The significance of motion sickness in the vestibular system.

In order to clarify the significance of motion sickness in the vestibular system, we compared the findings of experimental motion sickness between different kinds of subjects, some of which were already reported. Subjects were healthy adults, healthy children between the ages of 4 and 15 years, and patients with congenital and acquired labyrinthine loss. They were asked to walk while wearing horizontally and vertically reversing goggles. Equilibrium ataxia as well as motion sickness were evoked by horizontal reversal, but not by vertical reversal in healthy subjects. Kindergarten children exhibited severe ataxia, but little nausea. The frequency of severe ataxia decreased during growth, inversely as the frequency of nausea syndrome increased. Although a patient with acquired loss became severely ataxic, a patient with congenital loss did not show any ataxia at all. The present study suggests that vestibular cues are indispensable to the ego-spatial relationship in the brain, and once the ego-spatial relationship becomes inadequate, discomfort acts as a safety device to brake uncontrollable actions. Then, perception of the outer world may automatically adjust voluntary actions by affecting motor commands. The importance of visual cues for representing an alternative framework may differ between congenital and acquired labyrinthine loss.