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Séamas Weech, Nikolaus F. Troje; Does a convexity prior explain the facing-the-viewer bias in the perception of biological motion?. Journal of Vision 2013;13(9):187. doi: 10.1167/13.9.187.
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Point-light walkers generally contain no information about their orientation in depth, yet observers consistently prefer the facing-the-viewer (FTV) interpretation (Vanrie et al., 2004). Some research (Schouten et al., 2011) suggests that local stimulus properties elicit the bias: Presentation of the lower half of point-light figures elicits a pronounced FTV bias, while presentation of the upper half does not. Other research suggests high level causes: Male walkers generate a stronger FTV bias than female walker (Brooks et al., 2004). Interestingly, no FTV biases are observed with human silhouettes (Troje & McAdam, 2010). We hypothesise that the FTV bias is due to a convexity prior (Mamassian & Landy, 1998). Accordingly, the knees afford a FTV interpretation while the elbows, specifically when pointing back (as typical for women) rather than sideways (as in males), afford a facing-away interpretation. This requires visual structures other than occluding contours which are neither concave nor convex with respect to the line of sight. Here we asked observers to indicate perceived rotation directions (clockwise/counter clockwise) of a silhouette of a crouching human figure with knees pointing forward and elbows pointing back. Four conditions were included: silhouettes presented with no markers, with markers at the centre of the knees, with markers at the centre of the elbows, and with markers on both knees and elbows. We measured facing bias as the proportion of depth reversals from the ‘away’ to the ‘toward’ interpretation. The silhouette alone elicited a weak facing bias, as did the silhouette with both elbow and knee markers present. As predicted, silhouettes with knee markers only elicited a FTV bias while elbow markers alone elicited a notable facing away bias. These results help to interpret and unify various findings regarding the FTV bias, and support the idea that an experientially-driven convexity prior guides interpretations of depth-ambiguous figures.
Meeting abstract presented at VSS 2013
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