Patterned Protocols of Transcranial Magnetic Stimulation

For about two decades it has been well established that transcranial magnetic stimu-lation when applied repetitively (rTMS) is able to induce or modify cortical plasticity-like mechanisms during or beyond stimulation. As a general rule, higher frequencies distinctly above 1 Hz are expected to induce facilitation or potentiation, which is comparable with synaptic long-term potentiation (LTP) seen in experimental animals, and frequencies at or below 1 Hz inhibition or depression, which is comparable with long-term depression (LTD). For example, rTMS at 1 Hz given to the primary motor cortex (M1) for 25 min may suppress motor cortical excitability for about 25 min (Touge et al. 2001). By contrast, rTMS at 5 Hz given in separated short trains may facilitate motor cortical excitability for at least 30 min (Peinemann et al. 2004). The aftereffects of rTMS are believed to be plasticity like and share several common features of LTD and LTP (Ziemann 2004). These rules however may be violated or even reversed by a variety of factors such as duration of stimulation, homeostatic plasticity, or co-application of neuroactive drugs. Moreover, conventional forms of rTMS delivered at a regular fixed frequency usually require a lengthy stimulation and a high stimulus intensity to produce consistent aftereffects and commonly need to be interrupted for coil replacement because of overheating. Nevertheless, the aftereffects are still short-lived. In view of these difficulties, more efficient protocols, which are capable of producing consistent aftereffects with fewer stimuli or lower stimulus intensity, are demanded. Based on such demands, patterned rTMS protocols, e.g., theta burst stimulation (TBS), paired-pulse rTMS (pp rTMS), and more recently quadri-pulse stimulation (QPS), have been introduced.