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Walter Gerbino, Cristina Zabai, Federica Sgorbissa; The joint. Journal of Vision 2001;1(3):462. doi: 10.1167/1.3.462.
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When a concave 3D shape is perceived as two joined component objects, the joint is optically undetermined. No information on object completeness is locally available. In principle, the same joint might be perceived in different ways: as hiding an empty intersection volume bounded by the surfaces of the two indented objects (against horror vacui); as a volume entirely filled-in by parts of two indented objects (like in good woodwork); or as an indented object joined to a wholly convex object. Normally joints are perceived according to the last solution, that involves 3D amodal completion of one object only. Which of the two component objects is completed constitutes a problem for visual science. The pencil-and-block illusion (Gerbino 1997) and other demonstrations indicate that observer's knowledge of material penetrability cannot account for such a preference. We report data from a systematic investigation in which the following possible determinants were considered: color, relative size, role of support, minimization of occluded contours and surfaces, relative position along the vertical, relative orientation, and proximity to the observer. A multiple regression analysis of preferences in 40 conditions indicated that only the last three determinants play a significant role. The pencil-and-block illusion and analogous demonstrations are quite compelling when these three factors cooperate. Given a 3D composition, the object that happens to be higher, oblique, and closer to the observer is amodally completed and appears to penetrate the other. Two of the effective determinants, relative position and relative orientation, can be connected to the dynamics of falling, according to which higher and oblique objects tend to be more active than lower and vertical/horizontal objects. The third factor can be explained by an egocentric hierarchy in amodal completion, such that the filling-in of objects closer to the observer constrains amodal completion of more distant objects.
GerbinoW.(1997). Figural completion. In Taddei-FerrettiC. (ed.) “Biocybernetics of Vision: Integrative Mechanisms and Cognitive Processes”. Singapore: World Scientific, pp. 207–216.
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