Neurovestibular effects of bed rest and centrifugation.

Subjective body position, as influenced by 21-day 6 degrees -head-down bed rest and intermittent supine centrifugation, was investigated in two experimental groups. Treatment subjects were exposed to a daily dose of 1-hour of horizontal centrifugation, whereas control subjects were also put on the centrifuge, but were not spun. During the centrifuge runs subjects were exposed to a constant gradient with 1-G(z) at the heart and 2.5-G(z) at the feet. Subjects reported their subjective postural position (SPP) before, during and after each 1-hour centrifuge exposure. We find that subjects shift their frame of reference over the first week of bed rest and perceive the 6 degrees -head-down position as horizontal. Spinning shifts the SPP by about 30 degrees towards an upright position, consistent with the angle of the gravito-inertial vector positioned close to the vestibular system. The SPP estimates for the upper and lower body failed to reflect the massive gravity gradient between head and feet, and subjects generally reported a single tilt for the whole body.