Time-varying characteristics of visually induced postural sway.

To study potential time-varying dynamics of postural sway as measured via center-of-pressure (COP) under the feet, we applied time-frequency analysis to COP data from ten vestibularly impaired subjects and 13 nonimpaired controls, during quiet stance and in response to visual perturbation. This analysis revealed that 1) the spectral characteristics of COP change over time; 2) there are time-dependent and frequency-dependent differences in COP between impaired and nonimpaired populations during visual perturbation, and 3) there is no difference in COP during quiet stance (eyes opne) between impaired and nonimpaired populations for the parameters investigated. A novel finding of this research is that controls appear to adapt to constant frequency visual perturbation, while vestibularly impaired subjects do not. This difference could not have been observed with conventional Fourier analysis, which is commonly used in COP data analysis, because time is not a parameter of the spectrum and adaptation is, by nature, a time-varying process. These results suggest that time-frequency analysis of COP data is useful for studying temporal dynamics of postural control, and in particular the differences between vestibularly impaired subjects and healthy controls during visual perturbation.