Sensory Information necessary for the Size–Weight Illusion

Abstract

IT has been suggested^1,2, and repeated in textbooks³, that the size–weight illusion is due to differences in muscle tension while weights are being lifted. A large object is lifted with more initial force than a small object, and is judged lighter than a smaller object of the same weight because it can be raised more easily. Several authors^4–8 have investigated the differences in muscular effort which accompany errors of weight judgment, but the reports are inconsistent. (Some of these studies^1,5,6 were concerned with the “time error” rather than the size-weight illusion.) If certain tension changes were firmly established this would not be sufficient evidence that they were the cause of the illusion: both might be caused by some other factor. The illusion can be a result of pressure without lifting^9,10. Uznadze¹⁰ demonstrated this by placing pyramids of equal base areas, but different heights, on an arm at rest. I also found that the illusion was experienced to its normal extent when the weights were hung by wire loops from a supported hand¹¹.