Randomized, crossover, sham-controlled, double-blind study of transcranial direct current stimulation of left DLPFC on executive functions .

BACKGROUND: Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation technique commonly used to modulate cognitive functions; so-called "anodal" stimulation is considered to increase cortical excitability while "cathodal" stimulation is presumed to have the opposite result. Yet, a growing number of recent studies question the robustness of this polarity-dependent effect, namely because of the important inter-individual variability with regards to tDCS modulatory effects. A plausible reason for this heterogenous response may lay in task impurity issues in the evaluation of cognitive functions. OBJECTIVE: To address the question of task impurity the NIH-Examiner, a neuropsychological test battery that uses latent variables, which assess the common variance across multiple measures of a given concept, was administered to 24 healthy individuals following tDCS. This battery contains 11 tasks and provides latent variables for general executive functioning, fluency, cognitive control and working memory. METHODS: Anodal, cathodal, and sham stimulation (20 minutes, 1.5 mA) was administered over left dorsolateral prefrontal cortex and right supra-orbital area in a randomized, crossover, sham-controlled, double blind protocol. RESULTS: Factorial scores and task performance indices of executive function were not modulated by tDCS. CONCLUSIONS: Offline tDCS has limited impact on executive functions at both the task and factorial levels. This suggests that reducing task impurity does not increase the effectiveness of tDCS in modulating cognitive functions.

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

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.