The elevator illusion results from the combination of body orientation and egocentric perception

Abstract

Perception of body orientation and apparent location of objects are altered when humans are using assisted means of locomotion and the resultant of the imposed acceleration and gravity is no longer aligned with the gravitational vertical. As the otolithic system cannot discriminate the acceleration of gravity from sustained inertial accelerations, individuals would perceive the resultant acceleration vector (GiA) as the vertical. However, when subjects are aligned on the GiA, an increase in the magnitude of GiA induced a lowering of the apparent visual horizon (i.e. "elevator illusion"). The main aim of this study was to quantify the contribution of body and egocentric perception in the elevator illusion. While being exposed to 1G and 1.3G and aligned on the GiA acceleration, subjects (N=20) were asked (1) to set a luminous target to the subjective horizon, (2) to set a luminous target on "straight ahead" position (egocentric task) and (3) to rotate a tilting tube to their subjective perception of body orientation. Results showed that increasing GiA lowered horizon and egocentric settings and induces a backward body tilt perception. Moreover, the elevator illusion can be expressed as the additive combination of two processes: one that is dependent on body tilt perception, and the other that is dependent on egocentric perception. Both misperceptions in hypergravity may be considered to be a consequence of excessive shearing of the otolith organs. However large inter-individual differences in body tilt perception were observed. This last result was discussed in terms of the contribution of extravestibular graviceptors.