Fusion of visuo-ocular and vestibular signals in arm motor control

Keeping the finger pointing at an Earth-fixed object during body displacements can be achieved if compensatory arm movements counteract the effect of the rotation on the hand's position in space. Here we investigated the fusion of signals that originated from systems having different neurophysiological properties (i.e., the visuo-oculomotor and vestibular systems) in the production of such compensatory arm movements. To this end, we analyzed the subjects' performance in three conditions that differed according to the information they provided about relative target-body motion. This information originated either from the vestibular or visuo-oculomotor system, or from a combination of the two. To highlight the integration of visuo-oculomotor and vestibular signals, we compared the arm response to motion frequencies presumed to allow or not to allow optimal vestibular and oculomotor responses. When they could be used in isolation, the ocular signals allowed long-latency but precise kinematics control of the arm movement, whereas vestibular signals allowed accurate motor response early in the rotation but their contribution declined as body rotation developed. Optimal performance was obtained throughout the whole movement and for all rotation frequencies when the visuo-oculomotor and vestibular signals could be used together. This increase in hand-tracking performance could not be explained by a unimodal model or an additive model of vestibular and ocular cues, even when using weighted signals. Rather, the results supported a functional model in which vestibular and visuo-oculomotor signals have different influences on the temporal and spatial aspects of hand movement compensating for body motion.