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For over a century, scientists have known that the perception of an object’s weight can be influenced by its size. This phenomenon, known as the size–weight illusion, occurs when we perceive smaller objects with the same mass as heavier than larger ones. A common example of this is when a golf ball feels heavier than a beach ball of the same weight. But what about the perception of the weight of body parts, like hands? Do we experience the same illusions?
To explore this, researchers at Birkbeck University of London conducted a study to determine whether the size–weight illusion applies to body parts as it does to objects. Their research, published in the journal Cognition, focused on the effect of visual alterations to hand size on the perception of hand weight.
The team, led by Elisa Raffaella Ferrè, used a unique approach to manipulate hand size through a visual-tactile illusion. Participants were shown enlarged or shrunken images of their hands using specially designed magnifying and diminishing mirrors. Following this, the participants performed a psychophysical matching task to compare the weight of their actual hand to the perceived weight of a hanging object on their wrist.
The results were surprising. While the size–weight illusion typically suggests that smaller objects feel heavier, the study found that the opposite occurred with body parts. Participants consistently underestimated the weight of their hands when they perceived them as smaller, and overestimated the weight when they embodied a larger hand. This suggests that our perception of the weight of body parts does not follow the same rules as the perception of external objects.
Ferrè and her colleagues concluded that there are two distinct mechanisms in our brain for determining the weight of body parts and objects, one that processes the weight of external objects based on their size and another that processes the weight of body parts independently of size. “The size-weight illusion, which is so clear when perceiving objects, does not apply to body parts,” Ferrè explained.
The study offers important implications for understanding body perception, especially in the context of eating disorders. It opens up avenues for investigating how individuals with such conditions experience bodily distortions. “This research could help us understand the connection between eating disorders and altered body perceptions,” Ferrè added.
This groundbreaking study challenges long-standing assumptions about the perception of weight and size, highlighting how our brain processes the weight of body parts in a unique way compared to objects.
For more information, the full study is published in Cognition (2024), DOI: 10.1016/j.cognition.2024.105998.
