Head engagement during visuomotor tracking is determined by postural challenges and aging

Vision is important for various tasks, from visually pursuing moving objects to maintaining balance. People obtain visual information through eye movements performed either alone or in combination with head movements. Even when isolated eye movements can accommodate the amplitude of the desired gaze shift, humans still perform head movements, as they provide sensory signals that can lead to improved gaze estimates. However, head movements also create mechanical torques that could disturb balance. We here examined whether head engagement is determined by postural challenges when performing a visual pursuit task. Young participants tracked a target moving horizontally along different amplitudes (15°, 42°, 95°), while they were seated, standing on a firm and an unstable surface. Head engagement, which was the ratio of head rotation relative to the target’s amplitude, was larger when standing than sitting, but no systematic differences were found between firm and unstable surfaces. To further explore the interplay between head engagement and postural challenges, we conducted a second experiment where young and older participants performed a similar task, but now they were either allowed to move their head or instructed to limit head movements. Both tracking accuracy and postural sway increased when engaging the head. When allowed to move their head naturally, young participants engaged their head more when standing than sitting, but older adults reduced their head movements with more demanding postures. We suggest that head movements in young adults facilitate visual tracking, while limited head movements in older adults are selected to preserve balance.