Postural adjustment response to depth direction moving patterns produced by virtual reality graphics

Purpose. Human posture is controlled by a combination of vestibular, somato sensory and visual information. This paper is concerned with postural readj ustment responses induced by vection. In the visual control of posture, vis ually-induced perception of self-motion plays an important role and is call ed vection. Vection is difficult to measure quantitatively because it is a highly subjective phenomenon. Hypothesis. An optokinetic stimulus that move s in depth induces vection. We hypothesize that the magnitude of the visual ly-induced body sway is correlated with the degree of vection. Methods. A d epth optokinetic stimulus (DOKS) was projected onto a head-mounted display (HMD) worn by standing subjects. The DOKS consisted of a random dot pattern that was perceived three-dimensionally and moved in depth sinusoidally. Ve ction was estimated in two ways, a verbal assessment and a joystick maneuve r. In addition, visually-induced body sway was measured by monitoring five reference points on the body by two video-motion analyzers. Results. The ma gnitude of the subjective vection was highly correlated with visually-induc ed body sway and was strongly dependent on the velocity of the visual stimu lus. The ankle joint was pivoted during visually-induced body sway and acte d as a motion initiator. When the magnitude of body sway was large, the bod y movement was adjusted at the hip and head-neck joints, conclusions. The h igh correlation between vection and body sway suggests that vection can be estimated quantitatively by measuring visually-induced body sway.