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aspect, Held & Durlach (1992) state that the interfaces, by their restrictive character
(their lack of transparency), can reduce the feeling of presence.
Fourth, the realism factors that have to do with the realism of the environment,
in its perceptual and structural aspects, but also in its semantic aspects (coherence
of experience). Based on this conceptual framework, the authors have prepared two
questionnaires: the presence questionnaire, which directly measures the degree of sen-
sitivity of the subjects to the factors stated above, and the “immersive tendencies''
questionnaire, which tries to measure the general tendencies of the subjects to become
involved in a given context.
5.3.2 Physiological measurements
There are a certain number of physiological indicators, including skin conductance,
skin temperature, stretch receptors, rate of heart beat or even pupillometry, that have
been suggested as indicators of presence (Insko, 2003). For example, Strickland and
Chartier (1997) have illustrated the feasibility of measuring EEG (electroencephalo-
graphic) signals in a virtual reality headset. Skin conductance changes when palms
become sweaty, and is often associated to a stress response (as in the case of the
famous lie detector). The idea is that these indicators can provide continuous informa-
tion about the physiological response of the subjects to the stimuli or events occurring
in the virtual world. This type of measurement of presence is now used widely, in
conjunction with the subjective evaluations of presence (Wiederhold et al., 2001).
Recently, some studies have tried to directly measure the cerebral activity associated
with the feeling of presence (Hoffman et al., 2003; Hoffman et al., 2004). Beyond the
methodological progress that must be accomplished so that this type of measurement
reaches a level of analysis that is temporally more precise, it seems crucial to try to
co-relate this type of measurement to the actions of the subject and/or events of the
virtual world.
5.3.3 Behavioural measurements
5.3.3.1 Performance
We intuitively believe that performance will be a good indicator of presence. In other
words, the stronger the feeling of presence, the better the performance. However, the
situation is more complex due to inter-individual variability, mainly concerning senso-
rimotor skills. The idea of a positive presence/performance correlation has been directly
tested in a certain number of tasks. These tasks include visual search or visuo-manual
chasing tasks in virtual environments (for example, Pausch et al., 1997). However,
performance is not systematically correlated to presence in a number of cases (for
example, Slater & Wilbur, 1997). Thus, beyond performance, we can try to consider
the behaviours themselves, starting with reflex behaviours.
5.3.3.2 Reflex actions
Simple behaviours can indicate that the participants act as if they were in the virtual
world, i.e. in line with the properties and events of the virtual world. These behaviours
can include: extending arms to catch a virtual object, to socially interact with virtual
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