low baseline upper limb impairments seem to benefit more from tDCS than those with high baseline impairments. Regarding tDCS configuration, all stimulation types led to a significant improvement, but only tDCS applied during therapy, and not before therapy, yielded significant results. A positive dose-response relationship was identified for current/charge density and stimulation duration, but not for number of sessions.

CONCLUSION

Our results demonstrate that tDCS improves upper limb function post-stroke. However, its effectiveness depends on numerous factors. Especially chronic stroke patients improved, which is promising as they are typically least amenable to recovery.

SIGNIFICANCE

The current work highlights the importance of several patient-related and protocol-related factors regarding tDCS effectiveness.

Impact of Transcranial Direct Current Stimulation on Cognitive Function, Brain Functional Segregation, and Integration in Patients with Mild Cognitive Impairment According to Amyloid-Beta Deposition and APOE ε4-Allele: A Pilot Study

Anodal transcranial direct current stimulation (anodal-tDCS) is known to improve cognition and normalize abnormal network configuration during resting-state functional magnetic resonance imaging (fMRI) in patients with mild cognitive impairment (MCI). We aimed to evaluate the impact of sequential anodal-tDCS on cognitive functions, functional segregation, and integration parameters in patients with MCI, according to high-risk factors for Alzheimer’s disease (AD): amyloid-beta (Aβ) deposition and APOE ε4-allele status. In 32 patients with MCI ([¹⁸ F] flutemetamol-: n = 10, [¹⁸ F] flutemetamol+: n = 22; APOE ε4-: n = 13, APOE ε4+: n = 19), we delivered anodal-tDCS (2 mA/day, five times/week, for 2 weeks) over the left dorsolateral prefrontal cortex and assessed the neuropsychological test battery and resting-state fMRI measurements before and after 2 weeks stimulation. We observed a non-significant impact of an anodal-tDCS on changes in neuropsychological battery scores between MCI patients with and without high-risk factors of AD, Aβ retention and APOE ε4-allele. However, there was a significant difference in brain functional segregation and integration parameters between MCI patients with and without AD high-risk factors. We also found a significant effect of tDCS-by-APOE ε4-allele interaction on changes in the functional segregation parameter of the temporal pole. In addition, baseline Aβ deposition significantly associated negatively with change in global functional integrity of hippocampal formation. Anodal-tDCS might help to enhance restorative and compensatory intrinsic functional changes in MCI patients, modulated by the presence of Aβ retention and the APOE ε4-allele.

Dosage-Dependent Impact of Acute Serotonin Enhancement on Transcranial Direct Current Stimulation Effects

BACKGROUND

The serotonergic system has an important impact on basic physiological and higher brain functions. Acute and chronic enhancement of serotonin levels via selective serotonin reuptake inhibitor administration impacts neuroplasticity in humans, as shown by its effects on cortical excitability alterations induced by non-invasive brain stimulation, including transcranial direct current stimulation (tDCS). Nevertheless, the interaction between serotonin activation and neuroplasticity is not fully understood, particularly considering dose-dependent effects. Our goal was to explore dosage-dependent effects of acute serotonin enhancement on stimulation-induced plasticity in healthy individuals.

METHODS

Twelve healthy adults participated in 7 sessions conducted in a crossover, partially double-blinded, randomized, and sham-controlled study design. Anodal and cathodal tDCS was applied to the motor cortex under selective serotonin reuptake inhibitor (20 mg/40 mg citalopram) or placebo medication. Motor cortex excitability was monitored by single-pulse transcranial magnetic stimulation.

RESULTS

Under placebo medication, anodal tDCS enhanced, and cathodal tDCS reduced, excitability for approximately 60-120 minutes after the intervention. Citalopram enhanced and prolonged the facilitation induced by anodal tDCS regardless of the dosage while turning cathodal tDCS-induced excitability diminution into facilitation. For the latter, prolonged effects were observed when 40 mg was administrated.

CONCLUSIONS

Acute serotonin enhancement modulates tDCS after-effects and has largely similar modulatory effects on motor cortex neuroplasticity regardless of the specific dosage. A minor dosage-dependent effect was observed only for cathodal tDCS. The present findings support the concept of boosting the neuroplastic effects of anodal tDCS by serotonergic enhancement, a potential clinical approach for the treatment of neurological and psychiatric disorders.

Bilateral transcranial direct current stimulation attenuated symptoms of alcohol use disorder: A systematic review and meta-analysis

BACKGROUND

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.

METHOD

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.

RESULTS

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.

CONCLUSIONS

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.

Neuroplasticity and non-invasive brain stimulation in the developing brain

The brain is a dynamic organ whose growth and organization varies according to each subject's life experiences. Through adaptations in gene expression and the release of neurotrophins and neurotransmitters, these experiences induce a process of cellular realignment and neural network reorganization, which consolidate what is called neuroplasticity. However, despite the brain's resilience and dynamism, neuroplasticity is maximized during the first years of life, when the developing brain is more sensitive to structural reorganization and the repair of damaged neurons. This review presents an overview of non-invasive brain stimulation (NIBS) techniques that have increasingly been a focus for experimental research and the development of therapeutic methods involving neuroplasticity, especially Transcranial Magnetic Stimulation (TMS) and Transcranial Direct Current Stimulation (tDCS). Due to its safety risk profile and extensive tolerability, several trials have demonstrated the benefits of NIBS as a feasible experimental alternative for the treatment of brain and mind disorders in children and adolescents. However, little is known about the late impact of neuroplasticity-inducing tools on the developing brain, and there are concerns about aberrant plasticity. There are also ethical considerations when performing interventions in the pediatric population. This article will therefore review these aspects and also obstacles related to the premature application of NIBS, given the limited evidence available concerning the extent to which these methods interfere with the developing brain.

Modulating short-term auditory memory with focal transcranial direct current stimulation applied to the supramarginal gyrus

Previous studies have shown that transcranial direct current stimulation (tDCS) can affect performance by decreasing regional excitability in a brain region that contributes to the task of interest. To our knowledge, no research to date has found both enhancing and diminishing effects on performance, depending upon which polarity of the current is applied. The supramarginal gyrus (SMG) is an ideal brain region for testing tDCS effects because it is easy to identify using the 10-20 electroencephalography coordinate system, and results of neuroimaging studies have implicated the left SMG in short-term memory for phonological and nonphonological sounds. In the present study, we found that applying tDCS to the left SMG affected pitch memory in a manner that depended upon the polarity of stimulation: cathodal tDCS had a negative impact on performance whereas anodal tDCS had a positive impact. These effects were significantly different from sham stimulation, which did not influence performance; they were also specific to the left hemisphere - no effect was found when applying cathodal stimulation to the right SMG - and were unique to pitch memory as opposed to memory for visual shapes. Our results provide further evidence that the left SMG is a nodal point for short-term auditory storage and demonstrate the potential of tDCS to influence cognitive performance and to causally examine hypotheses derived from neuroimaging studies.

Inhibitory Effect of Apomorphine on Focal and Nonfocal Plasticity in the Human Motor Cortex

Dopamine is crucial for neuroplasticity, which is considered to be the neurophysiological foundation of learning and memory. The specific effect of dopamine on plasticity such as long-term potentiation (LTP) and long-term depression (LTD) is determined by receptor subtype specificity, concentration level, and the kind of plasticity induction technique. In healthy human subjects, the dopamine precursor levodopa (L-DOPA) exerts a dosage-dependent non-linear effect on motor cortex plasticity. Low and high dosage L-DOPA impaired or abolished plasticity, while medium-dose preserved and reversed plasticity in previous studies. Similar dosage-dependent effects were also observed for selective D1-like and D2-like receptor activation that favor excitatory and inhibitory plasticity, respectively. However, such a dosage-dependent effect has not been explored for a nonselective dopamine agonist such as apomorphine in humans. To this aim, nonfocal and focal motor cortex plasticity induction using paired associative stimulation (PAS) and transcranial direct current stimulation (tDCS) were performed respectively in healthy participants under 0.1, 0.2, 0.3 mg apomorphine or placebo drug. Transcranial magnetic stimulation-elicited motor-evoked potentials were used to monitor motor cortical excitability alterations. We hypothesized that, similar to L-DOPA, apomorphine will affect motor cortex plasticity. The results showed that apomorphine with the applied dosages has an inhibitory effect for focal and nonfocal LTP-like and LTD-like plasticity, which was either abolished, diminished or reversed. The detrimental effect on plasticity induction under all dosages of apomorphine suggests a predominantly presynaptic mechanism of action of these dosages.

Can Transcranial Direct Current Stimulation Enhance Poststroke Motor Recovery? Development of a Theoretical Patient-Tailored Model

New treatments that can facilitate neural repair and reduce persistent impairments have significant value in promoting recovery following stroke. One technique that has gained interest is transcranial direct current stimulation (tDCS) as early research suggested it could enhance plasticity and enable greater behavioral recovery. However, several studies have now identified substantial intersubject variability in response to tDCS and clinical trials revealed insufficient evidence of treatment effectiveness. A possible explanation for the varied and negative findings is that the physiologic model of stroke recovery that researchers have used to guide the application of tDCS-based treatments in stroke is overly simplistic and does not account for stroke heterogeneity or known determinants that affect the tDCS response. Here, we propose that tDCS could have a more clearly beneficial role in enhancing stroke recovery if greater consideration is given to individualizing treatment. By critically reviewing the proposed mechanisms of tDCS, stroke physiology across the recovery continuum, and known determinants of tDCS response, we propose a new, theoretical, patient-tailored approach to delivering tDCS after stroke. The proposed model includes a step-by-step principled selection strategy for identifying optimal neuromodulation targets and outlines key areas for further investigation. Tailoring tDCS treatment to individual neuroanatomy and physiology is likely our best chance at producing robust and meaningful clinical benefit for people with stroke and would therefore accelerate opportunities for clinical translation.

Acute effects of spaced cathodal transcranial direct current stimulation in drug resistant focal epilepsies

OBJECTIVE

To evaluate the safety and temporal dynamic of the antiepileptic effect of spaced transcranial direct current stimulation (tDCS) in different focal epilepsies.

METHODS

Cathodal tDCS with individual electrode placement was performed in 15 adults with drug resistant focal epilepsy. An amplitude of 2 mA was applied twice for 9 minutes, with an interstimulation interval of 20 minutes. Tolerability was assessed via the Comfort Rating Questionnaire and the frequency of interictal epileptiform discharges (IEDs) was sequentially compared between the 24 hours before and after tDCS.

RESULTS

TDCS led to a significant reduction in the total number of IEDs/24 h by up to 68% (mean ± SD: -30.4 ± 21.1%, p = 0.001) as well as in seizure frequency (p = 0.041). The maximum IED reduction was observed between the 3rd and 21st hour after stimulation. Favorable clinical response was associated with structural etiology and clearly circumscribed epileptogenic foci but did not differ between frontal and temporal epilepsies. Overall, the tDCS treatment was well tolerated and did not lead to severe adverse events.

CONCLUSIONS

The spaced stimulation approach proved to be safe and well-tolerated in patients with drug-resistant unifocal epilepsies, leading to sustained IED and seizure frequency reduction.

SIGNIFICANCE

Spaced tDCS induces mediate antiepileptic effects with promising therapeutic potential.

Effect of transcranial direct current stimulation on the psychomotor, cognitive, and motor performances of power athletes

In sports science, transcranial direct current stimulation (tDCS) has many unknown effects on neuromuscular, psychomotor and cognitive aspects. Particularly, its impact on power performances remains poorly investigated. Eighteen healthy young males, all trained in a jumping sport (parkour) performed three experimental sessions: anodal tDCS applied either on the left dorsolateral prefrontal cortex (dlPFC, cathode in supraorbital area) or on the primary motor cortex (M1, cathode on contralateral shoulder), and a placebo condition (SHAM), each applied for 20 min at 2 mA. Pre and post, maximal vertical and horizontal jumps were performed, associated to leg neuromuscular assessment through electromyography and peripheral nerve stimulations. Actual and imagined pointing tasks were also performed to evaluate fine motor skills, and a full battery of cognitive and psychomotor tests was administered. M1 tDCS improved jump performance accompanied by an increase in supraspinal and spinal excitabilities. dlPFC stimulation only impacted the pointing tasks. No effect on cognitive tests was found for any of the tDCS conditions. To conclude, the type of performance (maximal versus accurate) affected depended upon the tDCS montage. Finally, athletes responded well to tDCS for motor performance while results to cognitive tests seemed unaffected, at least when implemented with the present rationale.

Is tDCS a potential first line treatment for major depression?

Transcranial direct current stimulation (tDCS) is a novel treatment option for major depression which could be provided as a first-line treatment. tDCS is a non-invasive form of transcranial stimulation which changes cortical tissue excitability by applying a weak (0.5-2 mA) direct current via scalp electrodes. Anodal and cathodal stimulation leads to depolarisation and hyperpolarisation, respectively, and cumulative effects are observed with repeated sessions. The montage in depression most often involves anodal stimulation to the left dorsolateral prefrontal cortex. Rates of clinical response, remission, and improvements in depressive symptoms following a course of active tDCS are greater in comparison to a course of placebo sham-controlled tDCS. In particular, the largest treatment effects are evident in first episode and recurrent major depression, while minimal effects have been observed in treatment-resistant depression. The proposed mechanism is neuroplasticity at the cellular and molecular level. Alterations in neural responses have been found at the stimulation site as well as subcortically in prefrontal-amygdala connectivity. A possible mediating effect could be cognitive control in emotion dysregulation. Additional beneficial effects on cognitive impairments have been reported, which would address an important unmet need. The tDCS device is portable and can be used at home. Clinical trials are required to establish the efficacy, feasibility and acceptability of home-based tDCS treatment and mechanisms.

Noninvasive direct current stimulation for schizophrenia: a review

PURPOSE OF REVIEW

To provide an update of recent studies describing the effects of transcranial direct current stimulation (tDCS) on patients with schizophrenia, with particular focus on auditory verbal hallucinations (AVH), cognitive deficits, and negative symptoms.

RECENT FINDINGS

As a low-cost, easy-to-use neuromodulation technique, tDCS may have clinical implications for those suffering from treatment-persistent AVH, negative symptoms, and cognitive symptoms in schizophrenia. Over the past decade, tDCS has shown no effects for negative symptoms, except when used at a high frequency of sessions, and inconclusive results for AVH and cognitive symptoms. The treatment has little to no adverse effects.

SUMMARY

The studies reviewed here support the need for further investigation and empirical data regarding the use of tDCS. The underlying mechanisms of tDCS as well as the most effective stimulation parameters must be better understood. Findings support the need for increased duration and frequency of tDCS sessions. One of the next steps is the investigation of effects of concomitant nonpharmacological treatments with tDCS.

Evidence-Based Guidelines and Secondary Meta-Analysis for the Use of Transcranial Direct Current Stimulation in Neurological and Psychiatric Disorders

BACKGROUND

Transcranial direct current stimulation has shown promising clinical results, leading to increased demand for an evidence-based review on its clinical effects.

OBJECTIVE

We convened a team of transcranial direct current stimulation experts to conduct a systematic review of clinical trials with more than 1 session of stimulation testing: pain, Parkinson's disease motor function and cognition, stroke motor function and language, epilepsy, major depressive disorder, obsessive compulsive disorder, Tourette syndrome, schizophrenia, and drug addiction.

METHODS

Experts were asked to conduct this systematic review according to the search methodology from PRISMA guidelines. Recommendations on efficacy were categorized into Levels A (definitely effective), B (probably effective), C (possibly effective), or no recommendation. We assessed risk of bias for all included studies to confirm whether results were driven by potentially biased studies.

RESULTS

Although most of the clinical trials have been designed as proof-of-concept trials, some of the indications analyzed in this review can be considered as definitely effective (Level A), such as depression, and probably effective (Level B), such as neuropathic pain, fibromyalgia, migraine, post-operative patient-controlled analgesia and pain, Parkinson's disease (motor and cognition), stroke (motor), epilepsy, schizophrenia, and alcohol addiction. Assessment of bias showed that most of the studies had low risk of biases, and sensitivity analysis for bias did not change these results. Effect sizes vary from 0.01 to 0.70 and were significant in about 8 conditions, with the largest effect size being in postoperative acute pain and smaller in stroke motor recovery (nonsignificant when combined with robotic therapy).

CONCLUSION

All recommendations listed here are based on current published PubMed-indexed data. Despite high levels of evidence in some conditions, it must be underscored that effect sizes and duration of effects are often limited; thus, real clinical impact needs to be further determined with different study designs.

fMRI and transcranial electrical stimulation (tES): A systematic review of parameter space and outcomes

The combination of non-invasive brain stimulation interventions with human brain mapping methods have supported research beyond correlational associations between brain activity and behavior. Functional MRI (fMRI) partnered with transcranial electrical stimulation (tES) methods, i.e., transcranial direct current (tDCS), transcranial alternating current (tACS), and transcranial random noise (tRNS) stimulation, explore the neuromodulatory effects of tES in the targeted brain regions and their interconnected networks and provide opportunities for individualized interventions. Advances in the field of tES-fMRI can be hampered by the methodological variability between studies that confounds comparability/replicability. In order to explore variability in the tES-fMRI methodological parameter space (MPS), we conducted a systematic review of 222 tES-fMRI experiments (181 tDCS, 39 tACS and 2 tRNS) published before February 1, 2019, and suggested a framework to systematically report main elements of MPS across studies. Publications dedicated to tRNS-fMRI were not considered in this systematic review. We have organized main findings in terms of fMRI modulation by tES. tES modulates activation and connectivity beyond the stimulated areas particularly with prefrontal stimulation. There were no two studies with the same MPS to replicate findings. We discuss how to harmonize the MPS to promote replication in future studies.

Neuromodulation-Based Stem Cell Therapy in Brain Repair: Recent Advances and Future Perspectives

Stem cell transplantation holds a promising future for central nervous system repair. Current challenges, however, include spatially and temporally defined cell differentiation and maturation, plus the integration of transplanted neural cells into host circuits. Here we discuss the potential advantages of neuromodulation-based stem cell therapy, which can improve the viability and proliferation of stem cells, guide migration to the repair site, orchestrate the differentiation process, and promote the integration of neural circuitry for functional rehabilitation. All these advantages of neuromodulation make it one potentially valuable tool for further improving the efficiency of stem cell transplantation.

Serum Levels of Brain-Derived Neurotrophic Factor and Other Neurotrophins in Elite Athletes: Potential Markers of the Use of Transcranial Direct Current Stimulation in Sport

Transcranial Direct Current Stimulation (tDCS) is a non-invasive brain stimulation that may enhance mental and physical performance in sports, representing a potential new form of doping (“brain doping” or “electromagnetic doping”). This study aims to identify diagnostic biomarkers for detecting the possible abuse of tDCS in sport. Brain-Derived Neurotrophic Factor (BDNF) and other neurotrophins (NT, such as beta nerve growth factor, NGF) were pre-selected as potential candidates since their serum values have been observed to change following tDCS. Neurotrophins were measured using ELISA assays in 92 serum samples collected from elite athletes, classified by sex (males = 74; females = 18), age (range 17–25 n = 27, 26–35 n = 36, and over 35 n = 14; age not known n = 15), type of sports practiced (endurance n = 74; power n = 18), and type of sample collection (“in competition” n = 24; “out of competition” n = 68). Single nucleotide polymorphisms (rs6265, rs11030099, and rs11030100) were genotyped on 88 samples to determine their influence on the analytes' basal levels. Athletes older than 35 presented higher BDNF values than younger individuals (p < 0.05). Samples collected “in competition” showed higher BDNF concentrations than those collected “out of competition” (p < 0.05). The studied polymorphisms appeared to affect only on proBDNF, not altering BDNF serum concentrations. NT-3 and NT-4 were poorly detectable in serum. Our results suggest that BDNF can be considered as a first biomarker to detect the abuse of tDCS in sport doping. Further studies are necessary to assess whether proBDNF and beta NGF can also be considered suitable biomarkers to detect the recourse to electromagnetic brain stimulation in sports, especially in the case their serum levels can be monitored longitudinally. To the best of our knowledge, this is the first study aimed to pre-select serum biomarkers to identify the use of tDCS, and represents the first step toward the development of an indirect strategy, preferably based on the longitudinal monitoring of individual data, for the future detection of “brain doping” in sports.

Effects of Transcranial Direct Current Stimulation (tDCS) on Cognitive Performance and Cerebral Oxygen Hemodynamics: A Systematic Review

Background: There is increasing evidence to support the efficacy of transcranial direct current stimulation (tDCS) applications in cognitive augmentation and rehabilitation. Neuromodulation achieved with tDCS may further regulate regional cerebral perfusion affiliated through the neurovascular unit; however, components of cerebral perfusion decrease across aging. A novel neuroimaging approach, functional near-infrared spectroscopy (fNIRS), can aid in quantifying these regional perfusional changes. To date, the interaction of the effects of tDCS on cognitive performance across the lifespan and obtained fNIRS hemodynamic responses remain unknown.

Objective: This review aims to examine the effects of tDCS on cognitive performance and fNIRS hemodynamic responses within the context of cognitive aging.

Methods: Six databases were searched for studies. Quality appraisal and data extraction were conducted by two independent reviewers. Meta-analysis was carried out to determine overall and subgroup effect sizes.

Results: Eight studies met inclusion criteria. The overall effect size demonstrates that tDCS can alter cognitive performance and fNIRS signals, with aging being a potential intermediary in tDCS efficacy.

Conclusion: From the studies included, the effects of tDCS on cognitive performance and fNIRS metrics are most prominent in young healthy adults and appear to become less robust with increasing age. Given the small number of studies included in this review further investigation is recommended.

Transcranial direct current stimulation for spinal cord injury-associated neuropathic pain

Several types of pain occur following spinal cord injury (SCI); however, neuropathic pain (NP) is one of the most intractable. Invasive and non-invasive brain stimulation techniques have been studied in clinical trials to treat chronic NP following SCI. The evidence for invasive stimulation including motor cortex and deep brain stimulation via the use of implanted electrodes to reduce SCI-related NP remains limited, due to the small scale of existing studies. The lower risk of complications associated with non-invasive stimulation, including transcranial direct current stimulation (tDCS) and repetitive transcranial magnetic stimulation (rTMS), provide potentially attractive alternative central neuromodulation techniques. Compared to rTMS, tDCS is technically easier to apply, more affordable, available, and potentially feasible for home use. Accordingly, several new studies have investigated the efficacy of tDCS to treat NP after SCI. In this review, articles relating to the mechanisms, clinical efficacy and safety of tDCS on SCI-related NP were searched from inception to December 2019. Six clinical trials, including five randomized placebo-controlled trials and one prospective controlled trial, were included for evidence specific to the efficacy of tDCS for treating SCI-related NP. The mechanisms of action of tDCS are complex and not fully understood. Several factors including stimulation parameters and individual patient characteristics may affect the efficacy of tDCS intervention. Current evidence to support the efficacy of utilizing tDCS for relieving chronic NP after SCI remains limited. Further strong evidence is needed to confirm the efficacy of tDCS intervention for treating SCI-related NP.

Exploring and optimizing the neuroplastic effects of anodal transcranial direct current stimulation over the primary motor cortex of older humans

BACKGROUND

tDCS modulates cortical plasticity and has shown potential to improve cognitive/motor functions in healthy young humans. However, age-related alterations of brain structure and functions might require an adaptation of tDCS-parameters to achieve a targeted plasticity effect in older humans and conclusions obtained from young adults might not be directly transferable to older adults. Thus, our study aimed to systematically explore the association between tDCS-parameters and induced aftereffects on motor cortical excitability to determine optimal stimulation protocols for older individuals, as well as to investigate age-related differences of motor cortex plasticity in two different age groups of older adults.

METHODS

32 healthy, volunteers from two different age groups of Young-Old (50-65 years, n = 16) and Old-Old (66-80 years, n = 16) participated in this study. Anodal tDCS was applied over the primary motor cortex, with respective combinations of three intensities (1, 2, and 3 mA) and durations (15, 20, and 30 min), in a sham-controlled cross-over design. Cortical excitability alterations were monitored by single-pulse TMS-induced MEPs until the next day morning after stimulation.

RESULTS

All active stimulation conditions resulted in a significant enhancement of motor cortical excitability in both age groups. The facilitatory aftereffects of anodal tDCS did not significantly differ between age groups. We observed prolonged plasticity in the late-phase range for two protocols with the highest stimulation intensity (i.e., 3 mA-20 min, 3 mA-30 min).

CONCLUSIONS

Our study highlights the role of stimulation dosage in tDCS-induced neuroplastic aftereffects in the motor cortex of healthy older adults and delivers crucial information about optimized tDCS protocols in the domain of the primary motor cortex. Our findings might set the grounds for the development of optimal stimulation protocols to reinstate neuroplasticity in different cortical areas and induce long-lasting, functionally relevant plasticity in normal aging and in pathological conditions, which would require however systematic tDCS titration studies over respective target areas.

The effect of combined transcranial pulsed current stimulation and transcutaneous electrical nerve stimulation on lower limb spasticity in children with spastic cerebral palsy: a randomized and controlled clinical study

BACKGROUND

In the current study, we applied a combination of non-invasive neuromodulation modalities concurrently with multiple stimulating electrodes. Specifically, we used transcranial pulsed current stimulation (tPCS) and transcutaneous electrical nerve stimulation (TENS) as a novel strategy for improving lower limb spasticity in children with spastic cerebral palsy (SCP) categorized on levels III-V of the Gross Motor Function Classification System (GMFCS) with minimal side effects.

METHODS

Sixty-three SCP children aged 2-12 years, who were classified on levels III-V of the GMFCS were randomly assigned to one of two groups, resulting in 32 children in the experimental group and 31 children in the control group. The experimental group underwent a combination therapy of tPCS (400 Hz, 1 mA cerebello-cerebral stimulation) and TENS (400 Hz, max 10 mA) for 30 min, followed by 30 min of physiotherapy five times per week for 12 weeks. The control group underwent physiotherapy only 30 mins per day five times per week for 12 weeks. In total, all groups underwent 60 treatment sessions. The primary outcome measures were the Modified Ashworth Scale (MAS) and Modified Tardieu Scale (MTS). Evaluations were performed 3 days before and after treatment.

RESULTS

We found a significant improvement in MAS and MTS scores of the lower limbs in the experimental group compared to the control group in the hip adductors (Left: p = 0.002; Right: p = 0.002), hamstrings (Left: p = 0.001; Right: p < 0.001, and gastrocnemius (Left: p = 0.001; Right: p = 0.000). Moreover, MTS scores of R1, R2 and R2-R1 in left and right hip adduction, knee joint, and ankle joint all showed significant improvements (p ≤ 0.05). Analysis of MAS and MTS scores compared to baseline scores showed significant improvements in the experimental group but declines in the control group.

CONCLUSION

These results are among the first to demonstrate that a combination of tPCS and TENS can significantly improve lower limb spasticity in SCP children classified on GMFCS levels III-V with minimal side effects, presenting a novel strategy for addressing spasticity challenges in children with severe SCP.

TRIAL REGISTRATION

ChiCTR.org, ChiCTR1800020283, Registration: 22 December 2018 (URL: http://www.chictr.org.cn/showproj.aspx?proj=33953 ).

Corticospinal excitability enhancement with simultaneous transcranial near-infrared stimulation and anodal direct current stimulation of motor cortex

OBJECTIVES

Non-invasive brain stimulation (NIBS) is beneficial to many neurological and psychiatric disorders by modulating neuroplasticity and cortical excitability. However, recent studies evidence that single type of NIBS such as transcranial direct current stimulation (tDCS) does not have meaningful clinical therapeutic responses due to their small effect size. Transcranial near-infrared stimulation (tNIRS) is a novel form of NIBS. Both tNIRS and tDCS implement its therapeutic effects by modulating cortical excitability but with different mechanisms. We hypothesized that simultaneous tNIRS and tDCS is superior to single stimulation, leading to a greater cortical excitability.

METHODS

Sixteen healthy subjects participated in a double-blind, sham-controlled, cross-over designed study. Motor evoked potentials (MEPs) were used to measure motor cortex excitability. The changes of MEP were calculated and compared in the sham condition, tDCS stimulation condition, tNIRS condition and the simultaneous tNIRS and anodal tDCS condition.

RESULTS

tDCS alone and tNIRS alone both elicited higher MEP after stimulation, while the MEP amplitude in the simultaneous tNIRS and tDCS condition was significantly higher than either tNIRS alone or tDCS alone. The enhancement lasted up to at least 30 minutes after stimulation, indicating simultaneous 820 nm tNIRS with 2 mA anodal tDCS have a synergistic effect on cortical plasticity.

CONCLUSIONS

Simultaneous application of tNIRS with tDCS produces a stronger cortical excitability effect.

SIGNIFICANCE

The simultaneous tNIRS and tDCS is a promising technology with exciting potential as a means of treatment, neuro-enhancement, or neuro-protection.

Transcranial Random Noise Stimulation Acutely Lowers the Response Threshold of Human Motor Circuits

Transcranial random noise stimulation (tRNS) over cortical areas has been shown to acutely improve performance in sensory detection tasks. One explanation for this behavioral effect is stochastic resonance (SR), a mechanism that explains how signal processing in nonlinear systems can benefit from added noise. While acute noise benefits of electrical RNS have been demonstrated at the behavioral level as well as in in vitro preparations of neural tissue, it is currently largely unknown whether similar effects can be shown at the neural population level using neurophysiological readouts of human cortex. Here, we hypothesized that acute tRNS will increase the responsiveness of primary motor cortex (M1) when probed with transcranial magnetic stimulation (TMS). Neural responsiveness was operationalized via the well-known concept of the resting motor threshold (RMT). We showed that tRNS acutely decreases RMT. This effect was small, but it was consistently replicated across four experiments including different cohorts (total N = 81, 46 females, 35 males), two tRNS electrode montages, and different control conditions. Our experiments provide critical neurophysiological evidence that tRNS can acutely generate noise benefits by enhancing the neural population response of human M1.SIGNIFICANCE STATEMENT A hallmark feature of stochastic resonance (SR) is that signal processing can benefit from added noise. This has mainly been demonstrated at the single-cell level in vitro where the neural response to weak input signals can be enhanced by simultaneously applying random noise. Our finding that transcranial random noise stimulation (tRNS) acutely increases the excitability of corticomotor circuits extends the principle of noise benefits to the neural population level in human cortex. Our finding is in line with the notion that tRNS might affect cortical processing via the SR phenomenon. It suggests that enhancing the response of cortical populations to an external stimulus might be one neurophysiological mechanism mediating performance improvements when tRNS is applied to sensory cortex during perception tasks.

Motor cortex transcranial direct current stimulation effects on knee osteoarthritis pain in elderly subjects with dysfunctional descending pain inhibitory system: A randomized controlled trial

BACKGROUND

Although evidence has indicated a positive effect of transcranial direct current stimulation (tDCS) on reducing pain, few studies have focused on the elderly population with knee osteoarthritis (KOA).

OBJECTIVE

To evaluate whether tDCS reduces KOA pain in elderly individuals with a dysfunctional descending pain inhibitory system (DPIS).

METHODS

In a double-blind trial, individuals ≥ 60 years with KOA pain and a dysfunctional DPIS, we randomly assigned patients to receive 15 daily sessions of 2 mA tDCS over the primary motor cortex (anode) and contralateral supraorbital area (cathode) (M1-SO) for 20 min or sham tDCS. Change in pain perception indexed by the Brief Pain Inventory (BPI) at the end of intervention was the primary outcome. Secondary outcomes included: disability, quantitative sensory testing, pain pressure threshold and conditioned pain modulation (CPM). Subjects were followed-up for 2 months.

RESULTS

Of the 104 enrolled subjects, with mean (SD) age of 73.9 (8.01) years and 88 (84.6%) female, 102 finished the trial. In the intention-to-treat analysis, the active tDCS group had a significantly greater reduction in BPI compared to the sham group (difference, 1.59; 95% CI, 0.95 to 2.23; P < 0.001; Cohen's d, 0.58); and, also a significantly greater improvement in CPM-pressure in the knee (P = 0.01) and CPM-pain in the hand (P = 0.01). These effects were not sustained at follow-up. The intervention was well tolerated, with no severe adverse effects.

CONCLUSION

M1-SO tDCS is associated with a moderate effect size in reducing pain in elderly patients with KOA after 15 daily sessions of stimulation. This intervention has also shown to modulate the DPIS.

The neurobiology of prefrontal transcranial direct current stimulation (tDCS) in promoting brain plasticity: A systematic review and meta-analyses of human and rodent studies

The neurobiological mechanisms underlying prefrontal transcranial direct current stimulation (tDCS) remain elusive. Randomized, sham-controlled trials in humans and rodents applying in vivo prefrontal tDCS were included to explore whether prefrontal tDCS modulates resting-state and event-related functional connectivity, neural oscillation and synaptic plasticity. Fifty studies were included in the systematic review and 32 in the meta-analyses. Neuroimaging meta-analysis indicated anodal prefrontal tDCS significantly enhanced bilateral median cingulate activity [familywise error (FWE)-corrected p < .005]; meta-regression revealed a positive relationship between changes in median cingulate activity after tDCS and current density (FWE-corrected p < .005) as well as electric current strength (FWE-corrected p < .05). Meta-analyses of electroencephalography and magnetoencephalography data revealed nonsignificant changes (ps > .1) in both resting-state and event-related oscillatory power across all frequency bands. Applying anodal tDCS over the rodent hippocampus/prefrontal cortex enhanced long-term potentiation and brain-derived neurotrophic factor expression in the stimulated brain regions (ps <.005). Evidence supporting prefrontal tDCS administration is preliminary; more methodologically consistent studies evaluating its effects on cognitive function that include brain activity measurements are needed.

Inhibitory effect of tDCS on auditory evoked response: Simultaneous MEG-tDCS reveals causal role of right auditory cortex in pitch learning

A body of literature has demonstrated that the right auditory cortex (AC) plays a dominant role in fine pitch processing. However, our understanding is relatively limited about whether this asymmetry extends to perceptual learning of pitch. There is also a lack of causal evidence regarding the role of the right AC in pitch learning.  We addressed these points with anodal transcranial direct current stimulation (tDCS), adapting a previous behavioral study in which anodal tDCS over the right AC was shown to block improvement of a microtonal pitch pattern learning task over 3 days. To address the physiological changes associated with tDCS, we recorded MEG data simultaneously with tDCS on the first day, and measured behavioral thresholds on the following two consecutive days. We tested three groups of participants who received anodal tDCS over their right or left AC, or sham tDCS, and measured the N1m auditory evoked response before, during, and after tDCS. Our data show that anodal tDCS of the right AC disrupted pitch discrimination learning up to two days after its application, whereas learning was unaffected by left-AC or sham tDCS. Although tDCS reduced the amplitude of the N1m ipsilaterally to the stimulated hemisphere on both left and right, only right AC N1m amplitude reductions were associated with the degree to which pitch learning was disrupted. This brain-behavior relationship confirms a causal link between right AC physiological responses and fine pitch processing, and provides neurophysiological insight concerning the mechanisms of action of tDCS on the auditory system.

Magnetic resonance spectroscopy with transcranial direct current stimulation to explore the underlying biochemical and physiological mechanism of the human brain: A systematic review

A large body of molecular and neurophysiological evidence connects synaptic plasticity to specific functions and energy metabolism in particular areas of the brain. Furthermore, altered plasticity and energy regulation has been associated with a number of neuropsychiatric disorders. A favourable approach enabling the modulation of neuronal excitability and energy in humans is to stimulate the brain using transcranial direct current stimulation (tDCS) and then to observe the effect on neurometabolites using magnetic resonance spectroscopy (MRS). In this way, a well-defined modulation of brain energy and excitability can be achieved using a dedicated tDCS protocol to a predetermined brain region. This systematic review was guided by the preferred reporting items for systematic reviews and meta-analysis and summarises recent literature studying the effect of tDCS on neurometabolites in the human brain as measured by proton or phosphorus MRS. Limitations and recommendations are discussed for future research. The findings of this review provide clear evidence for the potential of using tDCS and MRS to examine and understand the effect of neurometabolites in the in vivo human brain.

Comparison of Treatment Outcome between Repetitive Transcranial Magnetic Stimulation (rTMS) and Transcutaneous Direct Current Stimulation (tDCS) in Intractable Tinnitus

Repetitive transcranial magnetic stimulation (rTMS) and transcutaneous direct current stimulation (tDCS) are non-invasive treatments for chronic tinnitus based on neuromodulation of cortical activity. Both are considered effective, but with heterogeneous results due to lack of established protocols. Because the target groups for both modalities overlap, it is difficult to recommend one of them. We tried to unify the inclusion criteria and treatment schedules to compare the two modalities. The medical charts of 36 patients who underwent rTMS as part of clinical routine were reviewed and data for 34 patients who underwent tDCS about 7 years later were collected prospectively. Both groups had chronic unilateral tinnitus refractory to medication. Patients were treated for 5 consecutive days, and tinnitus symptoms were evaluated by survey both at the end of the treatment schedule and 1 month after the treatment. The ratio of responders who showed >20% reduction in tinnitus handicap inventory scores were compared. At the end of the treatment, the rTMS group showed a rapid response compared to the tDCS group (rTMS, 30.6%; tDCS, 12.1%; p = 0.054). However, both groups showed a significant and similar reduction in tinnitus symptoms 1 month after the treatment (rTMS, 47.2%; tDCS, 36.4%; p = 0.618). As both groups showed comparable results for tinnitus reduction, tDCS may be superior in terms of cost-effectiveness.