Recurrence quantification analysis of postural fluctuations.

A technique (recurrence quantification analysis; RQA) for analyzing center of pressure (COP) signals is presented and applied to data obtained by having participants stand with the head forward or sideways and with eyes open or closed. RQA is suitable for short, nonstationary signals and quantifies dynamical (deterministic) structure and nonstationarity. Results indicated that vision affects the deterministic structure (in degree and complexity) of COP motions and that differential optical flow structure (radial versus lamellar flow) induced by spontaneous sway under different head orientations affects COP nonstationarity. Implications of these findings and the sensitivity of RQA to subtle time-evolutionary properties of the COP are discussed.