Ineffective sham-blinding assessment during 2mA transcranial direct current stimulation.

Non-invasive electrical stimulation methods are often used in experimental settings to investigate the possible modulation of antinociceptive mechanisms. Studies using transcranial direct current stimulation (tDCS) typically incorporate a fade-in, short-stimulation, fade-out sham (placebo) protocol, which is assumed to be indistinct from a 10-30min active protocol on the scalp. However, many studies report that participants can dissociate active stimulation from sham, even during low-intensity 1mA currents. In the present study we assessed whether delivery of a high-intensity 2mA current would exacerbate differences in the perception of active and sham protocols. Two protocols were delivered to 32 healthy, pain-free adults in a double-blinded, within-subjects design (active: 10min of 2mA, and sham: 20s of 2mA), with the anode over the left primary motor cortex and the cathode on the right forehead. Participants were asked “Is the stimulation on?” and “How sure are you?” at 30s intervals during and after stimulation. The differences between active and sham were more consistent and sustained during 2mA than during 1mA. We then quantified how well participants were able to track the presence and absence of stimulation (i.e. their sensitivity) during the experiment using cross-correlations. A good classifier of sensitivity during active tDCS was current strength, but exhibited only moderate specificity during sham. The accuracy of the end-of-study guess was no better than chance at predicting sensitivity. Our results from this methodological approach indicate that the traditional end-of-study guess poorly reflects the sensitivity of participants to stimulation, and may not be a valid method of assessing sham blinding. Further research should be carried out into inter-individual responses to sham-blinding and assessment methods in pain studies and the broader neurostimulation field. Wellcome Trust.