Effect of visual surrounding motion on body sway in a three-dimensional environment

Unidirectional motion of a uniplanar background induces a codirectional pos tural sway. It has been shown recently that fixation of a stationary foregr ound object induces a sway response in the opposite direction (Bronstein & Buckwell, 1997) when the background moves transiently. The present study in vestigated factors determining this contradirectional postural response. In the experiments presented, center of foot pressure and head displacements were recorded from normal subjects. The subjects faced a visual background of 2 x 3 m, at a distance of 1.5 m, which could be moved parallel to the in teraural axis. Results showed that when the visual scene consisted solely o f a moving background, the conventional codirectional postural response was elicited. When subjects were asked to fixate an earth-fixed foreground (wi ndow frame) placed between them and the moving background, a consistent pos tural response in the opposite direction to background motion was observed. In addition, we showed that this contradirectional postural response was n ot transient but was sustained for the 11 sec of background motion. We inve stigated whether this contradirectional postural response was the consequen ce of the induced movement of the foreground by background motion. Although induced movement was verbally reported by subjects when viewing an earth-f ixed target projected onto the moving background, the contradirectional swa y did not occur. These results indicate that foreground-background separati on in depth was necessary for the contradirectional postural response to oc cur rather than induced movement. Another experiment showed that, when the fixated foreground was attached to the head of the observer, the contradire ctional sway was not observed and was therefore unrelated to vergence. Fina lly, results showed that the contradirectional postural response was, in th e main, monocularly mediated. We conclude that the direction of the postura l sway produced by a moving background in a three-dimensional environment i s determined primarily by motion parallax.