No abstract

While the involvement of executive processes in mind wandering is largely undebated, their exact relationship is subject to an ongoing debate and rarely studied dynamically within‐subject. Several brain‐stimulation studies using transcranial direct current stimulation (tDCS) have attempted to modulate mind‐wandering propensity by stimulating the left dorsolateral prefrontal cortex (DLPFC) which is an important hub in the prefrontal control network. In a series of three studies testing a total of N = 100 participants, we develop a novel task that allows to study the dynamic interplay of mind wandering, behavioural varibility and the flexible recruitment of executive resources as indexed by the randomness (entropy) of movement sequences generated by our participants. We consistently find that behavioural variability is increased and randomness is decreased during periods of mind wandering. Interestingly, we also find that behavioural variability interacts with the entropy‐MW effect, opening up the possibility to detect distinct states of off‐focus cognition. When applying a high‐definition transcranial direct‐current stimulation (HD‐tDCS) montage to the left DLPFC, we find that propensity to mind wander is reduced relative to a group receiving sham stimulation.

Transcranial direct current stimulation (tDCS) has been proposed to be able to modulate different cognitive functions. However, recent meta‐analyses conclude that its efficacy is still in question. Recently, an increase in subjects’ propensity to mind‐wander has been reported as a consequence of anodal stimulation of the left dorsolateral prefrontal cortex (Axelrod et al., Proceedings of the National Academy of Sciences of the United States of America, 112, 2015). In addition, an independent group found a decrease in mind wandering after cathodal stimulation of the same region. These findings seem to indicate that high‐level cognitive processes such as mind wandering can reliably be influenced by non‐invasive brain stimulation. However, these previous studies used low sample sizes and are as such subject to concerns regarding the replicability of their findings. In this registered report, we implement a high‐powered replication of Axelrod et al. (2015) finding that mind‐wandering propensity can be increased by anodal tDCS. We used Bayesian statistics and a preregistered sequential‐sampling design resulting in a total sample size of N = 192 participants collected across three different laboratories. Our findings show support against a stimulation effect on self‐reported mind‐wandering scores. The effect was small, in the opposite direction as predicted and not reliably different from zero. Using a Bayes Factor specifically designed to test for replication success, we found strong evidence against a successful replication of the original study. Finally, even when combining data from both the original and replication studies, we could not find evidence for an effect of anodal stimulation. Our results underline the importance of designing studies with sufficient power to detect evidence for or against behavioural effects of non‐invasive brain stimulation techniques, preferentially using robust Bayesian statistics in preregistered reports.

In addition to lDLPFC stimulation, excitability changes in the MPFC should also be considered as a potential mechanism underlying clinical efficacy of bipolar montages. MDD-associated anatomical variations are not likely to substantially influence current flow. Individual modeling of tDCS protocols can substantially improve cortical targeting. We make recommendations for future research to explicitly test the contribution of lDLPFC vs. MPFC stimulation to therapeutic outcomes of tDCS in this disorder.

Transcranial direct current stimulation (tDCS) is a non‐invasive brain stimulation method that is frequently used to study cortical excitability changes and their impact on cognitive functions in humans. While most stimulators are capable of operating in double‐blind mode, the amount of discomfort experienced during tDCS may break blinding. Therefore, specifically designed sham stimulation protocols are being used. The “fade‐in, short‐stimulation, fade‐out” (FSF) protocol has been used in hundreds of studies and is commonly believed to be indistinguishable from real stimulation applied at 1 mA for 20 min. We analysed subjective reports of 192 volunteers, who either received real tDCS (n = 96) or FSF tDCS (n = 96). Participants reported more discomfort for real tDCS and correctly guessed the condition above chance‐level. These findings indicate that FSF does not ensure complete blinding and that better active sham protocols are needed.