Optical Push by Geographical Slant Affects Postural Sway

Alen Hajnal, Deanna Rumble, John F. Shelley-Tremblay, Wei Liu

Research output: Contribution to journalArticlepeer-review

2 Scopus citations


This study shows that perceived geographical slant affects postural stability. In 2 experimental conditions participants stood on a force platform that measured center of pressure (COP) during quiet stance while looking at a rigid, flat ramp surface of varying geographical slants. Using an otherwise identical procedure, participants in the second condition also provided verbal estimates of the steepness of the surface in degrees. Several measures of postural stability offered converging evidence that COP sway gradually increased as geographical slant decreased to 0 (horizontal ground). Specifically, COP was sensitive to changes in surface slant. Both the range and the standard deviation of COP showed the same trend of increased variability with decreasing geographical slant angles in both conditions. The area of the ellipse covering COP sway (based on a principal components analysis) showed the same tendency: ellipse area got larger for smaller, more horizontal slants. Nonlinear fractal dynamics of COP sway, as measured by the Hurst exponent of COP, 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 verbal estimates on any of the measures, suggesting that explicit awareness of slant does not bias postural stability above and beyond the effects of visual environment.

Original languageEnglish (US)
Pages (from-to)283-300
Number of pages18
JournalEcological Psychology
Issue number4
StatePublished - Oct 19 2014
Externally publishedYes

ASJC Scopus subject areas

  • Social Psychology
  • Computer Science(all)
  • Ecology, Evolution, Behavior and Systematics
  • Experimental and Cognitive Psychology


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