  Designing 3D User Interfaces (UI) requires adequate evaluation tools to
ensure good usability and user experience. While many evaluation tools are
already available and widely used, existing approaches generally cannot provide
continuous and objective measures of usa-bility qualities during interaction
without interrupting the user. In this paper, we propose to use brain (with
ElectroEncephaloGraphy) and physiological (ElectroCardioGraphy, Galvanic Skin
Response) signals to continuously assess the mental effort made by the user to
perform 3D object manipulation tasks. We first show how this mental effort
(a.k.a., mental workload) can be estimated from such signals, and then measure
it on 8 participants during an actual 3D object manipulation task with an input
device known as the CubTile. Our results suggest that monitoring workload
enables us to continuously assess the 3DUI and/or interaction technique
ease-of-use. Overall, this suggests that this new measure could become a useful
addition to the repertoire of available evaluation tools, enabling a finer
grain assessment of the ergonomic qualities of a given 3D user interface.
