Effects of contextual and local factors on Ponzo illusion magnitude

Ponzo illusion has been explained by considering either just the inducing elements present in a restricted area of the visual field, the same area in which the test elements are located, or the stimulus configuration as a whole in which even the most distal figural elements – i.e., the external converging lines, here called “Ponzo wedge”– play a crucial role. The two studies reported here aimed at showing that both global configurational characteristics and inducing elements locally interacting with the test stimuli can independently affect the illusory effect. This hypothesis was tested using stimuli in which graphic-inducing elements giving rise to a herringbone pattern (Coren & Girgus, 1978) were drawn in the same area of the test segments. Results of Exp. 1 confirmed the effect of the two factors. In particular, both factors proved to determine the illusion, since they induced illusory effects either in isolation or in the same/opposite direction. In Exp. 2 the relative weight of these two factors was evaluated in relation to the width of the angle of the inducing elements and to the distance of the test segments from the vertex. Results showed no linear relationships between the distance of the test segments from the external inducing elements and the weight of the Ponzo wedge factor. Received: 25 June 1996 / Accepted: 1 October 1997