EEG-triggered TMS reveals stronger brain state-dependent modulation of motor evoked potentials at weaker stimulation intensities

BackgroundCorticospinal excitability depends on the current brain state. The recent development of real-time EEG-triggered transcranial magnetic stimulation (EEG-TMS) allows studying this relationship in a causal fashion. Specifically, it has been shown that corticospinal excitability is higher during the scalp surface negative EEG peak compared to the positive peak of {micro}-oscillations in sensorimotor cortex, as indexed by larger motor evoked potentials (MEPs) for fixed stimulation intensity.\n\nObjectiveWe further characterize the effect of {micro}-rhythm phase on the MEP input-output (IO) curve by measuring the degree of excitability modulation across a range of stimulation intensities. We furthermore seek to optimize stimulation parameters to enable discrimination of functionally relevant EEG-defined brain states.\n\nMethodsA real-time EEG-TMS system was used to trigger MEPs during instantaneous brain-states corresponding to {micro}-rhythm surface positive and negative peaks with five different stimulation intensities covering an individually calibrated MEP IO curve in 15 healthy participants.\n\nResultsMEP amplitude is modulated by {micro}-phase across a wide range of stimulation intensities, with larger MEPs at the surface negative peak. The largest relative MEP-modulation was observed for weak intensities, the largest absolute MEP-modulation for intermediate intensities. These results indicate a leftward shift of the MEP IO curve during the {micro}-rhythm negative peak.\n\nConclusionThe choice of stimulation intensity influences the observed degree of corticospinal excitability modulation by {micro}-phase. Lower stimulation intensities enable more efficient differentiation of EEG {micro}-phase-defined brain states.