Does Pre-Sensorimotor Brain Activity Modulate TMS-Evoked Potentials?

Transcranial magnetic stimulation combined with electroencephalography (TMS-EEG) provides a direct measure of cortical responses to local brain perturbations and offers a powerful approach for investigating brain-state-dependent neural dynamics. However, it remains unclear to what extent ongoing sensorimotor brain activity influences TMS-evoked potentials (TEPs). To address this question, thirty healthy participants performed kinesthetic motor imagery of left- and right-index finger tapping while single-pulse TMS was applied over the left primary motor cortex. 64-channel EEG was recorded to characterize sensorimotor oscillatory activity and TEPs. Time-frequency analyses quantified event-related desynchronization (ERD) in the mu (8-12 Hz) and beta (13-30 Hz) frequency bands, while TEP amplitudes were examined within early latency windows (10-65 ms). Motor imagery induced robust bilateral mu- and beta-band ERD, confirming successful engagement of the sensorimotor network. However, ERD patterns were highly similar between left- and right-hand motor imagery, and neither frequentist nor bayesian analyses identified differences in TMS-evoked potentials between imagery conditions. Source reconstruction demonstrated rapid propagation of TMS-evoked activity from the stimulated motor cortex to distributed bilateral motor regions. Correlation analyses revealed that greater pre-stimulus beta-band ERD was associated with larger P60 amplitudes. Because the instructed motor imagery conditions did not produce distinguishable pre-stimulus brain states, an exploratory trial-by-trial analysis was subsequently performed to account for naturally occurring variability in sensorimotor lateralization to investigate the effect of variability in ERD on TRP. This analysis revealed an overall effect of trial-by-trial brain state on TEP amplitudes