The goal of the study was to investigate a rather neglected feature of body representation, i.e., hand size perception, in a large sample of young children, from 6 to 10 years old.

We found that children underestimate the size of their hands, while being accurate in judging objects size, a bias mirroring hand perception in adults11. This was true independently on the sensory modality, as underestimation was equally present for visual and haptic task, whenever it involved hands perception. Most importantly, the amount of underestimation increased with age, with younger children being relatively accurate compared to older children. In other words, it looks like the hand representation in the brain cannot keep up with the actual hand growth. Crucially, the increase of the underestimation was not the result of an increase in uncertainty in the response as older children less variability in their responses while being less accurate in the representation of their hand size.

Our data show that size underestimation emerges as a characteristic of body representations since a very young age. The question is why our brain represents, at a conscious level, our hands as smaller than they are.

One explanation could be in the over-representation in primary somatosensory and motor maps and the need for a compensation for it. Indeed, in both homunculi, hands occupy a very large cortical territory compared to other body parts, despite their physical size. However, this overrepresentation is not accompanied by biases in behaviour, as one would expect if such maps were used to control movements or organize perception. If we were to plan movements according to hand size as represented in M1 we would exhibit grasping errors.

Similarly, in the Weber Illusion a same distance between two tactile stimuli is perceived differently depending on whether the two stimuli are delivered on body part with lower or higher tactile acuity. For instance, two touches delivered on the fingers will be perceived as further apart as compared to the same touches delivered at the same distance from each other on the back. However, the hand over-representation in the primary somatosensory cortex can only partially explain this illusion18, as a much stronger bias would be predicted on the basis of the cortical magnification factor between different body part, which suggests the presence of a compensatory mechanism at a higher level. This hypothesis is supported by another perceptual effects whereby tactile distance perception is influenced by magnified or minified vision of the stimulated body part19, or illusory change in body part size as induced by proprioceptive stimulation20. Thus, it seems as if, in an attempt to compensate for a bias at the primary level, the brain exceeds on the opposite direction which results in an underestimation at a higher level of representation. The question then is why perception biases go in the direction of an underestimation.

Previous studies have shown how body representations at a different level are more prone to modifications in the direction of an extension/elongation, rather than the opposite (see van der Hoort & Ehrsson for different results). For example, Pavani and colleagues showed, using a Rubber Hand Illusion paradigm, how hands larger than the real one are easier to incorporate than smaller ones21. Similarly, body representations have been shown to be capable of incorporating long tools, elongating the arm length representation22,23, while short-term manipulation difficulty induce a perceive contraction24. One can make the hypothesis that given this directional advantage, a representation that underestimates can be quickly compensated when needed. However, this does not explain why non-veridical representations are present by default nor whether underestimation produces the elongation bias or vice versa.

Another hypothesis is that the underestimation is a more complex distortion that reflects multiple compensatory mechanisms. Since data on body size representations in children are not available, once again we can refer to the adult literature, which is richer. Hand representations are distorted in young adults and, in particular, we tend to represent our hands not simply as smaller25,26,27: fingers length is underestimated but not uniformly as the underestimation increases as we move from the thumb to the little finger. Meanwhile, knuckles distance is overestimated leading to a represented hand with short and more distant fingers than reality. This suggests that, if the underestimation is the result of an overcompensation the brains applies to the primary sensory and motor representations, this does not happen uniformly. It seems that less compensation is applied to the fingers that contribute the most to our dexterities, suggesting a role of motor experience in this dynamics.

Previous studies already showed how motor and sensory experience induce changes in body representation organization: for example, in the first months of life babies rely on canonical body representations for posture where, for example, the right hand occupies the right space and vice-versa for the left7,28.When the baby begins showing spontaneous reaches across the midline, behaviours such as the cross-hands effect, resulting from updated arm posture representation, start to appear. Similarly, one can make the hypothesis that the amount of sensory inputs received and motor skilfulness acquired with the hands affect their size perception.

Our data add a fundamental and contradictory new piece of information: we showed that underestimation increases with age, meaning that distortion increases with the acquisition of skillful motor control29. If we compare the amount of underestimation in the present sample of children and the adult literature30, we can see that the amount of underestimation is even higher from childhood to adulthood. This comparison has to be taken with caution because based on data from different tasks. One important difference between our task and Longo and Haggards’ is that we assess size estimation globally, while they investigated hand’s segments. It is possible that the global reduction in hand size estimation in children is the result of a distortion where fingers and palms are differently affected. What we capture here is a hand representation that seems not capable of following the entire hand in its global growth, although given the cross-sectional nature of our study we can not make strong inferences about the development of the hand representation but rather present snapshots of its state at different ages. Future research might focus on the development, using a longitudinal approach and studying single digits representations.

Another crucial result of our study is that hand distortion was present both for the visual and haptic condition. Previous studies on adults have proposed the existence of multiple body representations, based on the observation of differences in the amount and/or quality of the distortions in body representations and the sensory information they rely on (somatosensory vs. visual31,32,33,34,35). The debate about whether these representations are completely independent or interconnected is still ongoing, which makes relevant any study showing evidence in favor or contrary to a dissociation between vision-based and somatosensory-based representations33. Here we found a similar distortion (underestimation) in both visual and haptic tasks, a result that seems to contradict recently proposed theories according to which visual and somatosensory representations both show distortions, but in opposite directions to properly compensate for each other’s biases36. Our results show that such difference is not present when children judge hand size. Thus, they suggest that the hand size perception task employed here taps into the same (distorted) representation, that is accessible from the visual or somatosensory modalities, at least in children. The presence of a strong correlation between our two tasks results seem to exclude the alternative possibilities that two different representations are stored and used to judge hand size in either modality and both are equally distorted.

Finally, we showed that the distortions involve only hands’ size and not objects’, even when hands are used to create that judgement (that is, in the haptic condition). This points to a body specific distortion, in line with previous studies on Scale Errors on younger children. However we must consider a crucial difference between the two Experiments: while both required a comparison between a standard stimulus that is always present (Own hand in Experiment 1 and Right object in Experiment 2), the presentation modality of such standard stimulus differed, especially in the visual condition. By being attached to the rest of the body, information from the hand are always available through somatosensation, making the haptic condition quite similar between Experiment 1 and 2. This is not the case for the visual condition, in which we prevented children from seeing their own hand and forced them to rely on storage knowledge. This could suggest that the nature of the task (off-line in Experiment 1 and On-line in Experiment 2) could play a role in explaining our results. While we can not completely exclude this hypothesis, since we did not find any difference between the two conditions in both Experiments, we are quite confident that Experiment 2 constitutes a valuable control and supports our interpretation of a body-specific bias.

In conclusion, we showed, for the first time, that hand representation in children ages 6 to 10 is distorted and in particular that similarly to adults, children underestimate their hand’s size. Crucially the underestimation increases with age, is specific for the hand (as it is not present when judging an object size) and independent on the sensory modality provided.

These data provide a first insight into the development of body representations in childhood and show the appearance of a progressive divergence between the actual growth and the represented growth at a global level.