[Physiology of the caloric equilibrium reaction. Consequences from results of space experiments in Spacelab 1, December 1983].

A caloric nystagmus was observed during weightlessness in orbital flight. This cannot be reconciled with the classical convection theory of the caloric mechanism. Alternative hypotheses are discussed. It appears that a direct thermal effect on the canal afferents could only play a small part. It is suggested that a stronger effect is produced by the temperature-mediated volume change in the horizontal semicircular canal, the asymmetric stimulation of the canal ring leading to volume displacement towards the cupula. On earth, the convective torque produced by redistribution of endolymph mass may be considered as a secondary effect, comparable to a stimulus producing a nystagmus threshold response. Modulation of the caloric and rotatory nystagmus by body position and the role of the otoliths are also discussed.