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

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...