The slalom effect

Cesaro and Agostini (1998 Perception & Psychophysics 60 518 - 523) observed a dynamic illusion of direction and shape that has been named 'slalom effect'. When a dot moving along a straight horizontal trajectory intersects a static pattern of tilted lines, its trajectory is perceived as sinusoidal. The present work offers additional information on the factors involved in the genesis of the illusion. Three experiments were performed. In two experiments, we manipulated the distance between the inducing lines and the perceived depth displacement between the dot and the tilted lines. In the third experiment, the tilted lines were presented one at a time. In all the experiments, the size of the illusion was measured by the method of adjustment. The strength of the illusion is inversely proportional to the distance between the lines. Depth displacement and temporal presentation of the tilted lines do not cancel out the illusion. These results are consistent with an interpretation of the effect in terms of an integration of local distortions occurring at the intersections. Some variants of the illusion are presented and discussed.