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). We investigated how smooth pursuit eye movements (SPEM), which are slow eye movements used to track moving objects, influence LLRs of the upper and lower limb during mechanical interactions with moving objects. Seventeen participants stabilized their limb in anticipation of a collision with a virtual object approaching at 25 cm/s while standing. This task occurred while subjects either visually pursued the object or fixated a central location. Mechanical perturbations were applied at two time points: approximately 200ms and 60ms before the anticipated collision. On a random subset of trials, the robot applied a mechanical perturbation either 200ms (early) or 60ms (late) before the anticipated collision. As in previous studies LLRs were observed in leg muscles to upper limb displacement. Moreover, the leg LLRs were modulated by gaze, being larger during pursuit than fixation but only during late perturbations. This timing-specific modulation aligns with previous reports of policy transitions in feedback control roughly 60ms before impact. Upper limb LLRs were not impacted by gaze indicating a prioritization on postural control circuits. This work extends our understanding of the neural mechanisms underlying sensorimotor integration and highlights the sophisticated nature of the human motor control system in coordinating eye movements with whole-body postural responses.