Abstract
Introduction: Studies involving transcranial direct current stimulation (tDCS) typically include a “sham” (placebo) condition, with which performance during the “active” tDCS condition is compared. Sham tDCS usually involves only a few seconds of stimulation, and is assumed to be perceptually indistinct from active tDCS on the scalp. For this reason, tDCS is claimed to be an effective means of delivering double-blinded stimulation protocols. However, participants often show above-chance accuracy when asked retrospectively which condition involved sham. We aimed to probe the effectiveness of tDCS blinding in real-time during a reaction time experiment. Method: 32 adults were tested (pre-registered a priori sample size: d=0.45, ⊠=0.05, power=0.8). A forced-choice reaction time task was undertaken before, during and after active (10min of 1mA) and sham tDCS (20s of 1mA; both with 30s ramp-up/down). Conditions were applied on different days in a counterbalanced, double-blinded, within-subjects design. The anode was placed vertically over the left primary motor cortex (C3) to target the right hand, with the return on the right forehead. For 15min after the tDCS was initiated, 2 probe questions were interspersed within the task at 30s intervals (“Is the stimulation on?” & “How sure are you?”). Results: Active tDCS had no effect on reaction time compared to sham. Weighted responses were calculated for the probe questions, combining the yes/no guesses with confidence ratings. Confidence intervals were distinct for the anodal and sham conditions during most of the active stimulation period. This period began after the sham protocol had ended and lasted until the active stimulation had ramped down. Conclusion: Participants were clearly able to differentiate the active and sham conditions throughout the experiment, indicating that this method of blinding tDCS conditions may be ineffective. These results also add to the recent literature showing small, or no, overall behavioural effects of tDCS.
Acknowledgement: Wellcome Trust