Y. Gil, and V. Ratnakar. Proceedings of the 15th International Florida Artificial Intelligence Research Society Conference, page 413--418. Menlo Park, CA, AAAI Press, (2002)
S. Germesin, and T. Wilson. Proceedings of the 11th International Conference on Multimodal Interfaces and the 6th Workshop on Machine Learning for Multimodal Interfaces (ICMI-MLMI '09), Cambridge, MA, USA, page 7-14. (2009)