A new cue to distance in a case of amodal completion

ABSTRACT: We studied a well-defined set of line drawings that we call `completion by folding'. Such completion occurs when four quadrilaterals are arranged in such a way that two of them (called visible tails) look like a sheet of paper folded around an interposed rectangle. This is a type of amodal completion with unification which is produced even when current models do not foresee such a perceptual solution.We advance the hypothesis that such amodal completion depends on a 3-D interpretation. That is, the interposed rectangle may be seen as a parallelepiped. This hypothesis can account for those figures where the amodal unification occurs without the classical amodal completion constraints. In two experiments, we investigated the possibility that the geometrical horizontal distance between the two visible tails could be a new cue to relative distance. In experiment 1 we used the size-constancy effect to test the plausibility of the 3-D hypothesis. In experiment 2 we investigated the effect of the distances between the two tails on the degree of perceived depth. Experiment 1 confirmed the presence of the perceived depth, while results from experiment 2 confirmed a direct dependence of perceived depth on the physical distances of the two tails.