Abstract
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.
Highlights
On Earth, during static head tilt to the side, the eyes rotate around the line of sight in the direction opposite to the head tilt
A repeated measures ANOVA with 2 factors indicated that the Ocular counter-rolling (OCR) significantly increased with the angle of tilt [F(3,55) = 59.6, P < 0.001], and that there was a significant difference between the two tilt conditions [F(1,55) = 4.82, P = 0.03]
The results of this study indicate that OCR during head tilt is absent in microgravity and returns to normal within 2 hours of return from a short-duration spaceflight
Summary
On Earth, during static head tilt to the side, the eyes rotate around the line of sight in the direction opposite to the head tilt. This ocular counter-rolling (OCR) reflex allows the horizontal meridian of the retina to remain parallel to the horizon during head roll. Findings are inconsistent[4,5,6,7,8,9,10,11]: studies report OCR decreases after spaceflight in some subjects, and OCR increases or no changes in other subjects The inconsistency in these results may be due to the small number of crewmembers who were studied, the duration of weightlessness exposure, or the methods used, i.e. whole body tilt, head tilt, or eccentric rotation. The present study had three objectives: (a) use a normative population on Earth to compare the OCR in response to whole body tilt with the OCR in response to a head tilt relative to the body; (b) compare the OCR in response to head tilt relative to the body in astronauts before, during, and immediately after a short-duration spaceflight; and (c) compare the OCR in response to whole body tilt in astronauts before and immediately after a long-duration space flight
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