Abstract. Single-case,
longitudinal studies of the three-dimensional vestibulo-ocular response
(VOR) were conducted with two spaceflight subjects over a 180-day
mission. For reference, a control study was performed in the laboratory
with 13 healthy volunteers. Horizontal, vertical and torsional VOR was
measured during active yaw, pitch and roll oscillations of the head,
performed during visual fixation of real and imaginary targets. The
control group was tested in the head-upright position, and in the
gravity-neutral, onside and supine positions. Binocular eye movements
were recorded throughout using video-oculography, yielding eye position
in Fick co-ordinates. Eye velocity was calculated using quaternion
algebra. Head angular velocities were measured by a head-mounted rate
sensor. Eye/head velocity gain and phase were evaluated for the
horizontal, vertical and torsional VOR. The inclination of Listing's
plane was also calculated for each test session. Control group
gain for horizontal and vertical VOR was distributed closely around
unity during real-target fixation, and reduced by 30-50% during
imaginary-target trials. Phase was near zero throughout. During head
pitch in the onside position, vertical VOR gain did not change
significantly. Analysis of up/down asymmetry indicated that vertical
VOR gain for downward head movement was significantly higher than for
upward head movement. Average torsional VOR gain with real-target
fixation was significantly higher than with imaginary-target fixation.
No difference in phase was found. In contrast to vertical VOR gain,
torsional VOR gain was significantly lower in the gravity-neutral
supine position. Spaceflight subjects showed no notable
modification of horizontal or vertical VOR gain or phase during
real-target fixation over the course of the mission. However, the
up/down asymmetry of vertical VOR gain was inverted in microgravity.
Torsional VOR gain was clearly reduced in microgravity, with some
recovery in the later phase. After landing, there was a dip in gain
during the first 24 h, with subsequent recovery to near baseline over
the 13-day period tested. Listing's plane appeared to remain stable
throughout the mission. The findings reflect various functions of the
otolith responses. The reduced torsional VOR gain in microgravity is
attributed to the absence of the gravity-dependent, dynamic stimulation
to the otoliths (primarily utricles). On the other hand, the reversal
of vertical VOR up/down gain asymmetry in microgravity is attributed to
the offloading of the constant 1-g bias (primarily to the
saccules) on Earth. The observed increase in torsional VOR gain from
the 1st to the 6th month in microgravity demonstrates the existence of
longer-term adaptive processes than have previously been considered.
Likely factors are the adaptive reweighting of neck-proprioceptive
afferents and/or enhancement of efference copy.
