Transcranial Magnetic Stimulation (TMS)
Non-invasive brain stimulation
Transcranial Magnetic Stimulation (TMS) is a non-invasive method used to stimulate small regions of the brain. The procedure is performed by holding a magnetic field generating coil next to the head, over the brain area of interest. The coil is connected to a pulse generator, or stimulator, that delivers electric current to the coil at intervals controlled by the experimenter. The coil produces small electric currents in the region of the brain just under the coil via electromagnetic induction. TMS can either be used as a measurement tool to assess cortical excitability or indirect estimates of neurochemical concentration, or as an intervention to increase or decrease the activity within a given region.
TMS measurements can be obtained by holding the coil over the primary motor cortex (M1). TMS pulses cause neurons beneath the coil to depolarize, release an action potential, and this can go on to produce muscle activity (known as a motor evoked potential, MEP) which is then recorded using electromyography (EMG). The magnitude of muscle activity generated for a given TMS pulse strength forms a measure of cortical excitability in the stimulated region. Indirect measurements of certain neurochemicals (including GABA) can also be obtained using paired pulse TMS protocols. Depending on the timing and intensity of pulses, a first TMS pulse can either cause facilitation or inhibition of MEPs generated by the second pulse. The amount of facilitation or inhibition achieved using certain protocols has been proposed as a measure of the local concentration of certain neurochemicals.
Multiple TMS pulses can also be given repetitively over the course of several minutes in order to increase or decrease the activity from within a given brain area. By decreasing activity, TMS can be used to generate a temporary 'virtual lesion' in a specific are of the cerebral cortex. The functional role of the targeted area can then be assessed with carefully controlled experimental tests of behaviour. Under some circumstances, TMS can also be used to enhance neural processing by increasing the general excitability of a specific brain area.