In monocular vision, the horizontal/vertical aspect ratio (shape) of a fronto-parallel
rectangle can be based upon the comparison of the perceived directions of the rectangle's edges. In
binocular vision of a typical three-dimensional scene (when occlusions are present) this is not the
case: fronto-parallel rectangles would be perceived in a distorted fashion if an observer were to
base perceived aspect ratio on the perceived directions of the rectangle's edges. We
psychophysically investigated stereoscopically perceived aspect ratios of fronto-parallel occluding
and occluded rectangles for various distances and fixation depths. We found that observers did
not perceive the distortions as predicted on the basis of the above-mentioned comparison of the
perceived visual direction of the edges of the rectangle. Our results strongly suggest that the
mechanism that determines perceived aspect ratio is dissociated from the mechanism that
determines perceived direction. The consequences of the findings for the Kanizsa, Poggendorff,
and horizontal/vertical illusions are discussed