Neuro-otology in Space Medicine: Exploring the Pathogenic Mechanisms of Space Motion Sickness

<p>　In the weightlessness of space, a variety of neuro-otological symptoms occur, and those manifesting as symptoms of space motion sickness—most of which are the same as those of terrestrial motion sickness—are very important. From this perspective, in collaboration with the National Space Development Agency of Japan, we studied changes in autonomic nervous system activity and stress hormones and the incidence of motion sickness in healthy male volunteers by applying various acceleration loads to the subjects while moving their heads. We also examined the incidence of motion sickness in subjects when the timing or spatial position of visual field information presented via a virtual reality headset was shifted with respect to the subjects’ head movement. When head and eye movements became unexpectedly asynchronous, the incidence of motion sickness increased and, at the same time, stress hormones such as ACTH and ADH rapidly increased. After stimulation on 3 consecutive days, adaptation occurred and the incidence of motion sickness sharply decreased. In subjects whose sympathetic nervous system was abnormally excited by gentle Coriolis stimulation, motion sickness was induced at significantly higher rates by subsequent more-intense stimulation, suggesting the involvement of the autonomic nervous system in susceptibility to motion sickness. We believe that motion sickness results from information that is unanticipated and is asynchronous in space or time, and that it is also a state that induces a dynamic process of adaptation in the brain.</p>