Sham Transcranial Magnetic Stimulation Using Electrical Stimulation of the Scalp

Abstract

BACKGROUND: Most methods of sham, repetitive transcranial magnetic stimulation (rTMS) fail to replicate the look, sound, and feel of active stimulation in the absence of a significant magnetic field. OBJECTIVE/HYPOTHESIS: To develop and validate a new method of sham rTMS appropriate for a double-blind, placebo-controlled study with subject crossover. METHODS: The look and sound of active rTMS was replicated using a matched, air-cooled sham TMS coil. Scalp muscle stimulation associated with rTMS was replicated using large rubber electrodes placed over selected muscles. The intensity and pulse width of electrical stimulation necessary to match 1-Hz rTMS was developed in one sample of normal subjects. The sham technique was validated in back-to-back comparisons with active rTMS in new samples of normal subjects who were either naïve or experienced with rTMS. RESULTS: Subjects naïve to TMS could not tell which type of stimulation was active or sham or which was electrical or magnetic. Naïve subjects incorrectly picked sham stimulation as active, when forced to choose, because electrical stimulation felt more focused than magnetic stimulation. Subjects experienced with TMS could correctly identify sham and active stimulation. Experimenters could detect subtle differences between conditions. CONCLUSIONS: This method of sham rTMS closely mimics the look, sound, and feel of active stimulation at 1Hz without creating a significant magnetic field. It is valid for use with naïve subjects and in crossover studies. It can accommodate differences in scalp muscle recruitment at different sites of stimulation, and it could potentially be used with higher frequency stimulation.

Conflict of interest statement

Disclosure: The authors report no conflicts of interest.

Figures

Figure 1

Electrode placement is shown in the picture on the left. The active coil is shown in the middle picture and the sham coil is shown in the picture on the right. Differences in coil coverings between the active and sham coils were masked with Gaffer tape.

Figure 2. Stimulation Intensity Plot

Each data point is representative of the mean of three trials. The apparent relationship between TMS intensity and electrical stimulation was found to be linear over the intensity range that we explored (49% - 88% of MSO).