The Basis of Variability in Magnitude of the Response of Muscles to Transcranial Magnetic Stimulation of the Motor Cortex in Man

The magnitude of the compound motor evoked potential (cMEP) response of skeletal muscles to transcranial magnetic stimulation (TMS) in man may show a high degree of variability (Davey, Ellaway, Maskill, Anissimova, Rawlinson & Thomas, 1994). The variability occurs despite using a constant stimulating intensity and the efforts of the operator to maintain a constant stimulating coil position over the cranium. We have now investigated the basis of this variability.