Influence of motion parallax in the control of spontaneous body sway.

Visual control of postural sway during quiet standing was investigated in normal subjects to see if motion parallax cues were able to improve postural stability. In experiment 1, six normal subjects fixated a fluorescent foreground target, either alone or in the presence of full room illumination. The results showed that subjects reduced body sway when the background was visible. This effect, however, could be mediated not only by parallax cues but also by an increase in the total area of visual field involved. In experiment 2, other parameters such as image angular size and target distance were 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 two-LED target was placed either at 170 cm (maximum parallax) or at 85 cm (medium 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 reduced significantly, by approximately 20%, when the two targets were presented in depth (parallax present) as compared to when they were in the same plane (no parallax). The effect was only present in the lateral direction and for low frequency components of sway (up to 0.5 Hz). We confirmed in experiment 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 considerations based on these results support the existence of two modes of visual detection of body sway, afferent (retinal slippage) and efferent (extra-retinal or eye-movement based).