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.

Single gene polymorphisms as a predictor of noninvasive brain stimulation effectiveness (commentary on Pellegrini et al, 2021)

“…This recent work does not include any genotyping, but it does provide the simulation of the applied tDCS montage and the resulting differences in cortical excitability. While no significant effect of the anodal tDCS on the MEP amplitude was found in Pellegrini et al (2021c), a very significant effect was found in the present study (Pellegrini et al, 2021a). The authors do not comment on this discrepancy.…”

“…Still, the experiment itself and the statistical analysis deserve some constructive criticism. The experimental design used in this study is based on a recently published manuscript in Neuroscience Research (Pellegrini et al, 2021c). This recent work does not include any genotyping, but it does provide the simulation of the applied tDCS montage and the resulting differences in cortical excitability.…”

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 ; Machado et al, 2019 ). 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 ; Machado, S. et al, 2019 ; 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.…”

“…For anodal HD-tDCS, the anodal electrode was set at 2 mA, while the four return electrodes placed 3.5 cm apart from the anodal electrode were programmed to −0.5 mA. The previous study anticipated that −0.25 mA inhibitory input from the surrounding cortical areas may negate or override the focalized 1 mA current intensity over M1, which may influence overall M1 excitability (Pellegrini et al, 2020 ). Thus, the improvements in foot physical performance induced by anodal HD-tDCS may not be significant (Xiao et al, 2020 ).…”

Modulation of Interhemispheric Synchronization and Cortical Activity in Healthy Subjects by High-Definition Theta-Burst Electrical Stimulation

“…Positive outcomes of HD-tDCS were also reported in previous studies, displaying enhancement in cortical excitability [15], functional connectivity [16], and attention [17] in healthy subjects. In contrast, a number of studies revealed that there was no change in both cortical excitatory in healthy subjects [18] and motor performance in stroke patients [19]. Therefore, it is highly desired to develop alternative strategies for improving neuroplasticity and enhancing motor learning.…”