Depth adjacency and the Ponzo illusion

The effect of depth displacement of test bars from the induction wedge of the Ponzo illusion was investigated in two experiments. Either two wedges of opposite orientation were presented simultaneously, one at a near and the other at a far distance, or only one wedge was presented at either the near or the far distance. The test bars were stereoscopically either in the plane of the wedge or displaced from the wedge in distance. When the two wedges were presented simultaneously, the direction of the Ponzo illusion was determined by the wedge at the same perceived distance as the test bars. When onlyone wedge was present, stereoscopic displacement of the bars in front of, but not behind, the wedge decreased the magnitude of the illusion. The results are interpreted in terms of the adjacency principle.