The present paper focussed on the effects of the figure orientation on the magnitude of the Ponzo illusion. The Ponzo illusion figures used in this study were composed of the inducing figures and two induced lines. Experiment 1, in which inducing figures were two converging lines, examined whether there were systematic deviations of the illusion magnitudes according to the orientations of the inducing lines or not. The orientations of the converging lines were top-converging, right-converging, bottom-converging and left-converging. The illusion magnitude of the top-converging figure was maximum and the difference of the illusion magnitude between the top-converging and the bottom-converging figures was significant. The illusion magnitudes of the right-converging and the left-converging figures lay between the top- and the bottom-converging figures. In Experiment 2, the effects of the main factors, the orientations of the inducing figures, the distance between the standard and the variable lines, and the kinds of inducing figures were all significant. In addition, the effects of the orientation of the inducing figures were significant in the gradient texture and the subjective contour figures.
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