The Oppel-Kundt Illusion and Its Relation to Horizontal-Vertical and Oblique Effects.

The Oppel-Kundt illusion consists in the overestimation of the length of filled versus empty extents. Two experiments explored its relation to the horizontal-vertical illusion, which consists in the overestimation of the length of vertical versus horizontal extents, and to the oblique effect, which consists in poorer discriminative sensitivity for obliquely as opposed to horizontally or vertically oriented stimuli. For Experiment 1, Kundt's (1863) original stimulus was rotated in steps of 45° full circle around 360°. For Experiment 2, one part of the stimulus remained at a horizontal or vertical orientation, whereas the other part was tilted 45° or 90°. The Oppel-Kundt illusion was at its maximum at a horizontal orientation of the stimulus. The illusion was strongly attenuated with L-type figures when the vertical part was empty, but not enhanced when this part was filled, suggesting that the horizontal-vertical illusion only acts on nontextured extents. There was no oblique effect.