Virtual reality (VR) is emerging as one of the key new technological tools in a digital revolution sweeping across the health care industry. Immersive VR allows users to interact with a computer-generated world, where the users natural sensory perceptions are replaced with a digital three-dimensional (3D) alternative [1]. To create an immersive VR system, a computer is used to generate an image, a display system is required to project the image, and, finally, a tracker is required to update the image based on users’ movements. Traditionally, VR has been confined to laboratories as expensive and powerful computer(s) are needed to power it. VR, as we know it today, has been around for decades; the vision of VR was first realized by Ivan Sutherland in 1968 with the “Sword of Damocles” head-mounted display (HMD) and later by Morton Heilig with his multisensory Sensorama [2]. Failed attempts at VR systems by Nintendo (Virtual Boy) and Sega (Sega VR) in the 1990s and a further lack of development in the 2000s had many of its critics arguing that the technology was “dead” [3]. However, as noted by Olson et al [4], the video games industry has driven advances in graphics cards that are able to handle increasingly sophisticated 3D constructed environments. Furthermore, simultaneous developments in mobile phones and HMDs have made VR an accessible commodity for consumers. Lately, HMDs have markedly improved: an increased field of view (FoV), higher-resolution images, lightweight comfortable design, and an appealing price have added to its attraction [5]. Steed and Julier [6] described how they designed an immersive VR system around an Apple mobile phone (iPhone 4s), which had the computing power to act as a controller for a VR system. The implementation of gyroscope technology in mobile phones, which tracks user movements, has allowed HMDs to house mobile phones that act as the VR system itself. An example of the capabilities of gyroscope is the highly successful app “Pokemon Go” that tracks users’ movements as part of an augmented reality experience [7]. Collectively, these developments have brought VR back into the public domain. Furthermore, low costs, innovative apps, and an increasing accessibility have captured the imaginations of researchers who have proposed its use in the treatment and assessment of a wide range of health care issues.

Evidently, VR is a rapidly emerging field of research. Since 2016, new HMDs from Oculus, HTC, HP, Acer, Dell, and Sony and the arrival of a range of cheaper mobile phone alternatives have acted as a catalyst for a new wave of VR research. Despite recent investment in Oculus by Facebook, indicating VR is here to stay, its popularity among consumers is unlikely to affect the quantity of research around it. VR research has been continuously expanding in a time where it has not been at the forefront of digital consumerism. Oculus’ chief scientist Michael Abrash has suggested that in the next 5 years, we will see a new generation of VR products, which will operate with 4k screen resolution and with eye tracking that may allow for foveated rendering [8]. This prediction would appear to be coming partly true as companies race to produce HMDs with increasingly crisper resolution over their competition [9]. This suggests a potential new wave of VR products, thus, bringing down the prices of even the more sophisticated forms of VR today and making it an even more appealing tool for the health industry. With this in mind, now is the time to review the recent VR research, taking a view of what technology is being used and how it is being used.

In this review, we aim to critically assess the current state of head-mounted VR research in relation to mental health. By doing so, we look to determine which conditions are more susceptible to VR interventions, which conditions need more attention, and in what form VR interventions are most effective. Our secondary aim is to understand more about VR used over the past 5 years and compare it to the new generation of VR in terms of accessibility and specifications. As there are indications that VR HMDs can be used at home as a self-help resource to provide a valuable tool for public health, in this review, we will assess head-mounted VR health research to date to determine whether this has been tested.

HMD specifications are categorized into FoV, image resolution, and refresh rate (Hz). FoV refers to the view or surroundings a human can see without any eye movement. The human eye has a rotating FoV of up to 270° [10]; newer HMDs are attempting to create a FoV closer to that of the human eye. Currently, we can expect the Oculus Rift and HTC Vive to give an FoV of 110°, while some prototypes such as the Pimax 8k claim to offer an FoV of 200° [11]. Image or screen resolution refers to how clean and crisp the picture quality is; this is determined by the number of pixels in an image area and is reported by the number of pixels arranged horizontally and vertically [12]. For example, a screen resolution of 1280 ⨯ 720, which we refer to as 720p, is classified as high-definition (HD) ready. High-end HMDs today give a resolution between full HD (1080p) and QHD (1440p); again, both the Oculus Rift and HTC Vive offer a screen resolution of 2160 ⨯ 1200, which equates to 2,592,000 pixels per image. This method of reporting resolution has been key to selling televisions, which we see advertized as “full HD 1080p” or “4k.” The investment that companies put into screen resolution can be seen by HTC’s upgrade of the Vive to the Vive Pro, the two HMDs are essentially very similar with exception of the Vive Pro’s increased 2880 ⨯ 1600 resolution. The refresh rate reported as hertz is the number of times a screen can change image. We refer to this refresh rate as frames per second (FPS); an increased FPS will give a more fluid motion of images. FPS is particularly important as we want movements to be realistic; an environment should act according to the user, which means the reduction of any lag between the users’ input and the output of images. Furthermore, VR setups that operate below 90 FPS are more likely to induce nausea and disorientation [13].

We used a useful definition of mental health from a mental health foundation that defined it as:

The World Health Organization acknowledges that positive mental well-being is rooted within mental health; this state of well-being allows an individual to lead a fulfilling and productive life [15]. In this review, we aim to look at conditions that offer a scope to deliver psychological change that can make a meaningful difference to one’s mental health. We excluded severe mental illnesses that require a more complex approach to treatment.

Health care and VR first met in the 1990s as a simulation tool for colonoscopy and upper gastrointestinal tract endoscopy simulation within medical education [16]. VR would have remained as a simulation tool for physicians and surgeons, but its interactive nature suggested it as an applicable tool for psychological change. For example, in the therapy of phobias, adaptability of virtual worlds means that contextually relevant virtual worlds can be created that are used to enable systematic exposure to feared stimuli [17]. The ability to precisely control stimuli has allowed VR ecological validity in its assessment of behaviors, emotions, and cognitions [18]. As a result, established effective psychotherapeutic approaches, such as cognitive behavioral therapy (CBT), are recreated within VR alongside exposure techniques [19]. This exposure is particularly effective as the goal of VR is to produce an “illusion of reality” [5]; however, for the patients, despite knowing that the computer environment is not real—a computerized illusion—their brains perceive the images and sound as real stimuli [1]. The broad reach of VR has enabled its use for treating schizophrenia, posttraumatic stress disorder (PTSD), social and generalized anxiety disorders, specific phobias, eating disorders, substance abuse, attention-deficit/hyperactivity disorder, depression, pain management, and psychological stress, as well as its use as part of a wide range of poststroke rehabilitation.

Narrative syntheses were conducted on VR studies that were pertinent to areas of mental health [20]. The literature was critically assessed within the parameters of our review aims. Control and noncontrol studies were included, and studies varied from theory and assessment to treatment. Studies were featured if they appeared in peer-reviewed journals.

We set an inclusion timeframe from January 2012 to July 2017. A 5-year period was seen as sufficient enough to reflect the current state of the technology; this would allow us to assess studies that used both the new generation of HMDs and a range of older HMDs that have been in academia for the past decade. In line with this, studies were only included if they used an HMD. This meant excluding studies that used Cave Automatic Virtual Environment, the Computer-Assisted Rehabilitation Environment, and other projector systems without an HMD. As we focused on the systems used, if authors failed to disclose the type of VR used, or stated they used an HMD but provided no further information regarding its model or specifications, the study was excluded. Studies involving the use of two-dimensional (2D) virtual environments such as those seen in Second Life were excluded. Furthermore, augmented reality studies were not included as although augmented images are computer generated, the environment itself is not. Future VR may well feature an augmented experience within it [21]; however, at this time, the two technologies are separate, and this is reflected in this study’s direction.

Mental health conditions were categorized as behavioral conditions that showed the potential to be modified upon intervention. In accordance with the International Statistical Classification of Diseases and Related Health Problems [22], we were primarily concerned with the areas of “Neurotic, stress-related, and somatoform disorders,” “Behavioral syndromes associated with physiological disturbances and physical factors,” “Mood disorders,” and substance abuse. We included VR studies that featured any form of anxiety (generalized anxiety disorders, social anxiety disorders or specific phobias, and PTSD), depression, eating disorder (anorexia and binge eating), sleep disorders, and substance addiction or abuse. Pain management was also included as it has profound psychological and emotional consequences that can lead to depression and anxiety [23]. Severe mental disorders were excluded; a number of studies on psychosis were identified; however, due to its neurological origins as a state of brain development rather than a disease, psychosis was excluded from the study [24]. Autism was also excluded due it its neurodevelopmental origin. Finally, we excluded any rehabilitation studies, typically on stroke [25].

The search strategy implemented in this review was conducted in 5 stages.

A full list of identified studies can be found in Multimedia Appendix 2.