Disorientation of animals in microgravity.

Disorientation responses of animals exposed to microgravity produced by parabolic aircraft flights and also in-space experiments were reviewed. Disoriented postures in floating are largely species-dependent. Reflexive lowered tone of gravity-bearing extensor muscles by labyrinthectomy is not seen in mammals (hamster and monkey) and frog, though dorsiflexion of the neck and the trunk is not so remarkable in hamster and monkey as in frog. In weightlessness, fundamental vestibular reflexes may be affected (righting reflex in cat), but coordinated performance can be easily compensated by visual function (mouse, monkey and turtle). In normal birds and fish who can move three-dimensionally in their environments, exposure to parabolic flight microgravity induces irregular tumbling with the eyes open and regular looping with the eyes closed, although the loop direction is the opposite in these two animals; backward (inside) in pigeon and forward (outside) in fish. Most recently, however, it was found that normal fish (goldfish) tumbled backward when observed in prolonged microgravity in space, suggesting that microgravity effects on fish in aircraft-flight parabolas would differ from those in space. Sensory conflicts in normal fish diminish remarkably during 3-4 days of space microgravity, and the cerebellum may be involved in the recovery, lending support to the sensory conflict theory for the genesis of space motion sickness.