TMS of motor cortex produces muscle responses, termed motor-evoked potentials (MEPs), which provide a particularly useful metric for measuring cortical responses to TMS. The MEP size varies with the intensity of stimulus, with stronger TMS stimuli producing larger MEPs (van der Kamp et al., 1996). TMS-evoked MEPs are also facilitated if the subject voluntarily contracts the target muscle slightly (Hess et al., 1986, 1987; Andersen et al., 1999). Another stimulation paradigm, paired-pulse TMS, involves the application of a conditioning stimulus pulse prior to the test stimulus delivered, for example, over motor cortex. If the conditioning stimulus alters the MEP, then a functional interaction between the target of the conditioning stimulus and the location of the test stimulus is inferred.
