43 subjects observed Poggendorff figures with transverse line segments at a constant distance while the parallels were positioned closer or farther apart. At closer distances, no illusion occurred; at farther distances, the illusion diminished. The illusion no longer occurred when the parallels were located beyond the outside end of the transverse lines. Since illusions occurred only when the parallels physically touched the transverse line, this research emphasizes the importance of the intersection of figural elements in the production of the Poggendorff illusion.
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