Abstract
A valid sham control is important for determining the efficacy and effectiveness of repetitive transcranial magnetic stimulation (rTMS) as an experimental and clinical tool. Given the manner in which rTMS is applied, separately or in combination with self-regulatory approaches, and its intended impact on brain states, a valid sham control of this type may well serve as a meaningful control for biofeedback studies, where efforts to develop a credible control have often been less than ideal. This study examined the effectiveness of focal electrical stimulation of the frontalis muscle as a sham technique for blinding participants to high-frequency rTMS over the dorso-lateral prefrontal cortex (DLPFC) at durations, intensities, and schedules of stimulation similar to many clinical applications. In this within-subjects single blind design, 19 participants made guesses immediately after receiving 54 counterbalanced rTMS sessions (sham, 10 Hz, 20 Hz); 7 (13 %) of the guesses were made for sham, 31 (57 %) were made for 10 Hz, and 16 (30 %) were made for 20 Hz. Participants correctly guessed the sham condition 6 % (CI 1, 32 %) of the time, which is less than the odds of chance (i.e., of guessing at random, 33 %); correctly guessed the 10 Hz condition 66 % (CI 43, 84 %) of the time, which was greater than chance; and correctly guessed the 20 Hz condition 41 % (CI 21, 65 %) of the time, which was no different than chance. Focal electrical stimulation therefore can be an effective sham control for high-frequency rTMS of the DLPFC, as well as for active biofeedback interventions. Participants were unaware that electrical stimulation was, in fact, sham rTMS.