Basic Neurophysiology Measures

This chapter describes the rationale and method of several common TMS protocols. The stimulus–response curve evaluates corticospinal excitability, typically using the MEP amplitude across a range of stimulator intensities, from subthreshold to supramaximal, forming a sigmoidal response shape. Thus, the curve captures more information than single-intensity recordings, including motor threshold, maximal amplitude, and a curve slope measure. The cortical silent period (SP) is a phenomenon of electromyographic silence resulting from a TMS pulse during isometric contraction of the target muscle, which is sustained after the MEP, and its length is indicative of cortical inhibition. Central motor conduction time (CMCT) is the time for descending action potentials to arrive at the spinal motoneuron pool following the TMS pulse and is determined by subtracting the peripheral conduction time from the MEP latency. Motor mapping refers to the method of determining a spatial map on the cortex (neuronavigated) or scalp (without neuronavigation), which indicates the boundaries of the target muscle representation, or area, and other quantitative features such as the map center or peak amplitude. The concept of systematic mapping can be applied to the cortical expressive speech area.