Optical flow visual stimuli induce body sway and visually-induced motion sickness in virtual reality system: A preliminary study

Objectives: Visually-induced motion sickness (VIMS) remains an unsolved issue when using head-mounted displays (HMD) in immersive virtual reality (VR) systems. We investigate the influence of restricted visual field on sensorimotor incongruence of postural control in an immersive VR environment, as a surrogate marker of VIMS while standing steadily and viewing ambiguous optical-flow visual stimuli. Methods: Twenty-seven healthy participants wore HMD and viewed optical flow visual stimuli (called the plaid motion pattern) using a VR system. Visual stimuli were presented on a full-screen display, either only in the center or periphery of the visual field, or nowhere (Natural). To evaluate standing posture stability, we measured spatial (length and area of the center of the pressure (COP)) and temporal dynamics of body sway using a stabilometer. Subjective feelings of VIMS were assessed for each visual stimulus condition. Results: The full-screen condition significantly worsened the COP measurements and feelings. The COP areas of standing balance under the center of the visual field condition were significantly smaller than those under the full-screen condition; however, the periphery of the visual field condition was comparable to the full-screen condition. The condition effects for the conditions (natural, center of the visual field, periphery of the visual field and full-screen) were observed in the COP measurements and subjective feelings. Conclusion: Optical-flow visual stimuli can induce body sway, suggesting sensorimotor incongruence in postural control. Ancillary findings demonstrated that subsequent to full-screen presentation, the periphery of the visual field potentially contributes to spatial and temporal dynamics of body sway and the sensorimotor incongruence rather than the central area.