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Abstract

It is still highly controversial whether the Ebbinghaus (or Titchener) illusion affects grasping to the same extent as perception. Evidence for a stronger effect on perception has often been based on a perceptual task called "manual estimation". Here, we compared manual estimation to a standard perceptual measure as well as to grasping. The experiment was performed in a virtual environment in which the visual input as well as the haptic input was simulated.
Virtual target discs (diameter: 38, 40, or 42mm), surrounded by small or large discs (diameter: 10 or 58mm) were displayed stereoscopically on a monitor, generating the Ebbinghaus illusion. In the grasping task, 18 participants grasped the target. Haptic feedback was simulated by two robot arms (PHANToM TM). 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 disc to match the target disc.
We found illusion effects on manual estimation (2.0mm, SE 0.45mm) which were larger than both, the grasp effects (1.2mm, SE 0.33mm), as well as the effects on the standard perceptual measure (0.8mm, SE 0.14mm). The fact that the grasp effects were inbetween the effects on manual estimation and on the standard perceptual measure casts some doubt on the notion that grasping always shows smaller illusion effects than percpetion.