With respect to research question (1), areas of intervention of the studies analyzed most often involve the improvement of social and/or emotional skills. This result is expected because deficits in social and emotional skills are the two most important diagnostic criteria of ASD [1] and the deficits can greatly hinder one’s participation in various social functions. The use of immersive VR can help create a safe, controllable, and embarrassment-free environment, in which social and emotional skills can be practiced repeatedly across a variety of social contexts. Participants of such interventions are expected to gain the ability to better judge a given context and respond to it in a socially appropriate way. However, due to the broadness of both terminologies, there seems to be a lack of a common definition of the term social skills. For example, Politis et al. investigated conversation skill training using VR [53]. Seemingly, social conversation requires one to understand the social context as well as the emotion of other parties involved in the conversation—the ability of social perception [70]. This also requires emotional skills, such as recognition of others’ facial expressions and comprehension of emotions and the implicit reciprocal, which were not discussed explicitly in the article. Further, the studies analyzed dealt with a variety of interventions including the improvement of daily life skills, social and joint attention training, and facing general or specific phobias. All these areas of intervention could, following the articles looked into, significantly improve the life quality of individuals with ASD and help them better engage in social life.

 

With regard to the research question (2), the affordances of immersive VR in ASD intervention design have been partially discussed, but have yet to be fully investigated. The affordance of immersive VR that was most commonly mentioned in the articles included in our analysis was simulation fidelity, allowing participants to practice the skills and generalize to real-life situations. Unfortunately, generalization is difficult to measure and there was little evidence reported in the included articles. Another affordance that was not covered to a large extent but could potentially change the landscape of ASD intervention is the ability to deliver intervention remotely, especially when considering the current Coronavirus disease pandemic that is shaping the methods of intervention delivery. Moreover, measures of effectiveness can greatly benefit from immersive VR-enabled approaches. The computerized nature of VR allows for the collection of an enormous amount of data that can be analyzed to measure the interventional effectiveness as the intervention happens. The collection of driving performance data in the simulated cockpit in [48] is a great example of utilizing this affordance. Lorenzo et al. used computer vision algorithms to automatically recognize the participants’ facial expressions during the intervention [47], which aimed to assess the participants’ performance by examining the number of inadequate behaviors and/or facial expressions performed given the social situation. Physiological data, such as EDA [51] and electroencephalogram [46], was also captured in some of the studies, providing another way of measuring the participants’ responses to the interventions. It is expected to see such approaches being further developed to provide performance-based personalized intervention programs, given the great flexibility and customizability of VR scenarios.

 

Regarding research question (3) and the theoretical foundations of the studies investigated, clinical approaches such as graduated exposure and CBT have been adapted for guiding the design of the immersive VR-enabled interventions reported in [38,39]. CBT has shown to be effective in treating anxiety disorder, phobia, and depression among individuals with ASD by identifying unrealistic feelings or unhelpful thoughts, and thus changing their way of thinking and behaving [71]. In the case of applying CBT in an immersive VR environment, the computer-generated content can help the participants to better relax and break down the challenges into thoughts, feelings, and actions under the guidance of a therapist. Besides CBT, there are other clinical approaches, which may require a method of relaxing or the presentation of certain stimuli, and that can benefit from the utilization of immersive VR. Although not explicitly mentioned in many other included studies, the design of the ASD interventions clearly borrowed the idea from conventional learning and education. For example, Ip et al. created various scenarios that a primary school student will encounter on a school day [27], in which challenging situations would be introduced as the intervention progresses. The participants need to interpret the social contexts and choose a strategy to respond to these situations. The VR scenarios, as well as the humanoid virtual agents, were programmed to respond to the participants’ actions, providing instant feedback, which prompts the participants to reflect and rethink a new strategy if their actions were not socially appropriate in the contexts. These approaches of using immersive VR to allow free practicing and strategic reflection are very similar to Kolb’s experiential learning model (KELM) [72]. However, this might also bring questions regarding such approaches when being applied to individuals with ASD. Specifically, the ASD population may need more facilitation and scaffolding in the process, especially when they face a challenging situation. Adopting KELM requires learners to self-initiate the rethinking and reflecting process, which might not happen among individuals with ASD if there is insufficient facilitation. Ip et al. tried to address this issue by emphasizing the role of the trainers in their study [27]. Cox et al. adopted a similar approach; the participants were always accompanied by a driving coach in the virtual cockpit [48]. However, in some cases, facilitation from trainers might be limited by the VR environment. If HMDs were used as the enabling device, the participant would be totally immersed in the VR scenario and isolated from the surrounding. Whether the facilitation from trainers can still be effective in this case is yet to be fully investigated. Ip et al. tried to address the issue by placing program-controlled virtual agents with pre-scripted dialogues at the locations where the participants might need facilitation [60]. With the recent advancements of large language models (LLMs) and the development of ChatGPT (https://openai.com/blog/chatgpt (accessed on 26 May 2023))-like services, the solution can be further refined and is worth further investigation.

 

With regard to research question (4), we can summarize that the effectiveness of immersive VR-enabled interventions for individuals with ASD needs to be further examined and investigated in future studies. The limitations of the included articles are obvious; most of the studies reported involved a rather low number of participants and the experimental designs were not rigorous enough to draw a solid conclusion regarding the interventional effectiveness. Among the 15 articles included for the effect size calculation, five articles reported controlled trials [27,46,56,59,60]. The combined effect size (𝑑=0.285