Factors affecting the reliability of MEP amplitude

Transcranial magnetic stimulation (TMS)-induced motor evoked potentials (MEPs) have been used as an outcome measure for assessment of corticospinal excitability (CSE) changes in a range of research protocols. Larger MEP amplitude is an indicator of higher corticospinal excitability while the smaller MEP amplitude is an indicator of lower excitability. This chapter will discuss the reliability of TMS induced MEPs. Following a brief description of the need for the establishment of intrarater reliability in any experimental TMS study, the chapter will discuss the possible factors which may affect this reliability. These factors include 1. the number of recorded MEPs, 2. removal of the first few induced MEPs, 3. TMS Inter-pulse interval (IPI), 4. the intensity of the TMS stimuli, 5. normalization of MEPs to the size of M-wave and 6. background muscle contraction. This chapter not only provides a physiological basis for why these factors affect the test-retest reliability of the MEP amplitude but also provides a review of the literature on the studies which focussed on these factors and their effects on the reliability of MEP amplitude.