  The main aim of the work presented here is to contribute to computer science
advances in the multimodal usability area, in-as-much as it addresses one of
the major issues relating to the generation of effective oral system messages:
how to design messages which effectively help users to locate specific
graphical objects in information visualisations? An experimental study was
carried out to determine whether oral messages including coarse information on
the locations of graphical objects on the current display may facilitate target
detection tasks sufficiently for making it worth while to integrate such
messages in GUIs. The display spatial layout varied in order to test the
influence of visual presentation structure on the contribution of these
messages to facilitating visual search on crowded displays. Finally, three
levels of task difficulty were defined, based mainly on the target visual
complexity and the number of distractors in the scene. The findings suggest
that spatial information messages improve participants' visual search
performances significantly; they are more appropriate to radial structures than
to matrix, random and elleptic structures; and, they are particularly useful
for performing difficult visual search tasks.
