Queen's University

Weight versus size comparison an alterable illusion

 
2008-11-25

Psychologists have shown that the classic “size-weight illusion” – where we tend to judge the smaller of two equally weighted objects to be heavier – can be radically changed through experience.

“This demonstrates an important human adaptation that helps us to interact with objects in our environment,” says lead researcher Randy Flanagan, a member of the university’s Centre for Neuroscience Studies.

Published in the current edition of the journal, Current Biology, the findings show that this illusion can be completely inverted with training.

Over a 10-day period of practice lifting a specially constructed set of objects, whose weights varied inversely with volume, the participants’ illusion gradually weakened and then inverted such that the larger of two equally weighted objects was judged to be heavier.

This finding demonstrates that people judge weight, relative to expected weight based on size, and that these expectations can be altered by experience. Thus, people normally judge a small cube to be heavier than an equally weighted large cube because it is heavier than expected, for its size. However, when these expectations change, so do our weight judgments.

“Our fundamental expectation that weight should increase with size can be altered when the statistics of the world change,” says Dr. Flanagan, a member of the Queen’s Group in Sensory- Motor Integration who specializes in eye/hand movement. The researchers conclude that this tendency is neither “hard wired” (genetically determined as opposed to learned behavior) nor does it become crystallized during development.

Predictions about weight are not just used in guiding our actions, Dr. Flanagan continues. They also enable us to judge whether an object is heavier or lighter than expected, and to communicate this information to others. Whereas weight predictions used in manipulatory action are updated rapidly, predictions used in weight perception adapt far more slowly.

Also on the research team are Jennifer Bittner, from Queen’s, and Roland Johansson from Umea University in Sweden.

Funding for the study came from the Natural Sciences and Engineering Research Council of Canada, the Canadian Institutes of Health Research, and the Swedish Research Council.

 

 

 

 

 

 

 

 

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