Purpose: Visually guided motor behavior is assumed to be rather unreceptive to size illusions, indicating two different cortical processing streams for the purposes of perception and action (e.g., Aglioti, DeSouze & Goodale, 1995; Current Biology 5, 679-685). Having shown - in contrary - that grasping and perception are equally influenced by the Ebbinghaus / Titchener Illusion (ECVP 98), we tested whether this is also true for the Mueller-Lyer Illusion. Methods: Plastic bars (40, 43, 46 and 49 mm long, 5 mm wide) were positioned on top of a horizontally oriented monitor. Fins were presented on the monitor being directed either outwards or inwards, such that the fins and the bars resulted in the Mueller-Lyer Illusion. In the grasping task, twelve subjects grasped the bars and the maximal aperture between thumb and index finger was measured using an Optotrak (TM) system. In the perception task, the subjects adjusted the length of a comparison bar on the screen to match the length of the plastic bars. Results: There were strong effects of the Mueller-Lyer Illusion on grasping as well as on visual perception. The effect on grasping (3.5 +/- 0.5 mm) was even larger than on perception (2.1 +/- 0.3 mm). Conclusions: Our results show that grasping is influenced by visual illusions, indicating that the motor system is receptive to visual illusions.
CR: None Support: DFG grant FA119/15-1