Closed-loop adaptation of transcranial magnetic stimulation intensity with electroencephalography feedback

The effects of transcranial magnetic stimulation (TMS) on motor evoked potentials (MEPs) depend on brain state and stimulation intensity.

We implemented and evaluated a novel method for real-time TMS intensity adaptation during phase-specific stimulation.

Twenty-four participants underwent four randomized sessions of 1600 single TMS pulses each, examining four combinations of real-time 24 Hz phase-targeting (rising or falling flank of the oscillatory cycle) and online intensity adaptation (inverse or proportional scaling of stimulation intensity relative to pre-pulse oscillatory power) between 100% and 120% of the resting motor threshold, controlled by the oscillatory power of the stimulated motor cortex at the same frequency.

MEPs increased for 30 min in all conditions. The time-dependent increase was primarily modulated by the adaptation paradigm. Targeting the rising flank with an inverse adaptation paradigm produced the largest MEP increase (31 ± 5%). Across preprocessing variants, the direction of paradigm-dependent effects remained consistent across filtered datasets, supporting robustness of the adaptation effect to analytic choices.

Online adaptation of stimulation intensity during brain state-dependent TMS is a viable approach to enhance MEPs, suggesting stimulation intensity as a dynamic control parameter for aligning stimulation strength with underlying circuit states. This framework may support more individualized, state-contingent neuromodulation in both basic and clinical applications.