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Humans underestimate the weight of their hands by nearly 50%

New research sheds light on how we perceive our weight.

People perceive their hands to weigh much less than they actually do, underestimating their weight by nearly 50%, according to new research from Birkbeck.  

Led by academics Dr Elisa Raffaella Ferre, Reader in Cognitive Neuroscience, and Professor Matthew Longo, Professor of Cognitive Neuroscience, the research team developed a new research method to measure perceived hand weight. 

Participants in the study judged whether weights suspended from a wristband were heavier or lighter than perceived hand weight. Hand weight was underestimated on average by 49.4%, an effect the team has called “terrestrial weightlessness”. Moreover, inducing fatigue by hand exercise caused an increase in perceived hand weight. 

An object’s weight is usually calculated according to Isaac Newton’s laws and is equal to its mass multiplied by gravitational acceleration, but a person’s perception of their own body weight is constructed by their brain because their central nervous system is not informed by a sensory signal. 

Dr Ferre comments: “The sensation of our body’s weight caused by gravity presents an intriguing contrast to the weightlessness experienced in outer space. 

“Astronauts are often astonished when the experience of body weight re-emerges on returning to Earth. 

Our findings demonstrate that our everyday experience of body weight – weightedness - can be accurately quantified and it is strongly underestimated.” 

Denise Cadete, Birkbeck PhD student and co-author, explains:We believe this perceptual bias helps to make body movements seem effortless. 

A person’s brain could either encourage or discourage physical exertion by making their body seem light and actions therefore effortless, or heavy and actions thus effortful. 

“Phrases such as “my eyelids are getting heavy” which is used to mean that the speaker is sleepy, allude to a commonly experienced relation between their perceived body weight and motivation for action. 

We believe that perceived body weight could be an important component of a system that adapts with evolution to control our levels of behaviour. 

Existing research on body weight has largely focused on the social or medical aspects of how people feel about the overall weight of their body. Surprisingly, no research has investigated the more basic issue of how the brain constructs representations of the weight of individual body parts.   

Professor Longo says: “We investigated, for the first time, how the brain perceives the weight of body parts as spatially extended objects. Our work may have implications for the understanding of disorders of body weight, including anorexia nervosa and obesity.” 

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