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Baingio Pinna; New local and global shape illusions due to grouping. Journal of Vision 2007;7(9):203. doi: 10.1167/7.9.203.
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© ARVO (1962-2015); The Authors (2016-present)
The problem of perceptual organization concerns the perception of a world populated by phenomenal objects but not by differences of luminances and edges. The main question Wertheimer (1921) suggested is: how do the elements in the visual field ‘go together’ to form an integrated, holistic percept? He answered the question by discovering some ‘grouping principles’. The problem of grouping is strongly related but not superimposable to the problem of shape perception. The main questions we want to answer are: Can grouping influence shape perception? What is the relationship between grouping and shape perception? To answer these questions the role of several gestalt grouping principles (similarity, proximity, good continuation) were studied through psychophysical experiments in conditions where whole simple geometrical shapes (e.g. square, triangle) are composed by similar or different element components. The grouping of these components was varied in different directions and orientations by varying the gestalt principles to test if and how it affects the shape of both whole object and element components. The results revealed several new shape illusions demonstrating that grouping principles strongly influence the shapes of both the whole object and the element components. The resulting shape illusions occur under new conditions and are opposite to what expected from other well-known illusions: Oppel-Kundt, Helmholtz's square, Attneave's and Palmer's triangles, Giovannelli illusion. The results demonstrated that grouping is not only related to how individual elements create larger wholes separated from others, but to the more general problem of shape perception. The new phenomena link grouping to shape perception more deeply by suggesting that the latter can be likely reduced to the former. This likely happens because grouping principles induce new symmetric organizations in the entire set of element components that define the whole shape that in its turn determines the shape of each component element. Fondazione Banco di Sardegna, Alexander von Humboldt Foundation, PRIN ex 40% Cofin. es. 2005 (prot. 2005112805_002) and Fondo d'Ateneo (ex 60%)
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