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The half-Zollner Illusion

Addressed 2 questions asking: (1) whether the Zollner illusion requires concomitant presence of both left and right stacks, and (2) whether the illusion is mainly produced by the orientations of the segments or whether it is related to the geometry at their endpoints. Stimuli were presented with the long axis of a stack oriented vertically, horizontally or at any intermediate orientation defined by steps of pi /8 radians. Each orientation was presented once in random order. 34 adults having normal or corrected-to-normal vision were tasked to alter the figure until apparent collinearity was reached in the case of the 'half-Zollner' variants (Exps 2-5) or apparent parallelism of the stacks was reached in the standard illusion (Exp 1). Results suggest that the illusion itself is not a rotation of the stacks, but either a shear deformation in which segments of a stack slide with respect to one another, or an expansion of the stacks orthogonally to the segments. (PsycINFO Database Record (c) 2005 APA, all rights reserved)



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