Setup and Basic Procedures

Laboratory and clinical settings commonly require the assessment of corticospinal excitability to the hand. The excitability can be quantified using the motor-evoked potential (MEP), which can be recorded using electrodes placed over an intrinsic hand muscle, and typically occur around 20 ms after a single suprathreshold pulse of TMS over the contralateral motor cortex. The technique requires correct placement of the electrodes over the peripheral muscle, and the TMS coil over the scalp, above the optimal stimulation site (hotspot). This section illustrates the procedure for electrode positioning and hotspot finding and describes assessment of the motor threshold, defined as the minimum stimulator intensity for a given coil type and device that can result in an MEP. This can be assessed under resting and active conditions. In this manual, we focus on the intrinsic hand muscles; however, these procedures can be adapted to many other muscles of the body. A visual perception known as phosphenes can occur with stimulation over the visual cortex and requires correct positioning and orientation of the coil. Phosphene hotspot identification and threshold method are described.