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A comparison of two methods for estimating 50% of the maximal motor evoked potential
Julia B Pitcher1, Sebastian H Doeltgen1, Mitchell R Goldsworthy1
1Robinson Research Institute, School of Paediatrics & Reproductive Health, University of Adelaide, Adelaide 5005, Australia.
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 1 mV 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.
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