Academic journal article Perception and Psychophysics

Visual Control of Posture in Real and Virtual Environments

Academic journal article Perception and Psychophysics

Visual Control of Posture in Real and Virtual Environments

Article excerpt

In two experiments, we investigated the stabilizing influence of vision on human upright posture in real and virtual environments. Visual stabilization was assessed by comparing eyes-open with eyes-closed conditions while subjects attempted to maintain balance in the presence of a stable visual scene. Visual stabilization in the virtual display was reduced, as compared with real-world viewing. This difference was partially accounted for by the reduced field of view in the virtual display. When the retinal flow in the virtual display was removed by using dynamic random-dot stereograms with single-frame lifetimes (cyclopean stimuli), vision did not stabilize posture. There was also an overall larger stabilizing influence of vision when more unstable stances were adopted (e.g., one-foot, as compared with side-by-side, stance). Reducing the graphics latency of the virtual display by 63% did not increase visual stabilization in the virtual display. Other visual and psychological differences between real and virtual environments are discussed.

Two lines of evidence underscore the importance of visual input in the control of posture (balance). First, occluding vision increases standing body sway by 200%-300% (Begbie, 1967; Diener, Dichgans, Bacher, & Gompf, 1984; Edwards, 1946; Paulus, Sträube, & Brandt, 1984; Witkin & Wapner, 1950). Experimental degradation of both visual acuity (by placing semitransparent plastic foils over the eyes) and field of view (FOV) has been shown to increase standing sway, relative to unobstructed vision (Paulus et al., 1984), and naturally occurring visual deficiencies are considered to be a major risk factor for falls in the elderly (Harwood, 2001). second, the swinging worn paradigm has been used to demonstrate that displacement of the visual environment produces a compensatory postural response (Lee & Lishman, 1975). Presumably, the imposed visual motion is interpreted as self-motion, and a postural response is generated in an attempt to compensate for this perceived self-motion. Further solidifying this relationship between vision and posture is the finding that sinusoidal motion of the visual environment produces postural responses at the same frequency as the environmental motion (Bardy, Warren, & Kay, 1996,1999; Dijkstra, Gielen, & Melis, 1992; Dijkstra, Schöner, & Gielen, 1994; van Asten, Gielen, & van der Gon, 1988a, 1988b).

Given the aforementioned results establishing an important role for vision in maintaining balance, the present experiments were designed with two primary goals in mind: (1) to determine whether the stabilizing influence of vision is comparable in real and virtual environments and, if not, to determine potential causes for differences, and (2) to assess the hypothesis that optic flow, a 2-D motion-based stimulus, is an important visual cue used to maintain balance.

Postural Control in Virtual Reality

Much of our understanding of the visual control of posture has come through experiments using simulated visual displays (Bardy et al., 1996,1999; Bronstein & Buckwell, 1997; Cunningham, Nusseck, Teufel, Wallraven, & Bülthoff, 2006; Dijkstra et al., 1992, 1994; Kelly, Loomis, & Beall, 2005; Mitra, 2003; Mitra & Fraizer, 2004; van Asten et al., 1988a, 1988b). It is often assumed in these experiments that any variable affecting postural control in virtual reality (VR) would have a similar effect if implemented in a real environment (but for exceptions, see Cunningham et al., 2006; Stofrregen, Bardy, Merhi, & Oullier, 2004). However, visual stimulation in virtual displays can differ in potentially important ways from real-world viewing. Some of these differences, relative to real-world viewing, include reduced FOV, graphics update latency in response to observer movement, fixed accommodative distance, optical distortion, display quantization, and the added weight when a head-mounted display (HMD) is worn. …

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