# Immersive Phobia Therapy through Adaptive Virtual Reality and Biofeedback
## 2 Phobia Statistics and Therapy Methods
### Virtual Reality Exposure Therapy Systems Developed in the Academic Context  
The AcTiVity system [16], which was tested with 42 participants not affected by acrophobia, is used to assess how the user's avatar influences their sense of presence, immersion, and motion sickness. The subjects were divided into two groups--with and without an avatar representation of their body. The results indicated a higher presence in the group in which the participants tested the system with an avatar representation of their bodies because distances could be estimated based on their body height, and the method could be used for subjects who had acrophobic tendencies. In the acrophobia experiment presented by [PERSON] et al. [17], 15 subjects were gradually exposed to heights (in both virtual and real-world environments), from the first to the eighth floor of a building, for a period of 3 weeks, constantly assisted by a psychologist, who provided guidance and encouragement. The evaluation of the data showed that, after the therapy, both groups reached identical results, with differences regarding only the length of each session. In this aspect, virtual reality sessions were shorter than real-world sessions by 50%. [PERSON] et al. [18] used a low-budget virtual reality exposure system for acrophobia therapy and an in vivo treatment. In total, 29 subjects participating in the experiment had to rate their anxiety level on a scale from 1 to 10. When the therapist decided that the patient's anxiety level was sufficiently low, they adjusted the difficulty of exposure. The results showed a reduction in both anxiety and avoidance levels of the patients, concluding that virtual reality exposure therapy is at least as effective as in vivo exposure. A mountain and a city environment were presented in [19]. For that study, 10 subjects underwent real-world exposure for two months and another 10 subjects participated in virtual reality exposure therapy sessions for the same amount of time. The results showed that their anxiety levels significantly reduced from 64% to 40% in the case of the second group. ZeroPhobia [20] consists of six animated modules with explanatory voiceover and a virtual therapist; mobile VR games; and 360-degree videos, controlled by gaze direction. In total, 66 subjects tested the system, and the main conclusion drawn from the experiment was that the optimal duration of a VR session should be around 25 min, as increased exposure time necessarily increases therapy efficiency. In the experiment presented in [21], 10 subjects (5 with acrophobia and 5 in the control group) were exposed to a 3D virtual model of a city. They had to pass through a walkway between two buildings on the fifth floor and experience the view from the roof of a seven-story building. The research showed a positive correlation between anxious behavior (an increase of at least 15 beats per minute in heart rate compared with baseline measurements) and the reported fear level. A hilly landscape with a metal structure in the middle, containing four platforms situated at different heights above ground, was simulated in a CAVE (cave automaticvirtual environment) device [22]. The results of an experiment with 99 subjects showed that projection on the floor and walls increased immersion and anxiety, although it did not affect the sense of presence. Six acrophobic users underwent three e-virtual reality remote sessions (view from the balcony of an office), followed by three in vivo exposure sessions in the presence of a therapist [23]. The data analysis revealed that there was no difference between subjectively perceived anxiety rates (assessed by means of specific questionnaires) and physiological responses (heart rate measurements) under both conditions (virtual and in vivo). The VR simulator presented in [24] is not dedicated to therapy but to companies that want to see if their employees suffer from acrophobia. The virtual environment displays a moving platform and a virtual ladder placed on the top of a pole, at 11 m above ground.