Another important stimulation method is repetitive TMS (rTMS), which involves the delivery of trains of TMS pulses, often at high frequencies, to produce changes in cortical excitability that persist beyond the duration of the stimulus. The mechanisms through which these protocols alter excitability are unknown, but are believed to involve processes similar to synaptic long-term potentiation and long-term depression (Fitzgerald et al., 2003). In one of the earliest studies of the effects of rTMS, Pascual-Leone et al demonstrated that high-frequency (>5 Hz) rTMS trains generally increased cortical excitability, as measured via MEP size (Pascual-Leone et al., 1994). Significantly, these effects persisted for 3–4 minutes after the end of stimulation. In contrast, rTMS at frequencies of 1 Hz or below generally decreases cortical excitability (Chen et al., 1997). A recent review of studies of the effects of rTMS on cortical excitability (as measured with simultaneous EEG) notes that both low-frequency and high-frequency rTMS produce an approximately 30% change in TMS-evoked response (depression with low-frequency rTMS, and facilitation with high-frequency rTMS), with the excitability changes persisting for a mean of about 30
