Illusory figures from stereoscopically three-dimensional inducers depicting no occlusion event.

It has been claimed that an illusory figure is prevented from occurring when there is contrary evidence to occlusion, eg when the inducers are pictorial representations of 'complete' three-dimensional (3-D) objects. However, it was recently shown that illusory figures may also be induced by such pictorially 3-D inducers. Here, further evidence on this point is presented. Two experiments were carried out. The first was aimed at showing that an illusory figure can be induced even by stereoscopically 3-D 'complete' inducers; the second was aimed at checking whether inducers that are stereoscopically seen as closer than the induced figure can contribute to strengthen the illusion. The results show that stereoscopically 3-D inducing patterns can affect the illusion both in the absence of any occlusion cue and when there is contrary evidence to occlusion. These conclusions seriously challenge all the interpretations that regard occlusion, or interposition, cues as necessary for the formation of illusory figures.