SPATIAL CONTEXT AFFECTS THE POGGENDORFF ILLUSION

The Poggendorff illusion has often been explained as purely an interac tion between the parallels and the transversals. The present study dem onstrates that additional spatial context exerts an influence on this illusion. In Experiment 1, we examined the effects of a sur-rounding t ilted frame (complete and degraded versions) on collinearity adjustmen ts in the upright and rotated Poggendorff figures. The frame's orienta tion was always oblique. Relative to the no-frame condition, frames de creased error in collinearity adjustments in the upright Poggendorff f igure, and increased error in the rotated Poggendorff figure. In Exper iment 2, a circumscribing circle did not cause an orientation-inhibiti on effect (Ebenholtz & Utrie, 1982, 1983), suggesting that the effect of the frame on the Poggendorff illusion may not be closely related to the rod-and-frame effect. In Experiment 3, orientation of a central t exture modulated the magnitude of the illusion. The results do not ser ve to explain the mechanisms behind the Poggendorff illusion, but they do demonstrate the importance of visual reference frames in understan ding perceived misalignment.