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When lifting two objects of different volume but equal weight, people judge the smaller object to be heavier. This size-weight illusion, first documented in 1891 by Charpentier, is one of the best known and most powerful weight illusions.
Until recently, it was widely believed that the illusion stems from a mismatch between expected and actual sensory feedback – the idea is that the smaller of two equally weighted objects is judged to be heavier because it is heavier than expected.
To test this hypothesis we asked people to lift a large object and an equally weighed small object in alternation. As shown opposite, the objects were lifted with a small handle instrumented with force sensors and thus we were able to measure the fingertip forces used in lifting.In the first few lifts people use too much force for the large object and too little force for the small object. However, thereafter they scaled their forces perfectly for object weight. Thus, at the level of action control, the brain learned the true weights of the objects and there could be no mismatch between expected and actual sensory feedback.
However, people still experienced the size-weight illusion. This finding rules out the sensory-mismatch hypothesis and suggests that the illusion is caused by high-level cognitive and perceptual factors. The result also indicates that the sensorimotor system can operate independently of the cognitive/perceptual system.
For more information see: Flanagan JR, Beltzner MA (2003) Independence of perceptual and sensorimotor predictions in the size-weight illusion Nature Neuroscience 3: 737-741.
To read more about our ongoing work on weight prediction in action and perception, see Flanagan JR, Bittner JP, Johansson RS (2008) Experience can change distinct size-weight priors engaged when lifting objects and judging their weights. Current Biology 18: 1742-1747.
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