Sensory Organization Test (SOT) - will be used to measure change how well participants maintain balance under progressively more difficult test conditions, which either disrupt or remove visual and proprioceptive feedback. Visual and proprioceptive environments will be altered systematically for fixed support and sway-referenced support and surround conditions, and under normal (eyes open), absent (eyes closed), and sway-referenced vision (eyes sway-referenced). Under sway-referenced conditions, the platform on which the subject stands and/or the visual surround also move proportionally to their AP sway. During SRC the floor and visual surround are controlled by a servomechanism (Gain=1.0) to follow movements of the subjects' center of gravity. Sway-referencing eliminates accurate somatosensory and/or visual information used to orient the body. Performance under sway-referenced visual conditions shows the participant's ability to suppress conflicting visual inputs and to rely on alternative systems for maintaining equilibrium (table 1).