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

Abstract

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.

Are current flow models for transcranial electrical stimulation fit for purpose?

“…Yet we do know that the idiosyncratic properties of the head strongly influence how much of a fixed output current (e.g. 1mA) will reach the brain [1], and so we are in the extraordinary position of knowing that the effective dose of tES is highly variable across individuals (Figure 1), yet doing very little about it.The recent debate scrutinizing the efficacy, reliability and utility of tES [4,5,10] thus misses a vital point that likely contributes to variable outcomes. For a given stimulator output, current in a cortical target region may vary by up to 100% across individuals [7].…”

“…The recent debate scrutinizing the efficacy, reliability and utility of tES [4,5,10] thus misses a vital point that likely contributes to variable outcomes. For a given stimulator output, current in a cortical target region may vary by up to 100% across individuals [7].…”

“…It is easy to imagine that the inevitable variability in current delivered to the brain across individuals has so far been a key determinant for the variable effects of tES. What is puzzling, however, is that in most discussions about the variable effects of tES, this key point is absent [4,5,10]. As long as we do not know how much current is effectively delivered in an individual, the debate about the sources of variable responses to tES (including, for example, cortical state dependencies, individual traits, or genetic variables) is like posting a letter without a stamp nor address, and hoping it will arrive at its destination.…”

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

“…Tremblay et al (2016) investigated modulations of corticospinal excitability within motor cortex as induced by transcranial direct current stimulation (tDCS). Having delivered 1 and 2 mA stimulation for durations of 10 or 20 min, they assessed the outcomes by means of motor evoked potentials (MEPs).…”

“…A number of issues concerning the validity of tES research are brought to light by Tremblay et al (2016). As outlined by the authors themselves, current approaches lack sensitivity and place limitations on the reproducibility of effects.…”