Does transcranial electrical stimulation enhance corticospinal excitability of the motor cortex in healthy individuals? A systematic review and meta‐analysis

Abstract

Numerous studies have explored the effects of transcranial electrical stimulation (tES) - including anodal transcranial direct current stimulation (a-tDCS), cathodal transcranial direct current stimulation (c-tDCS), transcranial alternative current stimulation (tACS), transcranial random noise stimulation (tRNS) and transcranial pulsed current stimulation (tPCS) - on corticospinal excitability (CSE) in healthy populations. However, the efficacy of these techniques and their optimal parameters for producing robust results has not been studied. Thus, the aim of this systematic review was to consolidate current knowledge about the effects of various parameters of a-tDCS, c-tDCS, tACS, tRNS and tPCS on the CSE of the primary motor cortex (M1) in healthy people. Leading electronic databases were searched for relevant studies published between January 1990 and February 2017; 126 articles were identified, and their results were extracted and analysed using RevMan software. The meta-analysis showed that a-tDCS application on the dominant side significantly increases CSE (P < 0.01) and that the efficacy of a-tDCS is dependent on current density and duration of application. Similar results were obtained for stimulation of M1 on the non-dominant side (P = 0.003). The effects of a-tDCS reduce significantly after 24 h (P = 0.006). Meta-analysis also revealed significant reduction in CSE following c-tDCS (P < 0.001) and significant increases after tRNS (P = 0.03) and tPCS (P = 0.01). However, tACS effects on CSE were only significant when the stimulation frequency was ≥140 Hz. This review provides evidence that tES has substantial effects on CSE in healthy individuals for a range of stimulus parameters.

The Tolerability and Efficacy of 4 mA Transcranial Direct Current Stimulation on Leg Muscle Fatigability

“…For example, some subjects respond to tDCS, while some do not [55][56][57] and some subjects may have brain connectivity that allows the current to flow in the preferential direction [58], which may not be the case for others. Furthermore, M1 to supraorbital montages do not, in general, stimulate the motor system in a consistent way [6] and two supraorbital active electrodes may allow a great amount of current to pass through the orbit; in either of these montages, it is also possible that some structures located behind the eye (e.g., lower surface of the frontal cortex) are also affected by the stimulation. Another speculation is that a ceiling effect of the targeted motor cortex exists, which might increase transcallosal inhibition and enhance fatigability in the ipsilateral limb.…”

“…Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation technique [1][2][3][4] that can modulate the cortical excitability of targeted brain regions [5]. tDCS has been extensively tested in healthy subjects [6][7][8] and in people with cognitive [7] and motor impairments [9][10][11], and these studies suggest that tDCS can help improve cognitive and motor function. However, an important caveat is that the majority (~96%) of tDCS sessions to date have been limited to 2 mA or lower [12].…”

Effect of noisy galvanic vestibular stimulation in community-dwelling elderly people: a randomised controlled trial

“…The lack of decrease in the RMS area could be explained by the difference in stimulus intensity settings. In addition, we used a stimulation frequency band of 0.1–640 Hz on the basis of a previous study [ 16 , 34 – 37 ] although whether this band is optimal for nGVS remain unknown. The optimum stimulation condition (stimulation intensity and frequency band) for nGVS should be elucidated in future studies.…”

Prospects for transcranial temporal interference stimulation in humans: a computational study

“…Transcranial current stimulation (tCS) entails sending weak (≤ 2 mA) currents through the brain via electrodes placed on the scalp (see Reed and Cohen Kadosh, 2018, for a review). Applying direct current (tDCS) has been shown to affect the excitability of stimulated neurons (Nitsche and Paulus, 2000;Dissanayaka et al, 2017), while alternating currents (tACS) can also synchronize neuronal spikes and entrain them to the stimulation frequency (Zaehle et al, 2010;Tavakoli and Yun, 2017). At intensities commonly used for stimulation in humans, tCS does not produce sufficiently high field strengths to directly induce action potentials in neurons (Datta et al, 2009;Rampersad et al, 2014).…”