Effects of simulated viewpoint jitter on visually induced postural sway

In this study we examined the effects of simulated horizontal and vertical viewpoint jitter on the vection and postural sway induced by radial patterns of optic flow. During each trial, observers were exposed sequentially to 20 s periods of radially expanding flow, radially contracting flow, and static visual scenes. For half the trials, simulated viewpoint jitter was added to the radially expanding/contracting optic flow patterns. In experiment 1, we found that, while this jitter increased the backward postural sway induced by radial expansion, it actually decreased forward postural sway induced by radial contraction. However, in experiment 2 we found that jitter increased both the forward and backward vection induced by radially expanding and contracting flow patterns. We conclude that the processes involved in postural control are more sensitive to the sensory conflicts generated by viewpoint jitter than those involved in the perception of self-motion, and that the observed asymmetries in forward and backward sway are ecological in origin.