Do visual illusions affect grasping? Considerable progress in a scientific debate

When we set out to perform our preregistered study (Kopiske, Bruno, Hesse, Schenk, & Franz, 2016), our goal was to clarify whether or not grasping is affected by the Ebbinghaus illusion. This seemingly simple question has far-reaching theoretical consequences for our understanding of the functional architecture of the visual brain, and in particular for the two-visual systems hypothesis (TVSH; Milner and Goodale, 1995 and Milner and Goodale, 2006).We preregistered our design before collecting any data, painstakingly trying to avoid any methodological pitfalls that might compromise the interpretation. Two expert reviewers (at least one of them being a strong advocate of the TVSH) provided detailed input for improving our design and we adapted our study accordingly. Only after the design had been approved did we perform our large study with N = 144 participants and collected data in parallel in four different labs, intending to provide the best test to-date of whether or not grasping is affected by visual illusions, as proposed by the TVSH.However, Whitwell and Goodale (in this issue) argue that our study was methodologically weak and misguided from the outset because we presented only one Ebbinghaus display at a time, while the predictions of the TVSH could only be tested when simultaneously presenting a pair of two Ebbinghaus displays. In consequence, they think we missed our target and failed to contribute anything new. Here, we argue that this is far too grim a view. The methodological critique offered by Whitwell and Goodale (in this issue) is not justified, and the claim that nothing new has been contributed ignores that a de–facto consensus has been reached on a number of facts, as indirectly also acknowledged by Whitwell and Goodale (in this issue). These facts will in the future facilitate the scientific debate by narrowing down the contentious issues in need of clarification. We will first describe this de-facto consensus before we turn our attention to Whitwell and Goodale's (in this issue) main critique.