Consistency in exchange for inappropriately matched visual feedback? A comment on Franz and Gegenfurtner (2008)...

Franz and Gegenfurtner (2008) argue that the evidence for a division of labour within the visual system for action and perception is flawed because perception is often measured by manual estimation, which responds in general with a larger slope to a change of physical size than does adjusting. Therefore results obtained under manual estimation have to be corrected for this difference in slope: In a reanalysis of six studies grasping and perception were equally influenced by the illusion after this correction. However, closer inspection of methods reveals that visual feedback was confounded with conditions (suppressed vision while grasping vs. full vision while adjusting). We argue that studies can produce relevant and decisive data only when they (a) do not confound conditions with visual feedback, (b) do not allow online corrections of the action due to a direct comparison of the hand with the target, and (c) do not provide any risk of grasping being memory driven when the target is removed.