Control of Posture, Subjective Vertical, and Body Scheme in Changing Gravitoinertial Field

Spatial disorientation associated to decrease of sensorimotor performance may result in observers evolving in changing gravitoinertial field as created in aircrafts and other high dynamics modern vehicles. We have investigated the influence of body support orientation on the perception of egocentric and exocentric reference frames in 8 observers standing 80 cm off-center on a platform rotating at 120 deg.s. Standing support was either a fixed horizontal board or a swinging pendulum. The perceptive task consisted in adjusting the orientation of a visual rod to indicate geocentric (subjective vertical (SV) and horizontal (SH)) and egocentric (head, trunk and platform orientations) references. Subjects’ head and trunk, as well as rod orientations, were recorded with electromagnetic sensors (Polhemus Fastrack). The platform was equipped with sensors providing heel and toe vertical and lateral forces from both feet. Platform motion (when allowed to rotated in the swinging pendulum condition) was recorded with a potentiometer. We analyzed body postural reactions and compared veridical and perceived orientations in the two body support conditions. In the horizontal platform condition, to compensate the mechanical constraints caused by the centrifugal force, subjects leaned toward the axis of rotation adopting a hyperbolic body shape. Head was aligned with the gravitoinertial force. SH was not sensed as orthogonal to the SV. Foot pressure (vertical and lateral components) was higher under the outer than under the inner foot. In the swinging pendulum condition, SV and body were aligned with the gravitoinertial vector. SH and SV were orthogonal. Foot pressure was the same under both feet. In both conditions, head and body orientations were overestimated. The data suggest that, as the gravitoinertial vector evolves, the vestibular system induces compensatory postural adjustments. The hyperbolic body shape is thought to be due to body multisegmental coordination. Other postural adjustments, thought to be neither of vestibular nor of visual origin, occur to keep the projection of the combined forces in the body support area. Although observers provided erroneous cognitive answers, they maintained postural balance in both conditions suggesting sensorimotor control to be independent of posture related cognitive processes.