Excitation threshold of the motor cortex estimated with transcranial magnetic stimulation electroencephalography

The excitation threshold of the human motor cortex (M1) was estimated on the basis of electroencephalographic (EEG) responses evoked by transcranial magnetic stimulation (TMS). The hand area of M1 (n = 7) was stimulated at ten intensities. The four dominant peaks of the overall brain response were reliably determined when stimulation intensity was  40% of the motor threshold, the probability of identifying each peak ranging between 0.71 and 0.86. Therefore, the cortical electric field of 33–44 mV/mm, approximately being induced with this intensity, may be estimated as the threshold for evoking measurable brain activity by motor-cortex TMS. As TMS–EEG enables the assessment of cortical reactivity with excellent sensitivity, it seems justified to move over to lower stimulation intensities, for increased safety.