Predictable transcranial magnetic stimulation suppresses corticospinal excitability: a TMS experiment

Motor skill learning plays a crucial role in human functioning and is often studied using transcranial magnetic stimulation (TMS) to assess corticospinal excitability (CSE). CSE, which reflects the motor system's responsiveness, is compared across experimental conditions to determine whether a factor facilitates or inhibits motor skill learning. However, variability in stimulation predictability may confound interpretation of CSE measures. In this study, we examined the impact of TMS predictability on CSE by comparing motor-evoked potential (MEP) responses under predictable and unpredictable conditions. Twenty right-handed participants underwent TMS-EMG recordings while observing either a predictable visual cue (a moving white bar) or an unpredictable visual cue (a static white bar). MEP amplitudes were recorded from the first dorsal interosseous (FDI) muscle and normalized across participants. Results showed that unpredictable TMS produced significantly larger MEP amplitudes compared to predictable stimulation. The findings suggest that the predictability of TMS substantially modulates CSE, potentially confounding the results in previous TMS studies on action observation and motor facilitation. Ensuring consistent stimulation predictability across experimental conditions is therefore essential for accurately interpreting TMS-induced CSE changes.