Illusory motion causes postural sway

Visual stimuli simulating self-motion through the environment can induce potent postural adjustments in observers. This suggests a rather direct, stimulus-driven, mechanism subserving these visuo-vestibular interactions. Here we examine whether visual-motion induced sway can also be generated by an internal representation of visual motion, as apparent in the motion-aftereffect or whether any induced sway after adaptation is the result of a postural-aftereffect. We presented a random-pixel-array (87° x 56°) translating at ~3 deg/s leftwards or rightwards during adaptation. A static version of the random-pixel-array or a black screen was used as test pattern. The latter pattern did not generate a motion-aftereffect and was used to determine the sway caused by the postural-aftereffect. Observers, standing on a force plate collecting posturograhic data, initially received 40s adaptation, followed by 20s top-up adaptation epochs, interleaved by 14s test pattern epochs. Results show that a static test pattern induced more sway than a black test pattern. This suggests that the sway induced by the static test pattern is the result of the perceived motion in the motion-aftereffect and not a mere result of a postural-aftereffect. This is evidence that visuo-vestibular interactions observed in visual motion induced sway are the result of the actual visual experience.