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Séamas Weech, Nikolaus F Troje; Stick figures and point-light displays: Effects of inversion on the facing-the-viewer bias. Journal of Vision 2014;14(10):1024. doi: 10.1167/14.10.1024.
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Depth-ambiguous point-light walkers are most frequently seen as facing-the-viewer (FTV). Inverting the figures considerably reduces this FTV bias (Vanrie et al., 2004). The finding has been used to argue that the FTV bias depends on recognizing the stimulus as a person which is more difficult when the stimulus is inverted. Recent experiments indicate that the FTV bias is largely caused by a bias to perceive depth-ambiguous surfaces as convex (Weech and Troje, 2013). Based on this research, we hypothesized that the effect of inversion on FTV bias arises due to the difficulty with which coherent 3D shape is resolved from inverted point-light walkers. Without this shape, the stimulus appears flat and the convexity bias does not play out. If explicit, coherent shape is provided (as in stick figures) we would expect no effect of inversion on FTV bias. We measured the FTV bias in 30 participants for upright and inverted point-light walkers and stick figures. We depicted stimuli at frontal and three-quarter views and recorded observers perceived facing directions. We defined the FTV bias as the percentage of responses signaling a facing-towards interpretation. Participants accurately chose one of the two veridical interpretations at a rate of over 95% for both stimulus types. We found an interaction between stimulus representation and orientation: The inversion effect for stick figures (44%) was smaller than that for point-light walkers (55%). This result supports our hypothesis to a limited degree. Unexpectedly, both stimulus types generated reliable facing-away bias when inverted. Results are consistent with the hypothesis that the lower part of the stimulus takes precedence when subjects are making judgments of facing directions, given that the knees and elbows are opposing in terms of the facing direction implied when assumed to be convex.
Meeting abstract presented at VSS 2014
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