## 3 Presentation to the Participants
### Virtual Reality Systems/Devices  
Three-dimensional immersive virtual reality can be realized via sophisticated immersive virtual reality system with surrounding displays, or head-mounted virtual reality devices. Some studies did use such virtual reality systems/devices [28; 29; 30; 31; 32; 33; 34; 35; 36]. However, many studies used conventional computer monitors to display virtual reality scenes with one or more avatars [37; 38; 39; 40; 41; 42; 43; 44; 45; 46; 47]. Hence, there is certain degree of overlap between the virtual reality based presentation mode and the conventional computer based presentation mode. We should note that head-mounted display and virtual reality goggles may cause negative experiences such as dizziness and tiredness for some participants, as reported in [48]. [PERSON] et al. provided an excellent review on the impacts of the level of immersion in virtual reality systems on ASD treatment [49]. In [28], a head-mounted display (eMagin Z800 3 DVisor from eMagin Corporation at Bellevue, WA, USA) was used to present a virtual classroom containing multiple avatars to a participant. Different from virtual reality scenes provided by a conventional computer monitor, head-mounted display is capable of rendering the scenes in 3-dimension using stereoscopic images [50]. In addition to providing an immersive virtual reality environment, the head-mounted display can be integrated with head position monitor to detect eye gaze in the virtual visual field. More recently, researchers started to experiment with using consumer-off-the-shelf virtual reality goggles for autism treatment. In [48], the Oculus Rift head-mounted display was used to study the willingness and acceptance of using the device, and the sense of presence and immersion the participants experienced with the device. Oculus Rift was also used in [51] to create immersive 3D scenes. The term avatar has been used fairly liberally without a universal definition. In [25], [PERSON] et al. divided the avatars used in their study into three categories: (1) articulated avatar, where the body parts are represented using lines and simple shapes, which is similar to a stick figure; (2) pointing avatar, where the body can move along a single direction; (3) real avatar, where the avatar is a silhouette of a real person. In other papers, a virtual avatar is typically referred to as a synthetic human character [52]. [PERSON] et al. [52] further divided the avatar into two types: (1) animated avatar, which has an appearance of a 3-dimensional figure, (2) cartoon avatar, which is 2-dimensional like a cartoon character. Yet some literature calls a picture of a human face as an avatar [53]. Note that an avatar does not necessarily have to be human-like. In some programs, animal avatars were used, such as in the virtual pink dolphins project where avatars were dolphins [54; 55; 56]. While several different types of avatars, as well as pictures and videos of real persons have been used in many studies, few have compared their impacts to a participant regarding the effectiveness of the social skill learning. [PERSON] et al. reported such a study [52]. In the study, two experiments were conducted. In both experiments, a therapist had a conversation with an individual with ASD. In the first experiment, three presentations of the therapist were used: (1) a realtime video feed of the therapist; (2) an animated avatar that tracks the facial motion of the therapist, and (3) a cartoon avatar that also tracks the facial motion of the therapist limited to 2-dimension. The result shows that there is no apparent variation among participants responses (verbal and nonverbal behaviors) under these three different modes of presentations. The second experiment focused on three different modes of the animated avatar. Exactly because the animated avatar is synthesized based on the therapist's facial expressions and motions, the avatar can be altered to the way the authors' desired.