Informally, ``presence'' refers to the sense of ``being somewhere'': usually in the sense of being in a computer-generated or computer-mediated environment. A possible refinement to this definition, relating presence to the spatial perception literature, will be introduced in Section 3.3.3. The current section reviews what is known about presence and suggests why it may be of importance.
The view that presence is of primary importance to the study of virtual interfaces is not universally held. Ellis points out that increasing presence in one part of a task may decrease global performance (by inhibiting smooth transitions between distinct environments). He states that ``the design of a teleoperation or virtual environment system should generally focus on the efficient communication of causal interaction. In this view the sense of presence, that is of actually being at the simulated or remote workplace, is an epiphenomen[on] of secondary importance for design.'' [24]
Nevertheless, presence seems worthy of special attention for the following two reasons. The first reason is that presence is a common property of virtual interfaces. Hence, any attempt to explain the psychology of interfaces must explain presence.
The second reason is that while presence may not be directly related to the effectiveness of an interface, it appears to reflect many things which are. Thus, measuring presence may be a convenient means to assess the importance of these factors. Examples of factors which research implies increase the sense of presence include many which make an interface more natural to use (more intuitive). The literature covers, for instance: interactivity and pictorial realism [108]; update rate [7,108]; field-of-view (FOV) [36,82]; head-tracking and stereoscopic cues [39]; spatialized sound [40]; the addition of tactile cues [42]; and the degree to which the content is engaging [75]. Research reported in this dissertation (Appendix A; and at greater length in [41]) suggests that presence is related to the amount of meaning extracted from a display, with all other variables held constant. Presence in a virtual environment may also improve transfer of training to the real world [16].
The degree to which presence can measure the over-all effectiveness of an interface of course depends on the strength of the indirect link between presence and effectiveness. The strength of this link can only be established empirically, by examining cases in which good measures for both presence and over-all effectiveness are available.
Because it appears closely related to the nature and effectiveness of virtual interfaces, presence is a primary focus of this dissertation.