Abstract
A number of studies suggested that the effects of visual illusions on grasping are large at the start of the movement and then gradually decay while the movement unfolds. This effect was named “dynamic illusion effect” and led to intensive theorizing about the processes guiding visuo-motor behavior (e.g., Glover & Dixon, Perception and Psychophysics, 2002; Glover, Behavioral and Brain Sciences, in press). Using improved methodology, I tested whether the dynamic illusion effect exists. Participants grasped disks which were exposed to the Ebbinghaus \ Titchener illusion. The hand aperture of each grasp was analyzed at different, normalized time points. At each time point a correction for the responsiveness of the hand aperture to a physical variation of size was performed. Special care was taken to avoid possible artifacts which might arise from the hand already touching the target object. Different to our previous study (Franz & Scharnowski, VSS-2003), participants could see their hand and the stimuli all the time while grasping (closed-loop condition). Results show that the illusion effects are remarkably constant over time and that the dynamic illusion effect which was found by previous studies was most likely due to a contamination of the data by the hand already touching the target object.