Explanation of illusory contours in terms of strength of pattern and its spread effect

The generation of illusory contours is closely related to distinct parts of a pattern such as dots, line ends, and corner points. On the other hand, the remarkable property is that gaze at one point of the contours diminishes the illusion and a return of gaze to the whole pattern restores it. Therefore, illusory contours depend on local parts and the whole pattern formed by the parts, and fitting data on the two aspects is necessary to clarify underlying mechanisms. We have obtained such data from the experiments performed to elucidate other visual phenomena. On the basis of the data, the concepts of strength of pattern, strength of its spread effect, ridgelines of the spread effect, and a hollow of the spread effect are introduced and then various phenomena on illusory contours, including the Kanizsa triangle, are explained in terms of these concepts.