Evaluation of a novel sham condition for repetitive transcranial magnetic stimulation (rTMS)

In controlled clinical studies investigating repetitive transcranial magnetic stimulation (rTMS) as a potential treatment for psychiatric and neurological disorders, various forms of sham rTMS have been used as placebo controls. However, placebo rTMS has not been systematically established to date and several previous sham conditions have turned out as weak active conditions in subsequent trials. Here we investigate a new sham condition with a mu-metal shielded coil in a cross-over design in 20 healthy volunteers. The intensity of the shielded coil was calibrated so that the sound pressures of the sham and the active condition were equal. The resulting magnetic field was reduced to 9% in relation to the active condition. Both conditions did not significantly differ regarding effects on mood ratings. However, subjective sensations were significantly different. This study addresses important difficulties in finding the optimum sham condition for rTMS and reveals problems that need to be considered in future studies.