The three-dimensional vestibule-ocular reflex during prolonged microgravity

Single-case, longitudinal studies of the three-dimensional vestibule-ocular response (VOR) were conducted with two spaceflight subjects over a 180-day mission. For reference, a control study was performed in the laboratory wi th 13 healthy volunteers. Horizontal, vertical and torsional VOR was measur ed during active yaw, pitch and roll oscillations of the head, performed du ring visual fixation of real and imaginary targets. The control group was t ested in the head-upright position, and in the gravity-neutral, onside and supine positions. Binocular eye movements were recorded throughout using vi deo-oculography, yielding eye position in Fick co-ordinates. Eye velocity w as calculated using quaternion algebra. Head angular velocities were measur ed by a head-mounted rate sensor. Eye/head velocity gain and phase were eva luated for the horizontal, vertical and torsional VOR. The inclination of L isting's plane was also calculated for each test session. Control group gai n for horizontal and vertical VOR was distributed closely around unity duri ng real-target fixation, and reduced by 30-50% during imaginary-target tria ls. Phase was near zero throughout. During head pitch in the onside positio n, vertical VOR gain did not change significantly. Analysis of up/down asym metry indicated that vertical VOR gain for downward head movement was signi ficantly higher than for upward head movement. Average torsional VOR gain w ith real-target fixation was significantly higher than with imaginary-targe t fixation. No difference in phase was found. In contrast to vertical VOR g ain, torsional VOR gain was significantly lower in the gravity-neutral supi ne position. Spaceflight subjects showed no notable modification of horizon tal or vertical VOR gain or phase during real-target fixation over the cour se of the mission. However, the up/down asymmetry of vertical VOR gain was inverted in microgravity. Torsional VOR gain was clearly reduced in microgr avity, with some recovery in the later phase. After landing, there was a di p 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 th roughout 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/do wn gain asymmetry in microgravity is attributed to the off-loading of the c onstant l-g bias (primarily to the saccules) on Earth. The observed increas e in torsional VOR gain from the Ist to the 6th month in microgravity demon strates the existence of longer-term adaptive processes than have previousl y been considered. Likely factors are the adaptive reweighting of neck-prop rioceptive afferents and/or enhancement of efference copy.