The human postural control system serves two main functions: to maintain a stable upright posture, and to fix the orientation and position of the body segments that serve as a reference frame for perception and action. The question this research addresses is how various sources of information are integrated to form postural goals, how these goals are stabilized against perturbations, and how the goals are flexibly reorganized when the information or tasks change. In particular, the proposed research focuses on the contribution of vision to postural stability during standing and locomotion. In the experimental studies human participants will view computer-generated visual displays while their postural responses are monitored. The displays simulate simple scenes, such as a tunnel or hallway. The scenes are shown through a head-mounted display and postural responses are measured with a system that detects the position of infra-red markers on body segments of the participant. The research is motivated by a conceptualization of the postural control system as a two-level dynamical system. The low-level dynamics achieves postural stability and constant velocity of locomotion. The high-level dynamics achieves flexibility by tuning the parameters of the low-level dynamics to environmental constraints. This division between levels achieves flexibility by piecing together simple control strategies. A new contribution of this research is a concrete proposal as to how the high-level controller influences the low-level controller. The results of the studies will add to our understanding of human postural control, and may also point the way toward remedies for impairments of postural control (e.g., falling in the elderly).
