Accurate determination of motor evoked potential amplitude in TMS: The impact of personal and experimental factors.

Objective: Corticospinal excitability can be quantified using motor-evoked potentials (MEP) following transcranial magnetic stimulation (TMS). However, the inherent variability of MEPs poses significant challenges. We establish a framework using personal and experimental factors to select the optimal number of trials (n) required for reliable MEP estimates.

Methods: 47 healthy younger underwent single-pulse TMS over the left primary motor cortex (M1). Per participant, 550 MEPs were collected at intensities ranging from 110 % to 150 % of the resting motor threshold (rMT), in 10 % increments. Per intensity, we calculated n. We analyzed which personal and experimental factors affected n.

Results: n decreased with increasing TMS intensity, lower rMT baseline values, and exclusion of single-trial outliers. Sex had no significant effect.

Conclusions: Our study indicates that even when TMS is used as an outcome measure, custom-tailoring its protocol to study-related circumstances is key, as TMS intensity, outliers, baseline rMT, and the desired precision level affect the number of TMS trials needed to obtain a reliable MEP. Thus, we underscore the absence of a universal rule-of-thumb rule, although our predictive equations and online tool provide future TMS experimenters with the means to estimate the required number of TMS trials based on individual characteristics and specific experimental conditions.

Significance: Our predictive equations offer a tailored approach for selecting n, enhancing the reliability of TMS-derived corticospinal excitability measurements.