Egocentric perception through interaction among many sensory systems.

We perceive the egocentric position and velocity of ourselves by many senses, such as vision, proprioception and vestibular sense. Normally the information by these senses is in harmony. However, there are situations in which the information is inharmonious. When we watch a wide-screen monitor or we ride in an accelerating or turning vehicle, visual information conflicts with proprioceptive and vestibular information. Since human egocentric perception has been studied for each separate sense in the previous research, it is not clear how the integration among these senses contributes to perception of human orientation. In Experiment 1, we investigated the contribution of visual, proprioceptive and vestibular information in isolation and in combination to perception of direction of forward self-motion. An observer rode in small vehicle (vestibular information) or walked (proprioceptive information) through a narrow corridor. Many vertical bars were hung from a ceiling of the corridor. When the observer moved forward, she/he viewed expanding optic flow of the bars (visual information) through video cameras and a helmet-mounted stereoscopic display. By fastening the observer and/or the cameras at different angles, consistent or conflicting information about forward direction was given through each sense. It was found that when there was conflicting information about the direction of forward motion (a) vestibular information was more dominant than visual cute, (b) visual and proprioceptive information was linearly added with weighting, and (c) visual information was dominant for backward motion. In Experiment 2, we investigated sensory integration when we moved forward with linear acceleration. Direction of the acceleration was either forward or sideways, namely corresponding to a ride in an accelerating or turning vehicle. We developed a new method to measure sensation or self-motion more objectively by using the three-dimensional position-sensor system. Positions of observer's head, shoulder, waist and ankle were measured to find body tilt accompanied by sensation of self-motion. It was found that the body tilted towards the opposite direction of the self-acceleration and the angle of body tilt was in good agreement with the subjective amount of the accelerating sensation. The body tilt was even induced by solely visual information. This implied that visual information contributed to perception of self-acceleration as well as self-motion.