A comparison of two methods for estimating 50% of the maximal motor evoked potential

Objectives: Two commonly-used methods for setting stimulus intensities in transcranial magnetic brain stimulation studies were compared to determine which best approximated a motor evoked potential (MEP) of 50% of the maximal MEP amplitude (SI₅₀); a suprathreshold intensity relative to resting motor threshold (rMT) or adjusting the intensity to evoke an MEP amplitude of 1mV. Methods: Corticomotor stimulus-response curves and rMT for the right first dorsal interosseous (FDI) muscle of 176 subjects (aged 10-74. years) were retrospectively analysed. Results: Regardless of subject age or sex, SI₅₀ occurred at 127.5±11.3% rMT. Except in young children, MEPs of 1mV were significantly smaller than those evoked at SI₅₀. Conclusions: In the inactive FDI muscle, a stimulus intensity of 127-128% rMT consistently gives the best approximation of SI₅₀ in most subjects, except perhaps young children. Significance: Setting TMS stimulus intensities relative to rMT provides a less variable inter-subject comparator, with respect to individual differences in corticomotor input-output characteristics, than adjusting the stimulator output to give an absolute MEP magnitude.