The fade-in--short stimulation--fade out approach to sham tDCS--reliable at 1 mA for naïve and experienced subjects, but not investigators.

Slowly ramping down initial current intensity after a minimal interval of stimulation is the de facto standard for sham stimulation in transcranial electrical stimulation research. The aim of this study is to further investigate the effectiveness of this method of blinding.We have investigated the time course of the cutaneous perception during 10 min of anodal, cathodal, and sham transcranial direct current stimulation, probing the perceived strength and site of the perceived sensation. We have also utilized post-stimulation assessment and measurements of sleepiness prior to and after the intervention. Previous exposure to tDCS has also been taken into account: the experiment has been repeated in naïve and experienced subject groups, and a group consisting of investigators who use tDCS as a research tool.Although we have observed a general reduction in the perceived strength of the stimulation with time, we have not found the complete disappearance of the cutaneous perception during either the verum or the sham conditions. Experienced subjects were more likely to be able to differentiate between trials with stimulation and non-stimulation trials and to correctly identify sham and verum stimulation conditions.When taking only naïve and experienced subjects into account, there was no significant difference between the strength of the perceived stimulation in the verum and sham conditions. The fade-in - short stimulation - fade-out sham stimulation can be indistinguishable from verum stimulation, but not because it mimics the disappearance of the cutaneous sensations associated with the verum stimulation, but because these sensations persist also in the sham stimulation. The significance of this finding with potential confounding factors and limitations are discussed.