Three-dimensional illusory objects produced by rotation in depth.

Rotation of a Kanizsa triangle in depth around its vertical axis causes a perception of a three-dimensional object with a flat, rigid illusory triangle between the inducing discs. When the inducing discs of a Kanizsa triangle were made thicker, the illusory triangle between the discs also became thicker. In the experiments both computer animation and real inducers made of plastic were used. The method promoted border perception in a three-dimensional illusory figure. We suggest that the perception of three-dimensional illusory objects is due to a process which is also used in the perception of real three-dimensional objects.