Influence of motion parallax in the control of spontaneous body sway

Visual control of postural sway during quiet standing was investigated in n ormal subjects to see if motion parallax cues were able to improve postural stability. In experiment I. six normal subjects fixated a fluorescent fore ground target, either alone or in the presence of full room illumination. T he results showed that subjects reduced body sway when the background was v isible. This effect, however, could be mediated not only by parallax cues b ut also by an increase in the total area of visual field involved. In exper iment 2, other parameters such as image angular size and target distance we re controlled for. Twelve subjects fixated a two light-emitting diode (LED) target placed at 45 cm from their eyes in a dark room. A second similar tw o-LED target was placed either at 170 cm (maximum parallax) or at 85 cm (me dium parallax) from the fixated target, or in the same plane of the fixated target (0 cm, no parallax). It was Found that the amplitude of sway was re duced significantly, by approximately 20%. when the two targets were presen ted in depth (parallax present) as compared to when they were in the same p lane (no parallax). The effect was only present in the lateral direction an d for low frequency components of sway (up to 0.5 Hz). We confirmed in expe riment 3 on eight subjects with a design similar to that used in experiment 2 that the effect of motion parallax on body sway was of monocular origin since observed with monocular and binocular vision. Geometrical considerati ons based on these results support the existence of two modes of visual det ection of body sway, afferent (retinal slippage) and efferent (extraretinal or eye-movement based).