Stimuli

An adapted version of the Ponzo illusion was employed in the experiments. Depth perception was induced by two tilted lines that converged at the upper boundary of the display. The length of the lines was 28°, with a width of 0.2°. Two bars were placed at the center of the screen, one above the fixation cross, one below (Fig 1). The length of the upper bar was fixed at 6°, while the lower bar varied in length in order to measure the illusory effect.

Two converging lines serves as linear perspective cues. The Ponzo illusion is that the lower bar is perceived to be shorter than the upper bar even when their length is the same (as shown in the figure).