Spectral characteristics of visually induced postural sway in healthy elderly and healthy young subjects

Spectral analysis and time-frequency analysis were applied to anterior-posterior (A-P) center-of-pressure (COP) data collected during quiet stance and sinusoidal (0.25 Hz) moving visual scene perturbations from sixteen healthy elderly subjects and thirteen healthy young subjects, While the total energy of COP was larger in the elderly subjects than in the young subjects, energy-normalized spectra of the entire COP time series were similar between the two populations. Further time-dependent analysis of the spectral characteristics showed that the time-varying spectra were also similar, as revealed by time-frequency analysis, although some differences were observed. Specifically, time-dependent mean frequency and bandwidth responses to the stimulus showed similar increases in both young and elderly subjects with the introduction of the stimulus. Furthermore, time-frequency analysis showed that both groups exhibited an initial increase in sway at the stimulus frequency that then declined as the sinusoidal scene movement progressed. This reduction in sway during visual stimulation may indicate an "adaptation" to constant frequency visual perturbation that is invariant with age. This finding is in contrast to results reported recently for patients with vestibular impairments, who appear to lack such adaptation.