Many astronauts experience space adaptation syndrome, which may include illusions, loss of knowledge of limb position, nausea and vomiting. These symptoms may occur because of conflicting messages about body position and movement which the brain receives from the eyes, the balance organs of the inner ear and gravity sensing receptors in the muscles, tendons, and joints.
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The Space Adaptation Syndrome Experiment (SASE) consisted of seven investigations which studied adaptation of the human nervous system to weightlessness with an emphasis on the vestibular and proprioceptive systems. 1) The Sled/H-Reflex Experiment used the mini-sled to study changes in vestibular reflexes in order to determine how the absence of gravity affects human otolith organ function. 2) The Rotation/VOR Gain Experiment again used the mini-sled and recorded head and eye movements to determine why the vestibulo-ocular reflex (VOR) and other components of gaze control may be abnormal in weightlessness. 3) The Visual Stimulator Experiment examined the mechanism whereby the nervous system compensates for disrupted vestibular function in space. 4) The Proprioception (Relaxed) Experiment examined the degradation of proprioception in weightlessness. 5) The Proprioception (Active) Experiment examined similar degradation but following active muscular contractions. 6) Proprioception (Illusions) Experiment examined proprioceptive illusions and determined the effects of vision and tactile inputs on these illusions. 7) The Tactile Acuity Experiment examined the expanding of intervertebral discs and the resulting pressure blocks of nerve roots.
The crewmembers were subjected to limited vestibular testing in flight as well as pre and postflight. Major differences in individual "perceptual style" appeared in their reaction to the visual-vestibular stimuli in the rotating dome experiment, and especially in the extent to which non-directional tactile cues served to anchor the subjective vertical and body postural reactions. The ability of subjects to point to remembered target positions was degraded in space, which produced a tendency to point low in some subjects’ inflight. The eye movements and subjective response to sudden stops and head pitching following continuous spinning were measured both in space and on the ground. Although subjective duration of inflight rotation for the dumping tests was shorter than that for the preflight tests, the postrotatory nystagmus, with or without head pitch, was lengthened in time constant relative to preflight. Ground tests, in addition to the flight experiments, investigated the changes following weightlessness in subjective and oculomotor reactions to whole body tilt, the ability to balance with eyes open and closed; leg muscles strength and stamina as related to posture; visual field dependence; and the perceptual and oculomotor reactions to horizontal linear acceleration. Several of these tests, as well as post-flight measures of motion sickness susceptibility, revealed subtle evidence of neurovestibular alterations that lasted a week or more following the 10-day orbital exposure.
Watt D and Lefebvre L. Vestibular suppression during space flight. Journal of Vestibular Research.
Young LR, Oman CM, Merfeld D, Watt D, Roy S, DeLuca C, Balkwill D, Christie J, Groleau N, Jackson DK, et al. Spatial orientation and posture during and following weightlessness: human experiments on Spacelab Life Sciences 1. Journal of Vestibular Research.
1993. Fall; 3(3):231-9. [