The effect of prolonged and previous spaceflight missions on the ocular counter-roll

The gravitational sensors of the vestibular system, the otoliths, are responsible for the ocular counter-roll (OCR), a compensatory eye torsion sensitive to a lateral head roll with respect to gravity and the Gravito-Inertial Acceleration (GIA) vector during e.g., centrifugation. The OCR will ensure gaze stabilization and postural stability when making sharp turns during locomotion. In the international space station (ISS), the cosmonauts are subjected to a microgravity environment where the otoliths are very vulnerable because of their gravitational dependency. The lack of gravitational input will force the otoliths to adapt by a deconditioning, resulting in a decrease in gain of otolith-mediated reflexes, thus the OCR. We measured the OCR induced by off-axis centrifugation in a group of 27 cosmonauts before and after their 6-month space mission to the ISS to investigate the effect of prolonged spaceflight on the otoliths. Our results show a significant decrease early post-flight with a difference between novice and experienced cosmonauts. Novice cosmonauts or first-time flyers show to be more strongly affected compared to experienced or frequent flyers with a larger decrease of the OCR early post-flight. This strongly suggest that experienced space crew have acquired the ability to adapt faster after G-transitions and should therefore be sent for more challenging space missions, e.g., Moon or Mars, because they are noticeably less affected by microgravity regarding their vestibular system.