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We examined the potentially exploratory and performatory nature of postural sway. Subjects stood upright or leaned forward, with eyes open or closed. Postural data were analyzed using a statistical mechanics analysis of center of pressure (COP) trajectories, which examines the fractional Brownian nature of postural sway. Positive correlations (persistence) over short time scales are hypothesized to reflect exploratory behavior, and negative correlations (antipersistence) over long time scales are hypothesized to reflect performatory behavior. When leaning, subjects exhibited decreased levels of persistence (decreased correlation) and increased levels of antipersistence (increased correlation) than when upright. With eyes open, subjects showed decreased levels of persistence and decreased levels of antipersistence than with eyes closed. Effects of vision were more pronounced when leaning. Evidence for direction-specific exploration (based upon root mean square variability analysis) was considered. Task-specificity and trade-offs between biomechanical and task constraints in models of postural control were discussed.
Michael A. Riley, Suvobrata Mitra, and Michael T. Turvey are with the Center for the Ecological Study of Perception and Action, Department of Psychology, U-20 University of Connecticut, 406 Babbidge Road, Storrs, CT 06269. Thomas A. Stoffregen is with the Department of Psychology, University of Cincinnati, Cincinnati, OH 45221-0376.