Figure A1.

Stimuli Utilized for Collinearity Adjustments. The experimental display was a coincidentally occluded hexagon, with neither sides nor symmetry axes aligned along cardinal directions (Gerbino illusion, left). Control displays were the gray proximal shape in the absence of occluding triangles (middle) and the full regular hexagon (right). The observer adjusted the orientation of a probe made of two thin red lines (from +9° to −9°) to make it collinear with either the upper left or the lower right hexagon side (the probe for the lower right side is not shown in the figure). The orientation of both sides was 50° from the horizontal, counterclockwise. In the figure, red lines are objectively collinear with the relevant side, while gray lines mark the extreme of the ±9° range.