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The Weight Illusion: How the Brain Distinguishes Our Bodies from Objects


The findings in this study suggest that the perception of body weight is processed differently than the perception of objects, with important implications for understanding how the brain integrates sensory and motor information to form a coherent perception of the body and its environment. The inability to perceive the body accurately can lead to unhealthy eating behaviors and the maintenance of negative beliefs about one’s body, exacerbating the cycle of eating disorders.


The perception of the weight of an object is strongly influenced by its size, but how does this apply to the weight of parts of our body? Our body, in addition to being a fundamental part of our identity, is a physical object with measurable properties, such as volume, density, and weight.


During any movement, we exert effort to support the weight of our limbs, but unlike the weight of external objects, we are rarely consciously aware of this effort.


This raises the question: how does the brain determine the weight of body parts in comparison to external objects?


In physics, weight is derived from the product of an object’s mass and gravitational acceleration. However, in the human brain, there are no specific receptors for weight. Instead, the brain uses a combination of proprioceptive, vestibular, auditory, visual, and motor signals to calculate a representation of weight.


Understanding how the perception of volume influences the perception of weight can help us better understand how the brain calculates weight.


Distorted body perception is closely linked to eating disorders such as anorexia nervosa and bulimia. In these cases, individuals often perceive their bodies in an unrealistic way, believing they are overweight even when they are malnourished.

This distortion in the perception of body size and shape can arise due to psychological, social, and biological factors, usually beginning in adolescence or early adulthood.


The inability to perceive one’s body accurately can lead to unhealthy eating behaviors and the maintenance of negative beliefs about one’s body, exacerbating the cycle of eating disorders. For more than a century, research has shown that smaller objects of similar mass to larger objects feel heavier, a phenomenon known as the size-weight illusion (SWI).


Even when participants expect a smaller object to be heavier, they still perceive it as heavier. This illusion suggests that perceived size influences perceived weight, and several theories attempt to explain this phenomenon.


Surprisingly, the effect of size on weight perception persists even when the total volume of objects is held constant, altering only the perception of the space they occupy.


This raises the possibility that the perceived size of body parts may also influence the perception of their weight. Studies on body weight perception demonstrate that it is highly flexible and can change based on sensory signals, such as changes in gravity or auditory stimuli.

During microgravity situations, such as parabolic flights, participants report feeling their hands lighter, while in hypergravity conditions, their hands feel heavier.


Perceived body size also affects how we experience pain, motor control, and tactile perception. Studies show that visualizing an enlarged hand can improve motor performance and reduce pain. This change in perception is accompanied by a reconfiguration of brain resources allocated to the enlarged body part.


To explore how perceived hand size influences its perceived weight, researchers at the University of London manipulated the perceived hand size of 20 healthy participants (15 women and 5 men, average age 32) using visual-tactile illusions.


Participants viewed their hands through mirrors that made them appear larger, normal, or smaller. They then judged the weight of their hand compared to weights hanging from their wrists.

The results showed that participants tended to underestimate the weight of their hands more when they perceived them as smaller and less when they perceived them as larger. This indicates that the perceived size of the hand directly influences the perception of its weight.


Unlike external objects, the perception of the weight of body parts appears to follow a constant density model, where a larger hand is perceived as heavier due to the greater amount of perceived "material", while a smaller hand appears lighter.


These findings suggest that the perception of body weight is processed differently from the perception of the weight of objects, with important implications for understanding how the brain integrates sensory and motor information to form a coherent perception of the body and its environment.



READ MORE:


Perceived hand size and perceived hand weight

Denise Cadete, Vincenzo P. Marino, Elisa R. Ferrè, and Matthew R. Longo

Cognition, Volume 254, January 2025, 105998


Abstract:


The dimensions of objects and our body parts influence our perception of the weight of objects in our surroundings. It has been recently described a dramatic underestimation of the perceived weight of the hand. However, little is known on how perceived size informs the perceived weight of our own body parts. Here we investigated the effects of embodying an enlarged and a shrunken hand on perceived hand weight. We manipulated hand size using a visual-tactile illusion with magnifying and minifying mirrors. We then measured perceived hand weight using a psychophysical matching task in which participants estimate if a weight hanged on their wrist feels heavier or lighter than the experienced weight of their hand. Our results indicated that participants tended to underestimate the weight of their hand more when embodying a smaller hand, and less so when embodying a larger hand. That is, the perceived size of the hand plays a role in shaping its perceived weight. Importantly, our results revealed that the perception of the weight of body parts is processed differently from the perception of object weight, demonstrating resistance to the size-weight illusion. We suggest a model based on constant density to elucidate the influence of hand size in determining hand weight.

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