Oscillatory TMS-EEG-Responses induced by repetitive transcranial magnetic stimulation

Transcranial magnetic stimulation (TMS) is a noninvasive tool for interfering with brain activity. In transcranial magnetic stimulation studies, the stimulus intensity (SI) is usually normalized to the resting motor threshold (rMT), which produces a muscle response in 50% motor cortical (M1) stimulation. Because rMT is influenced by spinal cord excitability and coil-to-cortex distance, responses recorded from the cortex rather than peripheral muscles provide a more accurate state of cortical synchronization. Combining transcranial magnetic stimulation with electroencephalography allows measurement of whole brain cortical responsiveness to transcranial magnetic stimulation. We quantified changes in EEG oscillatory power and phase induced by transcranial magnetic stimulation using event-related spectral uptake (ERSP) and inter-trial coherence (ITC). We investigated the SI dependence of ERSP and ITC responses by stimulating the dominant M1 of 24 healthy volunteers using single-pulse TMS at 80% rMT, 100% rMT, and 120% rMT. We found SI-dependent ERSP and ITC responses in M1, the most pronounced of which was a broadband (1 -48 Hz) early ITC response, which is consistent with changes in SI. Our findings reveal the potential of oscillatory EEG responses as a measure of the synchronization status of cortical responses to SI.