A note on the influence of grouping on illusory contours

It is well known that dots superimposed on the Koffka cross change the shape of the illusory surface. It has been claimed, but never proved, that this effect is due to the shape of the virtual figure inwhich the dots organize. In this note it is proved that the shape of the virtual figure is indeed the crucial factor. In fact, the shape of the illusory surface changes if the way in which the dots organize is altered by adding other dots; if, then, the former organization is restored by adding further dots, the illusory shape is restored too, at least to a large extent.