Decreasing perceived optic flow rigidity increases postural sway.

Optic flow simulating self-motion through the environment can induce postural adjustments in observers. Some studies investigating this phenomenon have used optic flow patterns increasing in speed from center to periphery, whereas others used optic flow patterns with a constant speed. However, altering the speed gradient of an optic flow stimulus changes the perceived rigidity of such a stimulus. Optic flow stimuli that are perceived as rigid can be expected to provide a stronger sensation of self-motion than non-rigid optic flow, and this may well be reflected in the amount of postural sway. The current study, therefore, examined, by manipulating the speed gradient, to what extent the rigidity of an optic flow stimulus influences posture along the anterior-posterior axis. We used radial random dot expanding or contracting optic flow patterns with three different speed profiles (single-speed, linear speed gradient or quadratic speed gradient) that differentially induce the sensation of self-motion. Interestingly, most postural sway was observed for the non-rigid single-speed optic flow pattern, which contained the least self-motion information of the three profiles. Moreover, we found an anisotropy in that contracting optic flow produced more postural sway than expanding optic flow. In addition, the amount of postural sway increased with increasing stimulus speed, but for contracting optic flow only. Taken together, the results of the current study support the view that visual and sensorimotor systems appear to be tailored toward compensating for rigid optic flow stimulation.