Epidemiological studies report that 85% of stroke survivors show hemiparesis and a percentage ranging from 55% to 75% report upper limb functional impairment. Early rehabilitation in stroke patients with motor disorders can be effective to restore the affected function and to improve the performance of daily activities. Up to now, different methods and techniques have been used to recover motor functions in stroke patients. However, none of these has been conclusively proven to be more effective than the others. The clinical benefits induced by motor rehabilitation are closely related to "neuroplasticity". The main aim of the present study is to assess the efficacy of action observation treatment, based on Mirror Neuron System (MNS), in the rehabilitation of upper limb motor functions in acute stroke patients. The study will also aim at assessing neuroplasticity within areas belonging to the MNS through functional magnetic resonance (fMRI).