Spatial context affects the Poggendorff illusion

The Poggendorffillusion has often been explainedas purely an interactionbetween the parallels and the transversals. The present study demonstrates that additional spatial context exerts an influence on this illusion. In Experiment 1, we examined the effects of a surrounding tilted frame (complete and degraded versions) on collinearity adjustments iatheuprightandrotatedPoggendorfffigures. The frame’s orientation was always oblique. Relative to the no-frame condition, frames decreased error in collinearity adjustments in the upright-Poggendorff figure, and increased error in the rotated Poggendorfffigure. In Experiment 2, a circumscribing circle did not cause an orientation-inhibition effect (Ebenholtz & Utrie, 1982, 1983), suggestingthat the effect ofthe frame on the Poggendorif illusion may not be closely related to the rod-and-frame effect. In Experiment 3, orientation of a central texture modulated the magnitude ofthe illusion. The results do not serve to explain the mechanisms behind the Poggendorffillusion, but they do demonstrate the importance of visual reference frames in understanding perceived misalignment.