Transcranial direct current stimulation in stroke rehabilitation: a review of recent advancements

Post-stroke rehabilitation in the peri-pandemic COVID-19 era

The coronavirus disease 2019 (COVID-19), which arose in late 2019, caused extensive destruction, impacting a substantial proportion of the worldwide population and leading to millions of deaths. Although COVID-19 is mainly linked to respiratory and pulmonary complications, it has the potential to affect neurologic structures as well. Neurological involvement may manifest as minimal and reversible; however, a notable proportion of cases have exhibited pronounced neurological consequences, such as strokes. Endothelial inflammation, hypercoagulation, renin-angiotensin-aldosterone system alterations, and cardiogenic embolism are the pathophysiological mechanisms of stroke under COVID-19 circumstances. Physical activity and exercise have improved several aspects of post-stroke recovery, including cardiovascular health, walking capacity, and upper limb strength. They are commonly used to assist stroke survivors in overcoming their motor restrictions. Furthermore, stroke rehabilitation can incorporate a range of specific techniques, including body-weight-supported treadmill applications, constraint-induced movement therapy, robotic rehabilitation interventions, transcranial direct current stimulation, transcranial magnetic stimulation, and prism adaptation training. Under pandemic conditions, there were several barriers to neurological rehabilitation. The most significant of these were individual's fear of infection, which caused them to postpone their rehabilitation applications and rehabilitation areas being converted into COVID-19 units. The primary emphasis had turned to COVID-19 treatment. Several valuable data and views were gained in reorganizing rehabilitation during the pandemic, contributing to establishing future views in this regard.

Non-invasive brain stimulation on clinical symptoms in multiple sclerosis patients: A systematic review and meta-analysis

BACKGROUND

Non-invasive brain stimulation (NIBS) has demonstrated mixed effects on the clinical symptoms of multiple sclerosis. This systematic review and meta-analysis aimed to evaluate the effects of NIBS techniques on the most common symptoms of MS.

METHODS

A literature search was performed until October 2022 which included randomized controlled trials and quasi-experimental studies that used sham-controlled NIBS in patients with MS. We calculated the Hedge's effect sizes of each domain of interest and their 95% confidence intervals (95% CIs) and performed random effects meta-analyses.

RESULTS

A total of 49 studies were included in the systematic review (944 participants). Forty-four eligible studies were included for quantitative analysis, of which 33 applied transcranial direct current stimulation (tDCS), 9 transcranial magnetic stimulation (TMS), and 2 transcranial random noise stimulation (tRNS). We found a significant decrease in fatigue (ES:  - 0.86, 95% CI:  - 1.22 to - 0.51, p < 0.0001), pain (ES: - 1.91, 95% CI, - 3.64 to - 0.19, p=  0.03) and psychiatric symptoms (ES: - 1.44, 95% CI - 2.56 to - 0.32, p = 0.01) in favor of tDCS compared with the sham. On the other hand, there was no strong evidence showing tDCS effectiveness on motor performance and cognition (ES: - 0.03, 95% CI - 0.35 to 0.28, p = 0.83 and ES: 0.71, 95% CI, - 0.09 to 1.52, p = 0.08, respectively). Regarding TMS, we found a significant decrease in fatigue (ES: - 0.45, 95% CI: - 0.84 to -0.07, p = 0.02) and spasticity levels (ES: - 1.11, 95% CI: - 1.48 to - 0.75, p < 0.00001) compared to the sham. However, there was no strong evidence of the effectiveness of TMS on motor performance (ES: - 0.39, 95% CI - 0.95 to 0.16, p = 0.16). Finally, there was no significant evidence showing the effectiveness of tRNS on fatigue levels (ES: - 0.28, 95% CI: - 1.02 to 0.47, p = 0.46) and cognitive improvement (ES: - 0.04, 95% CI: - 0.6, 0.52, p = 0.88) compared with the sham.

CONCLUSIONS

Overall, most studies have investigated the effects of tDCS on MS symptoms, particularly fatigue. The symptom that most benefited from NIBS was fatigue, while the least to benefit was motor performance. In addition, we found that disability score was associated with fatigue improvement. Thus, these findings support the idea that NIBS could have some promising effects on specific MS symptoms. It is also important to underscore that studies are very heterogeneous regarding the parameters of stimulation, and this may also have influenced the effects on some specific behavioral domains.

Effect of transcranial direct current stimulation combined with respiratory training on dysphagia in post-stroke patients

BACKGROUND

Transcranial direct current stimulation (tDCS) has a considerable advantage in the rehabilitation treatment of dysphagia.

OBJECTIVE

The purpose of this study was to explore the effect of tDCS combined with respiratory training on dysphagia in post-stroke patients.

METHODS

From December 2017 to January 2019, 64 post-stroke patients who were hospitalized in the Department of Neurology of the Second Hospital of Hebei Medical University were enrolled in this study. They were randomly divided into control and treatment groups (n= 32). Patients in the two groups received routine swallowing rehabilitation training and respiratory training. On this basis, the patients in the treatment group received tDCS. The anode was placed in the movement area of the pharyngeal cortex on the unaffected side of the patients' bodies, and the cathode was placed in the upper orbital orbit on the opposite side. The current intensity was 1.5 mA, 20 min/time, 1 time/d, and 6 d/w. Before and after the treatment, the water swallow text, functional oral intake scale (FOIS), forced vital capacity (FVC) and peak expiratory flow (PEF) were assessed, and the correlation among them was evaluated.

RESULTS

There were no differences in all indexes before and after treatment. After treatment, water swallow text, FOIS, FVC and PEF were all better than before treatment, and the clinical efficacy in the treatment group was significantly better than that in the control group. FVC and PEF were positively correlated with water swallow text and FOIS.

CONCLUSION

tDCS combined with respiratory training may have a significant therapeutic effect on dysphagia in post-stroke patients.

Neural Mechanism Underlying Task-Specific Enhancement of Motor Learning by Concurrent Transcranial Direct Current Stimulation

The optimal protocol for neuromodulation by transcranial direct current stimulation (tDCS) remains unclear. Using the rotarod paradigm, we found that mouse motor learning was enhanced by anodal tDCS (3.2 mA/cm2) during but not before or after the performance of a task. Dual-task experiments showed that motor learning enhancement was specific to the task accompanied by anodal tDCS. Studies using a mouse model of stroke induced by middle cerebral artery occlusion showed that concurrent anodal tDCS restored motor learning capability in a task-specific manner. Transcranial in vivo Ca2+ imaging further showed that anodal tDCS elevated and cathodal tDCS suppressed neuronal activity in the primary motor cortex (M1). Anodal tDCS specifically promoted the activity of task-related M1 neurons during task performance, suggesting that elevated Hebbian synaptic potentiation in task-activated circuits accounts for the motor learning enhancement. Thus, application of tDCS concurrent with the targeted behavioral dysfunction could be an effective approach to treating brain disorders.

The Effect of Cerebellar tDCS on Static and Dynamic Balance of Inactive Elderly Men

The aim of this study was investigating the effect of cerebellar transcranial direct current stimulation (tDCS) on static and dynamic balance of inactive older adults. Twenty-four older adults participated in this study. All participants underwent static and dynamic balance tests. In the Experimental group, anode electrode was positioned at the O point in the cerebellum and cathode electrode was positioned on the left eye socket (FP1). In the control group, the anode and cathode electrodes were positioned at O and FP1 points, respectively, but the current stimulation was stopped after 30 s. Then, the posttest was performed. Data analysis was done using MANCOVA. There was a significant difference between the Experimental and control groups in static balance (p = .12) and dynamic balance (p = .18) and the performance was better in the experimental group. It can be concluded that tDCS can improve static and dynamic balance in inactive older adults.

Frequency-tuned electromagnetic field therapy improves post-stroke motor function: A pilot randomized controlled trial

BACKGROUND AND PURPOSE

Impaired upper extremity (UE) motor function is a common disability after ischemic stroke. Exposure to extremely low frequency and low intensity electromagnetic fields (ELF-EMF) in a frequency-specific manner (Electromagnetic Network Targeting Field therapy; ENTF therapy) is a non-invasive method available to a wide range of patients that may enhance neuroplasticity, potentially facilitating motor recovery. This study seeks to quantify the benefit of the ENTF therapy on UE motor function in a subacute ischemic stroke population.

METHODS

In a randomized, sham-controlled, double-blind trial, ischemic stroke patients in the subacute phase with moderately to severely impaired UE function were randomly allocated to active or sham treatment with a novel, non-invasive, brain computer interface-based, extremely low frequency and low intensity ENTF therapy (1-100 Hz, < 1 G). Participants received 40 min of active ENTF or sham treatment 5 days/week for 8 weeks; ~three out of the five treatments were accompanied by 10 min of concurrent physical/occupational therapy. Primary efficacy outcome was improvement on the Fugl-Meyer Assessment - Upper Extremity (FMA-UE) from baseline to end of treatment (8 weeks).

RESULTS

In the per protocol set (13 ENTF and 8 sham participants), mean age was 54.7 years (±15.0), 19% were female, baseline FMA-UE score was 23.7 (±11.0), and median time from stroke onset to first stimulation was 11 days (interquartile range (IQR) 8-15). Greater improvement on the FMA-UE from baseline to week 4 was seen with ENTF compared to sham stimulation, 23.2 ± 14.1 vs. 9.6 ± 9.0, p = 0.007; baseline to week 8 improvement was 31.5 ± 10.7 vs. 23.1 ± 14.1. Similar favorable effects at week 8 were observed for other UE and global disability assessments, including the Action Research Arm Test (Pinch, 13.4 ± 5.6 vs. 5.3 ± 6.5, p = 0.008), Box and Blocks Test (affected hand, 22.5 ± 12.4 vs. 8.5 ± 8.6, p < 0.0001), and modified Rankin Scale (-2.5 ± 0.7 vs. -1.3 ± 0.7, p = 0.0005). No treatment-related adverse events were reported.

CONCLUSIONS

ENTF stimulation in subacute ischemic stroke patients was associated with improved UE motor function and reduced overall disability, and results support its safe use in the indicated population. These results should be confirmed in larger multicenter studies.

CLINICAL TRIAL REGISTRATION

https://clinicaltrials.gov/ct2/show/NCT04039178, identifier: NCT04039178.

Transcranial Direct Current Stimulation Paired With Verb Network Strengthening Treatment Improves Verb Naming in Primary Progressive Aphasia: A Case Series

PURPOSE

There are few evidence-based treatments for language deficits in primary progressive aphasia (PPA). PPA treatments are often adopted from the poststroke aphasia literature. The poststroke aphasia literature has shown promising results using Verb Network Strengthening Treatment (VNeST), a behavioral therapy that focuses on improving naming by producing verbs and their arguments in phrases and sentences. Emerging research in poststroke aphasia and PPA has shown promising results pairing behavioral language therapy with transcranial direct current stimulation (tDCS).

METHOD

This study used a double-blind, within-subjects, sham-controlled crossover design to study the effect of anodal tDCS applied to left inferior frontal gyrus (IFG) plus VNeST versus VNeST plus sham stimulation in two individuals with nonfluent variant PPA and one individual with logopenic variant PPA. Participants received two phases of treatment, each with 15 1-hr sessions of VNeST. One phase paired VNeST with tDCS stimulation, and one with sham. For each phase, language testing was conducted at baseline, and at 1 week and 8 weeks posttreatment conclusion. For each participant, treatment efficacy was evaluated for each treatment phase by comparing the mean change in accuracy between baseline and the follow-up time points for naming trained verbs (primary outcome measure), untrained verbs, and nouns on the Object and Action Naming Battery. Mean change from baseline was also directly compared between tDCS and sham phases at each time point.

RESULTS

Results revealed a different pattern of outcomes for each of the participants. A tDCS advantage was not found for trained verbs for any participant. Two participants with nonfluent variant PPA had a tDCS advantage for generalization to naming of untrained verbs, which was apparent at 1 week and 8 weeks posttreatment. One participant with nonfluent variant also showed evidence of generalization to sentence production in the tDCS phase.

CONCLUSION

VNeST plus anodal tDCS stimulation of left IFG shows promising results for improving naming in PPA.

Potential of Transcranial Direct Current Stimulation in Alzheimer's Disease: Optimizing Trials Toward Clinical Use

Transcranial direct current stimulation (tDCS) is a safe and well-tolerated noninvasive method for stimulating the brain that is rapidly developing into a treatment method for various neurological and psychiatric conditions. In particular, there is growing evidence of a therapeutic role for tDCS in ameliorating or delaying the cognitive decline in Alzheimer's disease (AD). We provide a brief overview of the current development and application status of tDCS as a nonpharmacological therapeutic method for AD and mild cognitive impairment (MCI), summarize the levels of evidence, and identify the improvements needed for clinical applications. We also suggest future directions for large-scale controlled clinical trials of tDCS in AD and MCI, and emphasize the necessity of identifying the mechanistic targets to facilitate clinical applications.

Transcranial direct current stimulation combined with trunk-targeted, proprioceptive neuromuscular facilitation in subacute stroke: a randomized controlled trial

Background

Stroke is the foremost cause of death and disability worldwide. Improving upper extremity function and quality of life are two paramount therapeutic targets during rehabilitation.

Aim of the study

To investigate the effects of transcranial direct current stimulation (tDCS) combined with trunk-targeted proprioceptive neuromuscular facilitation (PNF) on impairments, activity limitations, and participation restrictions of subjects with subacute stroke.

Methodology

Fifty-four subjects with subacute stroke were divided into three groups using block randomization. All three groups received rehabilitation sessions lasting 90 min in duration, four times per week, for 6 weeks. Group 1 (n = 18) received conventional physical therapy (CPT); group 2 (n = 18) received CPT, trunk-targeted PNF, and sham tDCS; and group 3 (n = 18) received CPT, trunk-targeted PNF, and bihemispheric motor cortex stimulation with tDCS. Changes in motor impairment, motor activity, and health-related quality of life assessments were outcome measures.

Results

A two-way linear mixed model analysis revealed interaction effects (group × time) for all outcome measurements (Trunk Impairment Scale, Fugl-Meyer Assessment of Motor Recovery after stroke upper extremity subsection, Wolf Motor Function Test, 10-Meter Walk Test, and the Stroke-Specific Quality of Life scale; all p < 0.01 or lower). Overall, post-pre mean differences demonstrate more substantial improvement in the active tDCS group, followed by sham stimulation associated with the PNF group and the group that received CPT alone.

Conclusion

Trunk-targeted PNF combined with bihemispheric tDCS along with CPT engender larger improvements in upper extremity and trunk impairment, upper limb function, gait speed, and quality of life in the subacute stroke population.

Effect of Transcranial Direct Current Stimulation Combined with Rehabilitation on Arm and Hand Function in Stroke Patients: A Systematic Review and Meta-Analysis

Transcranial direct current stimulation (tDCS) is a noninvasive brain stimulation technique that may enhance motor recovery after stroke. We performed a systematic review and meta-analysis to assess the efficacy of tDCS combined with rehabilitation on arm and hand function after stroke. Electronic databases were searched from their inception to September 2021. We performed a systematic review of selected randomized controlled trials, and methodological qualities were measured using the PEDro (Physiotherapy Evidence Database) scale. We calculated the standardized mean difference for effect size using the Comprehensive Meta-Analysis 3.0 software. We selected 28 studies for the systematic review and 20 studies for the meta-analysis. The overall effect size was 0.480 (95% CI [0.307; 0.653], p < 0.05), indicating a moderate effect size of tDCS combined with rehabilitation for upper extremity function in stroke survivors. The tDCS with occupational therapy/physical therapy (0.696; 95% CI [0.390; 1.003], p < 0.05) or virtual reality therapy (0.510; 95% CI [0.111; 0.909], p < 0.05) was also significantly more effective than other treatments. This meta-analysis of 20 randomized controlled trials provides further evidence that tDCS combined with rehabilitation, especially occupational therapy/physical therapy and virtual reality therapy, may benefit upper extremity function of the paretic upper limb in stroke patients.

An Automated Workflow for the Electric Field Modeling of High-definition Transcranial Direct Current Stimulation (HD-tDCS) in Chronic Stroke with Lesions

Transcranial Direct Current Stimulation is a popular noninvasive brain stimulation (NIBS) technique that modulates brain excitability by means of low-amplitude electrical current (usually <4mA) delivered to the electrodes on the scalp. The NIBS research has gained significant momentum in the past decade, prompting tDCS as an adjunctive therapeutic tool for neuromuscular disorders like stroke. However, due to stroke lesions and the differences in individual neuroanatomy, the targeted brain region may not show the same response upon NIBS across stroke patients. To this end, we conducted a study to test the feasibility of targeted NIBS. The hand motor hotspot (HMH) for each chronic stroke participant was identified using Neuronavigated Transcranial Magnetic Stimulation (TMS). After identifying the HMH as the neural target site, we applied High-definition TDCS with the current delivered at 2mA for 20 minutes. To simulate the effects of HD-tDCS in the brain, especially with stroke lesions, we used the computational modeling tool (ROAST). The lesion mask was identified using an automated tool (LINDA). This paper demonstrates that the stroke lesions can be incorporated in the computational modeling of electric field distribution upon HD-tDCS without manual intervention.

Bilateral Motor Cortex tDCS Effects on Post-Stroke Pain and Spasticity: A Three Cases Study

Stroke patients frequently suffer from chronic limb pain, but well-suited treatment approaches have been not established so far. Transcranial direct current stimulation (tDCS) is a safe and non-invasive brain stimulation technique that alters cortical excitability, and it has been shown that motor cortex tDCS can reduce pain. Some data also suggest that spasticity may be improved by tDCS in post-stroke patients. Moreover, multiple sessions of tDCS have shown to induce neuroplastic changes with lasting beneficial effects in different neurological conditions. The aim of this pilot study was to explore the effect of multiple anodal tDCS (atDCS) sessions on upper limb pain and spasticity of stroke patients, using a within-subject, crossover, sham-controlled design. Brain damage was of similar extent in the three patients evaluated, although located in different hemispheres. The results showed a significant effect of 5 consecutive sessions of atDCS, compared to sham stimulation, on pain evaluated by the Adaptive Visual Analog Scales -AVAS-, and spasticity evaluated by the Fugl-Meyer scale. In two of the patients, pain was completely relieved and markedly reduced, respectively, only after verum tDCS. The pain improvement effect of atDCS in the third patient was considerably lower compared to the other two patients. Spasticity was significantly improved in one of the patients. The treatment was well-tolerated, and no serious adverse effects were reported. These findings suggest that multiple sessions of atDCS are a safe intervention for improving upper limb pain and spasticity in stroke patients, although the inter-individual variability is a limitation of the results. Further studies including longer follow-up periods, more representative patient samples and individualized stimulation protocols are required to demonstrate the efficacy and safety of tDCS for improving limb symptoms in these patients.

Cognitive Telerehabilitation with Transcranial Direct Current Stimulation Improves Cognitive and Emotional Functioning Following a Traumatic Brain Injury: A Case Study

OBJECTIVE

Cognitive deficits following a traumatic brain injury (TBI) are a leading cause of disability in young adults and there is a critical need for novel approaches to improve cognitive outcomes in TBI survivors. Transcranial direct current stimulation (tDCS) paired with cognitive remediation has emerged as a viable, cost-effective, noninvasive approach for treating cognitive impairments in a wide variety of neurological conditions. Here, we report the first case study utilizing remotely supervised tDCS (RS-tDCS) protocol paired with cognitive remediation in a 29-year-old man with persisting cognitive and emotional sequelae following TBI.

METHOD

Neuropsychological measures were administered before and after the patient completed 20 daily sessions of RS-tDCS (2.0 mA × 20 minutes, left anodal dorsolateral prefrontal cortex montage). During the daily stimulation period, he completed adaptive cognitive training. All treatment procedures were delivered at home and monitored in real time via videoconference with a study technician.

RESULTS

Following 20 RS-tDCS and cognitive training sessions, he had significant improvements (>1 SD) on tests of attention and working memory, semantic fluency, and information processing speed. Mood was also improved.

CONCLUSIONS

This is the first demonstration of at-home telerehabilitation with RS-tDCS and cognitive training to improve cognitive outcomes following TBI.

Promotion of Poststroke Motor-Function Recovery with Repetitive Transcranial Magnetic Stimulation by Regulating the Interhemispheric Imbalance

Repetitive transcranial magnetic stimulation (rTMS) is a noninvasive brain-stimulation technique that transiently modulates cerebral cortex excitability, achieving overall positive results in poststroke motor-function recovery. Excessive inhibition of the ipsilesional-affected hemisphere by the contralesional-unaffected hemisphere has seriously hindered poststroke motor-function recovery. Hence, intracortical disinhibition can be used as an approach to managing poststroke brain injury. This technique promotes neural plasticity for faster motor-function recovery. rTMS relieves unilateral inhibition of the brain function by regulatinga interhemispheric-imbalanced inhibition. This paper summarized 12 studies from 2016 to date, focusing on rTMS on motor function after acute and chronic stroke by regulating the interhemispheric imbalance of inhibitory inputs. Although rTMS studies have shown promising outcomes on recovery of motor functions in stroke patients, different intervention methods may lead to discrepancies in results. A uniform optimal stimulus model cannot routinely be used, mainly due to the stimulus schemes, stroke types and outcome-measuring differences among studies. Thus, the effect of rTMS on poststroke motor-function recovery should be investigated further to standardize the rTMS program for optimal poststroke motor-function recovery. More randomized, placebo-controlled clinical trials with standardized rTMS protocols are needed to ensure the effectiveness of the treatment.

Rewiring the Lesioned Brain: Electrical Stimulation for Post-Stroke Motor Restoration

Electrical stimulation has been extensively applied in post-stroke motor restoration, but its treatment mechanisms are not fully understood. Stimulation of neuromotor control system at multiple levels manipulates the corresponding neuronal circuits and results in neuroplasticity changes of stroke survivors. This rewires the lesioned brain and advances functional improvement. This review addresses the therapeutic mechanisms of different stimulation modalities, such as noninvasive brain stimulation, peripheral electrical stimulation, and other emerging techniques. The existing applications, the latest progress, and future directions are discussed. The use of electrical stimulation to facilitate post-stroke motor recovery presents great opportunities in terms of targeted intervention and easy applicability. Further technical improvements and clinical studies are required to reveal the neuromodulatory mechanisms and to enhance rehabilitation therapy efficiency in stroke survivors and people with other movement disorders.

Beyond the target area: an integrative view of tDCS-induced motor cortex modulation in patients and athletes

Transcranial Direct Current Stimulation (tDCS) is a non-invasive technique used to modulate neural tissue. Neuromodulation apparently improves cognitive functions in several neurologic diseases treatment and sports performance. In this study, we present a comprehensive, integrative review of tDCS for motor rehabilitation and motor learning in healthy individuals, athletes and multiple neurologic and neuropsychiatric conditions. We also report on neuromodulation mechanisms, main applications, current knowledge including areas such as language, embodied cognition, functional and social aspects, and future directions. We present the use and perspectives of new developments in tDCS technology, namely high-definition tDCS (HD-tDCS) which promises to overcome one of the main tDCS limitation (i.e., low focality) and its application for neurological disease, pain relief, and motor learning/rehabilitation. Finally, we provided information regarding the Transcutaneous Spinal Direct Current Stimulation (tsDCS) in clinical applications, Cerebellar tDCS (ctDCS) and its influence on motor learning, and TMS combined with electroencephalography (EEG) as a tool to evaluate tDCS effects on brain function.

Searching for the optimal tDCS target for motor rehabilitation

BACKGROUND

Transcranial direct current stimulation (tDCS) has been investigated over the years due to its short and also long-term effects on cortical excitability and neuroplasticity. Although its mechanisms to improve motor function are not fully understood, this technique has been suggested as an alternative therapeutic method for motor rehabilitation, especially those with motor function deficits. When applied to the primary motor cortex, tDCS has shown to improve motor function in healthy individuals, as well as in patients with neurological disorders. Based on its potential effects on motor recovery, identifying optimal targets for tDCS stimulation is essential to improve knowledge regarding neuromodulation as well as to advance the use of tDCS in clinical motor rehabilitation.

METHODS AND RESULTS

Therefore, this review discusses the existing evidence on the application of four different tDCS montages to promote and enhance motor rehabilitation: (1) anodal ipsilesional and cathodal contralesional primary motor cortex tDCS, (2) combination of central tDCS and peripheral electrical stimulation, (3) prefrontal tDCS montage and (4) cerebellar tDCS stimulation. Although there is a significant amount of data testing primary motor cortex tDCS for motor recovery, other targets and strategies have not been sufficiently tested. This review then presents the potential mechanisms and available evidence of these other tDCS strategies to promote motor recovery.

CONCLUSIONS

In spite of the large amount of data showing that tDCS is a promising adjuvant tool for motor rehabilitation, the diversity of parameters, associated with different characteristics of the clinical populations, has generated studies with heterogeneous methodologies and controversial results. The ideal montage for motor rehabilitation should be based on a patient-tailored approach that takes into account aspects related to the safety of the technique and the quality of the available evidence.

Different Brain Connectivity between Responders and Nonresponders to Dual-Mode Noninvasive Brain Stimulation over Bilateral Primary Motor Cortices in Stroke Patients

Noninvasive brain stimulation (NBS), such as repetitive transcranial magnetic stimulation (rTMS) or transcranial direct current stimulation (tDCS), has been used in stroke patients with motor impairment. NBS can help recovery from brain damage by modulating cortical excitability. However, the efficacy of NBS varies among individuals. To obtain insights of responsiveness to the efficacy of NBS, we investigated characteristic changes of the motor network in responders and nonresponders of NBS over the primary motor cortex (M1). A total of 21 patients with subacute stroke (13 males, mean age 59.6 ± 11.5 years) received NBS in the same manner: 1 Hz rTMS on the contralesional M1 and anodal tDCS on the ipsilesional M1. Participants were classified into responders and nonresponders based on the functional improvement of the affected upper extremity after applying NBS. Twelve age-matched healthy controls (8 males, mean age 56.1 ± 14.3 years) were also recruited. Motor networks were constructed using resting-state functional magnetic resonance imaging. M1 intrahemispheric connectivity, interhemispheric connectivity, and network efficiency were measured to investigate differences in network characteristics between groups. The motor network characteristics were found to differ between both groups. Specifically, M1 intrahemispheric connectivity in responders showed a noticeable imbalance between affected and unaffected hemispheres, which was markedly restored after NBS. The responders also showed greater interhemispheric connectivity and higher efficiency of the motor network than the nonresponders. These results may provide insight on patient-specific NBS treatment based on the brain network characteristics in neurorehabilitation of patients with stroke. This trial is registered with trial registration number NCT03390192.

A Functional Domain Based Approach in Neurocognitive Rehabilitation with Transcranial Direct Current Stimulation: A Case Report

Transcranial direct current stimulation (tDCS) is a novel brain stimulation technique which has kindled hope in alleviating motor, language as well as cognitive deficits in neuronal injury. Current case report describes application of tDCS in two phases using two different protocols in a patient with hypoxic injury. In the first phase anodal stimulation of dorsolateral prefrontal cortex improved the language fluency. Subsequently, after 6 months second phase application of anodal stimulation over posterior parietal region targeted arithmetic and working memory deficits. Individualising the treatment protocols of brain stimulation, based on the lesion and the functional deficits, for neuro-rehabilitation is emphasised.

Preclinical to Clinical Translation of Studies of Transcranial Direct-Current Stimulation in the Treatment of Epilepsy: A Systematic Review

Epilepsy is a chronic brain syndrome characterized by recurrent seizures resulting from excessive neuronal discharges. Despite the development of various new antiepileptic drugs, many patients are refractory to treatment and report side effects. Non-invasive methods of brain stimulation, such as transcranial direct current stimulation (tDCS), have been tested as alternative approaches to directly modulate the excitability of epileptogenic neural circuits. Although some pilot and initial clinical studies have shown positive results, there is still uncertainty regarding the next steps of investigation in this field. Therefore, we reviewed preclinical and clinical studies using the following framework: (1) preclinical studies that have been successfully translated to clinical studies, (2) preclinical studies that have failed to be translated to clinical studies, and (3) clinical findings that were not previously tested in preclinical studies. We searched PubMed, Web of Science, Embase, and SciELO (2002–2017) using the keywords “tDCS,” “epilepsy,” “clinical trials,” and “animal models.” Our initial search resulted in 64 articles. After applying inclusion and exclusion criteria, we screened 17 full-text articles to extract findings about the efficacy of tDCS, with respect to the therapeutic framework used and the resulting reduction in seizures and epileptiform patterns. We found that few preclinical findings have been translated into clinical research (number of sessions and effects on seizure frequency) and that most findings have not been tested clinically (effects of tDCS on status epilepticus and absence epilepsy, neuroprotective effects in the hippocampus, and combined use with specific medications). Finally, considering that clinical studies on tDCS have been conducted for several epileptic syndromes, most were not previously tested in preclinical studies (Rasmussen's encephalitis, drug resistant epilepsy, and hippocampal sclerosis-induced epilepsy). Overall, most studies report positive findings. However, it is important to underscore that a successful preclinical study may not indicate success in a clinical study, considering the differences highlighted herein. Although most studies report significant findings, there are still important insights from preclinical work that must be tested clinically. Understanding these factors may improve the evidence for the potential use of this technique as a clinical tool in the treatment of epilepsy.

Modulating Brain Connectivity by Simultaneous Dual-Mode Stimulation over Bilateral Primary Motor Cortices in Subacute Stroke Patients

Repetitive transcranial magnetic stimulation (rTMS) or transcranial direct current stimulation (tDCS) has been used for the modulation of stroke patients' motor function. Recently, more challenging approaches have been studied. In this study, simultaneous stimulation using both rTMS and tDCS (dual-mode stimulation) over bilateral primary motor cortices (M1s) was investigated to compare its modulatory effects with single rTMS stimulation over the ipsilesional M1 in subacute stroke patients. Twenty-four patients participated; 12 participants were assigned to the dual-mode stimulation group while the other 12 participants were assigned to the rTMS-only group. We assessed each patient's motor function using the Fugl-Meyer assessment score and acquired their resting-state fMRI data at two times: prior to stimulation and 2 months after stimulation. Twelve healthy subjects were also recruited as the control group. The interhemispheric connectivity of the contralesional M1, interhemispheric connectivity between bilateral hemispheres, and global efficiency of the motor network noticeably increased in the dual-mode stimulation group compared to the rTMS-only group. Contrary to the dual-mode stimulation group, there was no significant change in the rTMS-only group. These data suggested that simultaneous dual-mode stimulation contributed to the recovery of interhemispheric interaction than rTMS only in subacute stroke patients. This trial is registered with NCT03279640.

Transcranial direct current stimulation as a motor neurorehabilitation tool: an empirical review

The present review collects the most relevant empirical evidence available in the literature until date regarding the effects of transcranial direct current stimulation (tDCS) on the human motor function. tDCS in a non-invasive neurostimulation technique that delivers a weak current through the brain scalp altering the cortical excitability on the target brain area. The electrical current modulates the resting membrane potential of a variety of neuronal population (as pyramidal and gabaergic neurons); raising or dropping the firing rate up or down, depending on the nature of the electrode and the applied intensity. These local changes additionally have shown long-lasting effects, evidenced by its promotion of the brain-derived neurotrophic factor. Due to its easy and safe application and its neuromodulatory effects, tDCS has attracted a big attention in the motor neurorehabilitation field among the last years. Therefore, the present manuscript updates the knowledge available about the main concept of tDCS, its practical use, safety considerations, and its underlying mechanisms of action. Moreover, we will focus on the empirical data obtained by studies regarding the application of tDCS on the motor function of healthy and clinical population, comprising motor deficiencies of a variety of pathologies as Parkinson's disease, stroke, multiple sclerosis and cerebral palsy, among others. Finally, we will discuss the main current issues and future directions of tDCS as a motor neurorehabilitation tool.

Transcranial Electric Stimulation for Precision Medicine: A Spatiomechanistic Framework

During recent years, non-invasive brain stimulation, including transcranial electrical stimulation (tES) in general, and transcranial direct current stimulation (tDCS) in particular, have created new hopes for treatment of neurological and psychiatric diseases. Despite promising primary results in some brain disorders, a more widespread application of tES is hindered by the unsolved question of determining optimum stimulation protocols to receive meaningful therapeutic effects. tES has a large parameter space including various montages and stimulation parameters. Moreover, inter- and intra-individual differences in responding to stimulation protocols have to be taken into account. These factors contribute to the complexity of selecting potentially effective protocols for each disorder, different clusters of each disorder, and even each single patient. Expanding knowledge in different dimensions of basic and clinical neuroscience could help researchers and clinicians to select potentially effective protocols based on tES modulatory mechanisms for future clinical studies. In this article, we propose a heuristic spatiomechanistic framework which contains nine levels to address tES effects on brain functions. Three levels refer to the spatial resolution (local, small-scale networks and large-scale networks) and three levels of tES modulatory effects based on its mechanisms of action (neurochemical, neuroelectrical and oscillatory modulations). At the group level, this framework could be helpful to enable an informed and systematic exploration of various possible protocols for targeting a brain disorder or its neuroscience-based clusters. Considering recent advances in exploration of neurodiversity at the individual level with different brain mapping technologies, the proposed framework might also be used in combination with personal data to design individualized protocols for tES in the context of precision medicine in the future.

Transcranial direct current stimulation in post stroke aphasia and primary progressive aphasia: Current knowledge and future clinical applications

BACKGROUND

The application of transcranial direct current stimulation (tDCS) in chronic post stroke aphasia is documented in a substantial literature, and there is some new evidence that tDCS can augment favorable language outcomes in primary progressive aphasia. Anodal tDCS is most often applied to the left hemisphere language areas to increase cortical excitability (increase the threshold of activation) and cathodal tDCS is most often applied to the right hemisphere homotopic areas to inhibit over activation in contralesional right homologues of language areas. Outcomes usually are based on neuropsychological and language test performance, following a medical model which emphasizes impairment of function, rather than a model which emphasizes functional communication.

OBJECTIVE

In this paper, we review current literature of tDCS as it is being used as a research tool, and discuss future implementation of tDCS as an adjuvant treatment to behavioral speech-language pathology intervention.

METHODS

We review literature describing non-invasive brain stimulation, the mechanism of tDCS, and studies of tDCS in aphasia and neurodegenerative disorders. We discuss future clinical applications.

RESULTS/CONCLUSIONS

tDCS is a promising adjunct to traditional speech-language pathology intervention to address speech-language deficits after stroke and in the neurodegenerative disease, primary progressive aphasia. Limited data are available regarding how performance on these types of specific tasks translates to functional communication outcomes.

Remotely Supervised Transcranial Direct Current Stimulation Increases the Benefit of At-Home Cognitive Training in Multiple Sclerosis

OBJECTIVE

To explore the efficacy of remotely-supervised transcranial direct current stimulation (RS-tDCS) paired with cognitive training (CT) exercise in participants with multiple sclerosis (MS).

METHODS

In a feasibility study of RS-tDCS in MS, participants completed ten sessions of tDCS paired with CT (1.5 mA × 20 min, dorsolateral prefrontal cortex montage). RS-tDCS participants were compared to a control group of adults with MS who underwent ten 20-min CT sessions through the same remotely supervised procedures. Cognitive outcomes were tested by composite scores measuring change in performance on standard tests (Brief International Cognitive Assessment in MS or BICAMS), basic attention (ANT-I Orienting and Attention Networks, Cogstate Detection), complex attention (ANT-I Executive Network, Cogstate Identification and One-Back), and intra-individual response variability (ANT-I and Cogstate identification; sensitive markers of disease status).

RESULTS

After ten sessions, the tDCS group (n = 25) compared to the CT only group (n = 20) had significantly greater improvement in complex attention (p = 0.01) and response variability (p = 0.01) composites. The groups did not differ in measures of basic attention (p = 0.95) or standard cognitive measures (p = 0.99).

CONCLUSIONS

These initial findings indicate benefit for RS-tDCS paired with CT in MS. Exploratory analyses indicate that the earliest tDCS cognitive benefit is seen in complex attention and response variability. Telerehabilitation using RS-tDCS combined with CT may lead to improved outcomes in MS.

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

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

Vagus Nerve Stimulation and Other Neuromodulation Methods for Treatment of Traumatic Brain Injury

The objective of this paper is to review the current literature regarding the use of vagus nerve stimulation (VNS) in preclinical models of traumatic brain injury (TBI) as well as discuss the potential role of VNS along with alternative neuromodulation approaches in the treatment of human TBI. Data from previous studies have demonstrated VNS-mediated improvement following TBI in animal models. In these cases, VNS was observed to enhance motor and cognitive recovery, attenuate cerebral edema and inflammation, reduce blood brain barrier breakdown, and confer neuroprotective effects. Yet, the underlying mechanisms by which VNS enhances recovery following TBI remain to be fully elucidated. Several hypotheses have been offered including: a noradrenergic mechanism, reduction in post-TBI seizures and hyper-excitability, anti-inflammatory effects, attenuation of blood-brain barrier breakdown, and cerebral edema. We present other potential mechanisms by which VNS acts including enhancement of synaptic plasticity and recruitment of endogenous neural stem cells, stabilization of intracranial pressure, and interaction with the ghrelin system. In addition, alternative methods for the treatment of TBI including deep brain stimulation, transcranial magnetic stimulation, transcranial direct current stimulation, and focused ultrasound stimulation are discussed. Although the primary source data show that VNS improves TBI outcomes, it remains to be determined if these findings can be translated to clinical settings.

Sex differences in stroke therapies

Stroke is the fifth leading cause of death and acquired disability in aged populations. Women are disproportionally affected by stroke, having a higher incidence and worse outcomes than men. Numerous preclinical studies have discovered novel therapies for the treatment of stroke, but almost all of these have been shown to be unsuccessful in clinical trials. Despite known sex differences in occurrence and severity of stroke, few preclinical or clinical therapeutics take into account possible sex differences in treatment. Reanalysis of data from studies of tissue plasminogen activator (tPA), the only currently FDA-approved stroke therapy, has shown that tPA improves stroke outcomes for both sexes and also shows sexual dimorphism by more robust improvement in stroke outcome in females. Experimental evidence supports the inclusion of sex as a variable in the study of a number of novel stroke drugs and therapies, including preclinical studies of anti-inflammatory drugs (minocycline), stimulators of cell survival (insulin-like growth factor-1), and inhibitors of cell death pathways (pharmacological inhibition of poly[ADP-ribose] polymerase-1, nitric oxide production, and caspase activation) as well as in current clinical trials of stem cell therapy and cortical stimulation. Overall, study design and analysis in clinical trials as well as in preclinical studies must include both sexes equally, consider possible sex differences in the analyses, and report the differences/similarities in more systematic/structured ways to allow promising therapies for both sexes and increase stroke recovery. © 2016 Wiley Periodicals, Inc.

Impact of Transcranial Direct Current Stimulation (tDCS) on Neuronal Functions

Transcranial direct current stimulation (tDCS), a non-invasive brain stimulation technique, modulates neuronal excitability by the application of a small electrical current. The low cost and ease of the technique has driven interest in potential clinical applications. However, outcomes are highly sensitive to stimulation parameters, leading to difficulty maximizing the technique's effectiveness. Although reversing the polarity of stimulation often causes opposite effects, this is not always the case. Effective clinical application will require an understanding of how tDCS works; how it modulates a neuron; how it affects the local network; and how it alters inter-network signaling. We have summarized what is known regarding the mechanisms of tDCS from sub-cellular processing to circuit level communication with a particular focus on what can be learned from the polarity specificity of the effects.

Direct Current Stimulation Modulates LTP and LTD: Activity Dependence and Dendritic Effects

BACKGROUND

Transcranial direct current stimulation (tDCS) has been reported to improve various forms of learning in humans. Stimulation is often applied during training, producing lasting enhancements that are specific to the learned task. These learning effects are thought to be mediated by altered synaptic plasticity. However, the effects of DCS during the induction of endogenous synaptic plasticity remain largely unexplored.

OBJECTIVE/HYPOTHESIS

Here we are interested in the effects of DCS applied during synaptic plasticity induction.

METHODS

To model endogenous plasticity we induced long-term potentiation (LTP) and depression (LTD) at Schaffer collateral synapses in CA1 of rat hippocampal slices. Anodal and cathodal DCS at 20 V/m were applied throughout plasticity induction in both apical and basal dendritic compartments.

RESULTS

When DCS was paired with concurrent plasticity induction, the resulting plasticity was biased towards potentiation, such that LTP was enhanced and LTD was reduced. Remarkably, both anodal and cathodal stimulation can produce this bias, depending on the dendritic location and type of plasticity induction. Cathodal DCS enhanced LTP in apical dendrites while anodal DCS enhanced LTP in basal dendrites. Both anodal and cathodal DCS reduced LTD in apical dendrites. DCS did not affect synapses that were weakly active or when NMDA receptors were blocked.

CONCLUSIONS

These results highlight the role of DCS as a modulator, rather than inducer of synaptic plasticity, as well as the dependence of DCS effects on the spatial and temporal properties of endogenous synaptic activity. The relevance of the present results to human tDCS should be validated in future studies.

In-vivo Imaging of Magnetic Fields Induced by Transcranial Direct Current Stimulation (tDCS) in Human Brain using MRI

Transcranial direct current stimulation (tDCS) is an emerging non-invasive neuromodulation technique that applies mA currents at the scalp to modulate cortical excitability. Here, we present a novel magnetic resonance imaging (MRI) technique, which detects magnetic fields induced by tDCS currents. This technique is based on Ampere's law and exploits the linear relationship between direct current and induced magnetic fields. Following validation on a phantom with a known path of electric current and induced magnetic field, the proposed MRI technique was applied to a human limb (to demonstrate in-vivo feasibility using simple biological tissue) and human heads (to demonstrate feasibility in standard tDCS applications). The results show that the proposed technique detects tDCS induced magnetic fields as small as a nanotesla at millimeter spatial resolution. Through measurements of magnetic fields linearly proportional to the applied tDCS current, our approach opens a new avenue for direct in-vivo visualization of tDCS target engagement.

The effects of bi-hemispheric M1-M1 transcranial direct current stimulation on primary motor cortex neurophysiology and metabolite concentration

PURPOSE

The aim of the present study was to assess, in healthy individuals, the impact of M1-M1 tDCS on primary motor cortex excitability using transcranial magnetic stimulation and sensorimotor metabolite concentration using 1H-MRS.

METHODS

For both experiments, each participant received the three following interventions (20 min tDCS, 1 mA): left-anodal/right-cathodal, left-cathodal/right-anodal, sham. The effects of tDCS were assessed via motor evoked potentials (experiment 1) and metabolite concentrations (experiment 2) immediately after and 12 minutes following the end of stimulation and compared to baseline measurement.

RESULTS

No effect of M1-M1 tDCS on corticospinal excitability was found. Similarly, M1-M1 tDCS did not significantly modulate metabolite concentrations. High inter-subject variability was noted. Response rate analysis showed a tendency towards inhibition following left-anodal/right-cathodal tDCS in 50% of participants and increased GABA levels in 45% of participants.

CONCLUSION

In line with recent studies showing important inter-subject variability following M1-supraorbital tDCS, the present data show that M1-M1 stimulation is also associated with large response variability. The absence of significant effects suggests that current measures may lack sensitivity to assess changes in M1 neurophysiology and metabolism associated with M1-M1 tDCS.

The Effect of Electromagnetic Field Treatment on Recovery from Ischemic Stroke in a Rat Stroke Model: Clinical, Imaging, and Pathological Findings

Stroke is a leading cause of death and disability. Effects of stroke include significant deficits in sensory-motor skills and cognitive abilities. At present, there are limited effective interventions for postacute stroke patients. In this preliminary research we studied a new noninvasive, very low intensity, low frequency, electromagnetic field treatment (VLIFE), targeting a neural network, on an in vivo stroke rat model. Eighteen rats were divided into three groups: sham (M1) and two treatment groups which were exposed to VLIFE treatment for 4 weeks, one using theta waves (M2) and another using beta waves (M3); all groups were followed up for an additional month. Results indicate that the M2 and M3 treated groups showed recovery of sensorimotor functional deficits, as demonstrated by Modified Neurological Severity Score and forelimb placement tests. Brain MRI imaging results show a decrease in perilesional edema and lateral ventricle widening in the treated groups. Fiber tracts' imaging, following VLIFE treatment, showed a higher white matter integrity compared to control. Histological findings support neural regeneration processes. Our data suggest that VLIFE treatment, targeting a specific functional neural network by frequency rather than location, promotes neuronal plasticity after stroke and, as a result, improves clinical recovery. Further studies will investigate the full potential of the treatment.

Pairing broadband noise with cortical stimulation induces extensive suppression of ascending sensory activity

OBJECTIVE

The corticofugal system can alter coding along the ascending sensory pathway. Within the auditory system, electrical stimulation of the auditory cortex (AC) paired with a pure tone can cause egocentric shifts in the tuning of auditory neurons, making them more sensitive to the pure tone frequency. Since tinnitus has been linked with hyperactivity across auditory neurons, we sought to develop a new neuromodulation approach that could suppress a wide range of neurons rather than enhance specific frequency-tuned neurons.

APPROACH

We performed experiments in the guinea pig to assess the effects of cortical stimulation paired with broadband noise (PN-Stim) on ascending auditory activity within the central nucleus of the inferior colliculus (CNIC), a widely studied region for AC stimulation paradigms.

MAIN RESULTS

All eight stimulated AC subregions induced extensive suppression of activity across the CNIC that was not possible with noise stimulation alone. This suppression built up over time and remained after the PN-Stim paradigm.

SIGNIFICANCE

We propose that the corticofugal system is designed to decrease the brain's input gain to irrelevant stimuli and PN-Stim is able to artificially amplify this effect to suppress neural firing across the auditory system. The PN-Stim concept may have potential for treating tinnitus and other neurological disorders.

Visualizing simulated electrical fields from electroencephalography and transcranial electric brain stimulation: a comparative evaluation

Electrical activity of neuronal populations is a crucial aspect of brain activity. This activity is not measured directly but recorded as electrical potential changes using head surface electrodes (electroencephalogram - EEG). Head surface electrodes can also be deployed to inject electrical currents in order to modulate brain activity (transcranial electric stimulation techniques) for therapeutic and neuroscientific purposes. In electroencephalography and noninvasive electric brain stimulation, electrical fields mediate between electrical signal sources and regions of interest (ROI). These fields can be very complicated in structure, and are influenced in a complex way by the conductivity profile of the human head. Visualization techniques play a central role to grasp the nature of those fields because such techniques allow for an effective conveyance of complex data and enable quick qualitative and quantitative assessments. The examination of volume conduction effects of particular head model parameterizations (e.g., skull thickness and layering), of brain anomalies (e.g., holes in the skull, tumors), location and extent of active brain areas (e.g., high concentrations of current densities) and around current injecting electrodes can be investigated using visualization. Here, we evaluate a number of widely used visualization techniques, based on either the potential distribution or on the current-flow. In particular, we focus on the extractability of quantitative and qualitative information from the obtained images, their effective integration of anatomical context information, and their interaction. We present illustrative examples from clinically and neuroscientifically relevant cases and discuss the pros and cons of the various visualization techniques.

Timing-dependent priming effects of tDCS on ankle motor skill learning

Transcranial direct current stimulation (tDCS) has gained increasing interest in neurorehabilitation with its ability to modulate cortical excitability, and thereby influence neural plasticity and functional recovery. While the beneficial effects of tDCS on motor learning and function have been recognized, there is no clear consensus regarding the timing of the tDCS priming protocol in relation to the intervention especially with respect to lower limb motor learning. Depending on the time of priming in relation to the training task, the neural mechanisms of priming (gating vs. homeostatic plasticity) are different and thereby subsequently affect motor learning. Hence, the aim of this study was to examine the interaction of tDCS with subsequent vs. concurrent motor learning using an ankle visuomotor skill learning paradigm. Twelve healthy participants were tested under three stimulation conditions: (1) anodal tDCS prior to the motor task (tDCS-before), (2) anodal tDCS during the motor task (tDCS-during) and (3) sham tDCS during the motor task (tDCS-sham). Results revealed that tDCS application during practice of a skilled motor task increased motor performance compared to tDCS applied prior to motor practice. Both tDCS groups demonstrated enhanced motor learning when tested 24 hours after practice. We conclude that the priming effects of tDCS are timing dependent, and maybe a critical regulatory feature in determining outcomes of priming with tDCS.

Transcranial direct current stimulation reverses neurophysiological and behavioural effects of focal inhibition of human pharyngeal motor cortex on swallowing

The human cortical swallowing system exhibits bilateral but functionally asymmetric representation in health and disease as evidenced by both focal cortical inhibition (pre-conditioning with 1 Hz repetitive transcranial magnetic stimulation; rTMS) and unilateral stroke, where disruption of the stronger (dominant) pharyngeal projection alters swallowing neurophysiology and behaviour. Moreover, excitatory neurostimulation protocols capable of reversing the disruptive effects of focal cortical inhibition have demonstrated therapeutic promise in post-stroke dysphagia when applied contralaterally. In healthy participants (n = 15, 8 males, mean age (±SEM) 35 ± 9 years), optimal parameters of transcranial direct current stimulation (tDCS) (anodal, 1.5 mA, 10 min) were applied contralaterally after 1 Hz rTMS pre-conditioning to the strongest pharyngeal projection. Swallowing neurophysiology was assessed in both hemispheres by intraluminal recordings of pharyngeal motor-evoked responses (PMEPs) to single-pulse TMS as a measure of cortical excitability. Swallowing behaviour was examined using a pressure-based reaction time protocol. Measurements were made before and for up to 60 min post intervention. Subjects were randomised to active or sham tDCS after 1 Hz rTMS on separate days and data were compared using repeated measures ANOVA. Active tDCS increased PMEPs bilaterally (F_1,14 = 7.4, P = 0.017) reversing the inhibitory effects of 1 Hz rTMS in the pre-conditioned hemisphere (F_1,14 = 10.1, P = 0.007). Active tDCS also enhanced swallowing behaviour, increasing the number of correctly timed challenge swallows compared to sham (F_1,14 = 6.3, P = 0.025). Thus, tDCS to the contralateral pharyngeal motor cortex reverses the neurophysiological and behavioural effects of focal cortical inhibition on swallowing in healthy individuals and has therapeutic potential for dysphagia rehabilitation.

Origins of specificity during tDCS: anatomical, activity-selective, and input-bias mechanisms

Transcranial Direct Current Stimulation (tDCS) is investigated for a broad range of neuropsychiatric indications, various rehabilitation applications, and to modulate cognitive performance in diverse tasks. Specificity of tDCS refers broadly to the ability of tDCS to produce precise, as opposed to diffuse, changes in brain function. Practically, specificity of tDCS implies application-specific customization of protocols to maximize desired outcomes and minimize undesired effects. Especially given the simplicity of tDCS and the complexity of brain function, understanding the mechanisms leading to specificity is fundamental to the rational advancement of tDCS. We define the origins of specificity based on anatomical and functional factors. Anatomical specificity derives from guiding current to targeted brain structures. Functional specificity may derive from either activity-selectivity, where active neuronal networks are preferentially modulated by tDCS, or input-selectivity, where bias is applied to different synaptic inputs. Rational advancement of tDCS may require leveraging all forms of specificity.

Beyond the silence: bilateral somatosensory stimulation enhances skilled movement quality and neural density in intact behaving rats

It is thought that a close dialogue between the primary motor (M1) and somatosensory (S1) cortices is necessary for skilled motor learning. The extent of the relative S1 contribution in producing skilled reaching movements, however, is still unclear. Here we used anodal transcranial direct current stimulation (tDCS), which is able to alter polarity-specific excitability in the S1, to facilitate skilled movement in intact behaving rats. We hypothesized that the critical role of S1 in reaching performance can be enhanced by bilateral tDCS. Pretrained rats were assigned to control or stimulation conditions: (1) UnAno: the unilateral application of an anodal current to the side contralateral to the paw preferred for reaching; (2) BiAno1: bilateral anodal current; (3) BiAno2: a bilateral anodal current with additional 30ms of 65μA pulses every 5s. Rats received tDCS (65μA; 10min/rat) to the S1 during skilled reach training for 20 days (online-effect phase). After-effect assessment occurred for the next ten days in the absence of electrical stimulation. Quantitative and qualitative analyses of online-effects of tDCS showed that UnAno and BiAno1 somatosensory stimulation significantly improve skilled reaching performance. Bilateral BiAno1 stimulation was associated with greater qualitative functional improvement than unilateral UnAno stimulation. tDCS-induced improvements were not observed in the after-effects phase. Quantitative cytoarchitectonic analysis revealed that somatosensory tDCS bilaterally increases cortical neural density. The findings emphasize the central role of bilateral somatosensory feedback in skill acquisition through modulation of cortico-motor excitability.

Noninvasive strategies to promote functional recovery after stroke

Stroke is a common and disabling global health-care problem, which is the third most common cause of death and one of the main causes of acquired adult disability in many countries. Rehabilitation interventions are a major component of patient care. In the last few years, brain stimulation, mirror therapy, action observation, or mental practice with motor imagery has emerged as interesting options as add-on interventions to standard physical therapies. The neural bases for poststroke recovery rely on the concept of plasticity, namely, the ability of central nervous system cells to modify their structure and function in response to external stimuli. In this review, we will discuss recent noninvasive strategies employed to enhance functional recovery in stroke patients and we will provide an overview of neural plastic events associated with rehabilitation in preclinical models of stroke.

Non-invasive brain stimulation for enhancement of corticospinal excitability and motor performance

During the past 20 years, non-invasive brain stimulation has become an emerging field in clinical neuroscience due to its capability to transiently modulate corticospinal excitability, motor and cognitive functions. Whereas transcranial magnetic stimulation has been used extensively since more than two decades ago as a potential "neuromodulator", transcranial current stimulation (tCS) has more recently gathered increased scientific interests. The primary aim of this narrative review is to describe characteristics of different tCS paradigms. tCS is an umbrella term for a number of brain modulating paradigms such as transcranial direct current stimulation (tDCS), transcranial alternative current stimulation (tACS), and transcranial random noise stimulation (tRNS). Their efficacy is dependent on two current parameters: intensity and length of application. Unlike tACS and tRNS, tDCS is polarity dependent. These techniques could be used as stand-alone techniques or can be used to prime the effects of other movement trainings. The review also summarises safety issues, the mechanisms of tDCS-induced neuroplasticity, limitations of current state of knowledge in the literature, tool that could be used to understand brain plasticity effects in motor regions and tool that could be used to understand motor learning effects.