Perceived stability of objects changes when the body is tilted relative to gravity

Knowing an object's physical stability affects our expectations about its behaviour and our interactions with it. Objects topple over when the gravity-projected centre-of-mass (COM) lies outside the support area. The critical angle (CA) is the orientation for which an object is perceived to be equally likely to topple over or right itself, which is influenced by global shape information about an object's COM and its orientation relative to gravity. When observers lie on their sides, the perceived direction of gravity is tilted towards the body. Here we test the hypothesis that the CA of falling objects is affected by this internal representation of gravity. Observers sat upright or lay left- or right-side-down, and observed images of objects with different 3D mass distributions that were placed close to the right edge of a table in various orientations. Observers indicated whether the objects were more likely to fall back onto or off the table. The subjective visual vertical was also tested as a measure of perceived gravity. Our results show the CA increases when lying right-side-down and decreases when left-side-down relative to an upright posture, consistent with estimating the stability of rightward falling objects as relative to perceived and not physical gravity