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Gavin Buckingham, Jonathan S. Cant, Kai-Ling C. Kao, Melvyn A. Goodale; A dissociation between perception and action in the Material-Weight illusion. Journal of Vision 2009;9(8):1131. doi: 10.1167/9.8.1131.
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In the famous size-weight illusion, the smaller of two objects of equal weight is perceived as heavier than the larger one. Remarkably, however, people quickly learn to scale their grip and load forces to the actual rather than perceived weights of the objects, despite continuing to experience a robust size-weight illusion. In other words, there is a clear dissociation between perceived weight and the scaling of the forces required to pick up the objects. A similar illusion, where the material of the object (as revealed by its visual appearance) affects its perceived weight has also been described (the ‘material-weight illusion’ Seashore, 1899). In the current study, we investigated whether or not a similar dissociation between perceived weight and the scaling of grip and load forces could be demonstrated in this illusion. We recorded the grip and load forces when participants picked up objects of different materials (metal, wood, and polystyrene), constructed to have the same size and mass as one another. On each trial, we also recorded participants' judgments of the perceived weight of the objects. The classic perceptual effects of the ‘material-weight illusion’ were replicated; e.g., participants ranked the metal block as lighter than the blocks made of other materials. Nevertheless, we also found a similar dissociation to that seen in the earlier ‘size-weight illusion’ work between perception and action. In other words, after only one or two trials, all objects were gripped and lifted with the same forces, despite being perceived as having different weights.
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