EEG Responses to Transcranial Magnetic Stimulation with Different Intensities over the Motor Cortex

Transcranial Magnetic Stimulation (TMS) is a non-invasive brain stimulation technique that can modulate neural activity. While previous studies have primarily employed single-intensity TMS protocols, the impact of varying stimulation intensities (SI) on TMS-induced brain activity remains poorly understood. This study aims to investigate the characteristics of electroencephalogram (EEG) responses under different TMS intensities. Twenty-four healthy participants underwent TMS at varying intensities (sham stimulation, 80%, 100%, and 120% of the resting motor threshold (rMT)) over the left motor cortex, with concurrent EEG recording during stimulation. TMS-induced neural changes were comprehensively evaluated through multiple parameters, including EEG amplitude, oscillatory power, and phase characteristics, using TMS-evoked potentials (TEPs), event-related spectral perturbation (ERSP), and inter-trial coherence(ITC) analyses. The results revealed significant intensity-dependent changes in brain responses across multiple dimensions of analysis. Specifically, weaker stimulation (80% rMT) primarily induced phase-specific changes (ITC), whereas stronger stimulations (100% and 120% rMT) elicited more comprehensive neural responses, including significant alterations in TEP, ERSP and ITC. These findings demonstrate a clear dose-response relationship between TMS intensity and the magnitude of neural modulation, with higher intensities producing more pronounced changes in brain activity.