Holes in illusory conjunctions

To address the fundamental question "What are primitives of visual perception?" we tested the hypothesis of early topological perception employing illusory conjunctions. Through a series of experiments, which were well controlled for various feature errors, we consistently observed the illusory conjunction of holes, which are a typical kind of topological property. Subjects in our experiments perceived illusory hollow figures in which the conjoined holes underwent geometric transformations. This indicates that the holes were perceived as abstract topological entities available at an early stage, which provides further support for the hypothesis of topological perception.