Ocular Counter Rolling in Astronauts After Short- and Long-Duration Spaceflight.

Ocular counter-rolling (OCR) is a reflex generated by the activation of the gravity sensors in the inner ear that stabilizes gaze and posture during head tilt. We compared the OCR measures that were obtained in 6 astronauts before, during, and after a spaceflight lasting 4-6 days with the OCR measures obtained from 6 astronauts before and after a spaceflight lasting 4-9 months. OCR in the short-duration fliers was measured using the afterimage method during head tilt at 15°, 30°, and 45°. OCR in the long-duration fliers was measured using video-oculography during whole body tilt at 25°. A control group of 7 subjects was used to compare OCR measures during head tilt and whole body tilt. No OCR occurred during head tilt in microgravity, and the response returned to normal within 2 hours of return from short-duration spaceflight. However, the amplitude of OCR was reduced for several days after return from long-duration spaceflight. This decrease in amplitude was not accompanied by changes in the asymmetry of OCR between right and left head tilt. These results indicate that the adaptation  of otolith-driven reflexes to microgravity is a long-duration process.