Body representation, body localisation and body size perception: a study of bodily modulations

People are generally quite good at adapting to changes in body shape and size because of the flexibility of the body representation. By means of bodily illusions, it is possible to experimentally induce updating of body representation and, thus, manipulate the sense of self. The main aim of this thesis is to investigate the sense of self through bodily illusions. Firstly, we investigated the relationship between the sense of ownership and self-localisation (Study 1). The results from this study are taken to suggest that the proprioceptive drift (i.e. a bias in the localisation of a given body part) is more likely triggered by and related to the visual capture of touch than it is a reliable measure of a shift in the sense of body ownership. In fact, our data show that the proprioceptive drift occurs not only in the absence of a shift in the sense of ownership, but even in the absence of a body-like object. Secondly, we investigated self-localisation of body parts by means of a novel illusion, the Disappearing Hand Trick. In particular, we explored the role of vision and proprioception (Study 2), as well as the role of attention and motor acts (Study 3), in locating one’s own hands when visual and proprioceptive information regarding the body are incongruent. Our data (Study 2) are in line with previous research, confirming a predominant role of vision over proprioception. In addition, they show that, after a certain amount of time, proprioception is weighted more heavily than vision. That is, our results demonstrate that the cortical proprioceptive representations can be updated even when there is no real need to do it (i.e. no movement is required). This might be seen as an evolutionarily convenient response to keep the body ready for a possible quick reaction. In Study 3, we ruled out the possibility that this effect was only driven by spatial attention being directed towards the side of the space where the hand was actually located. In fact, no difference in the localisation accuracy was found when the direction of spatial attention was manipulated. Finally, by asking the participants to reach across for their hidden right hand (Study 3), we confirmed that a motor act accelerates the reliance on proprioception, most likely by aligning the motor and perceptual coordinates in order to plan the movement. In the first three studies, a modification of the body representation was intentionally induced – namely, the purpose of the illusions was to change how the participants perceived their body. However, we wondered whether this same change might also occur at a more implicit level and how rapidly this may occur. We designed two different studies in which we tried to manipulate the participants’ perceived body size by providing incongruent information about the position of their limb (Study 4) or by showing the participants images of unrealistic bodies (Study 5). In Study 4 we showed that incongruent proprioceptive information coming from the same joint does not affect the perceived size of that body part, but does lead to a more accurate estimation of its position. On the other hand, the results of Study 5 would seem to suggest that body perception is more vulnerable to change in women than in men after exposure to same-sex ideal bodies. Taken together, these results suggest that, by manipulating the body representation, both explicitly, by means of a variety of bodily illusion, and even implicitly, by generating subtle incongruence between one’s own real body and how the body ‘should’ be, we were able to shed some light on the mechanisms behind the computation of body position and size, both of which are important elements for the definition of the self.