Transcranial magnetic stimulation—a sandwich coil design for a better sham

Objective To improve the quality of TMS studies by developing a new sham condition. Methods We describe a novel and easily arranged TMS set-up of two standard TMS coils and a magnetic shield, stacked like a sandwich. In a first step we compare the magnetic field in the sham and verum conditions. In a second step we ask six subjects to rate the stimulation intensity. Results The magnetic field in the sham mode is reduced to about one eighth of that during verum stimulation. The attenuation of the magnetic field is not limited to the actual stimulation site but also effective at neighbouring brain areas, avoiding direct and indirect stimulation via connected neural pathways. This also minimizes stimulation of the skin, but as a consequence allows subjects to distinguish between verum and sham conditions when these are contrasted directly. The position of the coil system and the acoustic sensations are indistinguishable between sham and verum condition. Subjects are not able to discriminate TMS position and condition by external cues. Conclusions The proposed TMS setup is simple and allows verum and sham TMS without interaction of the researcher. If used with the magnetic shield, the magnetic field in the brain is attenuated most. Significance With the sandwich TMS coil system it is possible to improve the quality of TMS studies