Perception of small whole-body perturbations during standing balance in humans

Like breathing, standing balance is mostly effortless but the motion related to maintaining an upright posture can be perceived consciously. Because standing balance involves subcortical processes, the motion perceived while maintaining standing balance may differ from the motion detected by the balance control system. Human subjects standing on a robotic balance simulator were exposed to small whole-body imposed motions while balancing or braced. When balancing, subjects perceived the opposite direction of the imposed motion at small velocities. For larger velocities, they perceived the direction of the imposed motion in both balancing and braced conditions. This suggests that the balance control system detects small whole-body perturbations and evokes corrective responses during standing balance even when the imposed motion is not consciously perceived. For motion imposed at small velocities, standing humans perceive the sensory consequences of their own compensatory balance reactions instead of the actual perturbation.