Spatial disorientation in hypergravity: implication of oculomotor, otolithic and somatosensory cues

Travelling on board of locomotion vehicles often induce sensorial stimulations which provoke perceptive illusions. These erroneous perceptions of body position were defined in the literature as spatial disorientation, which is principally considered as otolithic origin. In fact, during a forward sustained linear acceleration, the resultant of the imposed acceleration and gravity is no longer aligned with the gravitational vertical. As other physical accelerometer, the otolithic system cannot discriminate the acceleration of gravity from sustained inertial acceleration, and will perceive this resultant, defined as gravito-inertial acceleration (GiA), as the new vertical. Elevator and oculogravic illusions are listed among the most experienced behavioural effects. These illusions are defined as erroneous visual perception of object position in hypergravity environment. This thesis takes an interest in i) sensorial information cues involved in perceptive illusions in hypergravity, ii) interindividuals differences in the estimation of visual horizon, iii) the role of egocentric and exocentric references implicated in elevator and oculogravic illusions. Our results highlight: (1) the fundamental role of gravito inertial intensity in the visual horizon perception, (2) the coupled effect of body and visual egocentric perceptions in visual horizon estimation, (3) inter-individuals differences, due to the modulation of gravito-inertial intensity, explained by otolithic cues, (4) the contribution of somesthetic cues, in particular for oculogravic illusion. All of the studies referred in this thesis suggest a sensory approach in otolithic and somesthetic terms, as well as an individual approach for visual horizon perception.