Transcranial Magnetic Stimulation 

Transcranial magnetic stimulation (TMS) represents a non-invasive brain stimulation method based on electromagnetic induction. Within certain guidelines, TMS is considered a safe and well tolerable neuromodulation technique (Lerner et al., 2019). By the application of brief electromagnetic pulses over the human head, electrical current transcends the cranial bone and depolarizes the underlying superficial neurons of the cortex perpendicular to the induced electrical current (Allen et al., 2007; Barker et al., 1985; Siebner et al., 2009). Extracerebral tissues (e.g., meninges) reduce the strength of the induced magnetic field (Klomjai et al., 2015), therefore it is not possible to exceed a stimulation depth of maximum six centimeters (dependent on the type of stimulation coil) (Roth et al., 2007).The rhythmic application of TMS pulses at a certain frequency is designated as repetitive transcranial magnetic stimulation (rTMS). Via rTMS it is feasible to induce neural consequences lasting longer than the actual stimulation period – induction of changes in neuroplasticity, (Hallett, 2007, 2000; Siebner and Rothwell, 2003). The induced neurophysiological effects strongly depend on the used rTMS parameters e.g., different stimulation protocols induce distinct changes in cortico-spinal excitability (Rossini et al., 2015). Low frequency protocols (≤ 1 Hz) are capable to evoke decreases, whereas with high frequency protocols (≥ 5 Hz) it is possible to induce increases in cortical excitability (Fitzgerald et al., 2006; Hoffman and Cavus, 2002; Pascual-Leone et al., 1994). However, these implications do not always apply and should therefore only be seen as rough guidelines (Fitzgerald, 2002; Fitzgerald et al., 2006; Klomjai et al., 2015; Rossini et al., 2015). Likewise the number of administered TMS pulses, the used stimulation intensity (Fitzgerald, 2002; Gamboa et al., 2010; Halawa et al., 2018; Nettekoven et al., 2014; Padberg et al., 2002) as well as the intrinsic state of the brain prior to stimulation (Siebner et al., 2004; Silvanto and Pascual-Leone, 2008; Weisz et al., 2012) might be relevant for the effect rTMS causes. Further research also highlights the significance of gene polymorphisms like the brain-derived neurotrophic factor (BDNF) as a predictor for respondence to non-invasive brain stimulation because of its connection to synaptic and cortical plasticity (Antal et al., 2010; Fidalgo et al., 2014; Polanía et al., 2018). If rTMS is administered over several days or weeks, therapeutic effects take place conceivably caused via long-term potentiation (LTP) or long-term depression (LTD) of synaptic strength (Chervyakov et al., 2015; Hallett, 2000; Lefaucheur et al., 2014). In the clinical context, rTMS is applied for a broad spectrum of psychiatric and neurologic disorders like schizophrenia, stroke, anxiety disorders or multiple sclerosis (Lefaucheur et al., 2020, 2014). With respect to guidelines for the clinical usage of rTMS, low frequency rTMS applied over the left temporo-parietal cortex may be effective for the treatment of tinnitus (Lefaucheur et al., 2020).

(Text taken and modified from "Towards a Moment of Silence: Individualization of Acoustic Stimulation and Repetitive Transcranial Magnetic Stimulation in Tinnitus" by Stefan Schoisswohl; 10.5283/epub.45974)