Getting a grip on illusions: replicating Stöttinger et al [Exp Brain Res (2010) 202:79-88] results with 3-D objects.

Studies using visual illusions to demonstrate a dissociation within the visual system can provide relevant and decisive data only if certain methodological points are taken into account. Although, our previous work (Stöttinger et al. in Exp Brain Res 202:88-97, 2010) followed these points, the task made use of only 2-D stimuli which may raise doubts concerning the nature of grasping in that experiment. We therefore replicated the study using a 3-D version of the empty space illusion. Consistent with the earlier study, that used 2-D stimuli, we found that grip aperture followed actual target size independent of illusory effects, while perceived length, as indicated by finger-thumb span, clearly was subject to the illusion. Therefore, the prior results cannot be due to the use of 2-D stimuli. Together, these two studies provide clear evidence for the perception versus action hypothesis.