Interrupted side affects regularity of geometrical figures.

When a side of a regular geometrical figure is interrupted, an illusory effect of irregularity can be perceived. The aim of our two experiments was to establish the influence of a spatial gap and of shrinkage by amodal completion on the perceived regularity of triangles. In the first experiment, we used an equilateral triangle with a gap in the base. The length of the sides was 8 deg. We manipulated the length (2, 2.5, 3 deg) and the position (0.6, 1.2, 1.8 deg from the base angles) of the gap. The adjustment method was adopted: ten subjects varied the horizontal position of the top angle until they perceived the shape to be regular. The results demonstrate that length and position of the gap influence the strength of the illusion. In the second experi- ment, we used, instead of a gap, an opaque rectangle. By amodal completion the base of the equilateral triangle was perceived to be complete. The results do not show a significant distortion of the triangle. A possible explanation is discussed, considering the role of the symmetry as the global factor, and the local effect of the inducer in relation to the Oppel Kundt illusion.