(p. 235) The Motor-evoked Potential in Health and Disease
The muscle twitch from motor cortex stimulation was the first phenomenon observed with TMS. Not only was this unambiguous evidence that the brain could be stimulated safely and painlessly through the head, but subsequent work demonstrated that it could be a quantitative measure of the state of a set of neurons in the CNS. It was soon clear that voluntary muscle activation or even the intention to move could enhance the motor-evoked potential (MEP). Later, the description of the cortical silent period and advent of paired-pulse techniques enabled the study of inhibitory processes and the effects of drugs, hormones, and diseases on cortical circuits. Changes observed in the motor cortex may reflect global alterations in cortical function and we can sometimes infer important facts about the mechanisms of disorders, for example migraine, and hormones with behavioral impacts that express themselves primarily through other regions. Additionally, the MEP promises to reveal highly specific information on the expression of the some of the genotypes associated with neurological and neurobehavioral disorders,
Motor-evoked potential conduction studies have enhanced our understanding of neurodevelopment in healthy children and those with developmental disorders. They have proven useful in the diagnosis of lesions in the central and proximal nerves and, while occupying a narrow diagnostic niche, provide a valuable tool for clinical neurophysiologists and anesthesiologists monitoring the integrity of central pathways. They have also been used to locate the motor cortex for neurosurgical planning.
Nearly all we know about the mechanisms of transcranial brain stimulation in its many forms has been inferred from this deceptively simple phenomenon, so distant from its source, and an understanding of its origin and the many factors that can affect it is important for any investigator who plans to use it. (p. 236)