IDENTIFICATION OF THE NONLINEAR STATE-SPACE DYNAMICS OF THE ACTION-PERCEPTION CYCLE FOR VISUALLY INDUCED POSTURAL SWAY

Human subjects standing in a sinusoidally moving visual environment di splay postural sway with characteristic dynamical properties. We analy zed the spatiotemporal properties of this sway in an experiment in whi ch the frequency of the visual motion was varied, We found a constant gain near 1, which implies that the sway motion matches the spatial pa rameters of the visual motion for a large range of frequencies. A line ar dynamical model with constant parameters was compared quantitativel y with the data, Its failure to describe correctly the spatiotemporal properties of the system led us to consider adaptive and nonlinear mod els. To differentiate between possible alternative structures we direc tly fitted nonlinear differential equations to the sway and visual mot ion trajectories on a trial-by-trial basis. We found that the eigenfre quency of the fitted model adapts strongly to the visual motion freque ncy. The damping coefficient decreases with increasing frequency, This indicates that the system destabilizes its postural state in the iner tial frame. This leads to a faster internal dynamics which is capable of synchronizing posture with fast-moving visual environments. Using a n algorithm which allows the identification of essentially nonlinear t erms of the dynamics we found small nonlinear contributions. These non linearities are not consistent with a limit-cycle dynamics, accounting for the robustness of the amplitude of postural sway against frequenc y variations, We interpret our results in terms of active generation o f postural sway specified by sensory information. We derive also a num ber of conclusions for a behavior-oriented analysis of the postural sy stem.