The perception we have of our own body is constructed by the brain and does not always match physical reality. Far from being a faithful mirror, this internal representation shows systematic distortions that, rather than being mere errors, may serve a functional purpose, especially in the context of motor control. The hand, a central organ in our interaction with the world, offers an ideal model for exploring these distortions.
In a recent study, a research team led by Elisa Ferrè investigated how 20 healthy adults perceive the volume and weight of their own hand. Using psychophysical tasks, participants estimated hand volume through adjustable visual representations and assessed weight by comparing it to suspended masses.
The results revealed an average overestimation of volume by 24% and an underestimation of weight by 25%. Based on these data, the researchers calculated the perceived density of the hand: approximately 0.75 g/cm³, 31% lower than the actual density. This suggests that the brain maintains a stable but inaccurate representation of hand density. Rather than estimating weight and volume independently, it adjusts both to fit this perceived density.
This reduced perception of weight – or perceived effort – may help the brain plan and execute movements more smoothly and efficiently, as if the hand were lighter and easier to move. This principle already inspires applications in robotics and rehabilitation, where reducing the sensation of effort helps improve motor control and the integration of prosthetic devices.
This study was published in the scientific journal Cortex, in the article Underestimation of human hand density, as a part of research project 41/20 - Luminous dancing fairies in weightlessness: How gravity shapes conscious experiences, supported by the BIAL Foundation.
Abstract
Our body is a 3D object, with physical properties such as volume, weight and density. Our brain has to represent these physical properties in the perception of one's own body and body parts. It has been shown that we have a distorted representation of hand size and hand weight. In this study, we investigated the perception of hand volume without experimental alterations. We found that people overestimate the volume of their hand on average by 24%, relative to its actual volume, and we replicated the hand weight underestimation by 25% relative to its actual weight. With a precise estimation of perceived hand volume and hand weight, we calculated perceived hand density. The mean perceived hand density was .75 g/cc, comparable to foam beads, an underestimation of 31% of actual hand density. Our findings suggest that the brain maintains a stable representation of hand density at a low level, with perceived hand weight and volume adjusting accordingly, rather than being estimated independently. Our results add to a body of evidence showing that the representation of our body parts is inherently distorted. This study contributes to the understanding of how volume, weight and density are estimated in the perception of body parts, and the relationship between the representations of physical bodily properties.
