Crossing the line: Estimations of line length in the Oppel-Kundt illusion

In the Oppel-Kundt illusion, one of the oldest and least understood geometrical visual illusions, a line subdivided by a series of short orthogonal ticks appears longer than an identical line without these. Paradoxically, bisecting a long line with a single tick leads to perceived shortening of the line. We have systematically investigated the effects of adding 1 to 12 ticks on perceived line length and results suggest that at least three mechanisms must be at work: (a) bisection, which reduces perceived length; (b) a filled extent effect, which is also apparent in the von Helmholtz illusion, though no satisfactory explanation for it exists; and (c) a local contour repulsion effect of the penultimate tick upon the perceived position of the end tick, but this effect, though significant, is too small to explain the Oppel-Kundt illusion in its entirety.