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 uncommonly large.

The goal of this study was to acquire reliable estimates of cortical excitability (MEP amplitude) with due respect to the notion that an initial transient-state might obscure a subsequent steady-state of cortical activity. Thus, the hypotheses were (i) that an initial transient state of increased excitability would precede a (ii) steady state with reliable MEP amplitudes. To address these questions we acquired repeated measures (n=21) of single pulse MEP amplitudes over the primary motor cortex with and without NBS and with various coils. Navigated Brain Stimulation (NBS) quantified and accounted for physical inaccuracies. We found a consistent decline of mean estimates of cortical excitability (MEP amplitude) over the first 20 stimuli. This was confirmed on a subject-by-subject analysis. Mean amplitudes estimates were significantly reduced in 19/21 cases, with up to a 30% difference. These effects could not be explained by physical parameters (correlation <0.1, location, orientation and tilt variability <2mm). Further, the „transient state“ was log-normal, the subsequent „steady state“ normally distributed. This leads us to argue that the acquisition of reliable measures of MEP-amplitudes is feasible, if they are dissociated from an initial transient state of excitability.