Manual size estimation: a neuropsychological measure of perception?

Manual size estimation (participants indicate the size of an object with index finger and thumb) is often interpreted as a measure of perceptual size information in the visual system, in contrast to size information used by the motor system in visually guided grasping. Because manual estimation is a relatively new measure, I compared it to a more traditional perceptual measure (method of adjustment). Manual estimation showed larger effects of the Ebbinghaus (or Titchener) illusion than the traditional perceptual measure. This inconsistency can be resolved by taking into account that manual estimation is also unusually responsive to a physical variation of size. If we correct for the effect of physical size, manual estimation and the traditional perceptual measure show similar illusion effects. Most interestingly, the corrected illusion effects are also similar to the illusion effects found in grasping. This suggests that the same neuronal signals which generate the illusion in the traditional perceptual measure are also responsible for the effects of the illusion on manual estimation and on grasping.