Transcranial Brain 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 short stimulation pulses in the range of 100 μs. Decreasing or increasing cortical excitability can be achieved by varying intensity, stimulation frequency, intervals, number of pulses, number of sessions, location of stimulation, directions of induced current flow and pharmacological manipulation. Alternatively transcranial electric stimulation techniques have been developed as cheap and efficient tools for modifying cortical plasticity. Transcranial direct current stimulation (tDCS) (Nitsche and Paulus, 2000) induces plastic aftereffects via membrane polarization: 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). Plastic aftereffects need a minimal stimulation duration time and may reverse with too long stimulation. TACS and tRNS induce less skin sensation than tDCS and accordingly can be blinded better. They are also less current flow direction sensitive as compared to rTMS or tDCS.