Variation in the response to transcranial magnetic brain stimulation in the general population

Objectives: 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. Methods: 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 15 ms (N=53). Results: 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 3 ms 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. Conclusions: These data should be useful for planning, analyzing, and interpreting TMS studies in healthy and patient populations.