Systematic assessment of duration and intensity of anodal transcranial direct current stimulation on primary motor cortex excitability

• Published in: The European journal of neuroscience Vol. 44; no. 5; pp. 2184 - 2190
• Main Authors: Tremblay, Sara, Larochelle-Brunet, Félix, Lafleur, Louis-Philippe, El Mouderrib, Sofia, Lepage, Jean-François, Théoret, Hugo
• Format: Journal Article
• Language: English
• Published: France Blackwell Publishing Ltd 01.09.2016
• Subjects: Adult; Analysis of Variance; Evoked Potentials, Motor; Female; Humans; inter-subject variability; Male; Motor Cortex - physiology; motor evoked potential variability; primary motor cortex; Pyramidal Tracts - physiology; transcranial direct current stimulation; Transcranial Direct Current Stimulation - adverse effects; Transcranial Direct Current Stimulation - methods; Transcranial Direct Current Stimulation - standards; Transcranial Magnetic Stimulation; inter-subject variability; motor evoked potential variability; transcranial direct current stimulation; primary motor cortex
• ISSN: 0953-816X; 1460-9568
• DOI: 10.1111/ejn.13321

Since the initial demonstration of linear effects of stimulation duration and intensity on the strength of after‐effects associated with transcranial direct current stimulation (tDCS), few studies have systematically assessed how varying these parameters modulates corticospinal excitability. Therefore, the objective of this study was to systematically evaluate the effects of anodal tDCS on corticospinal excitability at two stimulation intensities (1 mA, 2 mA) and durations (10 min, 20 min), and determine the value of several variables in predicting response. Two groups of 20 individuals received, in two separate sessions, 1 and 2 mA anodal tDCS (left primary motor cortex (M1)‐right supra‐orbital montage) for either 10‐ or 20‐min. Transcranial magnetic stimulation was delivered over left M1 and motor evoked potentials (MEPs) of the contralateral hand were recorded prior to tDCS and every 5 min for 20‐min post‐tDCS. The following predictive variables were evaluated: I‐wave recruitment, stimulation intensity, baseline M1 excitability and inter‐trial MEP variability. Results show that anodal tDCS failed to significantly modulate corticospinal excitability in all conditions. Furthermore, low response rates were identified across all parameter combinations. No baseline measure was significantly correlated with increases in MEP amplitude. However, a decrease in inter‐trial MEP variability was linked to response to anodal tDCS. In conclusion, the present findings are consistent with recent reports showing high levels of inter‐subject variability in the neurophysiological response to tDCS, which may partly explain inconsistent group results. Furthermore, the level of variability in the neurophysiological outcome measure, i.e. MEPs, appears to be related to response. This study suggests that none of the most frequently used anodal tDCS parameters effectively modify cortical excitability at a group level. Based on an objective classification of response rates, low levels of responders were obtained, with response rates ranging from 20 to 35%. Although none of the baseline measures were related to the magnitude of MEP changes, our result suggest that inter‐trial MEP amplitude variability may contribute to variability to response to anodal tDCS.