Do intentions for action penetrate visual experience?

A now-famous study by Aglioti et al. (1995) involves a graspable version of the Ebbinghaus illusion (Figure 1). Aglioti and colleagues constructed a 3D version of the illusion, using thin solid disks. Subjects were asked to pick up the central disk on the left if the two central disks appeared identical in size, and to pick up the central disk on the right if they appeared different in size. The experimenters varied the relative sizes of the two target disks randomly so that in some trials physically different disks appeared perceptually identical in size, while in other trials physically identical disks appeared perceptually different in size. In selecting a disk in either trial condition, Milner and Goodale observe,