Transcranial Evoked Potentials are Minimally Influenced by Afferent Feedback From Peripheral Muscle Contraction

Transcranial magnetic stimulation with electroencephalography (TMS-EEG) investigates cortical reactivity, yet the contribution of sensory feedback from muscle twitches to the recorded signals remains debated. We combined M1 TMS-EEG with electromyography, accelerometry, and peripheral electrical stimulation to dissociate central and peripheral contributions. We compared EEG responses across trials yielding large versus small motor outputs, while peripheral stimulation isolated pure afferent components. Bayesian analysis revealed that larger motor outputs covaried with distinct EEG morphologies uniformly across all time windows-including those too early for peripheral feedback-indicating that spontaneous fluctuations in baseline motor cortex excitability drive both responses. Conversely, isolated muscle contraction produced delayed (> 65 ms), markedly smaller EEG responses with distinct spatiotemporal profiles. We conclude that sensory feedback from muscle twitches contributes minimally to TMS-evoked EEG responses, which instead primarily reflect the baseline excitability state of the stimulated cortex.