The importance of perceived relative motion in the control of posture.

Two experiments investigated the role of optic flow in controlling posture. Both experiments measured postural sway in two virtual environments with different 3-D structure but the same optic flow. Observers attempted to maintain balance on one foot while viewing an object that appeared either rigid with respect to the environment or that appeared to move concomitantly with head movements. The apparent object motion concomitant with head motion was achieved by changing the perceived, but not physical, depth of the object. For both objects, the optic flow information was the same and only depth information was varied. Observers showed a decrease in stability (as measured by head sway) when viewing the object that appeared to move, suggesting that perceived relative motion, not optic flow, signals self-motion to the postural control system.