Introduction: The size-weight illusion (SWI) describes the observation that smaller objects are perceived as heavier than bigger objects that have the same weight after having been lifted. The origin of this illusion is still debated. A key question is what type of sensory information is needed to create a robust SWI. Previous research demonstrated that SWI occurs even when the object is neither seen nor felt during its lift (Buckingham & Goodale, 2010). This finding is surprising but also difficult to interpret given that some aspects of the previous study might have allowed observers to feel the difference between different objects. At the same time, full occlusion of the entire scene also reduced grip accuracy, thereby contaminating the data from load and grip forces. Methods: We re-examined the role of vision while aiming to avoid the potential problems of the previous study. We ensured that no haptic information about object identity was revealed during the grasp and also introduced a second vision condition that allowed vision of the grip-sensor, thereby allowing ideal grip accuracy, while still preventing vision of the object. Results: We found that under these conditions, the SWI was diminished whenever vision of the object during lift was reduced. Discussion: We will discuss the implications of our findings for our understanding of the role of vision and haptic feedback in creating the SWI.

Buckingham, G., & Goodale, M. A. (2010). PLoS ONE, 5(3), 3–6. https://doi.org/10.1371/journal.pone.0009709