Figure 2.

The five illusions tested. In each trial, participants adjusted one element of the stimulus in size, length or orientation to match it with a reference stimulus. The reference item for the Ebbinghaus (EB) was the white left disk and the adjustable element was the right white disk. For Müller-Lyer (ML), the reference item was the left line and the right line was the adjustable element. For Ponzo (PZ), the lower horizontal line was the reference item and the upper one was the adjustable element. For Ponzo-Hallway (PZh), the gray disk in the upper right corner was the reference item and the gray disk in the lower left corner was the adjustable element. In the Tilt illusion (TT), participants adjusted the orientation of the right grating to match the orientation of the inner grating of the stimulus presented on the left side of the image.