Visual field motion effects on the production of manual forces and displacements.

BACKGROUND: We have previously shown that subjects produce exaggerated arm forces when exposed to three times the normal gravitational acceleration (+3 Gz), and that this deficit is not related to direct mechanical effects, faulty proprioception, or increased cognitive load. Here we investigate whether it is related to vestibular activity. METHODS: Novice subjects observed a stationary, upward or downward moving visual field while producing pretrained arm forces (Exp. A, N = 12) or displacements (Exp. B, N = 12); a control group produced no motor responses and their arm EMG was registered (Exp. C, N = 12). RESULTS: Produced forces and EMG were higher with the moving than with the stationary field, irrespective of field direction (initial force +42%; peak force +20%, biceps brachii EMG +21%). Produced displacements were comparable with the moving and stationary field. DISCUSSION: The present pattern of findings is similar to that yielded previously in +3 Gz, which supports the existence of a common underlying mechanism. Specifically, we suggest that +3 Gz and vertical field motion stimulate the vestibular system, and that the observed exaggeration of produced force is due to vestibular modulation of descending volitional motor commands. The fact that displacements were not affected by +3 Gz and moving visual fields would then indicate that forces and displacements are controlled through distinct pathways which interact differently with the vestibular system.