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between the real and virtual worlds thus seems to be an important factor of the immer-
sive character of the virtual world, which will strongly condition the behaviour of the
user “in'' the virtual world. It naturally leads us to mention the concept of presence.
Until now, we have tried to remain objective in terms of the description of the immersive
character of a virtual environment. Now we come to the effects of immersion on the
behaviour of the subject. It is this subjective aspect of virtual environments that we will
call presence, which is thus the behaviour (understood in a broad sense) of the subject
in an immersive virtual environment (Biocca, 1997; Lombard & Ditton, 1997; Draper
et al., 1998; Ijsselsteijn et al., 2001; Slater, 2002; Burkhardt et al., 2003). Presence
is a multidimensional concept, involving psychological aspects (the classic meaning
given by the Anglo-Saxon authors of the 90s to this term, a feeling of being “in''
the virtual world) as well as sensorimotor aspects. Presence is thus understood as the
behavioural effects (broadly speaking) of immersive environments. It involves looking
for the motor, perceptual and cognitive position of the subject, between the real and
virtual worlds. It thus takes both the worlds into account, and must be defined as a
dynamic concept by nature. We are now going to try to prepare a status of the current
evaluation methods for presence, as well as the (more or less) implicit definitions of
presence that they use.
5.3.1 Questionnaires and subjective measurements
Since the middle of the 90's, researchers in the field of virtual reality have devel-
oped questionnaires aimed towards evaluating the subjective feeling of presence felt
by the users of immersive systems (Barfield & Weghorst, 1993; Witmer & Singer,
1998; Lessiter et al., 2001; Schubert et al., 2001, amongst others). Here, the authors
are interested in physical presence (even called spatial presence) and refer to it as a
psychological state in which the energy and attention of a subject are focussed on a
coherent unit of stimuli. Presence is described as the feeling of being (being situated)
in the virtual world or even as a feeling of non-mediation between the subject and the
virtual world (Lombard & Ditton, 1997). In this concept, we get a better idea about
the presence felt by the user with the help of questionnaires. Most often, they look like
visual analogue scales, asking the subject to evaluate aspects like the degree of control
that he has in the virtual environment, his feeling of being in the virtual environment,
or even its reality. Visual analogue scales are often used for this purpose.
Witmer and Singer (1998) have systematised this methodology. They suggested
that presence does not depend only on the immersive qualities of the devices used (for
example, Prothero et al., 1995), but also on the degree of involvement of the subject
vis-à-vis the virtual environment. Witmer and Singer (1998) list four categories of
factors related to the feeling of presence: First, the control factors that are related to
the interaction of the subject with the environment. Second, the sensory factors that
are related to the sensory richness mentioned above. Third, the distraction factors that
have to do more directly with the psychology of the user and concern the feeling of
isolation from the outside environment, the subject's motivation, or the awareness of
the subject about the control interfaces and sensory feedback. Regarding the latter
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