Visual ambiguity constrains motor prediction and corticospinal excitability in humans

Action observation (AO) provides sensory evidence that can engage predictive motor coding, yet how visual uncertainty shapes this process across behaviour and corticospinal excitability (CSE) remains unclear. We combined a behavioural timing task with AO-TMS across two experiments. In Experiment 1 (n = 56), uncertainty was manipulated parametrically through movement naturalness, partial blackout and shape congruence in goal-directed action videos, while participants produced time-locked responses to a defined target event. Precision was indexed by absolute response-time error. In Experiment 2 (n = 20), single-pulse TMS was delivered over left M1 during grasp and release phases of AO scenes selected from an independent cohort. FDI MEPs indexed CSE, with strict pre-EMG screening. Behaviorally, reduced naturalness, added occlusion and incongruent shapes each degraded temporal precision. Neurophysiologically, AO elicited a reliable overall facilitation of CSE relative to baseline, whereas condition-specific modulations associated with uncertainty were small and did not survive multiple-comparison correction. These results indicate that while perceptual uncertainty robustly affects behavioural prediction, corresponding changes in motor resonance are comparatively subtle under the present design. The study provides a reproducible AO-TMS protocol and controlled stimulus set that can support future high-powered tests of how uncertainty shapes predictive motor processes.