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We can also cite numerous works that demonstrate the role of attention in per-
ceptual processing, for example, the fact that a subject does not perceive certain visual
events when his attention is drawn to other elements of the scene (Mack&Rock, 1998).
Here, we will also observe the phenomenon of change blindness, the fact that
significant changes in a visual scene are not observed when they are accompanied by
transitory disturbances, like flickering of the image (O'Regan et al., 1999). In this
regard, it thus appears that the spatial visual representation is incomplete and modu-
lated by attention, since certain events occurring in a visual scene can go unobserved if
they are not the centre of interest for the activity of the subject. This example suggests
that it is not necessarily required to try and restore the whole visual scene, and that
it is always necessary to find the relevant information for a given activity. It is thus
necessary to carry out contextual studies (ergonomics) to define the relevant situation,
not only at the level of rendering but also at the level of the conditions of interac-
tion. We are now going to develop two central aspects of virtual reality systems for
which the problems that are presented examine the knowledge and the obscure areas
of Behavioural Sciences. We would like to state that we are more concerned about
making the reader aware of the fundamental problems raised by these aspects than
preparing an exhaustive review of the question.
4.3.3 The concept of “real time''
A main problem that arises is the time periods that still exist between the measurement
of the activity of the subject and the sensory feedback. By experimentally manipulating
these time periods and/or the different time periods between the different sensory pro-
cesses (for example: visual and haptic processes), we can try to provide virtual reality
techniques with the recommendations resulting from behavioural studies (particularly
at the level of the sensory stimulation techniques, motor interfaces and “real time''
interaction). Also note that this problem is not independent of the problem of realism
mentioned above. In fact, there is always a correlation between the complexity of a
virtual world and the calculation time required to generate this world. Thus, in line
with the development of the power of the digital and graphic processors, there will
always have to be a compromise between the requirements of “realism'' and “real
time''. At the experimental level, the effects of the delays on the execution of tasks,
like chasing objects with systems connected to head movements, hand movements or
even simulated driving tasks, have been widely studied for a long time.
For example, So and Griffin (1995) have shown, in tasks involving catching a
static target with a pointer connected to head movements, that the performance of
the operator went down for time periods of more than 70 milliseconds. Delays also
hamper the performance in tasks where the hand is used as a control “tool''. For
example, time periods of more than 100 ms seriously hamper performance in tasks
that involve keeping an object inside an area that moves in an unpredictable manner,
as well as in tasks that involve placing objects in determined areas (Liu et al., 1993).
In driving simulation tasks, Frank et al. (1988) have also observed a deterioration
of performance for the time periods of 170 ms. Thus, we already see that the effect
of the delays varies depending on the task and the control system involved (which
is necessarily connected in part to the neuro-biological aspects of the sensorimotor
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