The Facing-the-Viewer Bias in the Perception of Depth Ambiguous Human Figures
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Orthographically-projected biological motion point-light displays generally contain no information about their in-depth orientation, yet observers consistently prefer the facing-the-viewer (FTV) interpretation (Vanrie, Dekeyser and Verfaillie, 2004). This bias has been attributed to the social relevance of such stimuli (Brooks et al., 2008) although local stimulus properties appear to influence the bias (Schouten, Troje and Verfaillie, 2011). In the present study we investigated the cause of the FTV bias. In Experiment 1 we compared FTV bias for various configurations of stick-figures and depth ambiguous human silhouettes. The FTV bias was not present for silhouettes, but was strongly elicited for most stick-figures. We concluded that local attitude assignments for intrinsic structures of stick-figures are subject to inferences about the flexion of body surfaces, and that a visual bias that assumes surfaces to be convex drives the FTV bias. In Experiment 2 we manipulated silhouettes to permit local attitude assignments by using point-lights on emphasized flexion points. As predicted, the inclusion of intrinsic structures produced FTV bias for silhouettes. The results help to unify various findings regarding the FTV bias. We conclude that the FTV bias emerges during the 2 ½-D sketch stage of visual processing (Marr and Nishihara, 1978).