Bodies in the Brain : More than the weighted sum of their parts

Abstract

The main question of this thesis was: Can we dissociate multiple body representations in the healthy brain? Patient studies have already shown a dichotomy between the perceptual representation used for localizing a body part (body image) versus the motor representation used for moving a body part (body schema). Here it is shown for the first time that there is also a similar dichotomy in healthy individuals, as revealed by task dependency of bodily illusions. This dissociation between motor and perception tasks relied heavily on the high susceptibility of the body image, versus robustness of the body schema. To investigate whether the impermeability of the motoric body representation was dependent on the type of induction of the bodily illusion, feeling of agency was manipulated. Even active movement during induction could however not affect the body schema. Although motor actions themselves thus seemed robust to illusion, its possible mediating effect on subsequent perceptual body localization judgments was investigated. Results showed a reduction in the strength of the bodily illusion (in this case the rubber hand illusion) that was dependent on the number of movements made with the illuded hand. In other words, the more (new) proprioceptive information was provided about the illuded limb, the smaller the perceived relocation of the illuded hand (as measured by perceptual judgments). However, even though the strength of the illusion on the body image was reduced by preceding motor responses, it did not erase the effect of the illusion completely. Consequently, the main question Can we dissociate multiple body representations in the healthy brain? can be answered affirmatively. However, the main line of reasoning behind the used paradigm - significantly different responses are subserved by dissociable body representations - becomes problematic when significant different responses within the same type of task can be identified. In that case, this line of reasoning runs the risk of dissociating numerous body representations. Therefore a more dynamic angle is suggested, focusing on the question of how these different body representations are created, therefore looking more closely into the weighting of sensory information. Although this approach has its own pitfalls and does not answer the questions of how many body representations there are, it might lead to relevant new insight when combined with other fields of research and paradigms. As an example, a loss of proprioceptive influence on a visual afterimage is for the first time demonstrated, revealing a critical mediating role of feeling of ownership. This shows that studying the integration of proprioceptive and visual information, while keeping in mind the (dynamic) representation of the body and bodily sensations like feeling of ownership, might cast new light on the weighting of multisensory information and consequently on the way the representation of the body is built.