Abstract

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.

Highlights

  • Transcranial direct current stimulation is a popular, non‐invasive method of modulating cortical excitability

  • This study provides important information regarding the time course of cutaneous sensations during Transcranial direct current stimulation (tDCS), we argue that the question posed does not adequately probe whether participants can tell that the stimulation is active at present

  • We identified overlapping confidence intervals between the active and sham protocols throughout the first 90 s of stimulation, where participants were confident that the tDCS was switched on in both conditions

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Summary

Introduction

Transcranial direct current stimulation (tDCS) is a popular, non‐invasive method of modulating cortical excitability. In a typical sham protocol, the current is ramped‐up gradually, delivered for a short period at the same intensity as the active comparator (≤2 mins at 1–2 mA), followed by a fade‐out phase This brief stimulation period seems not to induce any substantial neuromodulatory effects (Nitsche et al, 2008) but enables the initial cutaneous sensations associated with stimulation, including tingling, itching, burning and headache Studies involving sham protocols rely on participants being unable to differentiate the sensations experienced during this brief sham period from a prolonged period of active stimulation (10–20 mins) This is of vital importance to tDCS research, because a failure of sham‐blinding could potentially encourage participants to modify their behaviour, even subconsciously, in response to knowing whether or not they are receiving an active protocol. The effectiveness of sham‐ blinding has been quantified in two broad ways in the prior literature: (a) by comparing the reported frequency and severity of cutaneous sensations during different protocols and (b) by asking participants to guess whether they had received active or sham tDCS (in the case of between‐groups designs), or which of multiple sessions had involved active or sham (in within‐group designs)

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