EFFECTS OF SPACEFLIGHT ON OCULAR COUNTERROLLING AND THE SPATIAL ORIENTATION OF THE VESTIBULAR SYSTEM

We recorded the horizontal (yaw), vertical (pitch), and torsional (rol l) eye movements of two rhesus monkeys with scleral search coils befor e and after the COSMOS Biosatellite 2229 Flight. The aim was to determ ine effects of adaptation to microgravity on the vestibule-ocular refl ex (VOR). The animals flew for 11 days. The first postflight tests wer e 22 h and 55 h after landing, and testing extended for 11 days after reentry. There were four significant effects of spaceflight on functio ns related to spatial orientation: (1) Compensatory ocular counterroll ing (OCR) was reduced by about 70% for static and dynamic head tilts w ith regard to gravity. The reduction in OCR persisted in the two anima ls throughout postflight testing. (2) The gain of the torsional compon ent of the angular VOR (roll VOR) was decreased by 15% and 50% in the two animals over the same period. (3) An up-down asymmetry of nystagmu s, present in the two monkeys before flight was reduced after exposure to microgravity. (4) The spatial orientation of velocity storage was shifted in the one monkey that could be tested soon after flight. Befo re flight, the yaw axis eigenvector of optokinetic afternystagmus was close to gravity when the animal was upright or tilted. After flight, the yaw orientation vector was shifted toward the body yaw axis. By 7 days after recovery, it had reverted to a gravitational orientation. W e postulate that spaceflight causes changes in the vestibular system w hich reflect adaptation of spatial orientation from a gravitational to a body frame of reference. These changes are likely to play a role in the postural, locomotor, and gaze instability demonstrated on reentry after spaceflight.