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

Purpose. Human posture is controlled by a combination of vestibular, somatosensory and visual information. This paper is concerned with postural readjustment responses induced by vection. In the visual control of posture, visually-induced perception of self-motion plays an important role and is called vection. Vection is difficult to measure quantitatively because it is a highly subjective phenomenon. Hypothesis. An optokinetic stimulus that moves in depth induces vection. We hypothesize that the magnitude of the visually-induced body sway is correlated with the degree of vection. Methods. A depth 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. Vection was estimated in two ways, a verbal assessment and a joystick maneuver. In addition, visually-induced body sway was measured by monitoring five reference points on the body by two video-motion analyzers. Results. The magnitude of the subjective vection was highly correlated with visually- induced body sway and was strongly dependent on the velocity of the visual stimulus. The ankle joint was pivoted during visually-induced body sway and acted as a motion initiator. When the magnitude of body sway was large, the body movement was adjusted at the hip and head-neck joints. Conclusions. The high correlation between vection and body sway suggests that vection can be estimated quantitatively by measuring visually-induced body sway.