Knowledge of one's own body size is a crucial facet of body representation, both for acting on the environment and perhaps also for constraining body ownership. However, representations of body size may be somewhat plastic, particularly to allow for physical growth in childhood. Here we report a developmental investigation into the role of hand size in body representation (the sense of body ownership, perception of hand position, and perception of own-hand size). Using the rubber hand illusion paradigm, this study used different fake hand sizes (60%, 80%, 100%, 120% or 140% of typical size) in three age groups (6- to 7-year-olds, 12- to 13-year-olds, and adults; N = 229). We found no evidence that hand size constrains ownership or position: participants embodied hands which were both larger and smaller than their own, and indeed judged their own hands to have changed size following the illusion. Children and adolescents embodied the fake hands more than adults, with a greater tendency to feel their own hand had changed size. Adolescents were particularly sensitive to multisensory information. In sum, we found substantial plasticity in the representation of own-body size, with partial support for the hypothesis that children have looser representations than adults.Perceiving one's own body is crucial for being able to perceive the world and act on it. But how do we do this? Imagine that I can see two hands resting on the table in front of me. One is mine, and one belongs to my friend. How do I tell which is which? This seems like an obvious question, but on consideration it is not. In fact, research has told us that adults use several different types of information, including multisensory visual, tactile, and movement cues; and stored knowledge about the form of their own hand. A more difficult question is how children manage to identify their own bodies in the midst of the constant growth and change which occurs in childhood. Very little is known about this. In particular, it is unclear how children balance the need for a consistent idea of their own body, and the need to be flexible as it grows. Further, new virtual reality technologies are emerging which can provide virtual bodies to children in games or educational settings. How might children accept and use these virtual bodies? This 3-year project addresses these issues by investigating how children and adults perceive their own bodies, and how this grounds the emerging sense of bodily self. We will run five carefully designed experiments, building on methods which we have previously used successfully with children. The project team have the theoretical and technical backgrounds necessary to carry out this pioneering work; our lab has suitable equipment; and the proposal includes previous published and pilot data showing the feasibility of the approach. We will experimentally examine own-body representation using the 'Rubber Hand Illusion'. The participant sees a fake hand on the table in front of them while their real hand is hidden. An experimenter strokes the hands at the same time. This makes the participant feel as if the fake hand is their own. Further, when asked to point underneath their own hand, they point near the fake hand. We have recently shown that 4 - 13-year-old children experience this illusion. Here, we will measure what happens when the size or shape of the fake hand is changed. If participants experience the illusion less in these cases, it shows that they have expectations for how their hand should look. Based on previous work, Experiments 1 and 2 will test the hypotheses that both children and adults will expect that a hand must be five-fingered; and that the hand must be approximately the right size. Crucially we will also determine whether there is plasticity in these body representations, enabling children to accept for example larger hands than their own to account for growth. Experiments 3-5 will examine how body representation may change given experience of a moving body. Do children learn more quickly from experience more than adults, for example requiring less movement experience to accept an oversized hand? How does touch information combined with movement in forming a sense of the bodily self? What are the limits of what children will accept as their own body in such an environment? The academic outputs of the project will provide vital new information on how children represent their bodies. This is an interdisciplinary project between Psychology and Computer Science, and its findings will be of major significance and interest across a range of disciplines - Psychology, Computer Science, Philosophy, and Cognitive Science. The work will also have non-academic impact. We will communicate findings to designers of virtual reality games, as well as healthcare practitioners developing bionic arms or using virtual rehabilitation programmes. Finally we will use the work as a springboard to invite volunteer children to a series of workshops examining the senses and movement. Through these we hope to encourage them towards STEM activities or careers.

We presented subjects with the rubber hand illusion and asked them their experiences. They were neurotypical adults, 6-7year olds and 12-13 year olds.