The somatogravic illusion during centrifugation: sex differences

Maintaining orientation in an environment with non-Earth gravity (1 g) is critical for an astronaut's operational performance. Such environments present a number of complexities for balance and motion. For example, when an astronaut tilts due to ascending or descending an inclined plane on the moon, the gravity vector will be tilted correctly, but the magnitude will be different from on earth. If this results in a mis-perceived tilt, then that may lead to postural and perceptual errors, such as mis-perceiving the orientation of oneself or the ground plane and corresponding errors in task judgment. Rotation on a centrifuge offers a unique opportunity to vary the direction of the gravity vector without physical tilt, that is, without co-activation of the semicircular canals during the simulated tilt. The tilt angle simulated is the tilt of the simple vector sum of gravity and the acceleration added by the centrifuge. Perceiving acceleration as tilt is the well-known somatogravic effect [Mach1875, Clark1951]. Sustained linear acceleration together with gravity creates a single gravito-inertial force (GIF). Under normal gravity conditions, sustained linear acceleration in the transverse plane can create an illusion of tilt - the somatogravic illusion - in which the entire GIF is interpreted as corresponding to gravity. However, the magnitude of the effect, i. e. the fraction of the GIF that is interpreted as gravity, has not been well quantified in the sagittal plane. We, therefore, varied the added acceleration to induce a somatogravic illusion and measured the perceptual effects using a haptic rod to indicate the perceived direction of gravity. Previous experiments measuring the somatogravity illusion have used a swinging gondola, hypergravity and a visual assessment method [Tribukait1999, Tribukait2006]. Our experiments used a haptic rod outside of the participant's view, ruling out effects of eye torsion, and a fixed chair mounted so that the participant sat upright and nose-out to provide a controlled and constant direction of otolith stimulation over a physiologically valid range of acceleration forces