Changes in gain of horizontal vestibulo-ocular reflex during spaceflight

Abstract

Background: The vestibulo-ocular reflex (VOR) is a basic function of the vestibular system that stabilizes gaze during head movement. Investigations on how spaceflight affects VOR gain and phase are few, and the magnitude of observed changes varies considerably and depends on the protocols used.

Objective: We investigated whether the gain and phase of the VOR in darkness and the visually assisted VOR were affected during and after spaceflight.

Methods: We measured the VOR gain and phase of 4 astronauts during and after a Space Shuttle spaceflight while the subjects voluntary oscillated their head around the yaw axis at 0.33 Hz or 1 Hz and fixed their gaze on a visual target (VVOR) or imagined this target when vision was occluded (DVOR). Eye position was recorded using electrooculography and angular velocity of the head was recorded with angular rate sensors.

Results: The VVOR gain at both oscillation frequencies remained near unity for all trials. DVOR gain was more variable inflight and postflight. Early inflight and immediately after the flight, DVOR gain was lower than before the flight. The phase between head and eye position was not altered by spaceflight.

Conclusion: The decrease in DVOR gain early in the flight and after the flight reflects adaptive changes in central integration of vestibular and proprioceptive sensory inputs during active head movements.

Keywords: Vestibular nystagmus; microgravity; vestibulo-ocular reflex; visual fixation; visual-vestibular interaction.

Fig.1

Left. Photograph showing a crewmember performing the experiment on board the Space Shuttle. The rate sensors housing unit is seen on the top of subject’s head. Directly in front of the subject is the cruciform target display used for head and eye calibrations, and for the presentation of the central target during head oscillations. Right. Photograph showing the head-mounted laser for head position calibration and the goggles that can occlude the subject’s vision. The black box is the controller used by the operator to control the visual display, the goggles occluding, and the cassette tape recorder that plays the frequency-modulated tones used for pacing the head oscillations. Photos credit: NASA.

Fig.2

Examples of head position recordings as a function of time accompanying a 0.33 Hz modulated tone (A) and a 1.0 Hz modulated tone (B) in the VVOR condition and the corresponding power spectral analyses. X-axis units are s for head position and Hz for power spectrum. Y-axis units are degrees.

Fig.3

Peak-to-peak amplitude of head movements about the yaw, pitch, and roll axes for both oscillation frequencies and visual conditions. Note that the head movements were predominantly in the yaw plane throughout all flight phases. Mean±SD of all subjects.

Fig.4

VVOR (A,B) and DVOR (C,D) gain during head oscillations at 0.33 Hz and 1.0 Hz as a function of flight day. Symbols represent individual subject’s values and continuous line is the median gain.

Fig.5

VVOR (A,B) and DVOR (C,D) phase during head oscillations at 0.33 Hz and 1.0 Hz as a function of flight day. Symbols represent individual subject’s values and continuous line is the median gain.