Does second-scale intertrial interval affect motor evoked potentials induced by single-pulse transcranial magnetic stimulation?

Objective To determine whether a second-scale intertrial interval (ITI) of single-pulse transcranial magnetic stimulation (TMS) affects the measured amplitude of motor-evoked potentials (MEPs) representing individual corticospinal excitability. This was performed to challenge the common assumption of time invariance of such amplitudes. Methods Navigated TMS was used to map the dominant hemisphere of nine healthy subjects for the cortical representation focus of the contralateral thenar muscle, and resting motor threshold (MT) was determined. Single-trial MEP amplitudes were analyzed from trains of 30 responses induced at an intensity of 120% of the MT, and constant ITIs were investigated at 1, 2, 3, 5, and 10 seconds as well as randomized at ranges of 1-3 seconds, 3-5 seconds, and 5-10 seconds. MEP responses were divided into three blocks of 10 consecutive responses within each stimulation train. Repeated samples ANOVA was used to assess whether the individual characteristic MEP amplitudes were time invariant, i.e., not affected by the different ITIs and stimulus blocks. Results The individual single-trial MEP amplitudes were affected significantly ( P < 0.05) by the ITI (8/8 subjects), block number (5/8 subjects), and ITI by block number interaction (6/8 subjects). One subject was excluded as the sphericity of the variances could not be confirmed. Consequently, the found time variant nature of the individual single-trial MEP amplitudes affected the estimates (means) of individual characteristic MEP amplitudes. This was also observed as a significant block number effect ( P < 0.05) across all subjects. Conclusions The individual characteristic MEP amplitudes are time variant, contrary to the common assumption. Hence, individual characteristic MEP amplitude estimates should be used cautiously, as erroneous conclusions could be made when assuming those as time invariant.