The Effects of Transcranial Electrical Stimulation on Human Motor Functions: A Comprehensive Review of Functional Neuroimaging Studies

The effects of transcranial random noise stimulation on motor function: A comprehensive review of the literature

The present review considers all papers published on the topic up to the end of the year 2022. Transcranial random noise stimulation (tRNS) is a non-invasive neuromodulation technique introduced about 15 years ago whose use is becoming increasingly widespread in neuroscience. It consists of the application over the scalp of a weak, white noise-like current, through electrodes having a surface of several square centimetres, for a duration ranging from seconds to minutes. Despite its relatively low spatial and temporal resolution, tRNS has well defined effects on central motor excitability, which critically depend on stimulation parameters. These effects seem to be chiefly based on an effect on neuronal membrane sodium channels and can last much longer than the stimulation itself. While the effects at the cellular level in the motor cortex are becoming progressively clear, much more studies are needed to understand the effects of tRNS on motor behaviour and performance, where initial research results are nevertheless promising, in both basic and applied research.

The sound stimulation method and EEG change analysis for development of digital therapeutics that can stimulate the nervous system: Cortical activation and drug substitution potential

INTRODUCTION

The purpose of this study is to propose a treatment method and the effect on the nervous system of digital therapeutics, which is a new treatment method to replace surgery and drug prescription for the treatment and prevention of diseases.

METHODS

The 20 subjects who participated in the experiment, including men and women, had an average age of 26 ± 2.40 years. The proposed treatment method used three types of sound stimulation and air or bone conduction sound transmission methods to induce total of 6-time EEG electroencephalogram(EEG) changes. EEG was measured with 200 sampling rate each in the P4, Cz, F8 and T7 channel located in the parietal, central, frontal and temporal lobes, respectively, according to the 10/10 system. A total of 2 min of data were created by extracting EEG signals with less noise from the measured data and the extracted data were applied with a 1-40 Hz Butterworth filter and a 50 Hz notch filter with a quality factor of 30. After that, EEG are subdivided into delta (0.5-4 Hz), theta (4-8 Hz), alpha (8-13 Hz), beta (13-30 Hz), and gamma (30-45 Hz) bands. Finally, EEG changes in response to sound stimuli were analyzed using power spectral density and T-test validation in the frequency band.

RESULTS

When a sound stimulus of less than 1 KHz was stimulated by air conduction, brainstem activation was induced and the reticular activation system was activated. In addition, a great potential for replacing drugs was confirmed by inducing changes in the nervous system similar to drugs used for sedation.

CONCLUSION

These results will be able to expand the concept of digital therapeutics, and it is expected that it will be developed as a safer treatment method that can replace surgery and drugs.

Effects of Preceding Transcranial Direct Current Stimulation on Movement Velocity and EMG Signal during the Back Squat Exercise

This study aimed to evaluate the effects of preceding anodal transcranial direct stimulation (a-tDCS) over the dorsolateral prefrontal cortex (DLPFC) during the back squat exercise on movement velocity and surface electromyographic (sEMG) activity. Thirteen healthy, well-trained, male firefighters (34.72 ± 3.33 years; 178 ± 7.61 cm; 76.85 ± 11.21 kg; 26.8 ± 4.2 kg·m−2; back squat 1-repetition maximum 141.5 ± 16.3 kg) completed this randomised double-blinded sham-controlled crossover study. After familiarisation and basal measurements, participants attended the laboratory on two occasions separated by 72 h to receive either Sham or a-tDCS (current intensity of 2 mA for 20 min). Immediately after stimulation, participants completed three sets of 12 repetitions (70% of 1-RM) with three minutes of recovery between sets monitored with a linear position transducer. The sEMG of the rectus femoris (RF) and vastus lateralis (VL) of both legs were recorded. No significant differences were observed between a-tDCS and Sham interventions on mean concentric velocity at any set (p > 0.05). Velocity loss and effort index were significantly higher (p < 0.05) in set 3 compared to set 1 only in the a-tDCS group. The right-leg RM and right-leg VL elicited the greatest muscle activation during set 1 after a-tDCS and Sham, respectively (p < 0.05). Our results revealed that a-tDCS over the DLPFC might impact movement velocity or fatigue tolerance in well-trained individuals. Notwithstanding, significant differences in dominant-leg muscle activity were found both in a-tDCS and Sham.

Biological and Neurobiological Mechanisms of Transcranial Direct Current Stimulation

Objective: Changes in cortical excitability and neuroplasticity are important parts of the neuropathology and pathophysiology of many neuropsychiatric disorders. Noninvasive brain stimulation is a high-potential therapeutic approach to modify cortical activities. One of the most popular of these techniques is transcranial direct current stimulation (tDCS). However, the biological and neurobiological effects of tDCS should be better clarified to enable its optimal use in clinical and therapeutic practices. In this paper, we summarize the neurophysiological and physiological effects and mechanisms of action of tDCS. Method : An update literature review was conducted on the biological responses of tDCS reported in human, in vitro and in vivo studies, with a focus on cellular cascades related to neuroplasticity, neuronal reorganization and inflammation caused by applied direct current electric fields. Results: The regulatory mechanisms of tDCS on motor and cognitive functions can be described by membrane polarization and transmembrane potential with a main subsequent effect on neurotransmission systems, neuronal excitability, synaptic microenvironment and neuronal connectivity to neuronal reorganization and neurogenesis in association with synaptic plasticity as well as inflammatory processes. In general, the effects of tDCS may include acute- or after-effects and direct or indirect effects and can be examined at different levels including the neurochemical, the neuroelectrical and the brain oscillatory levels. Conclusion: A deep understanding of the molecular and cellular responses to tDCS is very important and crucial. This therapeutic technique can be utilized in various clinical trials with a perspective of being routinely suggested and presented to patients with different pathological conditions influencing the central or peripheral nervous system.

Neuroimaging guided tES to facilitate complex laparoscopic surgical tasks - insights from functional near-infrared spectroscopy

Fundamentals of Laparoscopic Surgery (FLS) is a prerequisite for board certification in general surgery in the USA. In FLS, the suturing task with intracorporeal knot tying is considered the most complex task. Transcranial direct current stimulation (tDCS) of the dorsolateral prefrontal cortex (PFC) has been shown to facilitate FLS surgical skill acquisition where 2mA tDCS for 15min with the anode over F3 (10/10 EEG montage) and cathode over F4 has improved performance score in an open knot-tying task. Since PFC has a functional organization related to the hierarchy of cognitive control, we performed functional near-infrared spectroscopy (fNIRS) to investigate PFC sub-domain activation during a more complex FLS suturing task with intracorporeal knot tying. We performed fNIRS-based analysis using AtlasViewer software on two expert surgeons and four novice medical students. We found an average cortical activation mainly at the left frontopolar PFC across the experts, while the average cortical activation across the novices was primarily at the left pars opercularis of the inferior frontal gyrus and ventral premotor cortex, inferior parietal lobule, and supramarginal gyrus. Here, the average cortical activation across the novices included not only the cognitive control related brain regions but also motor control complexity related brain regions. Therefore, we present a computational pipeline to identify a 4x1 high-definition (HD) tDCS montage of motor complexity related PFC sub-regions using ROAST software.Clinical Relevance-A computational pipeline for fNIRS-guided tES to individualize electrode montage that may facilitate FLS surgical training in our future studies.

The Efficacy of Transcranial Direct Current Stimulation in Enhancing Surgical Skill Acquisition: A Preliminary Meta-Analysis of Randomized Controlled Trials

The application of transcranial direct current stimulation (tDCS) to targeted cortices has been found to improve in skill acquisition; however, these beneficial effects remained unclear in fine and complicated skill. The aim of the current meta-analysis was to investigate the association between tDCS application and the efficacy of surgical performance during surgical skill training. We included randomized controlled trials (RCTs) investigating the efficacy of tDCS in enhancing surgical skill acquisition. This meta-analysis was conducted under a random-effect model. Six RCTs with 198 participants were included. The main result revealed that tDCS was associated with significantly better improvement in surgical performance than the sham control (Hedges’ g = 0.659, 95% confidence intervals (95%CIs) = 0.383 to 0.935, p < 0.001). The subgroups of tDCS over the bilateral prefrontal cortex (Hedges’ g = 0.900, 95%CIs = 0.419 to 1.382, p < 0.001) and the primary motor cortex (Hedges’ g = 0.599, 95%CIs = 0.245 to 0.953, p = 0.001) were both associated with significantly better improvements in surgical performance. The tDCS application was not associated with significant differences in error scores or rates of local discomfort compared with a sham control. This meta-analysis supported the rationale for the tDCS application in surgical training programs to improve surgical skill acquisition.

Brain stimulation by tDCS as treatment option in Autism Spectrum Disorder-A systematic literature review

Autism Spectrum Disorder (ASD) is a neurodevelopmental disorder characterized by deficits in social communication and interaction as well as stereotypical and repetitive behavior. Transcranial direct current stimulation (tDCS) has been proposed as a new intervention method in ASD with the potential to improve cognitive, motor and social communication abilities by targeting specific underlying neuronal alterations. Here, we report results of a systematic literature review on tDCS effects on EEG and behavioral outcomes, and discuss tDCS as treatment option for ASD. PsychInfo, PubMed, ScienceDirect, Web of Science, https://clinicaltrials.gov and the German Clinical Trials Register (Deutsches Register Klinischer Studien) were searched systematically for randomized, sham-controlled clinical trials of tDCS in individuals with ASD, and information regarding study designs and relevant results was extracted. Six eligible studies were identified. The dorsolateral prefrontal cortex (DLPFC) was targeted in four trials, with core ASD symptoms and working memory as outcome measures. One study targeted the primary motor cortex (M1) with motor skills as outcome, and one study targeted the temporoparietal junction (TPJ) with social communication skills as outcome measure. Comparison of the implemented study designs showed high methodological variability between studies regarding stimulation parameters, trial design and outcome measures. Study results indicate initial support for improved cognitive and social communication skills in ASD following tDCS stimulation. However, systematic and comparison studies on the best combination of stimulation intensity, duration, location as well as task related stimulation are necessary, before results can be translated into routine clinical application.

Decreasing the Surgical Errors by Neurostimulation of Primary Motor Cortex and the Associated Brain Activation via Neuroimaging

Acquisition of fine motor skills is a time-consuming process as it is based on learning via frequent repetitions. Transcranial electrical stimulation (tES) is a promising means of enhancing simple motor skill development via neuromodulatory mechanisms. Here, we report that non-invasive neurostimulation facilitates the learning of complex fine bimanual motor skills associated with a surgical task. During the training of 12 medical students on the Fundamentals of Laparoscopic Surgery (FLS) pattern cutting task over a period of 12 days, we observed that transcranial direct current stimulation (tDCS) decreased error level and the variability in performance, compared to the Sham group. Furthermore, by concurrently monitoring the cortical activations of the subjects via functional near-infrared spectroscopy (fNIRS), our study showed that the cortical activation patterns were significantly different between the tDCS and Sham group, with the activation of primary motor cortex (M1) and prefrontal cortex (PFC) contralateral to the anodal electrode significantly decreased while supplemental motor area (SMA) increased by tDCS. The lowered performance errors were retained after 1-month post-training. This work supports the use of tDCS to enhance performance accuracy in fine bimanual motor tasks.

Multimodal Assessment of Precentral Anodal TDCS: Individual Rise in Supplementary Motor Activity Scales With Increase in Corticospinal Excitability

Background

Transcranial direct current stimulation (TDCS) targeting the primary motor hand area (M1-HAND) may induce lasting shifts in corticospinal excitability, but after-effects show substantial inter-individual variability. Functional magnetic resonance imaging (fMRI) can probe after-effects of TDCS on regional neural activity on a whole-brain level.

Objective

Using a double-blinded cross-over design, we investigated whether the individual change in corticospinal excitability after TDCS of M1-HAND is associated with changes in task-related regional activity in cortical motor areas.

Methods

Seventeen healthy volunteers (10 women) received 20 min of real (0.75 mA) or sham TDCS on separate days in randomized order. Real and sham TDCS used the classic bipolar set-up with the anode placed over right M1-HAND. Before and after each TDCS session, we recorded motor evoked potentials (MEP) from the relaxed left first dorsal interosseus muscle after single-pulse transcranial magnetic stimulation(TMS) of left M1-HAND and performed whole-brain fMRI at 3 Tesla while participants completed a visuomotor tracking task with their left hand. We also assessed the difference in MEP latency when applying anterior-posterior and latero-medial TMS pulses to the precentral hand knob (AP-LM MEP latency).

Results

Real TDCS had no consistent aftereffects on mean MEP amplitude, task-related activity or motor performance. Individual changes in MEP amplitude, measured directly after real TDCS showed a positive linear relationship with individual changes in task-related activity in the supplementary motor area and AP-LM MEP latency.

Conclusion

Functional aftereffects of classical bipolar anodal TDCS of M1-HAND on the motor system vary substantially across individuals. Physiological features upstream from the primary motor cortex may determine how anodal TDCS changes corticospinal excitability.