Effects of spaceflight on ocular counterrolling and the spatial orientation of the vestibular system

Abstract

We recorded the horizontal (yaw), vertical (pitch), and torsional (roll) eye movements of two rhesus monkeys with scleral search coils before and after the COSMOS Biosatellite 2229 Flight. The aim was to determine effects of adaptation to microgravity on the vestibulo-ocular reflex (VOR). The animals flew for 11 days. The first postflight tests were 22 h and 55 h after landing, and testing extended for 11 days after reentry. There were four significant effects of spaceflight on functions related to spatial orientation: (1) Compensatory ocular counterrolling (OCR) was reduced by about 70% for static and dynamic head tilts with regard to gravity. The reduction in OCR persisted in the two animals throughout postflight testing. (2) The gain of the torsional component 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 nystagmus, 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. Before 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. We postulate that spaceflight causes changes in the vestibular system which 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.

The Vestibular System: A Newly Identified Regulator of Bone Homeostasis Acting Through the Sympathetic Nervous System

“…Similar to aged subjects, remodeling of synaptic connections in vestibular sensorial hair cells and an increase in the number and the size of synaptic knobs have been reported in adult rats during shortterm space missions [111,112]. The vestibulo-spinal and vestibulo-ocular reflexes are also altered in these conditions [113][114][115].…”

Complementary gain modifications of the cervico-ocular (COR) and angular vestibulo-ocular (aVOR) reflexes after canal plugging

“…Under anesthesia and sterile surgical conditions, a head mount was implanted on the skull of the animals to provide painless head fixation in stereotaxic coordinates during testing (Yakushin et al 2000). Two weeks later, two coils were implanted on the left eye to measure horizontal, vertical, and torsional eye position (Judge et al 1980; Dai et al 1994). Later, all six semicircular canals were plugged in one rhesus (M3002) and two cynomolgus (M9357, M17115) monkeys by opening each bony canal via the mastoid cavity and packing the canal with fascia and bone chips (Suzuki et al 1991; Cohen et al 1983; Raphan et al 1983; Yakushin et al 1995, 1998).…”

“…Eye movements were calibrated off-line based on induced eye velocities during rotation at 30°/s in light. We assumed that yaw and pitch aVOR gains of the canal-plugged monkeys were 1.0, and roll gain was 0.6 (Robinson 1963; Crawford and Vilis 1991; Henn et al 1992; Dai et al 1994; Yue et al 1994; Telford et al 1996). Chair yaw and pitch positions and the three components of eye position were stored with a resolution of 16 bits at 1 kHz for off-line analysis.…”

Cerebellum and Gravity: Altered Earth’s Gravity Perception Under Pathological Conditions and Response to Altered Gravity in Space

“…It appears that astronauts experience the loss of the gravitational frame of reference. Experiments with primates suggest that exposure to microgravity results in changes in the vestibular system, which reflect adaptation of spatial orientation from a gravitational to body frame of reference (Dai et al 1994).…”