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 typically administered by tilting the coil 45–90° off the scalp, with one or two wings of the coil touching the scalp. Lack of cortical effects has not been verified. mWe compared sham manipulations in their thresholds for eliciting motor-evoked potentials (MEPs) in human volunteers and in intracerebral measurements of voltage induced in the prefrontal cortex of a rhesus monkey. mThree types of sham (one-wing 45° and 90° and two-wing 90° tilt) induced much lower voltage in the brain than active TMS (67–73% reductions). However, the two-wing 45° 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. mSome sham TMS conditions produce substantial cortical stimulation, making it critical to carefully select the sham manipulation for clinical trials.