Ocular torsion as a test of the asymmetry hypothesis of space motion sickness.

Disconjugate eye torsion induced by 0 G and 1.8 G during parabolic flight was studied in nine former astronauts in 1990 and eight in 1991, four of whom were included in the previous experiment. The astronauts could be divided into two statistically significant groups on the basis of low and high scores of disconjugacy. When their histories of space motion sickness (SMS) were later revealed, all of the low scorers had not been sick on previous space flights; all the high scorers had had SMS. These data give support to the hypothesis that SMS in one-half or two-thirds of astronauts is due to an otolith, probably utricular, asymmetry in those persons.