FREQUENCY-DEPENDENCE OF THE ACTION-PERCEPTION CYCLE FOR POSTURAL CONTROL IN A MOVING VISUAL ENVIRONMENT - RELATIVE PHASE DYNAMICS

When standing human subjects are exposed to a moving visual environmen t, the induced postural sway displays varying degrees of coherence wit h the visual information. In our experiment we varied the frequency of an oscillatory visual display and analysed the temporal relationship between visual motion and sway. We found that subjects maintain sizeab le sway amplitudes even as temporal coherence with the display is lost . Postural sway tended to phase lead (for frequencies below 0.2 Hz) or phase lag (above 0.3 Hz). However, we also observed at a fixed freque ncy, highly variable phase relationships in which a preferred range of phase lags is prevalent, but phase jumps occur that return the system into the preferred range after phase has begun drifting out of the pr eferred regime. By comparing the results quantitatively with a dynamic al model (the sine-circle map), we show that this effect can be unders tood as a form of relative coordination and arises through an instabil ity of the dynamics of the action-perception cycle. Because such insta bilities cannot arise in passively driven systems, we conclude that po stural sway in this situation is actively generated as rhythmic moveme nt which is coupled dynamically to the visual motion.