VESTIBULAR PLASTICITY FOLLOWING ORBITAL SPACEFLIGHT - RECOVERY FROM POSTFLIGHT POSTURAL INSTABILITY

Results of previous studies suggested that the vestibular mediated pos tural instability observed in astronauts upon return to earth from orb ital spaceflight may be exacerbated by an increased weighting of visua l inputs for spatial orientation and control of movement. This study w as performed to better understand the roles of visual and somatosensor y contributions to recovery of normal sensori-motor postural control i n returning astronauts. Preflight and postnight, 23 astronaut voluntee rs were presented randomly with three trials of six sensory organizati on test (SOT) conditions in the EquiTest system test battery. Sagittal plane center-of-gravity (COG) excursions computed from ground reactio n forces were significantly higher on landing day than preflight for t hose test conditions presenting sway-referenced visual and/or somatose nsory orientation cues. The ratio of summed peak-to-peak COG sway ampl itudes on the two sway-referenced vision tests (SOTs 3 + 6) compared t o the two eyes closed tests (SOTs 2 + 5) was increased on landing day, indicating an increased reliance on visual orientation cues for postu ral control. The ratio of peak-to-peak COG excursions on sway-referenc ed surfaces (SOTs 4, 5 & 6) to an earth fixed support surfaces (SOTs 1 , 2 & 3) increased even more after landing suggesting primary reliance on somatosensory orientation cues for recovery of postflight postural stability. Readaptation to sway-referenced support surfaces took long er than readaptation to sway-referenced vision. The increased reliance on visual and somatosensory inputs disappeared in all astronauts 4-8 days following return to earth.