g. > 30 ms) is most probably related to physiological response. Oscillations induced by TMS have been reported in previous studies. Paus et al. (2001) observed that single pulses over M1 induced a brief period of synchronized activity in the beta range within the vicinity of the stimulation
site. Fuggetta et al. (2005) further observed that oscillations in the alpha and beta ranges were induced, for supra-threshold stimulation of M1, over the motor, premotor and parietal cortex ipsilateral to the stimulation Target Selective Inhibitor Library ic50 site. It was suggested that either the pulse activated ‘idling neurons’ that began to oscillate with alpha and/or beta frequencies, or more probably, that the TMS pulse synchronized spontaneous activity of a population of neurons (resetting hypothesis, Paus et al., 2001; Fuggetta et al., 2005; Van Der Werf & Paus, 2006), via a local (cortical) pacemaker or a thalamic pacemaker (Fuggetta et al., 2005). In addition, an alteration Forskolin cell line of inhibitory
mechanisms might also play a role (Brignani et al., 2008). The oscillations induced by single-pulse TMS might be of physiological nature and reveal the ‘natural rhythms’ of different regions (Rosanova et al., 2009). Indeed, when stimulated, each region tended to preserve its own natural frequency (alpha over the occipital cortex, beta over the parietal and fast beta/gamma over the frontal). Based on these previous studies, we suggest that each single pulse aligns the phase of active, but non-synchronized, oscillators (resetting hypothesis). Within this framework, two mechanisms can explain our results on the effect of cTBS. An increase (respectively a decrease) of TMS-induced oscillations after cTBS could reveal an increase (respectively a decrease) in the number of active oscillators at baseline (i.e. before the single-pulse TMS), while the percentage of synchronization between these oscillators Immune system remains unchanged.
Alternatively, the same observation can be related to a decrease (respectively increase) of percentage of synchronization at baseline (i.e. before the single-pulse TMS) while the number of active oscillators remains unchanged (see Fig. 7). In other words, cTBS might affect the number of active oscillators without affecting their relative synchronization, or it might alter the relative synchronization of an unchanged number of oscillators. In fact, the hypothetical cTBS effects on the number of active oscillators and on the percentage of synchronization are not mutually exclusive, but as discussed below, the analysis on cTBS modulation of eyes-closed EEG provides evidence in support of the second scenario. We found that cTBS tends to decrease power in the high beta band, and relatively increases power in theta band during eyes-closed resting.