This study investigated how reward probability affects corticospinal excitability during a rewarded time-sensitive task. Participants pressed buttons based on visual cues and feedback ("10-yen" or "no pay") while mentally estimating a 2.5 s interval. Transcranial magnetic stimulation (TMS) measured motor evoked potential (MEP) amplitudes. Results showed that the variability of MEP amplitudes shifted from a random process at rest to an autoregressive process during the task. Low reward probability increased MEP magnitude, while high reward probability transformed MEP variability. This indicates that reward probability differentially alters the magnitude and variability of expectation-related corticospinal excitability.
