A technique (recurrence quantification analysis; RQA) for analyzing center
of pressure (COP) signals is presented and applied to data obtained by havi
ng 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 orientation
s affects COP nonstationarity. Implications of these findings and the sensi
tivity of RQA to subtle time-evolutionary properties of the COP are discuss
ed. (C) 1999 Elsevier Science B.V. All rights reserved.