Anteroposterior dynamic balance reactions to circular movement of the visual field without ocular pursuit.

It has been hypothesised that during natural vision, retinal image motion and eye motion information exert antagonistic effects on balance that cancel each other out. In a previous study, we found that pursuit movements of the eye unaccompanied by background slip on the retina induced strong, stereotyped anteroposterior sway in dynamic balance. In the present work, we investigated the influence of background retinal image slip alone on anteroposterior dynamic balance. We expected that retinal slip without ocular pursuit would induce large anteroposterior sway. Circular slip without rotation of the whole visual field image on the retina was obtained using a rotating prism. In this study, carried out on monocular vision, eye movement was prevented by fixating a stationary target seen through a small aperture in the prism. Anteroposterior dynamic balance conditions were elicited by a rocking platform. Strong, stereotyped anteroposterior sway accompanied by a visual illusion of target motion was observed. The induced sway was synchronised to the stimulus and led to a significant disturbance of balance. The similarity in magnitude of the effects of both retinal image motion studied here and eye motion information obtained previously, and their opposing phases, strongly support the initial hypothesis that the antagonistic effects of the two sources of information provided by normal ocular pursuit could interact in an additive algebraic mode.