Sham TMS: Intracerebral measurement of the induced electrical field and the induction of motor-evoked potentials

Testing the therapeutic potential of transcranial magnetic stimulation (TMS ) in controlled trials requires a valid sham condition. Sham TMS is typical ly administered by tilting the coil 45-90 degrees off the scalp, with one o r two wings of the coil touching the scalp. Lack of cortical effects has no t been verified. We compared sham manipulations in their thresholds for eliciting motor-evok ed potentials (MEPs) in human volunteers and in intracerebral measurements of voltage induced in the prefrontal cortex of a rhesus monkey. Three types of sham (one-wing 45 degrees and 90 degrees and two-wing 90 deg rees tilt) induced much lower voltage in the brain than active TMS (67-73% reductions), However, the two-wing 45 degrees sham induced values just 24% below active TMS, This sham was about half as potent in inducing MEPs over the motor cortex as active TMS. Some sham TMS conditions produce substantial cortical stimulation, making i t critical to carefully select the sham manipulation for clinical trials. B iol Psychiatry 2001;49: 460-463 (C) 2001 Society of Biological Psychiatry.