Postural Responses to Two Technologies for Generating Optical Flow

Abstract

The perception and control of stance are frequently studied in virtual environments, where computer-generated videographic displays are used to simulate the optical consequences of body sway. Generally, the intent of such studies is to understand how posture is controlled outside of virtual environments (i.e., in daily life). Accordingly, the validity of such studies will depend upon the extent to which postural responses to videographically generated optical flow resemble postural responses to optical flow that is generated in other ways. We conducted a direct test of postural responses to optical flow generated using two technologies: physical displacement of the visible surroundings (using a moving room), and videographic projection of computer-generated graphics. We attempted to make the two displays as similar as possible in terms of visual angle, optical texture, and the amplitude and frequency of oscillation. The results revealed several significant differences in postural responses to the two types of stimuli. This raises questions about the extent to which postural control in virtual environments can be generalized to postural control in the real world.

Visual control of posture in real and virtual environments

“…1 Subsequently, Cunningham et al (2006) used a large FOV virtual display (220º horizontal 50º vertical) to test postural responses to a swinging room while varying the room oscillation amplitude. In contrast to Stoffregen et al (2004), they found a clear increase in body sway with increasing room oscillation amplitude. There are, however, multiple differences that could account for the discrepant results.…”

“…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;Stoffregen, 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. Recently, Stoffregen et al (2004) compared postural responses to real and virtual swinging rooms and matched the FOV in the two different viewing environments. When the real display was viewed, the coupling between postural sway In two experiments, we investigated the stabilizing influence of vision on human upright posture in real and virtual environments.…”

“…Recognizing that the display quantization of the virtual scene could also be responsible for the observed differences, Stoffregen et al (2004) manipulated the size of the texture elements in the two displays. They expected that larger texture elements, which are less subject to spatial aliasing, would produce a larger postural response to the virtual environment.…”

“…There are, however, multiple differences that could account for the discrepant results. First, Cunningham et al used a wide range of room motion amplitudes, from 2.5 to 80 cm, whereas Stoffregen et al (2004) used amplitudes between 2 and 22 cm. Second, Cunningham et al used a frequencymodulated oscillatory room motion and a somewhat different method of data analysis.…”