The transcranial static magnetic stimulation, a new non-invasive brain neuromodulatory technique: using TMS-EEG to understand its mechanisms.

Transcranial static magnetic stimulation (tSMS) was recently added to the family of non-invasive brain stimulation techniques. It offers promising perspectives being the only neuromodulatory technique that can produce changes in cortical excitability without inducing electric currents. Recent studies demonstrated that the application, for different time intervals, of static magnetic fields on the primary motor cortex (M1) can produce both short and long-lasting effects, consisting in a reduction of motor cortex excitability. Nevertheless, the mechanisms underpinning this new neuromodulatory technique are still mostly unclear. In this study, we use a combination of Transcranial Magnetic Stimulation (TMS) and Electroencephalography (EEG) to explore local and global cortical excitability modulation after a 15 minutes session of tSMS on the supplementary motor area (SMA). Single pulse TMS was delivered over the right SMA, before and after the tSMS sessions, while EEG data were recorded from 60 channels. For each session, indexes of global and local cerebral excitability were obtained, computed as global and local mean field power (Global Mean Field Power, GMFP and Local Mean Field Power, LMFP) on mean TMS-evoked potentials (TEPs), for three temporal windows: 0-50, 50-100 and 100-150 milliseconds. The global index was computed on all 60 channels. The local indexes were computed in different clusters correspondent to the SMA and to the left and right M1 and premotor cortex (PMC). We hypothesize a change in cortical excitability after the tSMS session, not only in the area directly stimulated, but also in the regions functionally and structurally connected to it. These results could prompt the use of this simple, portable and low-cost technique even in clinical and rehabilitation protocols.