Repetitive transcranial magnetic stimulation to the motor cortex leads to a sequential increase in phase synchronization and power of TMS-evoked electroencephalographic recordings

BackgroundHigh-frequency (10 Hz) repetitive transcranial magnetic stimulation (rTMS) to the primary motor cortex (M1) is used to treat several neuropsychiatric disorders, but its main mechanism of action remains unclear. ObjectiveTo probe four cortical hubs used for rTMS (M1; dorsolateral-prefrontal cortex, DLPFC; anterior cingulate cortex, ACC; posterosuperior insula, PSI) with TMS coupled with high-density electroencephalography (TMS-EEG) and measure cortical excitability and oscillatory dynamics before and after active and sham rTMS to M1. MethodsBefore and immediately after active or sham M1-rTMS (15 min, 3,000 pulses at 10 Hz), single-pulse TMS evoked EEG were recorded at the four targets in 20 healthy individuals. Measures of cortical excitability and oscillatory dynamics were extracted at the main frequency bands ( [8-13 Hz], low-{beta} [14-24 Hz], high-{beta} [25-35 Hz]). ResultsComparing active and sham M1 rTMS, M1 TMS-EEG demonstrated an increase in high-{beta} synchronization in electrodes around M1 stimulation area and remotely in the contralateral hemisphere (p=0.026). The increase in high-{beta} synchronization (48-83 ms after TMS-EEG stimulation) was succeeded by an enhancement in low-{beta} power (86-144 ms after TMS-EEG stimulation) both locally and in the contralateral hemisphere (p=0.006). No significant differences were observed in TMS-EEG responses probing DLPFC, ACC, or PSI. ConclusionM1-rTMS engaged a sequence of enhanced phase synchronization, followed by an increase in power occurring within M1, that spread to remote areas and was measurable after the end of the stimulation session. These results are relevant to understanding the M1 neuroplastic effects of rTMS and associated changes in cortical activity dynamics.