Fluctuations of the TMS induced MEP: a steady-state and reliable estimates

Transcranial magnetic stimulation (TMS) and muscle evoked potentials (MEP) are a common procedure in neurological clinical practice and neuro-scientific research. The MEP amplitude is an exciting measure of cortical excitability, yet also severely limited by notorious sweep-to-sweep variability. For example, in contrast to measures of conduction time, some subjects show MEP-amplitude sweep-to-sweep variability between 0 and 4 mV. Others are quite stable. This variability might be of physiological or physical origin, or both. Interestingly, a common observation is that the initial few responses are most often unc…