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humanoid avatars, etc. They can also include even simpler behaviours, such as avoid-
ance behaviour or shutting of eyes when a virtual object approaches, or even surprised
reactions (Held & Durlach, 1992). It thus appears that a behavioural approach makes
it possible to objectify the concept of presence. The fact remains that reflex behaviours
are simple sensorimotor couplings that are not even peculiar to human beings (for
example: shutting eyes when an object approaches). It is thus not demonstrated that
they are unequivocal indicators of the subject's experience of the virtual world. We
believe that it is necessary to deal with more “evolved'' behaviours.
5.3.3.3 Sensorimotor control
The general idea expressed by numerous authors is that presence can be correlated
to the fact that the behaviour of subjects is similar to the behaviour that they would
exhibit in the real world, under similar circumstances (Ijsselsteijn et al., 2000). We can
thus look for sensorimotor invariants in the behaviour of the subject, by adopting an
analysis of the subject's behaviour at the level of the sensorimotor coupling models. We
would thus like to suggest a “sensorimotor'' approach to the phenomenon of presence.
Thus, the general assertion is that the identification of sensorimotor invariants in the
subject's behaviour (consider Fitts' law linking speed and precision of the gesture) is a
behavioural marker of the feeling of presence.
5.4 CONCLUSION
We have tried to clarify the concepts of immersion and presence using a few examples
and a few suggestions for classification. Immersion can be defined as all the objec-
tive characteristics of a virtual environment that aim towards providing a user with
sensory stimulations and possibilities to carry out actions in this virtual environment.
Presence is defined as all the behaviours, from verbal contacts to complex behaviours,
going through physiological reactions, which are observed when the user faces this
environment. It is defined with respect to two environments; the real environment and
the virtual environment. The analysis of the subject's behaviour thus makes it possible
to determine if he is acting in accordance with the real world or the virtual world.
We can thus reasonably examine the benefit of this approach. At the basic level,
the feeling of presence presents the problem of the mechanisms of perception of the
environment and even the bases of our experience of reality. These aspects have been
dealt with in chapter 4 of this volume, dedicated to relations between virtual reality
and Behavioural Sciences.
At the level of virtual reality applications, a main question concerns the transfer
of knowledge acquired in a virtual reality situation to the “real'' world. We can con-
sider the example of professional training. It is evident that it is important to give the
“trainees'' experiences that are, if not dangerous, at least difficult to experience under
real conditions (for example: flight simulators). It seems reasonable to consider that
the analysis of the subject's behaviour will help to describe his involvement in the task
(his “presence'' in the virtual world). Following the same logic, we consider that the
analysis of behaviour will thus help to carry out the transfer of skills acquired in the vir-
tual reality situation to the real world. It is however more difficult to determine the
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