General remarks on the role of the vestibular system in weightlessness.

Different methods are described to experimentally achieve weightlessness. Since the function of the otolith system depends on the presence of contact forces opposing gravity, it is disabled in weightlessness and may send misleading positional information to the brain. Without the contributions of the otolith system it is difficult in space to distinguish self-motion from object motion. Furthermore, the disintegration of information from the neck position receptors from those of the otolith system can lead to additional illusory positional sensations. Since the function of the semicircular canal system in previous space flights was found to be essentially undisturbed, the vestibular experiments in the Spacelab-D1 missions concentrated more on the otolith system. The function of other orientational cues from the visual system, the semicircular canal system and proprioception can be studied in isolation from the otolith system in space. In the Spacelab-D1 mission, the space vestibular sled was used as a device for studying linear acceleration. The vestibular helmet "permitted" video and EOG recordings of all eye movements and provided caloric and optokinetic stimulation. Various factors contributing to static and dynamic forms of space sickness are identified.