Brain-State-Resolved Consistency of Corticospinal Responses with EEG–TMS

Background

Transcranial magnetic stimulation (TMS) over the primary motor cortex (M1) elicits motor-evoked potentials (MEPs), a neurophysiological marker of corticospinal excitability. Ongoing brain activity at the time of stimulation, such as the phase and power of the sensorimotor mu rhythm (8–13 Hz), has a significant impact on MEP amplitudes. However, it remains unclear whether these endogenous excitability states also influence the consistency of MEP amplitudes across repeated trials.

Objectives

We investigated whether instantaneous mu dynamics modulate not only the magnitude but also the consistency of corticospinal responses to TMS.

Methods

Twenty-nine healthy participants received 1200 single TMS pulses over the left M1 during simultaneous EEG recording. Trials were stratified based on pre-stimulus mu power, phase, and interhemispheric M1–M1 functional connectivity. Brain-state-resolved MEP variability was quantified using the coefficient of variation (CV) within subsets of trials defined by similar pre-stimulus mu dynamics.

Results

Trial subsets characterized by high mu power or high M1–M1 functional connectivity were associated with reduced MEP variability, indicating more consistent corticospinal output. In contrast, the mu phase did not significantly influence response consistency. Brain-state-resolved MEP variability showed greater stability across sessions compared to MEP variability estimated from random trial subsampling.

Conclusions

Pre-stimulus mu dynamics shape not only magnitude but also consistency of corticospinal responses to TMS. We show that corticospinal response consistency reflects a structured, brain-state-dependent property of the sensorimotor network. These findings contribute to our mechanistic understanding of brain-state-dependent neuromodulation and may be leveraged to reduce variability and improve efficacy to TMS.

Highlights

• Ongoing sensorimotor mu dynamics shape both magnitude and consistency of MEPs.

• Trial subsets characterized by high mu power were associated with reduced MEP variability.

• Mu phase modulated MEP amplitude but did not influence MEP consistency.

• Brain-state-resolved estimates of MEP variability were more reliable across sessions.

• Future TMS protocols may reduce effect variability by targeting stable excitability states.