Influence of motion parallax in the control of spontaneous body sway

Abstract

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).

Mirror versus stationary cross feedback in controlling the center of foot pressure displacement in quiet standing in elderly subjects

“…In other words, the contrasting effects observed in the mirror feedback condition according to the ML and AP directions could be referred to the different thresholds of visual detection of the body movements in the ML and AP directions (25). It remains however that manipulating the distance between the observer and the mirror or the reflected image size would probably modify the effects observed in the present experiment (22,23,26).…”

Age-dependent modulation of sensory reweighting for controlling posture in a dynamic virtual environment

“…It is possible that visual anticipation of the randomly approaching objects increased visual anchoring and may have upweighted the visual contribution to the control of posture resulting in excessive sway variability. Visual fixation or anticipation of an unpredictable event in the visual domain strengthens the anchoring of vision to the environment with positive effects on postural stability when the visual environment is reliable (Guerraz et al 2000;Laurens et al 2010). However, when the visual input is attenuated, i.e., during visual field oscillations or in the absence of peripheral visual cues, this anchoring process may compromise postural stability (Glasauer et al 2005;Laurens et al 2010).…”

Visual contribution to postural stability: Interaction between target fixation or tracking and static or dynamic large-field stimulus

“…In another study [17], subjects stood on a stable surface and fixated a LED at a distance of 2.5 m, either in darkness or superimposed on a stationary large-field pattern. In this study, both lateral and AP COP displacements were significantly reduced by the stationary large-field pattern, which corroborates our findings.…”