Postural Sway and Sway-adaptation During Exposure to Optic Flow: the Effect of Stimulus Periodicity and Concurrent Cognitive Tasks

Stable upright stance is achieved through an active postural control process that requires the accurate integration of sensory feedback signals from the visual, graviceptive and proprioceptive systems. Previous studies have shown that this integration process may involve "reweighting", whereby the relative contributions of the various sensory signals are dynamically altered in order to minimize reliance on unreliable signals. In addition, evidence suggests that feedback control by itself cannot explain experimental observations of postural behavior. In light of these observations, the current study proposed that a predictive mechanism exists within the postural control system that can identify the highly repetitive pattern within a predictable sensory input, and can use this information to facilitate the sensory reweighting process. The objectives of the current work were to: (1) uncover experimental evidence of such a predictive mechanism, through examination of postural sway responses in healthy young adults exposed to various types of predictable and unpredictable moving visual scenes; (2) examine how this predictive mechanism manifested itself in individuals who were particularly reliant on visual sensory information; and (3) determine if this predictive mechanism was influenced by cognitive tasks, which are thought to interact with the sensory reweighting process.Data revealed that in healthy young adults predictable stimuli elicited improved sway performance compared to unpredictable stimuli, as indicated by significant decreases in both overall sway magnitude, and the time required for sensory reweighting to occur. This effect was enhanced during the performance of a concurrent cognitive task, but was not observed in visually dependent individuals, apparently due to an inability to perform sensory reweighting. Taken together, these observations support the existence of a predictive component to postural control that can alter the dynamic reweighting of sensory inputs during exposure to predictable stimuli. These findings may have implications for the design of experiments involving moving visual scenes, as well as for the treatment of individuals suffering from certain balance disorders