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Abstract
The aim of this study is to describe the variability and other characteristics of
the motor evoked potential (MEP) to transcranial magnetic stimulation (TMS) in a large
database.
One hundred fifty one subjects, including 17 sib pairs, free of neurological or psychiatric
disease and on no neuroactive medications were studied with uniform techniques. Nineteen
were studied on 3 occasions. Measures included MEP threshold (N=141) during rest and
voluntary muscle activation and the response to paired TMS (subthreshold conditioning
stimulus) at interstimulus intervals (ISIs) of 3, 4, 10, and 15ms (N=53).
There was a large variability in all the measures. Approximately 40-50% of this appeared
to come from within-subjects variation or experimental error. The MEP threshold data
were skewed downward, but normalized with log transformation. The paired-pulse ratios
(conditioned/unconditioned MEP) were normally distributed except those from the 3ms
ISI which had no lower tail and could not be normalized. There were subjects showing
inhibition and others showing facilitation at all ISIs. There were no correlations
in any of the data with age or sex, but MEP thresholds were highly correlated within
sibs.
These data should be useful for planning, analyzing, and interpreting TMS studies
in healthy and patient populations.