Stereokinetic effects with sharp and fuzzy illusory contours.

A black, 8-shaped pattern, whose centre of gravity is in the centre of a rotating disc, appears to split into two black discs rotating with phenomenal independent motion, orientation stability, and sliding of one on the other. The type of observed movement, the order of overlapping, and the extent of the stereokinetic depth in relation to the contour type and different dimensions of the pattern were investigated. The experimental data show that a fuzzy contour facilitates the stereokinetic effect. Furthermore, the extent of the stereokinetic depth has been found to be greater in case of fuzzy contour and vertical orientation of the pattern. These results are in contrast with the models in which this effect is considered as a problem of minimisation of distances or velocities. A vector model of the observed movement and an interpretation of the overlapping based on an energy approach are proposed. A tentative explanation of the stereokinetic depth for our patterns is put forward.