Perception of comfort during active and sham transcranial direct current stimulation: a double blind study

A limited number of studies have assessed the tolerability and comfort experienced while undertaking transcranial direct current stimulation (tDCS).

This study intended to assess tolerability and the level of comfort experienced in a large sample of participants undertaking tDCS for 30 min at 2 mA of current strength. Moreover, we assessed whether sham and active stimulation are indistinguishable.

One-hundred and forty-nine participants underwent 195 tDCS sessions. The delivery of stimulation was double-blind. Participants were asked: (i) to rate comfort levels using a visual analog scale; (ii) to report any symptom experienced during the period of tDCS stimulation; (iii) to indicate, at the end of the session, whether the tDCS stimulation was active or sham.

No adverse effects occurred. However, comfort levels were significantly higher in the sham than in the active stimulation condition (primarily in Study 2). A comparable number of symptoms were experienced in the active and in the sham conditions. However, in the majority of symptoms reported, a greater proportion of participants complained in the active than in the sham stimulation condition. Ancillary analyses indicated that with smaller electrodes more symptoms were experienced. However, this occurred in a comparable way in both active and sham stimulations. Finally, participants could not reliably distinguish the type of stimulation received.

The present study adds and complements the growing literature suggesting that tDCS is a well-tolerated and safe neurostimulation tool. Moreover, at least under the stimulation parameters used in the present study, neurostimulation can be successfully administered using a double-blind procedure without participants being able to reliably assess whether the stimulation received is either active or sham.