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Alen Hajnal, Deanna Rumble, John F. Shelley-Tremblay, Wei Liu; Postural Sway is Affected by Visually Perceived Geographical Slant. Journal of Vision 2013;13(9):211. doi: https://doi.org/10.1167/13.9.211.
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Past research has shown that visual inspection of nearby targets is tied to more stable posture during quiet stance (Bonnet, Temprado, & Berton, 2010). The current study seeks to answer an even more basic question, namely, how ground surface inclination (geographical slant) influences postural stability. The present contribution shows that perceived geographical slant may significantly influence postural stability even in the absence of definite visual targets. In two experiments participants stood on a force platform that measured center of pressure (COP) during quiet stance while looking at a rigid surface of varying geographical slants. Using an otherwise identical procedure, participants in the second experiment also provided verbal estimates of the steepness of the surface in degrees. Several measures of postural stability offered converging evidence that postural sway gradually increased as geographical slant decreased to zero (horizontal ground). Specifically, the anterior-posterior component (y-coordinate) of COP was sensitive to changes in surface slant. Both the range and the standard deviation of COPy showed the same statistically significant trend of increased variability with decreasing slant angles in both experiments. The area of the ellipse covering COP sway (based on a principal components analysis) showed the same tendency: ellipse area got larger for smaller slants. Finally, the nonlinear fractal dynamics of postural sway, as measured by the Hurst exponent of COPy pointed in the same direction: more fractal patterns, known to be correlated with increased muscle activity and decline in postural stability were measured for shallower surface slants. There were no effects of Experiment in any of the measures, suggesting that explicit perception (verbal estimation) of slant does not bias postural stability above and beyond the effects of visual environment. Future studies are planned to tease apart the implicit effects of distance and slant to gauge the overall contribution of visual environment to postural stability.
Meeting abstract presented at VSS 2013
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