Consistency in exchange for inappropriately matched visual feedback? A comment on Franz and Gegenfurtner (2008) “Grasping visual illusions: Consistent data and no dissociation”

Abstract

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.

Keywords:

• Visual illusions
• Vision-for-perception
• Vision-for-action
• Visual feedback
• Condition mismatch

Notes

¹ One could argue that the difference in illusion effect is due to a general difference in response functions depending on availability of visual feedback. Therefore we analysed slopes of response functions while adjusting a neutral line—either with or without feedback. Data revealed that slopes were pretty much the same: 0.91 (SE = 0.04) with and 0.86 (SE = 0.10) without feedback. The ratio of slopes was 0.91/0.86 = 1.05, which is very similar to the ratio of 1.03 between a grasping and a perception task found by Franz et al. (2001). For this relationship of 1.03 they argued that it was close enough to 1 to justify a direct comparison of absolute values between these two tasks.

² Of course, it can be argued that participants use some kind of cross-modal online correction during grasping based on proprioceptive feedback of their hand. However, if such cross-modal online correction really occurs, it will occur in perceptual tasks as well (e.g., in manual estimation tasks).