Microgravity effect on the vestibulo-ocular reflex is dependent on otolith and vision contributions

Background: We studied whether microgravity influences horizontal and vertical vestibule-ocular reflex (VOR), and what the otolith contributes to VOR in the absence of gravity, in six cosmonauts during and after space missions. Method: VOR was elicited by active yaw and roll head movement at a frequency of about 0.2 Hz. Results: The various individual quantitative changes (increase, decrease, and left-right asymmetry) found in the horizontal VOR evoked by yaw head movements during the adaptation period to microgravity suggested central reprogramming of mechanisms controlling VOR; i.e., a non-specific effect of microgravity on VOR. At the same time, horizontal and vertical VOR's were recorded during roll head movements, which were not obvious before flight. In the cosmonaut who participated in a long-term flight, the increased activity of vertical canals turned to unidirectional (down-ward) eye movements, independent of the head movement direction, lasting during the whole mission. These VOR changes probably resulted from the absent adequate otolith stimulation and reduced otolith influence upon semicircular canal function. Conclusion: Thus, a specific effect of microgravity on VOR was observed during roll head movements, when the interaction between semicircular canals and otoliths should be more pronounced, mainly in the vertical plane. The stability of the ''space'' pattern of interactions in the readaptation period depended on the time spent in microgravity. We suggest that in visual-vestibular interactions revealed in VOR evoked by head movements with open eyes, vision dominates when a conflict arises between ''space'' and ''terrestrial'' patterns of sensory interactions.