Il senso dell'azione e dell'apparenza corporea nei disturbi funzionali del movimento

Functional Movement Disorders (FMD) are part of the spectrum of functional neurological disorders. Recent research findings implicate three key processes in the pathophysiology of FMD: abnormal attentional focus; abnormal beliefs and expectations; and abnormalities in sense of agency (SoA). These three processes have been combined in a recent neurobiological model of FMD, suggesting that abnormal predictions related to movement are triggered by abnormally focused attention; the resulting movement is generated without the normal sense of agency that accompanies voluntary movement, being the FMD itself felt as involuntary. This gap might be related to altered self-recognition of bodily actions that consists of two fundamental components: the sense of agency (SoA), that is the subjective experience of being in control of own actions and the sense of body ownership (SoBo), that is the feeling of the body as part of the self. Here we investigated whether SoA, SoBo and their relationship are altered in FMD. Differently from previous studies on the implicit component of SoA, we focused on the explicit component of SoA, which more closely resembles the clinically described lack of explicit control of the motor symptom in FMD. With the use of an ad-hoc paradigm (Kalchert e Ehrsson, 2012), based on the rubber hand illusion (RHI). To this porpuse, the first step was to build up a suitable task and test it in. Experiment 1: 13 young participants. We applied three different conditions: active synchronous condition, passive congruent condition and control condition. Namely, subjects gave higher scores to ownership and agency after synchronous than asynchronous movements in the active condition. Moreover we found higher scores of agency after active compared to passive movements. With regards to the proprioceptive drift, however, the data were not clear. Moreover, we realized that there was no way to separate agency from ownership. In particular, there was no 8 condition in which there was only agency without ownership. We decided to add a condition in which the artificial hand was rotated 180° and therefore in an incongruent position with respect to the subject’s own hand. This creates a sense of agency, but since the two hands are in an incongruent position, the sense of body ownership is not induced. Experiment 2: we applying the paradigm with this additional condition to 24 healthy subjects, we confirmed the previous results and we were also able to separate agency from ownership. Experiment 3: in order to test if the task could be executed also by 4 patients with essential tremor, we recruited and we found that they were able to perform the task, they appeared to have a pattern similar to controls. Experiment 4: twenty-one patients with diagnosis of FMD. The results showed that: synchronous movements determined a strong sense of agency and ownership; passive movements suppressed agency but not ownership; the anatomically implausible position of the rubber hand eliminated ownership but not agency; asynchronous movements abolished both agency and ownership. This pattern of responses suggesting that FMD patients maintain an explicit sense of agency for normal voluntary movements and that the sense of body ownership is preserved. The latter finding is in line with a previous study using the static RHI.