Inter- and intra-individual variation in the response to TMS

The variable amplitude of motor cortex is a striking aspect of the muscle response to transcranial magnetic stimulation. It is easy to produce large motor-evoked potentials (MEPs) in some healthy subjects, while others' cortico-muscular pathways seem barely excitable, even by the strongest available stimuli. MEP amplitude and other measures also vary widely within individuals over time. The factors of these differences among and within individuals are age, gross anatomy of the individuals, genetic factors, and physiological differences associated with behavioural and other traits such as personality, conditions like migraine. The MEP varies over time within individuals at rest under laboratory conditions. These variations can be short term or long term. Differences among neurologically normal individuals have important implications for research using TMS. These differences open doors to new fields of study to neurophysiologists in the treatment and etiology of brain disease.