The time course of ineffective sham‐blinding during low‐intensity (1 mA) transcranial direct current stimulation

Studies using transcranial direct current stimulation (tDCS) typically compare an active protocol relative to a shorter sham (placebo) protocol. Both protocols are presumed to be perceptually identical on the scalp, and thus represent an effective method of delivering double‐blinded experimental designs. However, participants often show above‐chance accuracy when asked which condition involved active/sham retrospectively. We assessed the time course of sham‐blinding during active and sham tDCS. We predicted that participants would be aware that the current is switched on for longer in the active versus sham protocol. Thirty‐two adults were tested in a preregistered, double‐blinded, within‐subjects design. A forced‐choice reaction time task was undertaken before, during and after active (10 min 1 mA) and sham (20 s 1 mA) tDCS. The anode was placed over the left primary motor cortex (C3) to target the right hand, and the cathode on the right forehead. Two probe questions were asked every 30 s: “Is the stimulation on?” and “How sure are you?”. Distinct periods of non‐overlapping confidence intervals were identified between conditions, totalling 5 min (57.1% of the total difference in stimulation time). These began immediately after sham ramp‐down and lasted until the active protocol had ended. We therefore show a failure of placebo control during 1 mA tDCS. These results highlight the need to develop more effective methods of sham‐blinding during transcranial electrical stimulation protocols, even when delivered at low‐intensity current strengths.