Or, put differently, failure of the blinding condition (although, see 28 and 29). This is potentially a major issue for sham-controlled studies, as observed results may reflect peripheral effects rather than the influence of stimulation on the cortex. Such feelings were significantly higher in active conditions as compared to sham, for example tingling (89% active vs. 26 with 131 subjects in 277 tDCS sessions. Cutaneous feelings are common in tDCS as seen in a prospective comparison conducted by Kessler et al. Indeed, in NIBS studies, participants can report perceptual sensations such as visual disturbances, and cutaneous feelings 26, 27. However, the efficacy of sham-controlled approaches has been called in to question 23, 24, 25. It is assumed that sham stimulation controls for any potential unrelated effects of the direct cortical stimulation 22. Typically, the length of this period of active stimulation is dependent on the stimulation length used for the active condition 22. The most common method of blinding in tDCS studies is the sham control method 21, which mimics the typical initial sensations (i.e., itching or tingling) induced by tDCS by delivering active stimulation for a short period at the beginning (and sometimes again at the end), then either no stimulation or very reduced pulses (which allow some continued sensation) for the rest of the session. Key factors identified by the authors to be addressed include poor methodological design, under-powered samples which give rise to inflated results and, most importantly for the present study, the inadequate blinding of control conditions. 3 provide recommendations to insure the reliability, reproducibility, and validity of effects of NIBS studies. 19 found only 45% of participants (n = 56) responded to anodal transcranial current stimulation as expected. Given this broad interest there is growing demand for clarity on the efficacy of brain stimulation across the extensive stimulation parameter space.Īlthough tDCS has been employed extensively, some reviews and meta-analyses 18, 19, 20 have suggested limited to no effects of transcranial stimulation, with a key focus being on large variability across participants. A variety of cognitive operations have previously been shown to be influenced by tDCS including: motor 10 and speech motor learning 11, working memory 12, 13, response selection 14, multitasking 15, and attention 16, 17. Interest has grown in this brain stimulation approach as it can lead to cognitive enhancement both within clinical settings, such as for the treatment of drug-resistant depression 5, 6, as well as in commercial settings via do-it-yourself devices such as the foc.us headset 7, 8, 9. tDCS typically involves passing electrical current through two electrodes placed on the scalp a cathode and an anode 4. There are multiple methods of NIBS, one of the most common being transcranial direct current stimulation (tDCS 3).
In 2020 the brain stimulation industry was projected to be worth an estimated three billion dollars 2, a figure that is almost certain to increase in the future. Non-invasive brain stimulation (NIBS) is a popular tool for investigating causal relationships between activity in cortical regions and behaviour 1. However, it is important to control for and understand the possible inadequacies of sham-controlled methods. Our findings support objective intervention as the strongest predictor of the observed effects of mind-wandering in both re-analyses, over and above that of subjective intervention. In addition, we examine another key contrast from a different tDCS mind-wandering study that employed similar methodology. sham-which was not examined in the reanalysis. Here we consider the subjective and objective intervention effects in a key contrast from that data set-2 mA vs. Previous work evaluated subjective effects in an earlier study which reported a mind-wandering and tDCS data set and concluded that subjective belief drove the pattern of results observed. Thus, the implementation of measures to systematically evaluate subjective expectation regarding stimulation is needed. However, this method cannot provide insight into the effect of unblinding on observed stimulation outcomes. A common approach to assess blinding success is the inclusion of correct guess rate. Blinding in non-invasive brain stimulation research is a topic of intense debate, especially regarding the efficacy of sham-controlled methods for transcranial direct current stimulation (tDCS).