The vestibular system, or the balance system, is the sensory system that provides input about one's movement and orientation in space. This system is located in the inner ear where two balance-sensing organs reside, the semicircular canals and the otoliths. Each organ senses a different awareness relative to gravity. The semicircular canals sense head angular acceleration, such as when an individual is rotating; whereas, the otoliths sense both head linear acceleration and head tilt relative to gravity. As a result, during rotation at a constant velocity about an axis tilted at various degrees relative to being upright, only the otolith organs are stimulated by the change in head orientation relative to gravity. When the otoliths sense motion, they send neurosensory signals that control eye movement and to the muscles that control posture. Since gravity is equivalent to a constant linear acceleration, the absence of gravity in space can greatly affect this vestibular system, especially the role played by the otolith organs.
The objective of this experiment was to assess the changes in otolith information processing following adaptation to microgravity by comparing otolith-induced eye movements and self-motion perception before and early postflight. This objective was met by recording eye movement and self-motion perception during Off-Vertical Axis Rotation (OVAR) to accurately reflect the neural processing of otolith signals by the brain. This experiment also determined the time course of recovery of otolith-tilt responses back to baseline levels by repeating postflight data collection at regular intervals.
This experiment was designed not only to understand the fundamental mechanisms by which the central nervous system processes otolith information to derive compensatory eye movements and motion perception, but also to understand the functional organization and plasticity of the central nervous system when exposed to a novel environmental situation, such as microgravity. This experiment helped provide information for postural instability in astronauts after flight and in patients suffering from similar symptoms on Earth.
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This experiment collected data preflight and postflight. The otolith data was recorded while the subject was rotated clockwise and counterclockwise at various angles and speeds using the Off-Vertical Axis Rotation (OVAR) chair. Subjects experienced off-vertical axis rotation in darkness at an angular velocity of 45 degrees/s (0.125 Hz) at 0 degrees, 10 degrees, and 20 degrees of tilt and at 180 degrees/s (0.5 Hz) at 0 degrees and 20 degrees of tilt. Subjects were rotated in both the clockwise and counter-clockwise directions. Oculomotor responses were recorded using infrared video-oculography, and perceived motion was evaluated using verbal reports and a joystick with four degrees of freedom (pitch and roll tilt, front-back and lateral translation). The sessions were scheduled three times preflight to establish a baseline value for each subject and repeated as soon as possible after return to Earth, for a total of four sessions within the first twelve days of landing.
Results from experiments with similar objectives conducted by the same investigators have been published. The citations for these publications are listed in the Experiment References section.
Clement G, Pierre D., Reschke MF, Wood SJ, Clement G. Human ocular counter-rolling and roll tilt perception during off-vertical axis rotation after spaceflight. J Vestib Res. 2007; 17(5-6):209-15.[
Clement G, Reschke M, Wood S. Neurovestibular and sensorimotor studies in space and earth benefits. Curr Pharm Biotechnol. August 2005; 6(4):267-283.[
Wood SJ, Reschke MF, Sarmiento LA, Clement G. Tilt and translation motion perception during off-vertical axis rotation. Exp Brain Res. September 2007; 182(3):365-377.[
3-D displacement (horizontal, vertical, torsion) of both right and left eyes
Stimuli (chair velocity, tilt angle and position)
Verbal description of subjective motion perception