Poggendorff illusion with subjective contours

The Poggendorff illusion is a puzzle. Numerous mechanisms have been proposed to account for it, including angle expansion caused by lateral inhibition in the visual cortex, spatial blurring, assimilation to cardinal axes, apparent depth induction, distance mis-estimation, and extrapolation errors. Several such factors may be operational at once. Replacing luminance edges by subjective contours enables us to exclude some of the proposed accounts. When the Poggendorff's rectangle is demarcated by subjective contours, the illusion remains. Here, we investigate an unexpected reversal of the normal direction of the illusion that occurs when instead it is the diagonals that are subjective. This occurs whether the rectangle's contours are real or subjective, but only for acute angles of intersection. Eliminating the rectangle altogether reveals a persisting misalignment too, supporting extrapolation error, assimilation to cardinal axes and angle expansion as explanatory factors. However, no single explanation accounts for all the data, supporting a multifactorial origin of the illusion and its variants.