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

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.

Figure 1

Goggles used to measure OCR on board the Space Shuttle. With his head stabilized to the goggles by a dental bite, the astronaut tilted his head relative to his body. He then viewed an inverted “T” that appeared as an afterimage on the retina (see insert). With his head back in the vertical position (aligned with his body), he matched the retinal afterimage with a reference target using a thumb-wheel. The display was set to an arbitrary unit of 1000. Photo courtesy of NASA.

Figure 2

The NASA Johnson Space Center Off-Vertical Axis Rotation chair was used to tilt the ISS crewmembers 25° to the right or to the left.

Figure 3

OCR during whole body tilt (in red) and during head tilt (in blue) in the control group (N = 7). Average of the subjects’ three trials at each of the body or head tilt angles. The green symbols/line represent the data from Miller that were obtained during whole body tilt using photographic methods for measuring eye torsion (N = 100). Second order polynomial least-square curve fitting. Mean ± SD. *P < 0.05 (between whole body tilt and head tilt at ±45°).

Figure 4

OCR during head tilt relative to the body before (A), during (B), immediately after (R + 0), and after 3 days (R + 3) after short-duration spaceflight. Blue symbols: Mean ± SD of OCR of the 6 Space Shuttle crewmembers averaged across 3 trials and all 6 preflight (A) and inflight (B) sessions. Mean ± SD of OCR of each of the 6 crewmembers averaged across 3 trials at each of the neck tilt angles at R + 0 (C) and R + 3 (D). Second order polynomial least-square curve fitting. Red symbols: Mean ± SD of OCR measured by Vogel & Kass and Young & Sinha on 8 crewmembers before and after 2 Space Shuttle missions using photography for measuring eye torsion during whole body tilt. Green symbols: Mean ± SD of OCR measured by Cheung et al. on 6 participants in normal gravity (A) and microgravity during parabolic flight (B) using eye coils for measuring eye torsion during head tilt.

Figure 5

Asymmetry ratio of OCR during head tilt of 15°, 30°, and 45° before (Pre), during (In), and after (R + 0, R + 3) a short-duration spaceflight. Mean ± SD, N = 6. The mean ± SD asymmetry ratio obtained in the control group (N = 7) during head tilt is shown for comparison.

Figure 6

(A) Right and left OCR during body tilt of ±25° relative to gravity before and after a long-duration spaceflight. Mean ± SD of the 6 ISS crewmembers averaged across all 3 preflight sessions (Pre) and during each post-flight session. *P < 0.05 relative to preflight; ^#P < 0.05 relative to R + 1. (B) Asymmetry ratio of OCR of the 6 ISS crewmembers. Mean ± SD. Gray lines show individual data.

Figure 7

Overall results of 11 OCR studies performed before and after spaceflight. Each data point represents the percentage of subjects showing an OCR decrease postflight relative to preflight for the duration of the flight corresponding to that particular study. Study reference and number of subjects tested are indicated next to each data point. Results of the present Space Shuttle and ISS studies are shown in red. Note the logarithmic scale on the x-axis (Flight Duration). The blue line depicts a logarithmic function fitted to the data (r² = 0.69).