The effects of transcranial direct current stimulation on corticospinal and cortico-cortical excitability and response variability: Conventional versus high-definition montages

Abstract

Response variability following transcranial direct current stimulation (tDCS) highlights need for exploring different tDCS electrode montages. This study compared corticospinal excitability (CSE), cortico-cortical excitability and intra-individual variability following conventional and HD anodal (a-tDCS) and cathodal (c-tDCS) tDCS. Fifteen healthy young males attended four sessions at least one-week apart: conventional a-tDCS, conventional c-tDCS, HD-a-tDCS, HD-c-tDCS. TDCS was administered (1mA, 10-minutes) over the primary motor cortex (M1), via 6x4cm active and 7x5cm return electrodes (conventional tDCS) and 4x1 ring-electrodes 3.5cm apart in ring formation around M1 (HD-tDCS). For CSE, twenty-five single-pulse transcranial magnetic stimulation (TMS) peakto-peak motor evoked potentials (MEP) were recorded at baseline, 0-minutes and 30-minutes post-tDCS. For cortico-cortical excitability, twenty-five paired-pulse MEPs with 3-millisecond (ms) inter-pulse interval (IPI) and twenty-five at 10ms assessed short-interval intracortical inhibition (SICI) and intracortical facilitation (ICF) respectively. MEP standardised z-values standard deviations represented intra-individual variability. No significant differences were reported in CSE between conventional and HD a-tDCS, but significant differences between conventional and HD c-tDCS 0-minutes post-tDCS. Intra-individual variability was significantly reduced in conventional tDCS compared to HD-tDCS for a-tDCS (0-minutes) and c-tDCS (30-minutes). No significant changes were reported in SICI and ICF. These novel findings highlight current technical issues with HD-tDCS, suggesting future tDCS studies should utilise conventional tDCS to minimise intraindividual variability, ensuring tDCS after-effects are true changes in CSE and cortico-cortical excitability.

Can genetic polymorphisms predict response variability to anodal transcranial direct current stimulation of the primary motor cortex?

“…The results of this current study were only reported on a specific subgroup of the population and via one tDCS electrode configuration and dosage. Recruiting just healthy young males for this study was an attempt to minimize potential participant factors as sources of inter‐individual variability (Pellegrini et al., 2020). Therefore, whether these current results can be extrapolated to other populations remains unclear.…”

“…Sixty‐one healthy male volunteers aged (mean ± SD ) 26.82 ± 7.62 years provided written informed consent to attend two experimental sessions. Sample‐size was calculated (with 80% power) based on results of a pilot conducted on 15 subjects (Pellegrini et al., 2020). In this pilot study, changes in CSE following a‐tDCS were reported with a Cohen's d moderate effect size of 0.45 (Pellegrini et al., 2020).…”

“…Sample‐size was calculated (with 80% power) based on results of a pilot conducted on 15 subjects (Pellegrini et al., 2020). In this pilot study, changes in CSE following a‐tDCS were reported with a Cohen's d moderate effect size of 0.45 (Pellegrini et al., 2020). At 5% significance level, for changes in CSE following a‐tDCS, a moderate effect size of 0.45 required sample‐sizes between 17 and 26 (Portney & Watkins, 2009).…”

Systematic Review of the Impact of Transcranial Direct Current Stimulation on the Neuromechanical Management of Foot and Ankle Physical Performance in Healthy Adults

“…Specifically, the variance in experimental outcomes in response to tDCS was caused by several factors associated with high inter-individual variability, including the organization of local circuits, basal level of function, psychological state, level of neurotransmitters and receptor sensibility, baseline neurophysiological state, and genetic aspects (López-Alonso et al, 2015;Pellegrini et al, 2018a,b;. In addition, those previous studies used different experimental designs and stimulation protocols (i.e., different montages), potentially contributing to the variance in the results or even the reversal effect (Datta et al, 2009;Angius et al, 2018;Hassanzahraee et al, 2020;Pellegrini et al, 2020). For example, Hassanzahraee et al (2020) observed the reversal of corticospinal excitability of anodal tDCS with a current intensity of 1.0 mA when the stimulation duration was over 26 min, which may provide certain implications for preventing excessive brain activation.…”

A Novel Highly Durable Carbon/Silver/Silver Chloride Composite Electrode for High-Definition Transcranial Direct Current Stimulation

“…Due to the charge accumulation at the electrode/electrolyte interface, these inert electrodes cannot deliver high current density. Therefore, the area of inert electrodes has to be quite large (25~35 cm 2 ), which severely sacrifices the spatial resolution of tDCS [ 33 , 34 , 35 ]. In general, the current density of inert electrodes is less than 0.08 mA cm −2 .…”