Cortical cellular actions of transcranial magnetic stimulation.

Transcranial magnetic stimulation (TMS) can be used in two different ways to manipulate cortical information processing, either by applying a single pulse around the time point of expected task processing or by persistently shifting cortical excitability by repetitive stimulation (rTMS). Single pulses applied when specific cortical processing takes place always impair cortical function due to increased noise or enhanced inhibition, both resulting in decreased signal-to-noise ratio, while repetitive stimulation may allow to weaken or improve cortical processing depending on the type of stimulation. The opposite effects of low- ( approximately 1 Hz) and high-frequency rTMS (5-20 Hz), as well as the opposing effects of continuous versus intermittent theta-burst trains, lowering or raising cortical excitability respectively, have mainly been attributed to synaptic plasticity. As reviewed in this article, in a series of electrophysiological, immunohistochemical and molecular-biological animal experiments we obtained evidence for modulation of inhibitory cortical activity as a further reason of changing cortical excitability following rTMS.