Motor Intention Drives Corticospinal Facilitation during Interception Planning

Intercepting a moving target requires integrating visual motion information with motor preparation, yet how action intention and gaze strategy jointly shape corticospinal excitability during interception planning remains unclear. We applied single-pulse transcranial magnetic stimulation over primary motor cortex to probe corticospinal excitability while participants either prepared to intercept a moving target or passively viewed its motion. Targets moved at one of two speeds, and participants judged target speed relative to a reference after each trial. Across blocks, participants were instructed to either smoothly pursue the target or fixate their eyes on the interception zone. Motor-evoked potentials were elicited at baseline, after task instruction, early during target motion, or shortly before target arrival. Task goal and gaze strategy interacted to influence both perceptual judgments and corticospinal excitability. Passive viewing led to overestimation of target speed and was associated with sustained suppression of corticospinal excitability, particularly during fixation. In contrast, preparing to intercept produced a robust facilitation of corticospinal excitability immediately before movement onset, regardless of gaze strategy. Smooth pursuit improved perceptual accuracy and reduced interception timing error compared with fixation but did not independently influence corticospinal facilitation. These findings demonstrate that the intention to act is the primary determinant of the transition from corticospinal suppression to facilitation during interception planning, whereas eye movements modulate perceptual estimates and support behavioral performance.