Intra subject variation and correlation of motor potentials evoked by transcranial magnetic stimulation

Characterising intra and inter-subject variability of motor-evoked potential (MEP) measurements from transcranial magnetic stimulation (TMS) is key to its development as a diagnostic tool.

We performed three experiments to elucidate MEP variability within subjects: (i) repeated measurements at different levels of stimulation and muscle activation, (ii) simultaneous measurements at pairs of ipsilateral and contralateral muscles, (iii) simultaneous measurements of contralateral muscles when one is activated.

Cube root transformation of MEP data produces approximately constant coefficient of variation with Gaussian distribution and no significant autocorrelation between repeat measurements. After adjustment of intersubject variability, correlation between simultaneous muscle pairs was insignificant (p = 0.36). Highly significant effects were observed due to increase in intensity of stimulation: (0.07, 0.23) mV, p < 0.0001, muscle type: (p < 0.009) and activation of ipsilateral muscle: (0.10, 0.49) mV, p < 0.0001.

Corticospinal effect is dominated by intersubject variability in simultaneous measurements on normal population.