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control systems, which we will not develop here, but which justify a neuroscientific
approach to the problem).
It must also be noted that the time periods can have several sources; delays due
to the sampling frequency of the movement measurement system, transmission delays
between different devices, and calculation periods, as well as the refresh rate of the
visual feedback. (Liu et al., 1993) have also studied the effect of the refresh rates of
visual stimulation. One usually observes a fall in performance, in chasing tasks, for
refresh rates lower than 10 images per second (Hz).
For these low refresh rates, one usually observes a deterioration of the chasing
movements which become jerky, suggesting that the subject then takes up a task
of reduction of successive errors and not a “continuous'' chasing task. The problem
becomes even more complex if we consider that the effect of the delays is also related
to the characteristics of the movements involved, mainly their amplitude, as well as
the angular dimensions of the display device. Particularly in the case of head move-
ments (connecting device widely used in the immersive environments in which we try
to update the 3D image according to the position of the observer's head), the delays
will introduce position errors that are proportional to the speed of the head movement
(Bryson & Fisher, 1990). In this context, So and Griffin (1995) have shown a chasing
error proportional to the speed of the target for time periods between 40 and 200ms.
It must also be noted that some authors have observed, in the case of significant time
periods, that a reaction of the subjects then consists in limiting their movements, which
is not really the objective sought in an interactive visual system. It is thus necessary
to remember that the effects of time periods in the “action-perception'' coupling will
quite obviously depend on the type of movement (mainly, amplitude and speed) that
the operators will have made. Another aspect of the problem is the range of the field
of vision. In fact, the size of the display device can affect movements (mainly, head
movements) made by the subject and thus the errors of moving the image caused by
the time periods. By comparing visualisation headsets with field widths lower than 60
degrees and panoramic devices, some authors have observed that the subjects made
lesser head movements with “wide angle'' devices, which naturally reduced the effect
of time periods (Woodruff et al., 1990).
4.4 CONCLUSION
We have tried to provide a methodological answer to the problems presented by the def-
inition of an “efficient'' virtual reality system, by suggesting that virtual reality systems
are developed through a dialogue between Engineering technologies and Behavioural
Sciences (Cognitive and sensorimotor). We have only presented a very partial view
of these problems, the purpose being to educate the scientific and industrial users as
well as the designers of virtual reality systems about the potential benefit of a scientific
and basic approach to these problems. The reader has been able to assess that the
solutions to these problems are, at best, incomplete and that it is difficult to offer a
unique methodology to resolve them today, due to the multi-dimensional nature of the
objective of the virtual reality systems.
However, we stand by the idea of a strong and necessary interaction between
Behavioural Sciences and virtual reality ... It is therefore a question of establishing a
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