Self-induced motion sickness in unperturbed stance.

Motion sickness typically occurs when the body is subjected to externally imposed motions, but there are situations in which sickness occurs in the absence of imposed motion. We report a new and unanticipated instance of the latter. Subjects in a study of spontaneous standing postural sway sometimes reported dizziness and motion sickness. Reports of sickness were correlated with changes in postural sway. We consider possible implications of these findings for two current theories of motion sickness etiology: the sensory conflict theory and the postural instability theory.