THE GRAVITY REFERENCE RESPONSE, THE ROTAT ION SENSA- T ION, AND OTHER ILLUSORY SENSATIONS EXPERIENCED IN A IRCRAFT AND SPACE FL IGHT*

Abstract. An analysis of the gravitational and inertial forces which act during aircraft flight upon the vestibular systems of the aircraft occupants reveals that in the absence of a visual horizon, certain illusory sensations are predictable for various acceleration environments. Horizontal forward applied acceleration results in a climbing (backward tilting) sensation; conversely, horizontal rearward applied acceleration results in a diving (forward tilting) sensation. During any attempt to achieve weightless-ness in aircraft parabolic flight, special care has to be taken to avoid unintended longitudinal (x-axis) accelerations. Recent flight tests established that the 'rotation sensation ' (Dzendolet, 1971; Gerathe-wohl, 1956) during entry into parabolic flight can be attributed tothe existence ofunintended longitu-dinal accelerations. However, the 'inversion illusion ' (Graybiel and Kellogg, 1966) felt by some human subjects at 0 g seems to be different from the rotation sensation and could be caused by the diminished pressure forces of the otoliths on the maculae. The 'inversion illusion ' of man correlates well with the blind fish diving behavior observed uring aircraft parabolic flight (yon Baumgarten etal., 1969, 1972). It is suggested that the fish low g diving response and the human inversion illusion are due to the substitution of a predominately shearing force of low magnitude as a vestibular eference in place of a normal, predominately pressure force. This hypothesis indicates that vestibular senses alone cannot provide meaningful postural orientation tosimulated or actual gravity of a magnitude below that of Earth's gravity. 1