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Erik C. Chang, Melvyn A. Goodale; Size-weight illusion dissociates from grip forces when objects lifted from other hand. Journal of Vision 2006;6(6):861. doi: 10.1167/6.6.861.
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When lifting two visible objects of the same weight but different volumes, the smaller object is judged to be the heavier. It has been suggested that the illusion arises from a mismatch between expectation and sensory feedback. To clarify the role of sensory feedback, we measured estimates of heaviness and precision-grip forces when participants lifted objects (1331 cm3 and 147 cm3; 0.33 kg each) that were resting either on a table or on the palm of their other hand. Testing order was counterbalanced across participants and object size was alternated randomly between trials. A robust size-weight illusion, as evidenced by the ratio of small-to-large heaviness estimates (average: 1.42), was observed in both the table and palm conditions, with the table condition showing a slightly stronger illusion (1.55 vs. 1.29). Participants who first lifted the objects from the table (without first lifting the object from the palm of their other hand) generated peak grip forces that were significantly greater for the larger of the two objects. After that initial lift, however, the grip forces applied to the two objects rapidly converged and remained so for the rest of the experiment. In contrast, participants who first lifted objects from the palm applied equivalent grip forces to both objects on the very first trial and continued to do so for the remainder of the experiment. These results suggest that even though the lifting hand can use information from the supporting hand to scale grip forces accurately, the size-weight illusion is only slightly diminished.
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