Figure 4

Measurement of the illusion magnitude. The size of one object (the reference object) was kept constant and the size of the other object (the test object) was varied. The percentage of the test object being judged as larger was plotted against the relative size of the test object (illustrative data from the Ponzo illusion in (A) and the Ebbinghaus illusion in (B)). The data were fitted with a logistic function, and the illusion magnitude was quantified according to the threshold point at which the test object was judged as larger for half of the trials (dashed lines). Error bars represent SEM (N = 20).