Mechanisms underlying visually induced body sway.

We investigate the relationship between visually induced perceptual illusions of body motion (vection) and visually induced postural responses (VEPRs). Ten standing healthy subjects were tested in two visual conditions known to induce directionally opposite VEPRs: subjects fixated either a static head-mounted or an earth-fixed visual display in front of a horizontally translating visual background. The VEPR was in the direction of background motion when fixating the head-mounted display but transiently reversed in the earth-fixed condition. In contrast, vection occurred in only one direction (opposite to background motion) and developed later than VEPRs. The different time course and in-congruency between direction of VEPRs and direction of vection suggests that perceptual and postural responses are not causally related. However, since vection did increase VEPR magnitude in the direction of background motion, we postulate that VEPRs might be mediated by two different mechanisms: (1) a short latency system, driven by transient visual stimuli and sensitive to visual geometry (parallax-no parallax), responsible for automatic postural sway adjustments and (2) a longer latency, vection-enhanced postural mechanism, related to the conscious perception of self-motion during longer duration (locomotor, vehicular) body displacements.