Not Quite All in Our Head: Intervention is a Better Predictor of tDCS Mind-Wandering Effects than Subjective Beliefs About Experimental Results

Abstract

AbstractBackgroundEstablishing adequate blinding for non-invasive brain stimulation research is a topic of extensive debate, especially regarding the efficacy of sham control methods for transcranial direct current stimulation (tDCS) studies. Fassi and Cohen Kadosh [1] assessed the influence of subjective participant belief regarding stimulation type (active or sham) and dosage on behaviour using data from Filmer et al. [2] who applied five stimulation protocols (anodal 1.0mA, cathodal 1.0mA, cathodal 1.5mA, cathodal 2.0mA and sham) to assess the neural substrates of mind wandering. Fassi and Cohen Kadosh [1] concluded that subjective belief drove the pattern of results observed by Filmer et al. [2].ObjectiveFassi and Cohen Kadosh [1] did not assess the key contrast between conditions in Filmer et al. (2019) – 2mA vs sham – rather they examined all stimulation conditions. Here, we consider the relationship between objective and subjective intervention in this key contrast.MethodsWe replicated the analysis and findings of both Filmer et al. [2] and Fassi and Cohen Kadosh [1] before assessing 2mA vs. sham via Bayesian ANOVA on subjective belief regarding stimulation type and dosage.ResultsOur results support objective intervention as the strongest predictor of stimulation effects on mind-wandering when 2mA vs sham was examined, over and above that of subjective intervention.ConclusionsThe conclusions made by Filmer et al. [2] are confirmed. However, it is important to control for and understand the possible effects of subjective beliefs in sham-controlled studies. Best practice to prevent these issues remains the inclusion of active control conditions.