Three dimensional (3D) display technologies such as immersive virtual reality systems (VR), or non-immersive VR system using stereo-glasses (Ware, 1993) have significantly improved in display quality in recent years. Such constant improvement in designing virtual environments is mainly due to an attentive observation of how human interacts with these devices and to a precise analysis of which components increase at most the user's immersion feeling. An example of this trend is the introduction of the head tracking system as a solution for the parallax distortion perceived by the user. Likewise the advantages in using either the head mounted displays (HMD) or the retro-projection has been showed by the increased sensation of immersion and the better convergence/accommodation effect reported by the subjects. Yet Volumetric Displays have been recently proved being a valid solution for the relevant limitations concerning depth perception which usually affect the user's performance in VR environments (Grossman, 2006).

It appears evident, form these few examples, that VR displays is a tool designed for different human interactions (e.g. medical surgery, flight simulation, communication, human research) whose configuration is continuously corrected in response of the outcomes coming of the same interaction. Thus the aim in designing VR devices, is to reach as much as possible what Korteling et al. called "functional validity" i.e. "the extent to which the behaviour [..]of a person in the simulator resembles his or her behaviour on the real task under the same condition" (Korteling 1999).

The figure below attempts to represent the dual approach of our team, either practical and scientific. It is characterized by a closed loop relation in which the human performance is either the factor of interest and the factor of comparison (with the real world). Consequently, an attentive analysis of the human interaction is an exclusive condition for drawing a measure of "functional validity" of the tool and improving its quality (design correction).



In accordance with a practical approach whose aim is to design the most efficient VR application (e.g. for training or simulation), a good functional validation might be reached by correcting the VR design as soon as a satisfactory users/ virtual environment is obtained. That is what Hettinger and Haas defined "User Centred Design".Likewise, having to face with research aims, the validation of our tool will be extremely relevant not only to describe the specific human ways of interaction in virtual environments rather to investigate why human interact such a way. Hence, with the advent of VR and novel robotic interfaces, it has became possible, to create sophisticated computer-controlled environment. Such control over the physics of the virtual world with which subjects interact has allowed detailed tests on computational models of planning, control and learning (Wolpert, 2000).

Thus, our current and future works are headed for two parallel aims: