Monitoring Changes in TMS-Evoked EEG and EMG Activity During 1 Hz rTMS of the Healthy Motor Cortex

Abstract

Repetitive transcranial magnetic stimulation (rTMS) is a non-invasive brain stimulation technique capable of inducing neuroplasticity as measured by changes in peripheral muscle electromyography (EMG) or electroencephalography (EEG) from pre to post stimulation. However, temporal courses of neuromodulation during ongoing rTMS are unclear. Monitoring cortical dynamics via TMS-evoked responses using EMG (motor-evoked potentials; MEPs) and EEG (transcranial-evoked potentials; TEPs) during rTMS might provide further essential insights into its mode of action - temporal course of potential modulations. The objective of this study was to first evaluate the validity of online rTMS-EEG and -EMG analyses, and second to scrutinize the temporal changes of TEPs and MEPs during rTMS. As rTMS is subject to high inter-individual effect variability, we aimed for single-subject analyses of EEG changes during rTMS. 10 healthy human participants were stimulated with 1000 pulses of 1 Hz rTMS over the motor cortex while EEG and EMG were recorded continuously. Validity of MEPs and TEPs measured during rTMS was assessed in sensor and source space. Electrophysiological changes during rTMS were evaluated with model fitting approaches on a group- and single-subject level. TEPs and MEPs appearance during rTMS was consistent with past findings of single pulse experiments. Heterogeneous temporal progressions, fluctuations or saturation effects of brain activity were observed during rTMS depending on the TEP component. Overall, global neuronal brain activity increased over the course of stimulation. Single-subject analysis revealed inter-individual temporal courses of global neuronal activity. The present findings are in favor of dose-response considerations and attempts in personalization of rTMS protocols.Significance StatementRepetitive transcranial magnetic stimulation (rTMS) is a non-invasive brain stimulation method capable to induce neuroplasticity. Usually, changes in peripheral muscle or brain activity from pre-to-post stimulation serve as an indicator for TMS-related consequences. Monitoring temporal changes of these indicators during rTMS might provide essential information on how many pulses are necessary to induce modulation respectively change cortical excitability. In the present study, we show different temporal progressions, fluctuations, and effect plateaus of cortical excitability dependent on the indicator of interest. In general, global brain activity increased with the number of applied pulses. Further, inter-individual differences in the temporal course of global brain activity were observed. Our results suggest a consideration of dose-response dependencies and advocates approaches in customizing rTMS protocols.