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Nikolaus Troje; Perceptual biases in biological motion perception and other depth-ambiguous stimuli. Journal of Vision 2010;10(7):792. doi: 10.1167/10.7.792.
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© ARVO (1962-2015); The Authors (2016-present)
Biological motion stick-figures rendered orthographically and without self-occlusions do not contain any information about the order of their elements in depth and therefore are consistent with at least two different in-depth interpretations. Interestingly, however, the visual system often prefers one over the other interpretation. In this study, we are investigating two different sources for such biases: the looking-from-above bias and the facing-the-viewer bias (Vanrie et al. 2004). We measure perceived depth as a function of the azimuthal orientation of the walker, the camera elevation, and the walker's gender, which have previously been reported to also affect the facing bias (Brooks et al, 2008). We also compare dynamic walkers with static stick-figure displays. Observers are required to determine whether 0.5 s presentations of stick-figures are rotating clockwise or counter-clockwise – basically telling us in that way which of the two possible in-depth interpretations they are perceiving. In contrast to previous work, this measure is entirely bias-free in itself. Data collected with this method show that the facing-the-viewer bias is even stronger than previously reported and that it entirely dominates the viewing-from-above bias. Effects of walker gender could not be confirmed. Static figures which imply motion result in facing biases which are almost as strong as obtained for dynamic walker. The viewing-from-above bias becomes prominent for the profile views of walkers, for which the facing-the-viewer bias does not apply, and for other depth ambiguous stimuli (such as the Necker cube). In all these cases, we find a very strong bias to interpret the 2D image in terms of a 3D scene as seen from above rather than from below. We discuss our results in the context of other work on depth ambiguous figures and look at differences between the initial percept as measured in our experiments and bistability observed during longer stimulus presentations.
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