Left/Right asymmetries in the contribution of body orientation to the perceptual upright

INTRODUCTION The direction of the orientation at which objects and characters are most easily recognized, the perceived upright has been modelled as a weighted vector sum of the directions defined by the body's long axis (egocentric), gravity, and visible cues (Dyde et al. 2006, Exp. Brain Res.). This model predicts symmetrical responses such that subjects lying left or right side down relative to gravity should exhibit mirror symmetric patterns of responses. Such symmetry is also expected if torsional eye orientation dependent upon body orientation relative to gravity or visual orientation relative to the body is included in the model. METHODS Nineteen subjects drawn from researchers and students at York University participated. The Oriented Character Recognition Test (OCHART - described in Dyde et al. 2006) was administered while subjects viewed several orientations of visual background while either upright, left side down, or right side down relative to gravity. OCHART identifies the perceptual upright using the perceived identity of letters. RESULTS Responses revealed a systematic difference between the response pattern when lying left side down and lying right side down. This asymmetry can be modelled by a leftwise bias in the perceived orientation of the body relative to its actual orientation. DISCUSSION The asymmetry in the effect of body orientation is reminiscent of the left-leaning asymmetry in determining the direction of light coming from above (Mamassian Goutcher 2001 Cognition 81:B1). The asymmetry might reflect a similar tendency to perceive the body as tilted