P169 Does anodal transcranial direct current stimulation elicit consistent effects?

Abstract

Introduction It is well known that transcranial direct current stimulation (tDCS) is capable of modulating cortical excitability. However, a source of growing concern has been the observed inter-individual variability of tDCS responses, and more recently, the intra-individual variability over repeated expositions. Few studies have investigated the reproducibility of tDCS effects over repeated sessions by assessing whether individuals respond in a predictable manner. The findings of these investigations showed to be inconsistent, and lack control groups to determine whether variability was related to the measuring tool (i.e. TMS). Objective The main objective of this study was to determine inter- and intra-individual variability of anodal tDCS (atDCS) effects on primary motor cortex (M1) excitability when applied over multiple sessions. In addition, we assessed whether tDCS response consistency was dependent on the stimulation parameters (i.e. current intensities). Subjects & methods Twelve subjects participated in a randomized crossover counterbalanced study, in which they received atDCS over M1 in three separate conditions (2 mA, 1 mA, sham) each repeated three times separated by 48 h. tDCS was delivered using a standard montage with the active electrode centered over M1 and the reference over the right supra-orbital area. Motor evoked potentials were recorded before and after stimulation (up to 30 min). Time and day of testing was maintained consistent within participants. To estimate the reliability of tDCS effects across multiple sessions, the Intra-class Correlation Coefficient (ICC) was calculated. Results AtDCS significantly increased cortical excitability at the group level in all sessions, except for the 1 mA condition. The ICC revealed fair to good reliability of tDCS effects. Given the distribution of individual responses showed important variability for the sham condition, we established a cut-off value based on this data to classify responses and to track individual changes across sessions for all conditions. Using this value, an intra-individual consistent response pattern was observed only in the 2 mA condition. Conclusion 2 mA anodal tDCS results in consistent intra- and inter-individual increases of M1 excitability.