## 4 How Could Interactions in VRET Be Improved? Unlike existing studies, which seem to consider VR a simple medium to help patients narrate their trauma in an imaginary scenario of exposure therapy [3; 50], we argue that VRET should be regarded as a full-fledged therapy because VR technologies can effectively enable the application of so-called active methods of learning. Indeed, it has been established that exposure therapy operates through a learning mechanism [28; 66]. Learning has evolved through several models, and current assumptions are that better learning results from a subject's active participation in the process. During an active learning experience, subjects are engaged in activities that involve concrete encounters. They think about what they learn, relate it to their past experiences, and apply their learning to their daily lives [70; 71]. They must have a sense of progress and control over their learning [70], in accordance with the principles of VR, which suggest a continual loop of the user's actions and the environment's feedback [18]. However, findings indicate that notwithstanding the potential of VR to allow the subject to be actively involved during VRET, the exposure remains a passive learning process for the patient. Therefore, we propose two main foundations to consider when designing VEs for VRET:  
1. Learning as the primary mechanism of VRET
2. The application of key concepts of VR (immersion, presence, and interaction) in VRETThese foundations could lead to the possibility of considering the therapy process as an SG because this type of game facilitates the effective application of a learner-centred model of learning [72]. SGs consist of activities that allow users to learn while having fun [73; 74; 75; 76]. They are characterized by a utilitarian purpose, compounded by a playful purpose through the process of gamification [77]. From the perspective of an SG, we understand that a VRET tool can be regarded as an environment in which the patient seeks to achieve a given objective by means of game mechanics (that is, rules that define what is achievable or not at a specific moment). In this way, the player/patient is required to perform various actions that capture their attention. Thus, the player's engagement in the game is emphasized. As such, by being combined with VR technologies, SGs constitute an effective and natural way for learners to achieve a high level of interaction with the system [72]. One could even push the issue further by considering the VR application as an SG that adapts to the patient during the exposure. Indeed, SG players have game goals that can be achieved by solving one or more challenges in specific contexts. Because all users of the same SG have different learning skills, they may feel bored or frustrated with a game depending on whether it is too easy or too difficult for them. It appears that, for each player, a better balance between one's performances and learning activities would lead to a better transfer of skills, determined by the technique of dynamic difficulty adjustment (DDA) in games. DDA techniques are commonly based on a player's in-game performances. However, several studies have investigated the possibility of an adjustment according to the more objective characteristics of each player, including physiological measures [78; 79; 80; 81]. If applied to SGs intended to treat mental disorders, such methods would yield compelling results because physiological parameters are good indicators for the evaluation of a patient's emotional state [49]. Physiological reactions involved in stress responses associated with PTSD include the reactivity of the heart rate [82]. These responses to anxiety-provoking stimuli encapsulate the symptoms of PTSD in hyper-arousal and \"reliving\" (intrusions, flashbacks, etc.) [83]. Such responses arise from the activation of the subject's fear network when they are confronted with stimuli reminiscent of their trauma and result in an increased heart rate [83].