Transcranial Magnetic and Electric Stimulation

Repetitive transcranial magnetic stimulation (rTMS) allows increasing or decreasing the excitability of corticospinal or cortico-cortical pathways depending on the intensity and frequency of ultrashort stimulation pulses. Prolonged repetitive transcranial magnetic (rTMS) protocols are being refined by varying many parameters such as frequency, duration, intensity, mono- versus biphasic and variable pulse shape width and intervals. This talk focusses on the relevance of mono- versus biphasic stimulation and on the importance of intervals for the direction and efficacy of aftereffects. Cathodal stimulation hyperpolarizes, while anodal stimulation depolarizes the resting membrane potential, whereby the induced after-effects depend on polarity, duration and intensity of the stimulation. Transcranial alternating current (tACS) (Antal et al, 2008) and random noise stimulation (tRNS) intend to interfere with ongoing cortical oscillations (Terney et al., 2008). Using these techniques, we can induce and modify differently neuroplastic changes with different advantages and disadvantages of tDCS, tACS and tRNS. Plastic aftereffects need a minimal stimulation duration time and may reverse with too long stimulation. Whereas in the normal stimulation duration range of about 10 minutes tDCS allows for excitability increase and decrease, tACS and tRNS induce only excitability increases in particular with higher frequencies between 100 and 600 Hz. The latter induce less skin sensation than tDCS and accordingly can be blinded better. These effects are strongly modified by neuropharmacological co-application: L-DOPA leads to a focusing effect in analogy to its otherwise found U-shaped dose dependency. Dopamine agonists may reverse anodal excitatory tDCS into inhibition, SSRI provide the opposite effect. In conclusion continuous transcranial electrical stimulation techniques allow for targeted modulation of cortical plasticity.