Towards a novel platform to study embodiment

Body representation defines the general representation of our body in our brain. It is composed of a complex sub-division of representations that can be regrouped into two main concepts: the body image and the body schema. Whereas the body image is a conscious aggregation of our feelings and emotions towards our body, the body schema is an implicit model of the metric properties of our body integrating limb dimensions and position in space. A pathological body schema resulting from an amputation can provoke phantom limb pain from the missing limb. A solution to restore a healthy body representation in amputees could be to equip them with prostheses that could replicate the sensations of their former limb. An obstacle to this solution is the abandonment rate of prostheses from their users, who mainly report the lack of feedback from the device. Those inputs direct towards the necessity to promote the embodiment of the prostheses. Embodiment is a complex phenomenon that takes place when we perceive that we own a body part and that we are responsible for its action, senses respectively referred as ownership and agency. However, little is known on the brain process of embodiment so far. Therefore there is a need to cast light on its grey areas to be able to understand the necessary characteristics that a prosthesis must have to be fully embodied and thus integrated into the user's body schema. The present work presents the development and validation of components of a platform aimed at the comprehensive study of the brain processes underlying embodiment. A first component of the platform was developed integrating motion capture and virtual reality into a virtual environment to study the embodiment and its effect on the body schema. The platform was validated and the study showed through the assessment of tactile distance perception that experiencing an elongated arm in immersive virtual reality effectively modifies the body schema. It was additionally found that in this study the body schema alteration was mainly due to the sense of vision whereas the role of the visuo-tactile congruent feedback - typically eliciting ownership - appeared irrelevant. The second component of the platform consisting in an robot-aided transcranial magnetic stimulation system was developed and validated through the test of control approaches for subjects' movement tracking and compensation. A hybrid force-position control and a selective impedance control were found to be the best choices for our future applications. Finally, the author obtained theoretical and practical hands-on knowledge on the use of electroencephalography to monitor the brain activity through a study of a preferred hand posture in the body schema. Results did not provide any evidence of such a hypothesis from an electrophysiological point of view.