Further evidence to support disconjugate eye torsion as a predictor of space motion sickness.

Disconjugate eye torsion in hypo- and hypergravity of parabolic flight was examined in four former astronauts and four previously tested ex-astronauts to replicate an earlier study and to further test the asymmetry hypothesis of otolith function. Results in the new subjects supported the asymmetry hypothesis and confirmed previous findings that those with low scores of torsional disconjugacy on the KC-135 did not suffer space motion sickness in their prior Shuttle missions while those with high scores did. Tilting subjects with high disconjugacy scores slightly to one side and the other failed to find a position that decreased disconjugacy in hypergravity, leading to the conclusion that a simple planar asymmetry about the y-axis was probably not the cause of the observed torsional differences in the two eyes. Disconjugacy increased at 0 G with increasing parabolas, much more so in subjects who had suffered SMS. Because of this, 10 to 20 parabolas were deemed to be a more certain discriminator than a fewer number.