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Stephen Palmer, Karen Schloss; A configural shape illusion. Journal of Vision 2009;9(8):917. doi: 10.1167/9.8.917.
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© ARVO (1962-2015); The Authors (2016-present)
A new illusion - the configural shape illusion (CSI) - is reported in which the shape of a rectangle is systematically distorted by an attached/adjacent contextual region, when both are seen as part of a single configuration. In particular, the rectangle's perceived aspect ratio changes in a direction consistent with the aspect ratio of the whole configuration. We measured the magnitude of this illusion in two ways. First, observers adjusted the height and width of a separate, unattached rectangle to match those dimensions of a rectangle that was part of various configurations. Second, observers adjusted the height and width of the rectangle within various configurations to appear perfectly square. Systematic CSIs were present using both procedures, but their magnitude depended on the spatial and color relations between the rectangle and the adjacent context. The results are consistent with the hypothesis that the illusion is greater to the extent that the elements in the configuration are strongly related by virtue of standard grouping factors, including connectedness, proximity, good continuation, lightness similarity, hue similarity, and shape similarity. Somewhat surprisingly, the illusion was stronger when the contextual region was smaller, suggesting that the magnitude of the illusion may be governed more by the proportion of the entire configuration occupied by the target rectangle than by the aspect ratio of the entire configuration itself. Similar effects are apparent for the aspect ratio of an oval, although the distortions are less pronounced. The relation between the CSI and the occlusion illusion (Kanizsa, 1979; Palmer, Brooks & Lai, 2007) will also be discussed.
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