Our body is a 3D volumetric physical object, and like any other object, it has measurable properties such as size, volume, weight and density. Yet, the way we perceive the physical properties of our body parts is quite different from the way we perceive the physical properties of any other objects in the world. We are fairly accurate in perceiving the size of objects (Norman et al., 2022), even though it can be influenced by other properties, such as orientation (Shepard, 1990; Tyler, 2011), its relative size to another object (Gentaz & Hatwell, 2004) or how familiar they are (Maltz et al., 2021). A key aspect of how we perceive our body parts lies in the presence of a dedicated body map within the brain, something unique to our own bodies and not applicable to external objects. This internal map consistently displays systematic distortions that are reliably observed across healthy individuals (Longo, 2017, 2022). In fact, when perceiving our own bodies, we experience consistent distortions in the perceived size (Linkenauger et al., 2015; Longo et al., 2015; Longo & Haggard, 2010), volume (Sadibolova et al., 2019) and weight (Ferrè et al., 2023) of body parts (Sadibolova et al., 2019).

Perception of hand size and shape is systematically distorted, with fingers perceived to be 20–30% shorter than their actual length, while hand width is perceived to be 60–80% wider than it truly is (Longo & Haggard, 2010). It has also been shown that there is a gradient of finger length underestimation, with the little finger being the most underestimated and the thumb the least. Critically, in a case of congenital limb absence, these distortions still occur (Longo et al., 2012), indicating that the shape and size of phantom limbs are represented in a consistent and potentially innate configuration, even without the limb itself or any visual or somatosensory input from it. Perception of body part length is also distorted: the actual length tends to be overestimated for less sensitive areas, such as the arms, legs, and torso (Linkenauger et al., 2015; Sadibolova et al., 2019). Similarly, we have demonstrated that we significantly and systematically underestimate the weight of our own hand, perceiving it to be 49% lighter than its actual weight (Ferrè et al., 2023). Taken together, these studies indicate that we have a heavily distorted perception of our hand.

Our body has physical properties such as volume, weight and density. However, no receptors can directly convey these properties; instead, our central nervous system processes different types of information to construct our perception of them. Volume, weight and density are interconnected concepts, as we rely on one to estimate the others. That is, we estimate density based on volume and weight, or infer volume from weight and density. While this relationship is well-established for the physical properties of the body, it remains unclear whether similar principles apply to how we perceive these properties, and whether there is a direct relationship between perceived volume, perceived weight and perceived density. To investigate this, it is essential to establish a baseline for perceived hand volume. Recent findings suggest that alterations in the perception of hand size change weight estimation: when we perceive a larger hand, we perceive its weight as being closer to its actual weight (less underestimation) than when we perceive a shrunken hand (more underestimation) (Cadete et al., 2025). In that study, magnifying and minifying mirrors were used to induce the feeling of having an enlarged, a normal and a shrunken hand. Perceived hand weight was quantified using a psychophysical staircase procedure, which showed that the hand was consistently perceived as lighter than its actual weight across all hand size manipulations. However, the shrunken hand was perceived as significantly lighter than the enlarged hand. This pattern of results is coherent with a constant density model: when experiencing a change in hand size, perceived hand weight is estimated as if the hand's density remains constant. This means that a larger hand would be perceived as heavier because it would contain more of the same “hand stuff” rather than dispersing the same mass over a larger area. This constant-density model helps explain how size, weight, and density are integrated in the perception of our own hand.

Here we systematically investigated the perceived volume, weight and density of the hand. To determine perceived hand volume, we employed a psychophysical staircase procedure in which participants judged whether the volume of a wooden block was smaller or larger than their left hand on each trial. Using cubes allowed us to create an abstract measure of hand volume, independent of the hand's shape. For perceived hand weight, we replicated the weight estimation task developed by Ferrè et al. (2023). This approach enabled us to analyse any correlation between perceived hand volume and weight and to calculate perceived hand density, using the mathematical formula of density as mass divided by volume.