The neural mechanisms of body representation: new insights to facilitate prosthesis' incorporation in upper-limb amputees

One of the biggest challenges in prosthetics is to design artificial hands which feel like patients' own body part. Although in the last years neuroscience, technology and medicine put a great effort in developing innovative prostheses, the improvements have been almost exclusively restricted to their functional aspects. A significant proportion of patients choose not to use their prosthesis or decide to wear it only for cosmetic purposes. In most cases, the prosthesis is perceived as an extracorporeal tool and not as part of their body. Only little attention has been devoted, so far, in promoting artificial limbs' acceptance within patients' body representation. The aim of the research conducted in this doctoral program was to expand the current understanding on the mechanisms of body representation and promoting the embodiment of artificial limbs. Five studies are presented in this thesis: in three of them, an experimental paradigm for testing the plasticity of the body representation (i.e. the Rubber Hand Illusion – RHI) has been employed on groups of healthy volunteers. In another study, a multimodal approach with behavioral and neurophysiological investigations was conducted on a group of healthy participants to find the optimal posture of a hand prosthesis through which it would be easier to embody. In the remaining one, an upper limb amputee implanted with intraneural electrodes has been longitudinally tested to investigate the level of embodiment over two different neurally-interfaced prostheses. To this aim, two objective paradigms based on multisensory integration (i.e. the Visuo Tactile Integration – VTI and the Temporal Order Judgement – TOJ) have been exploited. The results suggest that the amount of practice with a prosthesis and the richness of somatosensory feedback are crucial for facilitating its integration with one's body representation. The optimal posture for a prosthesis to be embodied is the one performed with the thumb-down and other fingers-up. Furthermore, when designing a prosthesis which employ sensory substitution feedback, the richness of the somatosensory stimuli is another aspect to take into consideration to facilitate its embodiment. Finally, a new and objective way to measure the onset and the initial evolution of the sense of ownership over an artificial limb has been discovered: the Non-Specific Skin Conductance Response (NS-SCR).