## 1 Introduction  
Virtual reality (VR) has been defined as the \"use of interactive simulations created with computer hardware and software to present users with opportunities to engage in environments that appear and feel similar to real-world objects and events,\" [1]. In computer science, definitions of VR emphasize the possibility and ability to combine software with hardware to create a fully immersive experience [2]. In health care, the term is used in a slightly different way; to describe both non-immersive and immersive experiences that create an alternative reality [3]. Its main applications seem to be in the field of motor learning, whereas only few randomized controlled studies address its effects on other cognitive functions [4]. Its advantages rest on the possibility to fine-control, personalize and hierarchize tasks. Furthermore, it gives the possibility of providing multimodal feedback in real time; indeed, the stimulation of multiple perceptual channels, implemented by the use of auditory and visual feedback, increases the patient's awareness of his performance and allows a sense of global wellbeing [5]. Indeed, VR represents a very promising technology for neurorehabilitation. Computers elaborate a simulation of the real world using real-time graphics, through which the subject can interact with the environment [6]. There are different types of VR, including (a) immersive virtual reality; (b) desktop virtual reality; (c) projection virtual reality (or semi-immersive virtual reality); and (d) simulation virtual reality [7]. VR ranges from non-immersive to fully immersive, according to the degree to which the user is isolated from the physical surroundings while interacting with the virtual environment [8]. Usually, immersive systems involve computer interface devices, such as head-mounted displays (HMD) or projection screens surrounding the subject (cave systems), fiber-optic wired gloves, position tracking devices, and audio systems providing 3D sound. Immersive virtual reality in particular provides an immediate, first-person experience and a deep \"sense of presence\" [9], i.e., the perception to be immersed in a different world created by the components of software and hardware [10]. The level of presence is a subjective feeling and depends on user experience [11]. Compared to immersive VR systems, semi-immersive devices do not provide the constant update of the visual information according to the participant's head movements. On the other hand, they are more immersive than typical 3D monitors, particularly in terms of the range of sensory modalities accommodated [12; 13]. The use of VR in neurorehabilitation has grown considerably, and experimental evidence suggests that this technology could favor functional recovery in neuropsychological disorders [8; 14]. With respect to other tools in neurorehabilitation, VR has a number of peculiarities. Among others, the possibility of creating tailor-made training programs, so that the rehabilitation process can be individualized and adapted to each patient's specific needs. Moreover, VR can foster active involvement, thanks to the possibility of creating new and appealing environments [14]. Very often, VR gives therapists the possibility to individualize treatment needs, as well as the opportunity for repeated learning trials, while gradually increasing task complexity and/or decreasing therapist support and feedback [1]. Immersive and semi-immersive VR systems provide the opportunity to practice cognitive and motor activities that cannot be practiced within the clinical or the educational environment, performing simulations of real-life scenarios and activities [5; 15]. In many cases, clinicians need special software development tools for the design and coding of interactive simulated environments to achieve specific rehabilitation goals [1].