Sham TMS – A New Approach

Discussion: Sham TMS is a tradeoff between competing quality criteria. So far, the applied sham conditions accepted a reduced cortex stimulation effect to imitate the sensations of verum stimulation. We here focused on the development of a sham condition with only minor cortex stimulation. This was achieved by using a mu-metal shield between a double-coil arrangement. With two coils of the same type and a rigid joint between them, the acoustic artifacts and the vibrations felt on the skin are nearly identical for sham and verum stimulation. The only criterion not fulfilled is the identical scalp sensation arising from scalp muscle stimulation under verum. However, this can be realized by synchronous electrical stimulation of the skin [5]. Methods: Two planar figure-of-eight coils are attached back-to-back (see Fig. 1). Between the coils, a mu-metal shield is inserted and fixed with spacers (2 cm distance from shield to each coil). For verum stimulation, the coil adjacent to the subject‘s head is triggered and for sham stimulation the coil facing into the air. To probe the physical properties of this coil complex, we measured the induced voltages (Upp) in a pair of wireloops located in front of the verum coil, for the sham and verum stimulation with and without the mu-metal shield (fixed stimulator output of 40% maximal output). Sham weaker Yes No Yes (1⁄2 of verum) Coil tilted at 90°