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 occurred when the transversal line-segment was oriented 90.degree. with respect to true vertical or true horizontal. Magnitude of illusion increased as the transversal line-segment deviated from these positions. There seems to be a stability of horizontal and vertical orientations. The hypothesis that visual acuity plays a role in the perception of the Poggendorff illusion was also proposed.