Non-invasive brain stimulation and short-term cortical plasticity

Non-invasive brain stimulation (NIBS) techniques have been widely used to study central nervous system pathophysiology and have revealed new information about the functional roles of specific brain structures. This review first outlines the basic technologies and neural mechanisms associated with NIBS. Transcranial magnetic stimulation (TMS) was first utilized to obtain motor-evoked potentials, which reflect corticospinal tract function. Later, studies using repetitive TMS and patterned TMS-like theta-burst stimulation reported the presence of aftereffects, which can enhance or inhibit cortical excitability depending on the stimulus frequency or continuous/intermittent stimulation. Over the past two decades, transcranial electrical stimulation has become increasingly popular for neuromodulation, which includes direct current stimulation (tDCS) and alternating current stimulation (tACS). While tDCS modulates cortical excitability depending on the stimulation polarity, tACS exerts its effects in a frequency-phase-dependent manner. Although entrainment may be an important underlying mechanism, the recorded aftereffects are likely due to spike-timing-dependent plasticity. Second, recent innovations using NIBS to induce short-term cortical plasticity are highlighted. State-dependent or informed open-loop stimulation can be used to explore the interactions between endogenous cortical oscillations and TMS. Electroencephalographic oscillations prior to TMS stimulation reflect instantaneous cortical excitability depending on the stimulation power and phase, and repetitive TMS at a certain EEG state can enhance cortical excitability. Finally, temporal deafferentation of the peripheral nerve induced by a modified ischemic nerve block can modulate sensorimotor function after tourniquet deflation. Overall, NIBS techniques are promising tools for investigating human brain functions and developing new protocols for neurorehabilitation.