Grasp effects of the Ebbinghaus illusion are ambiguous

The assumption that the Ebbinghaus/Titchener illusion deceives perception but not grasping, which would confirm the two-visual-systems hypothesis (TVSH) as proposed by Milner and Goodale (The visual brain in action, 1995), has recently been challenged. Franz et al. (Exp Brain Res 149:470-477, 2003) found that the illusion affects both perception and grasping, and showed that the effect of the illusion on the peak grip aperture (PGA) cannot be accounted for by different sizes of the gap that separates the central target disk from the surrounding flankers. However, it is not yet clear if the presence of flankers per se influences grasping. We therefore compared kinematic parameters of prehension, using the Ebbinghaus illusion, and a neutral control condition where normal subjects grasped a disk without any flankers. In accordance with the well-known effects of the illusion on perceived size, the PGA was smaller when the target disk was surrounded by large flankers, and larger when it was encircled by small flankers. However, the largest PGA values were reached in the neutral control condition. Hence the presence of flankers leads to a general reduction of the PGA, possibly because the flankers are regarded as obstacles. This 'reduction effect' casts doubts on how appropriate it is to directly compare perceptual measures and PGA values when using the Ebbinghaus illusion. Even smaller effects of the illusion on the PGA compared to larger perceptual effects cannot be unequivocally interpreted.