## 2 Related Work
### Social Presence  
Human beings have an innate need for social connectedness. Social interactions are said to influence the internal states of a being, such as the capacity to perceive and understand others' thoughts, intentions, and associated behaviors, thus altering the decision-making process [43]. The concept of social presence is essential for successful computer mediated environments like online learning, online assisted shopping, teleconferences, gaming, assisted driving, and so on [44].  
Unlike telepresence and self-presence, social presence requires the actual experience in a virtual environment (VE) mediated by a co-present entity. Social presence describes a person's state in an environment--the state of consciousness in a VE and the sense of being in a place [45]. \"The minimum level of social presence occurs when users feel that a form, behavior, or sensory experience indicates the presence of another intelligence. The amount of social presence is the degree to which a user feels access to the intelligence, intentions, and sensory impressions of another\" [46]. More definitions on social presence can be found in [47].  
Presence, per se, is a complex concept to describe and measure. A standardized instrument for assessing presence was proposed in [48]. Their tool identified six dimensions for presence: social richness, realism, transportation, immersion, social actor within medium, and medium as social actor.  
In a review of theories on social presence, it was found that social presence consists of three dimensions with regard to the interaction and behavior during VE experiences: co-presence, psychological involvement, and behavioral engaging [10]. [PERSON] et al. introduced the concept of social presence referring to both the participants' feeling of connectedness and the perceived psychological distance during the interaction [49]. Social presence is more of an interpersonal relation, with verbal and non-verbal language being a key element for the interaction in VE [50], and is essential in a shared VE interaction with the sense of belonging to more than just a group, but of being fully immersed in the place [45]. Immersion is the quantifiable technological capacity of a medium to deliver \"an inclusive, surrounding and vivid\" [45] illusion of reality to all the participants involved in the interaction. The perception of the VE increases with the capacity to be present and engaged in virtual reality; thus, the environment delivered by the VR headset appears more intuitive, realistic, and attractive.  
In their extensive review, [PERSON] et al. [50] showed that social presence may be influenced by several factors which contribute to the overall impact on the participants. Studies showed that face-to-face interactions are preferred to computer-mediated communication. The quality of the partner's representation in a VR interaction has a great impact on one's \"sense of social presence.\" The availability of visual representation and the visual realism of the virtual environment directly influences the level of social presence in comparison with experiences where participants are not able to see their partner's avatar and are therefore interacting with an invisible partner. The more the virtual representation acts (communicates, behaves) and looks like a human being, the greater the realism factor, and the stronger the impact on social presence [50]. Behavioral patterns of an individual in VE and the extent to which the same or similar behavior or responses are shown in normal life circumstances can be observed and correlated with certain aspects of immersion [45]. Higher levels of social presence are perceived when participants have the opportunity to interact directly with the partner [51], even more when the partners are both involved in manipulating a virtual object together and receive haptic feedback [50].