To Pass or not to Pass: More a Question of Body Orientation than Visual Cues

Abstract

This study investigated the influence of pitch body tilt on judging the possibility of passing under high obstacles in the presence of an illusory horizontal self-motion. Seated subjects tilted at various body orientations were asked to estimate the possibility of passing under a projected bar (i.e., a parking barrier), while imagining a forward whole-body displacement normal to gravity. This task was performed under two visual conditions, providing either no visual surroundings or a translational horizontal optic flow that stopped just before the barrier appeared. The results showed a main overestimation of the possibility of passing under the bar in both cases and most importantly revealed a strong influence of body orientation despite the visual specification of horizontal self-motion by optic flow (i.e., both visual conditions yielded a comparable body tilt effect). Specifically, the subjective passability was proportionally deviated towards the body tilt by 46% of its magnitude when facing a horizontal optic flow and 43% without visual surroundings. This suggests that the egocentric attraction exerted by body tilt when referring the subjective passability to horizontal self-motion still persists even when anchoring horizontally related visual cues are displayed. These findings are discussed in terms of interaction between spatial references. The link between the reliability of available sensory inputs and the weight attributed to each reference is also addressed.

Body Orientation Affects the Perceived Size of Objects

“…Despite its importance, people often make errors when determining an object's size which can depend on the size of the observer themselves ( Stefanucci & Geuss, 2009 ), where the object is positioned in the field of view ( Bertamini et al, 1998 ), and the exposure time ( Thomas & Cantor, 1976 ). Misperceived size can result in some undesirable and potentially dangerous errors, for example, when a person needs to quickly evaluate how much to move to avoid an approaching object or to navigate through a narrow doorway ( Bourrelly et al, 2014 ).…”