The caloric vestibular reaction in space. Physiological considerations.

Caloric stimulus testing was performed as part of the vestibular research program during the European Spacelab 1 mission in Nov/Dec 1983. Contrary to prediction according to the classical endolymph flow theory originally forwarded by Bárány, caloric nystagmus was elicited in both tested astronauts. The intensity of the response was found comparable to that measured on earth. The theoretical consequences of these findings are discussed and possible mechanisms are considered. The direct volume displacement hypothesis is favoured as the primary effect responsible for the observed vestibulo-ocular nystagmus. Estimated differential pressure conditions resulting in the endolymph canal support this hypothesis and are in agreement with the observed response intensity. It is further speculated that interaction in the central vestibular system between canal and otolith signals be responsible for the well-known body position modulation of the observed nystagmus.