In this chapter I discuss the physiology of TMS and describe some of the common techniques that have been applied by those using TMS. I will not describe the details of each method, but outline the general principles and limitations. Most of the work on the basic mechanisms of these techniques has been performed on the motor cortex, where the response to each stimulus is easy to quantify as the amplitude of an MEP response. However, it is thought that the same general principles will apply to stimulation of other areas of cortex, although this may be difficult to prove in practice.

Single pulse transcranial stimulation

Although the majority of studies use TMS to activate the brain, the older method of transcranial electrical stimulation (TES) is still used occasionally. As described below, comparison of the effects of TMS and TES can help distinguish whether an intervention changes cortical excitability at the site of stimulation or at a distant projection target (such as the spinal cord).

Transcranial electrical stimulation of the corticospinal output of the hand area of motor cortex

The corticospinal system forms the largest output of the motor cortex. Experiments in primates have shown that single pulse electrical stimulation of the surface of the exposed cortex activates this output both directly, through depolarization of corticospinal axons in the immediately subcortical white matter, and indirectly via excitatory synaptic input from other cortical neurones (Patton & Amassian, 1954).