Dynamics of cortical excitability in stimulus-response mapping for overt and covert movements is locked to visual stimulus: an LRP-TMS study.

Visual perception enables goal-directed movement control by mapping sensory input onto motor representations. While neural mechanisms of visuomotor integration have been extensively studied, the temporal dynamics of this process during real and mentally simulated movements remain poorly understood, particularly regarding stimulus-driven versus response-driven motor cortex contributions. We used transcranial magnetic stimulation (TMS) and electroencephalography (EEG) to investigate cortical activity during physical and imagined hand movements in a stimulus-response task. Single-pulse TMS was delivered to the primary motor cortex at 100, 200, and 400 ms following visual stimulus presentation to probe corticospinal excitability. Motor cortex facilitation during early preparation was found to be stimulus-locked rather than motor response-locked, indicating that visual cues drive initial motor cortex activation. Both motor execution (ME) and kinesthetic motor imagery (kMI) showed similar facilitation dynamics during this early stage, supporting functional equivalence during preparatory processing. However, ME and kMI diverged at later response stages: ME showed elevated excitability whereas in kMI it returned to baseline. EEG analyses confirmed this dissociation at later stages. Notably, kMI did not produce significant hemispheric lateralization, whereas ME generated robust lateralized readiness potentials whose duration correlated with pre-response motor excitability and behavioral performance. These findings challenge conventional response-driven conceptualizations of motor imagery and highlight stimulus-driven mechanisms in visuomotor processing during both overt and imagined movements.