Rehabilitation of paretic stroke patients, aimed to improve function of the impaired upper limb, uses a wide range of intervention programs. A new rehabilitative approach, called Action Observation Therapy (AOT), based on the discovery of mirror neurons, has been used to improve motor functions of adult stroke patients and children with cerebral palsy. Recently, Virtual Reality (VR) provided the potential to increase the frequency and the effectiveness of rehabilitation treatment and offered challenging and motivating tasks. The purpose of the present project is to design a randomized, controlled, six-month follow-up trial (RCT) for evaluating whether action observation (AO) added to standard VR (AO+VR) is effective in improving upper limb function in patients with stroke, compared with a control treatment consisting in observation of naturalistic scenes (CO) devoid of action content, followed by VR training (CO+VR). The AO+VR treatment may represent an extension of the current rehabilitative interventions available for recovery after stroke and the outcome of the project could allow to include this treatment within the standard sensorimotor training or in individualized tele-rehabilitation.

Stroke is the leading cause of disability among adults, and more than 60% of stroke survivors have motor deficits, particularly related to the upper limb. Stroke rehabilitation usually involves intensive motor training aimed to promote adaptive plasticity, by reducing motor deficits and developing new motor learning strategies. It has recently been proposed that the systematic use of action observation (AO) followed by imitation (Action Observation Therapy - AOT) is an effective way to improve motor functions and to promote upper limb recovery in patients with motor disorders. During a typical AOT session, a series of daily life actions (e.g., grasping a key and inserting it into a lock) are practiced for about 2/5 weeks (with a frequency of 3-5 daily sessions per week). During each rehabilitation session, patients are instructed by the therapist to observe a specific action performed by an actor, presented as a short video-clip on a monitor, and afterwards to reproduce the previously observed action with the paretic limb. In each video, a single motor act is usually presented as observed from different perspectives (e.g., subjective, front or side view). This therapy is based on the neural model of the Mirror Neuron System (MNS), originally discovered in the monkey premotor and parietal cortex, formed by visuomotor neurons that become active both when a monkey performs a goal-directed motor act and when it simply observes the same o a similar motor act performed by the experimenter. A comparable MNS has also been identified in humans using different electrophysiological and neuroimaging techniques. In humans, the two main nodes of the MNS are the inferior parietal lobule (IPL) and the ventral premotor cortex (PMv), plus the caudal part of the inferior frontal gyrus (IFG).

AOT is considered particularly useful for activating the motor system in those conditions in which intensive motor training is not feasible, because of the severity of the impairment of motor functions or due to the presence of pain, inflammation, muscle fatigue. In the last years, new Virtual Reality-based (VR) rehabilitation treatments have been introduced, in order to present rehabilitation exercises in more practical and friendly setting. These treatments are generally well accepted by the patients because they offer several advantages: relatively low cost (in particular for semi-immersive versions), engaging environment, real-time personalization of exercises and greater adaptability to the patient's clinical features and progress, as well as possibility to record motor performance and to acquire and provide feedback to the patient in real time. Furthermore, VR exercises usually require a minimal therapist supervision, thereby facilitating home-based form of rehabilitation.

Several studies support the application of VR methods in the rehabilitation of the hemiplegic upper limb in patients with stroke. Recent literature reviews provided evidence for improvement of upper limb motor function and daily life activity after VR-based training, as compared to vicarious standard interventions. However, clinical evidence based on rigorous RCT on the effect of combined use of observation of actions followed by their immediate imitation in a VR environment (AO+VR therapy) are lacking, especially in the case of rehabilitation applied during the chronic phase after the stroke.

The main hypothesis is that, for the recovery of motor function of hemiplegic stroke patients, the combined rehabilitation treatment (AO+VR therapy) is more effective than a control treatment (Control Observation - CO) based on observation of videos without motor content (e.g., environmental natural scenes ), followed by the execution of actions in VR (CO+VR control therapy),.

In sum, the planned trial will examine the following hypotheses:

1. AO+VR is an effective tool to promote upper limb control in paretic stroke patients, and its effects are higher than CO+VR control treatment.
2. motor performance, cognitive level, and structural brain damage assessed before treatment are correlated to the degree of improvement determined by the AO+VR intervention;
3. AO+VR intervention determines, as compared to CO+VR control treatment, plastic functional changes of the MNS activity.

Inclusion Criteria:

1. primarily motor symptoms with unilateral upper limb paresis (controlled via standard neurological examination);
2. residual movement ability of the paretic upper limb, controlled by Medical Research Council (MRC) index > 2 and < 4, active use of the hemiplegic limb, from minimal (mainly for assistance tasks to the preserved limb) to discrete (characterized by coarse manipulation and an inability to perform precision grip);
3. sufficient cooperation and cognitive understanding to participate in the activities, controlled by the investigator recruiting the patient.

Exclusion Criteria:

1. severe cognitive impairment (score <20 on MMSE);
2. presence of severe forms of unilateral spatial neglect (Bells Test, cut-off =/> 50% ).
3. presence of severe anosognosia;
4. presence of severe language comprehension deficits assessed by clinical examination;
5. presence of severe untreated psychiatric disorders;
6. sensory impairment hindering participation and/or not compensated visual deficits of central origin;
7. drug-resistant epilepsy;
8. presence of cognitive disability (IQ < 65) controlled by administration of Wechsler Adult Intelligence Scale IV (WAIS-IV) (Wechsler, 2008).

Participants and their caregivers will be informed about the study aims and procedures, but they will be blinded to group allocation. In the case the patient asks for the presence of the caregiver, she/he will be seated near the participant but out of her/his view, without interfering during the treatment session. The therapist performing the intervention will not be blinded to the group allocation. Outcome assessments will be administered and scored by a member of the staff blinded to group allocation.