AbstractA topic of debate in the early 2000's surrounded whether pictorial illusions (e.g., Müller-Lyer [ML] illusion) influence grasping. The general conclusion from this literature was that pantomime-grasps are 'tricked' by illusions, whereas naturalistic grasp are immune – a result attributed to actions mediated via functionally and anatomically distinct visual pathways. It is, however, important to note that pantomime- and naturalistic grasps differ not only in terms of their visual properties but also because the former does not entail haptic feedback. This is an important distinction because recent work by our group has shown that haptic feedback supports an absolute calibration. In the present study, participants complete pantomime-grasps following a brief memory-delay to target objects embedded in fins-in and fins-out ML figures. Notably, the target object was removed during the memory delay. In the no-haptic feedback condition, the target remained unavailable to physically grasp, whereas in the haptic feedback condition the experimenter placed the physical object between participants' thumb and forefinger at the end of their response – a manipulation providing haptic feedback related to object size. Results yielded a main effect for ML configuration such that PGAs for the fins-in figure were smaller than the fins-out. Additionally, an interaction between ML configuration and feedback condition indicated that the illusory effect was larger for the no-haptic than the haptic feedback condition. Thus, although haptic feedback was not sufficient to ameliorate illusory effects in pantomime- grasping, results show that haptic feedback does influence a calibration to veridical object size.
Acknowledgments: Supported by NSERC.