## 2 Background
### Virtual Reality  
Virtual reality is a technology that helps in anxiety disorder treatment [26]. However, virtual reality aims to immerse the person by taking them to another place while they have not moved physically. Moreover, confounding the human brain requires special equipment and software that are responsible for creating the illusion: mainly, platforms to run the application on desktop and mobile, hardware sensors for motion tracking, input devices for motion recognition, and head-mounted displays or stereoscopic displays (HMDs) to display the virtual environment [14; 27]. A virtual reality experience consists of four elements including virtual world, immersion, sensory feedback, and interactivity.  
The virtual world element describes the given medium content. The virtual world's existence can be in a different form, the originator's mind, computer software, or on paper reality [28]. However, a virtual world based on computer software is an object's representation inside the simulation. By viewing the virtual world using software that fetches objects and interacts with them by physical immersion, the user will experience the true meaning of virtual reality [16].  
The _immersion_ element implies the process of overwhelming the user in an alternate reality [28]. Mental immersion and physical immersion are the two ways in which the term \"immersion\" can be used. The state of deeply engaging the user intellectually is the definition of mental immersion. On the other hand, physical immersion is defined as using technology to stimulate the body's senses artificially. Physical immersion is referred to as the VR system property that substitutes or enhances the stimulus participant's senses.  
_Sensory feedback_ is a fundamental element in virtual reality. In VR systems based on the physical position of the participants, sensory feedback is provided. A high-speed computer is required to accomplish instant interactive feedback. The participant's movement must be tracked by the VR system to base the sensory output on the correct participant position. Position tracking can be defined as an object's computerized position or location sensing in the physical world. Usually, the participant's head and hand are tracked by the VR system. However, body joints can also be tracked in advanced VR systems. To accomplish tracking in VR systems, various technologies can be employed and input devices such as a head-tracking system or a joystick.  
_Interactivity_ elements provide authenticity for virtual reality, where the user finds themselves responding to their actions. Therefore, interactivity is an important element of virtual reality. Moreover, interactivity is concerned with two forms: one affects the world within the computer software, the other persuades the user of that change. Changing object location and moving around in the virtual world are both interactivity examples the user can usually perform.  
Researchers in [6] reviewed several recent works. They described that VRET implementation for a specific phobia usually applies the same treatment protocols used in traditional treatments of a phobia. These include an explanation of the treatment nature for the patient before the start that the phobic level of the patient must be determined in advance by information-gathering procedures and that the gathered information will be a base for establishing stimulus hierarchy levels.