Monday, May 16, 2005

The Poggendorff Illusion

Fig. 1. The Poggendorff illusion and its behavior. (A) When an obliquely oriented straight line is interrupted by a vertical occluder, the line segment on the right appears to be shifted downward with respect to the line segment on the left. (B) A similar effect occurs when the orientation of the interrupted line is reversed. In this case, the collinear extension on the right appears to be shifted upward. (C) When an oblique line is interrupted by parallel horizontal lines, the oblique line segments appear to be shifted horizontally with respect to each other. (D) The magnitude of the effect increases as the interrupted line is made more vertical. (E) The magnitude also increases as the width of the interruption increases. (F) The illusion is largely abolished when only the acute components of the stimulus are presented, but the effect is maintained when only the obtuse components are shown. (G) The illusion is diminished when the standard configuration is rotated so that the interrupted line is horizontal.

A new explanation for this illusion has just been published.

The Poggendorff illusion explained by natural scene geometry by Catherine Q. Howe, Zhiyong Yang, and Dale Purves in Proc. Natl. Acad. Sci. USA.
One of the most intriguing of the many discrepancies between perceived spatial relationships and the physical structure of visual stimuli is the Poggendorff illusion, when an obliquely oriented line that is interrupted no longer appears collinear. Although many different theories have been proposed to explain this effect, there has been no consensus about its cause. Here, we use a database of range images (i.e., images that include the distance from the image plane of every pixel in the scene) to show that the probability distribution of the possible locations of line segments across an interval in natural environments can fully account for all of the behavior of this otherwise puzzling phenomenon.

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