TEMPORAL STABILITY OF THE ACTION-PERCEPTION CYCLE FOR POSTURAL CONTROL IN A MOVING VISUAL ENVIRONMENT

When standing human subjects are exposed to a moving visual environmen t, the induced postural sway forms a stable temporal relationship with the visual information. We have investigated this relationship experi mentally with a new set-up in which a computer generates video images which correspond to the motion of a 3D environment. The suggested mean distance to a sinusoidally moving wall is varied and the temporal rel ationship to induced sway is analysed (1) in terms of the fluctuations of relative phase between visual and sway motion and (2) in terms of the relaxation time of relative phase as determined from the rate of r ecovery of the stable relative phase pattern following abrupt changes in the visual motion pattern. The two measures are found to converge t o a well-defined temporal stability of the action- perception cycle. F urthermore, we show that this temporal stability is a sensitive measur e of the strength of the action-perception coupling. It decreases as t he distance of the visual scene from the observer increases. This fact and the increase of mean relative phase are consistent with predictio ns of a linear second-order system driven by the visual expansion rate . However, the amplitude of visual sway decreases little as visual dis tance increases, in contradiction to the predictions, and is suggestiv e of a process that actively generates sway. The visual expansion rate on the optic array is found to decrease strongly with visual distance . This leads to the conclusion that postural control in a moving visua l environment cannot be understood simply in terms of minimization of retinal slip, and that dynamic coupling of vision into the postural co ntrol system must be taken into account.