Establishing our orientation in the world is necessary for almost all aspects of perception and behavior. Gravity usually defines the critical reference direction. The direction of gravity is sensed by somatosensory detectors indicating pressure points and specialized organs in the vestibular system and viscera that indicate gravity's physical pull. However, gravity's direction can also be sensed visually since we see the effects of gravity on static and moving objects and also deduce its direction from the global structure of a scene indicated by features such as the sky and ground. When cues from either visual or physical sources are compromised or ambiguous, perceptual disorientation may result, often with a tendency to replace gravity with the body's long axis as a reference. Orientation cues are compromised while floating in the weightlessness of space (which neutralizes vestibular and somatosensory cues) or while suspended at neutral buoyancy in the ocean (which neutralizes somatosensory cues) and the ability to sense orientation cues may also be compromised in the elderly or in clinical populations. In these situations, enhancing the visual cues to orientation may be beneficial. In this chapter, we review research using specially constructed virtual and real environments to quantify the contribution of various visual orientation cues. We demonstrate how visual cues can counteract disorientation by providing effective orientation information.
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