Postural responses to simulated moving environments are not invariant for the direction of gaze.

Postural responses were measured for observers instructed to maintain an erect position when viewing a visual scene, which stimulated motion of the observer relative to a corridor with texture on the walls. Both the direction and amplitude of the postural responses appeared to depend on the fixation direction of the observer. This result indicates that postural responses are not invariant with respect to fixation direction. Our results suggest that not one of the frequently suggested features in the optical flow in particular is used to maintain posture but rather that a combination of several optical flow components, that have been suggested in the literature, is relevant.