Deterministic and stochastic postural processes: effects of task, environment, and age.

Upright standing is always environmentally embedded and typically co-occurs with another (suprapostural) activity. In the present study, the authors investigate how these facts affect postural dynamics in an experiment in which younger (M age = 20.23 years, SD = 2.02 years) and older (M age = 75.26 years, SD = 4.87 years) participants performed a task of detecting letters in text or maintaining gaze within a target while standing upright in a structured or nonstructured stationary environment. They extracted the coefficients of drift (indexing attractor strength) and diffusion (indexing noise strength) from the center of pressure (COP) time series in anteroposterior (AP) and mediolateral (ML) axes. COP standard deviation decreased with drift and increased with diffusion. The authors found that structure reduced AP diffusion for both groups and that letter detection reduced younger SDAP (primarily by diffusion decrease) and increased older SDML (primarily by drift decrease). For older and younger participants, ML drift was lower during letter detection. Further, in older letter detection, larger visual contrast sensitivity was associated with larger ML drift and smaller SDML, raising the hypotheses that ML sway helps information detection and reflects neurophysiological age.