## 4 Immersive Virtual Reality: Hardware and Software Perspectives  
### Overview of Immersive Virtual Reality Hardware  
In healthcare, the most immersive experiences are often associated with the use of HMD devices. Therefore, this hardware overview will primarily focus on HMDs and accessories that can be utilized to enhance immersion. The leading companies currently offering VR hardware for immersive experiences are primarily targeting entertainment, business applications and areas such as education and training [45, 85, 86, 87, 88]. Their focus is on providing devices that serve the general consumer and enterprise market, rather than specifically addressing the needs of patients. Nevertheless, researchers are increasingly using these companies' hardware to perform therapeutic interventions with patients and support the treatment of various health conditions. It appears that _Meta's_ headset series are one of the most widely used immersive VR devices in chronic pain and musculoskeletal research. For example, VR headsets developed by _Meta_ (formerly _Oculus_/_Facebook Technologies_) have been used in studies involving experimental groups  
Figure 4: (**a**) Representation of the most commonly mentioned categorization of VR based on immersion in healthcare. Currently, in healthcare, VR is classified on a single continuum of immersion depth, ranging from non-immersive VR (often involving systems such as _Wii games_) to immersive VR, typically associated with the use of HMDs. (**b**) Our proposed categorization of virtual reality based on immersion levels. We propose that the depth of immersion should be divided into two dimensions: the immersion depth of the exteroceptive and interoceptive systems. Additionally, immersion depth should also account for whether it is influenced by the number of senses stimulated or the quality of virtual stimulation. of patients with various chronic pain conditions, including those with chronic low back pain [49; 89; 90; 91; 92], chronic neck pain [93; 94], knee osteoarthritis [95] as well as participants experiencing different types of chronic pain across various body regions [96; 97; 98; 99]. In studies conducted on patients with chronic pain, researchers have also used the _Pico_[99; 100], _HTC Vive_[101; 102; 50; 103], _Samsung_[96; 104; 105], and _Vuzix_[106] headsets. These companies typically offer a range of headset options rather than a single model, each tailored to different use needs and experiences [85; 86; 88]. Each VR headset varies across numerous specifications, significantly impacting the user experience. For instance, higher resolution per eye contributes to clearer and more detailed visuals, while refresh rate affects the smoothness and responsiveness of the display. The field of view (FOV), which represents the observable extent of the virtual environment influences the level of immersion. Headsets also differ in their ability to adjust interpupillary distance (IPD) to accommodate various users' eye spacing. Weight and comfort of headsets are critical considerations, as heavier devices increase discomfort and pressure loads [107]. Battery performance is another important factor, varying between devices and determining their suitability for prolonged use without interruptions. Tracking capabilities also differ significantly, with some headsets supporting full-body, eye, face, or hand tracking through integrated sensors and cameras within the headset, meaning they can track body movements without the need for additional external sensors [85; 88]. VR headsets can be categorized as either standalone or wired. Standalone models offer portability and freedom of movement due to integrated hardware, while wired headsets deliver higher performance but are limited by a direct connection to a PC [108]. Along with the purchase of VR headsets, companies often provide handheld controllers to facilitate interaction within the virtual environment. Additionally, a variety of optional accessories are available for separate purchase, enhancing the VR experience.