The human response to artificial gravity in a weightless environment: Results from the Neurolab centrifugation experiments

Abstract

On Earth, afferent otolith activity encodes head tilt with respect to gravity, as well as head linear acceleration. The vector sum of head acceleration and gravity, termed gravito-inertial acceleration (GIA), is used by the vestibular system to generate the perception of vertical, ocular counter-rolling (OCR), and vertical or roll components of nystagmus, which orient the axis of eye velocity towards alignment with the GIA. Head tilt is not sensed by the otoliths in microgravity, but the otoliths are still activated by translation. It has therefore been proposed that the brain reinterprets tilt-related otolith information as translation during space flight. During the 1998 Neurolab mission, 4 astronauts were exposed to inter-aural (IA) and body vertical (Z-axis) centripetal accelerations of 0.5 and 1-g by rotation in an off-axis centrifuge. During rotation, binocular 3D eye movements were measured using a video technique with the subject in darkness and while viewing an optokinetic stimulus. On Earth, 1-g centrifugation tilts the GIA 45° with respect to the head, and pre-flight perception of roll tilt was 35°. By flight day 5, all astronauts perceived ≈90° of roll tilt in response to a constant 1-g IA linear acceleration. This tilt angle was under-reported early in flight, and over-estimated on return to Earth, possibly due to adaptation of otolith and/or somatosensory function during transition to the new gravitational states. Tilt perception was proportionally lower during 0.5-g centrifugation, both pre-flight (20°) and in-flight (46°). Similar results were obtained for pitch tilt perception during Z-axis centrifugation. The magnitude of OCR in response to the GIA during IA centrifugation was unchanged in microgravity, and was proportional to the centripetal acceleration. This suggests that both OCR and roll tilt perception are strongly dependent on the magnitude of IA linear acceleration, as in microgravity there is no body vertical (Z-axis) linear acceleration. The axis of eye velocity during both horizontal optokinetic nystagmus and angular acceleration in darkness tended to align with the GIA, as on Earth. In space, low frequency otolith information is not reinterpreted as translation, and constant 1-g linear acceleration is sensed as being equivalent to gravity.