"The ‘sense of being there’ in the environment depicted by the virtual reality system."
"Simply put, presence in VR is the sensation of being in the space of a given experience, of sharing that space with characters, of being there. That sensation in turn leads to the need to be active, to have intentions, to play a role."
From "Place illusion and plausibility can lead to realistic behaviour in immersive virtual environments" by Mel Slater
"The origin of the concept of presence as a ‘feeling of being there’ is rooted in teleoperator systems, and is the feeling of being at the place of a remote physical robot that the user is operating (telepresence) (Minsky 1980). In the early 1990s this idea was transplanted to virtual reality, where instead of being at the remote physical environment, the participant was in a virtual environment with a sense of being at the place depicted by the virtual displays (Held & Durlach 1992; Sheridan 1992).
We reserve the term ‘place illusion’ (PI) for the type of presence that refers to the sense of ‘being there’. This terminology is used in order to avoid confusion, to make it clear that we refer specifically and only to the strong illusion of being in a place and not to other multiple meanings that have since been attributed to the word ‘presence’. It is the strong illusion of being in a place in spite of the sure knowledge that you are not there. Since it is a qualia there is no way to directly measure it. However, indirect assessments based on questionnaires, physiological and behavioural responses have been used, all of which in some way compare responses with those that would have been expected in real experiences.
Note that a system with valid sensorimotor actions that have the same range as in physical reality is not sufficient to support SCs that approximate those of physical reality. Imagine a participant viewing a virtual environment through a head-tracked HMD, but with a very narrow field of view (say 10° horizontally instead of the more than 180° of natural vision). Now since the HMD has head tracking, the participant can look around the scene and visually perceive by the full range of head movements possible with natural vision. But since the visual capture is significantly less than that in normal vision the participant would have to learn how to perceive in a different way, with greater head movements and patterns of head movements to obtain the same information compared to normal. This is not to say that eventually with sufficient movement and after a sufficiently long enough learning period these SCs would not become ‘normal’.
What is left of the distinction between immersion and PI? PI occurs as a function of the range of normal SCs that are possible. But we have also defined immersion in terms of the range of SCs that are possible, and an immersion hierarchy formed by the extent to which one system can be used to simulate another. Apparently PI like immersion has become essentially a property of the physics of the situation. By definition if a person perceives the virtual world making use of motor actions to perceive in the same way as perceiving the real world, but on the other hand knows that this is a virtual reality, then this must give rise to PI (how could it not?). But this would also be the most immersive system. So, are PI and immersion now the same from this point of view?
Suppose there are two participants who each in turn enter into an immersive system (for example, the Cave system). Person A experiences a low level of PI and B a high level. Clearly, if immersion and PI were identical then this could not occur. But in fact it could occur (irrespective of individual differences between A and B). Suppose person B stands in more or less the same position and simply looks around, whereas person A moves around, looks closely at objects, touches them and so on. Person A will quickly reach the bounds of resolution of the system, and see pixels. Moreover, A will expect, touch and feel nothing, except when bumping into the physical Cave walls—a break in PI, since this is a perception from outside the virtual environment (Garau et al. 2008). Person A is probing the bounds of perception to a much greater extent than B, and therefore PI will have the opportunity to break more often.
Immersion provides the boundaries within which PI can occur. Only in physical reality is it not normally possible to break those boundaries (except in illness or brain damage that disrupts the motor and perceptual process). In any kind of virtual system there will always be limits beyond which SCs fail to be applicable. PI occurs to the extent to which participants probe the boundaries of the system—the more they probe, the greater the chance for PI-breaks. This is further discussed in §7." http://rstb.royalsocietypublishing.org/content/364/1535/3549.short
Note SCs refers to Sensorimotor Contingencies: "Immersive systems can be characterized by the sensorimotor contingencies (SCs) that they support. SCs refer to the actions that we know to carry out in order to perceive, for example, moving your head and eyes to change gaze direction, or bending down and shifting head and gaze direction in order to see underneath something.