Abstract
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 occurred when the transversal line-segment was oriented 90° 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.

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