Abstract
We actively maintain postural equilibrium in everyday life, and, although we are unaware of the underlying processing, there is increasing evidence for cortical involvement in this postural control. Converging evidence shows that we make appropriate use of ‘postural anchors’, for example static objects in the environment, to stabilise our posture. Visually evoked postural responses (VEPR) that are caused when we counteract the illusory perception of self-motion in space (vection) are modulated in the presence of postural anchors and therefore provide a convenient behavioural measure. The aim of this study is to evaluate the factors influencing visual appraisal of the suitability of postural anchors. We are specifically interested in the effect of perceived ‘reality’ in VR the expected ‘stability’ of visual anchors. To explore the effect of ‘reality’ we introduced an accommodation-vergence conflict. We show that VEPR are appropriately modulated only when virtual visual ‘anchors’ are rendered such that vergence and accommodation cues are consistent. In a second experiment we directly test whether cognitive assessment of the likely stability of real perceptual anchors (we contrast a ‘teapot on a stand’ and a ‘helium balloon’) affects VEPR. We show that the perceived positional stability of environmental anchors modulate postural responses. Our results confirm previous findings showing that postural sway is modulated by the configuration of the environment and further show that an assessment of the stability and reality of the environment plays an important role in this process. On this basis we propose design guidelines for VR systems, in particular we argue that accommodation-vergence conflicts should be minimised and that high quality motion tracking and rendering are essential for high fidelity VR.
Highlights
It is generally accepted that human bipedal upright stance is achieved by feedback mechanisms that generate appropriate actions to correct for natural body-sway motion detected by our sensory systems [1] [2]
In the second experiment we investigate whether an explicit evaluation of the positional stability of the reference points present in the scene modulates Visually evoked postural responses (VEPR) during visual motion stimulation
We conclude that conflicting accommodationvergence cues for the moving background significantly reduce VEPR amplitude
Summary
It is generally accepted that human bipedal upright stance is achieved by feedback mechanisms that generate appropriate actions to correct for natural body-sway motion detected by our sensory systems [1] [2]. Generated visual motion, which causes the perception of illusory self-motion (vection), typically leads to a postural response to counteract the perceived motion. These visually evoked postural responses (VEPR) have been documented under a range of conditions [6,7,8,9,10,11] and depend on stimulus characteristics such as motion speed [12], stimulus texture [13] or position of the stimulus within the visual field [8, 14]. Guerraz et al [15] argue that, in order to induce consistent and directionally specific responses, the relative motion of objects in the environment must reflect the motion signals that a moving observer would experience in a stable 3-D environment
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