How Eating Disorders Shape Perception of Body Part and Weight of Object

Researchers have examined how humans perceive the size and weight of body parts, specifically the hand, and compared this to their perception of objects. They found that the two processes differ. For objects, smaller items tend to feel heavier, while for body parts like the hand, the opposite occurs. This discovery is crucial for understanding how individuals with anorexia nervosa and other eating disorders perceive the size and weight of their bodies (1^✔ ✔Trusted Source
Perceived hand size and perceived hand weight

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Brain’s Perception of Weight

According to Newton’s law, weight is the product of an object's mass and gravity. But how does the brain interpret the weight of objects and body parts? For over a century, it has been known that an object's size affects our perception of its weight. For instance, when lifting a small object and a large object of similar mass, the smaller one feels heavier—such as a golf ball compared to a beach ball. This phenomenon, called the size-weight illusion, demonstrates how perceived size strongly influences the perceived weight of an object.

Little is known, however, whether the same is also true for the weight of parts of our body. With the support of the BIAL Foundation, a team of researchers from Birkbeck University of London sought to answer this question by assessing the effects of embodying an enlarged hand and a shrunken hand on perceived hand weight in a sample of 20 healthy participants.

“We manipulated hand size using a visual-tactile illusion with magnifying and diminishing mirrors”, explains researcher Elisa Raffaella Ferrè. “We then measured the perception of hand weight using a psychophysical matching task, in which participants estimated if a weight hung on their wrist feels heavier or lighter than the experienced weight of their hand.”

Different Mechanisms in Weight Perception for Body Parts and Objects

The results were presented in the article “Perceived hand size and perceived hand weight”, published in the scientific journal Cognition in January 2025, and indicated that the participants tended to underestimate the weight of their hand more when embodying a smaller hand and less when embodying a larger hand.

This study showed that the size-weight illusion, which occurs in the perception of objects, does not apply to body parts, as an enlarged hand felt heavier and not lighter, even though it weighs the same, and the reverse logic for the small hand. The results thus point to two different mechanisms in weight perception, one for body parts and the other for objects.

“Investigating how individuals with eating disorders experience the bodily size-weight illusion, as presented in this study, can deepen our understanding of these disorders and their connection to bodily distortions”, emphasises Elisa Rafaella Ferrè.

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Reference:

1. Perceived hand size and perceived hand weight - (https://www.sciencedirect.com/science/article/abs/pii/S0010027724002841?via%3Dihub)

Source-Eurekalert