Effects of the Ebbinghaus Illusion on grasping in a virtual environment

It is an open question whether the Ebbinghaus (or Titchener) illusion affects perception more than grasping. Evidence for a stronger effect on perception has often been based on a perceptual task called 'manual estimation'. We compared manual estimation to a standard perceptual measure as well as to grasping. Virtual target discs (diameter: 38, 40, or 42 mm), surrounded by small or large discs (diameter: 10 or 58 mm) were displayed stereoscopically on a monitor, generating the Ebbinghaus illusion. In the grasping task, ten participants grasped the target. Haptic feedback was simulated by two robot arms (PHANToM ™). In the manual-estimation task, participants indicated the size of the target using index finger and thumb (without seeing their fingers). In the standard perceptual task, they adjusted a comparison to match the target. We found illusion effects on manual estimation (2.3 mm, SE 0.45 mm) which were larger than both the grasp effects (1.1 mm, SE 0.29 mm) as well as the effects on the standard perceptual measure (0.7 mm, SE 0.11 mm). This suggests that manual estimation shows relatively large illusion effects