The size of motor-evoked potentials: influencing parameters and quantification

This article discusses parameters influencing the size of motor-evoked potentials (MEPs) in normal and pathological conditions, and the methods of meaningful quantification of the MEPs. MEPs are widely used to study the physiology of corticospinal conduction in healthy subjects and in patients with diseases of the central nervous system. The characteristics of MEP size are, stimulus intensity, coil positioning, and facilitation. MEPs show variability in size and shape from one stimulus to the next, even if the stimulus parameters are kept constant. This article describes the triple stimulation technique (TST), which was developed to eliminate the effects of phase cancellation from the MEPs, to allow for a better quantification. Pathological conditions may modify the parameters discussed in the article and influence the size of the MEPs by lesions of motor neurons or of their axons, central conduction velocity slowing, or conduction block.