Spatial orientation and balance control changes induced by altered gravitoinertial force vectors

To better understand the mechanisms of human adaptation to rotating environ ments, we exposed 19 healthy subjects and 8 vestibular-deficient subjects ( "abnormal"; four bilateral and four unilateral lesions) to an interaural ce ntripetal acceleration of Ig (resultant 45 degrees roll-tilt of 1.4g) on a 0.8-m-radius centrifuge for periods of 90 min. The subjects sat upright (bo dy z-axis parallel to centrifuge rotation axis) in the dark with head stati onary, except during 4 min of every 10 min, when they performed head saccad es toward visual targets switched on at 3- to 5-s intervals at random locat ions (within +/- 30 degrees) in the earth-horizontal plane. Eight of the no rmal subjects also performed the head saccade protocol in a stationary chai r adjusted to a static roll-tilt angle of 45 degrees for 90 min (reproducin g the change in orientation but not the magnitude of the gravitoinertial fo rce on the centrifuge). Eye movements, including voluntary saccades directe d along perceived earth- and head-referenced planes, were recorded before, during, and immediately after centrifugation. Postural center of pressure ( COP) and multisegment body kinematics were also gathered before and within 10 min after centrifugation. Normal subjects overestimated roll tilt during centrifugation and revealed errors in perception of head-vertical provided by directed saccades. Errors in this perceptual response tended to increas e with time and became significant after approximately 30 min. Motion-sickn ess symptoms caused approximately 25% of normal subjects to limit their hea d movements during centrifugation and led three normal subjects to stop the test early. Immediately after centrifugation, subjects reported feeling ti lted 10 degrees in the opposite direction, which was in agreement with the direction of their earth-referenced directed saccades. Postural COP, segmen tal body motion amplitude, and hip-sway frequency increased significantly a fter centrifugation. These postural effects were short-lived, however, with a recovery time of several postural test trials (minutes). There were also asymmetries in the direction of postcentrifugation COP and head tilt which depended on the subject's orientation during the centrifugation adaptation period (left ear or right ear out). The amount of total head movements dur ing centrifu gation correlated poorly or inversely with postcentrifugation postural stability, and the most unstable subject made no head movements. T here was no decrease in postural stability after static tilt, although thes e subjects also reported a perceived tilt briefly after return to upright, and they also had COP asymmetries. Abnormal subjects underestimated roll-ti lt during centrifugation, and their directed saccades revealed permanent sp atial distortions. Bilateral abnormal subjects started out with poor postur al control, but showed no postural decrements after centrifugation, while u nilateral abnormal subjects had varying degrees of postural decrement, both in their everyday function and as a result of experiencing the centrifugat ion. In addition, three unilateral, abnormal subjects, who rode twice in op posite orientations, revealed a consistent orthogonal pattern of COP offset s after centrifugation. These results suggest that both orientation and mag nitude of the gravitoinertial vector are used by the central nervous system for calibration of multiple orientation systems. A change in the backgroun d gravitoinertial force (otolith input) can rapidly initiate postural and p erceptual adaptation in several sensorimotor systems, independent of a stru ctured visual surround.