A Case Study of Human Roll Tilt Perception in Hypogravity

Increased gravito-inertial acceleration, or hypergravity, such as produced in a centrifuge or in an aircraft coordinated turn, causes humans to systematically overestimate their roll tilt in the dark. This is known as the "G-excess" illusion. We have previously modified a mathematical observer model of dynamic orientation perception to replicate these illusory tilt perceptions. This modified model also made a novel, previously untested, prediction that humans would underestimate acute roll tilt in reduced gravitational environments (hypogravity).

In the current study, we used aircraft parabolic flight to test this prediction in a single subject. Roll tilt perception was reported using a subjective visual vertical task in which the subject aligned an illuminated line, presented in a head mounted display, with their perceived direction of down. The same subject made reports during hypogravity parabolas (0.165 G and 0.38 G, corresponding to lunar and Martian gravity, respectively), hypergravity maneuvers (1.6 G during a pull out maneuver and 1.2 G during a coordinated turn), and 1-G control conditions (both on the ground and in straight and level flight). As hypothesized, the subject significantly underestimated roll tilt in the hypogravity environments by approximately 40% compared to 1-G reports while overestimating roll tilt in the hypergravity environments.

The amount of underestimation observed was quantitatively consistent with that predicted a priori by the modified observer model. We propose the term "G-shortage" illusion for the underestimation of roll tilt in hypogravity. This illusion may have implications for aircraft pilots and astronauts.Clark TK, Young LR. A case study of human roll tilt perception in hypogravity. Aerosp Med Hum Perform. 2017; 88(7):682-687.