Vestibular guidance of active head movements.

Vestibular sensors provide precise and timely information about head velocity in space. It is well established that this information is used to stabilize eyes, head and body against movements from outside, i.e., passive movements. Here, we investigate whether vestibular information also helps to monitor and guide active head movements during gaze shifts. We measured head movements during large gaze shifts toward briefly flashed targets in humans with complete vestibular loss (vestibular subjects) and in healthy controls before and after increasing their head moment of inertia. Whereas normally head movements oscillate neither in vestibular subjects nor in controls, the increase in head moment of inertia caused marked head oscillations only in vestibular subjects. We conclude that vestibular information plays an important role in the on-line guidance of active head movements and helps to correct for unexpected changes such as additional torque imposed by an increase in moment of inertia.