Is amodal completion necessary for the formation of illusory figures?

Kanizsa's hypothesis suggests that the creation of an anomalous surface is due to the amodal completion of the inducers. In the present paper a new pattern that is able to disconfirm this explanation is presented. According to the Helmholtz-Ratoosh law amodal completion only occurs when the borders of two adjacent surfaces meet forming T-shaped junctions. When the borders of the two adjacent surfaces have Y-shaped junctions, amodal completion is absent. However, when a pattern inducing an anomalous figure has the latter figural characteristics, in spite of the absence of amodal completion, an illusory figure is still visible. In this paper a set of experimental results (carried out by means of a magnitude estimation procedure as well as the method of constant stimuli) supporting the aforementioned observations is presented.