Smooth pursuit eye movements contribute to long-latency reflex modulation in the lower extremity

Somatosensory-mediated reactions play a fundamental role in adapting to environmental changes, particularly through long-latency responses (LLRs), rapid corrective muscle responses (50–100 ms) following limb perturbations that account for limb bio-mechanics and task goals. We investigated how smooth pursuit eye movements (SPEMs), which are slow eye movements used to track moving objects, influence LLRs of the upper and lower limb during mechanical interactions with moving objects. In the first experiment, participants stood and stabilized their arm against a colliding virtual object. This occurred while subjects either visually pursued the moving object or fixated a central location. The robot occasionally applied a mechanical perturbation to the arm either 200 ms or 60 ms before the anticipated collision. As in previous studies, LLRs were observed in leg muscles to a perturbation of the upper limb. Moreover, leg LLRs were modulated by gaze, being larger during pursuit than fixation, but only during the late perturbations. This timing-specific modulation aligns with previous reports of policy transitions in feedback control roughly 60 ms before impact. Upper limb LLRs were not significantly impacted by gaze. This lack of modulation could reflect the context of upright stance, so we conducted a second experiment that was the same in all ways except that the subjects remained seated. Again, the upper limb LLRs were not impacted by gaze. The selective impact of gaze modulation on stance control highlights the sophisticated nature of coordinating eye movements, arm control, and whole body postural responses. NEW & NOTEWORTHY Smooth pursuit eye movements are used to track moving objects. We show for the first time that smooth pursuit eye movements contribute to modulation of long-latency reflexes in the lower limb during virtual object interactions in upright stance. These findings suggest a neurophysiological link between predictive control of eye movements and feedback control of upright stance.