Virtual Reality (VR) is starting to be used in psychological therapy around the world. However, a thorough understanding of the reason why VR is effective and what effect it has on the human psyche is still missing. Most research on this subject is related to the concept of presence. This paper gives an up-to-date overview of research in this diverse field. It starts with the most prevailing definitions and theories on presence, most of which attribute special roles for the mental process of attention and for mental models of the virtual space. A review of the phenomena thought to be effected by presence shows that there is still a strong need for research on this subject because little conclusive evidence exists regarding the relationship between presence and phenoma such as emotional responses to virtual stimuli. An investigation shows there has been substantial research for developing methods for measuring presence and research regarding factors that contribute to presence. Knowledge of these contributing factors can play a vital role in development of new VR applications, but key knowledge elements in this area are still missing. 183

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The aim of the present study was to evaluate the effectiveness of low-budget virtual reality exposure versus exposure in vivo in a within-group design in 10 individuals suffering from acrophobia. Virtual reality exposure was found to be at least as effective as exposure in vivo on anxiety and avoidance as measured with the Acrophobia Questionnaire (AQ), and even more effective on the Attitude towards Heights Questionnaire (AHQ). The present study shows that virtual reality exposure can be effective with relatively cheap hardware and software on stand-alone computers currently on the market. Further studies are recommended, in which virtual reality exposure is compared with in vivo exposure in a between-group design, thus enabling investigation of the long-term effects of virtual reality treatment.