THE MOTOR CORE OF BODY OWNERSHIP

The present work deals with one of the most fascinating aspect of consciousness, the awareness of the bodily-self, and in particular with the notion of body ownership. The goal of this thesis is to combine the phenomenological conception of the bodily-self, mainly grounded on the concept of embodiment, with the scientific investigation of the physiological bases of the sense of body ownership. Chapter 1 introduces the theoretical subject and its experimental investigation in both pathological and healthy brain, arguing that the very core of the self, as claimed by neuro-phenomenology, is determined by the dynamic relationship between body representation and motor system, which enables human beings to properly act in the world and to build a coherent sense of self. In this view, body representation and the possibility to move are conceived as the two main factors allowing human beings to reach a conscious experience of the self. Can this theoretical insight meet scientific evidence? In other words, are we able to measure the contribution of the sensory-motor system in generating body ownership? This aspect of human experience was investigated using two experimental approaches. In Chapter 2, given that the fundamental characteristics of subjectivity can sometimes be illuminated through the study of their pathological distortions, we experimentally tested how body metric representation can be susceptible to plastic remapping after tool-use training, in healthy subjects (Experiment 1) and in a particular sample of patients in which an ischemic episode in the right hemisphere caused a disruption of body ownership, along with primary sensory-motor dysfunction (Experiment 2). These patients show a sort of complementary disease to the well-described symptoms of patients in classical somatoparaphrenia (delusion of dis-ownership), by showing a pathological embodiment of someone else’s limb (delusion of ownership). As is often the case with neuropsychological syndromes, a large amount of theoretical questions arises from the observation of this condition. First of all, which is the phenomenal experience involved? Is that condition just a confabulatory manifestation? Does this subjective and sometimes illusory sense of body ownership influence objective measures of the sensory-motor system? Which is the relationship between sensory-motor impairment and body ownership disruption? Chapter 2 addresses these questions by demonstrating that the intentional motor processing which underlines the action execution with an alien limb experienced as own, induces body representation remapping, even in absence of actual proprioceptive feedback. Chapter 3 investigates how a multisensory illusion that alters the sense of body ownership, the rubber hand illusion (RHI), affects the activity of the motor system in a sample of healthy subjects. During this experimental manipulation, subjects report a feeling of ownership over an artificial, rubber hand and a concurrent feeling of dis-ownership over their real hand. What is the mechanism underlying the self-attribution of the rubber hand to one’s own body? And again, which relationship is there between the functionality of the sensory-motor system and the feeling of body ownership? Are the qualia of ownership/dis-ownership reflected at the level of the motor system? We found that the sensation for the hand disappearing is reflected in a lower excitability of the motor pathways that govern the movement of the same hand, providing the first neurophysiological evidence that our conscious experience to have or not a body has a fundamental counterpart in the activity of the sensory-motor system. In conclusion, even if usually taken for granted, our conscious belief to own a body has a very multi-level nature. Both in presence of brain injuries and experimental manipulation, the sense of body ownership can be altered; only in these cases we are able to realize the complexity of its structure and how the brain plays an essential part in its generation. The results reported here are discussed in the context of phenomenological and cognitive literatures in an effort to understand the role of both body awareness and of sensory-motor activity as the building stones of the human conscious experience of being in the world.