Poggendorff illusion as a function of orientation of transversal and parallel lines.

Adult human subjects (8 male, 8 female undergraduates) with normal vision were required to judge various orientations of the Poggendorff illusion. The transversal and parallel line-segments of the illusion were manipulated to produce the orientations to be judged. Minimum illusion occured when the transversal line-segment was oriented 90 degrees with respect to true vertical or true horizontal. Magnitude of illusion increased as the transversal line-segment deviated from these positions. The findings suggested that there is a stability of horizontal and vertical orientations. In addition, the hypothesis that visual acuity plays a role in the perception of the Poggendorff illusion was proposed.