Cite as:
Bressanelli D, Massironi M, 2006, "A new optical-geometrical illusion" Perception 35 ECVP Abstract Supplement

A new optical-geometrical illusion

D Bressanelli, M Massironi

Our aim is to describe and explain a new and unusual geometrical optical illusion with three levels of distortion. The illusory figure was made up of three juxtaposed bands of the same width--a parallelogram and two rectangular trapezoids--which, when appropriately juxtaposed, appeared to be of different widths. The illusion seems to share some aspects with other well-known illusions, such as Shepard's, Jastrow's, and Sander's. We hypothesised that the effect would depend on the combined action of the following factors: (i) global: the band shape, and 3-D perceptual assumption; (ii) local: the degree of contrast in length in the nearer side of different bands, which is transferred to the band widths; (iii) the inability to account for the whole projective transformation. To test these hypotheses, an experiment was conducted in which the shape of three stimuli was manipulated through affine transformation as well as variation of side lengths. The task of participants was to evaluate the width of the bands, by using four different methods of measurement. The results revealed a robust and stable illusory effect and confirmed the factors hypothesised by us.

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