Casting a Shadow on the Ponzo Size illusion

In the classic Ponzo illusion the perceived size of a stimulus is affected by lines in the background suggesting a linear perspective in depth. A stimulus or object close to the vanishing point appears further away and therefore larger than an object of the same size placed in the foreground. However, introducing a static or moving shadow cast by the object creates the impression that the object floats above the ground, thereby changing perceived size (Yonas, Goldsmith & Hallstrom, 1978; Kersten, Mamassian & Knill, 1997). Here we investigate how strong a cast shadow of a stimulus ball affects perceived size. A total of N=38 observers participated in an online study with a 4 by 2 within-subjects design. We tested the effect of four vertical shadow offsets (0, 46, 92, and 184 pixels) and two positions of the reference ball (64, and 164 pixels). An analysis of variance (ANOVA) with repeated measurement revealed that shadow offset (F(3,111)=13.17, p