This work has been done in the Laboratory of Computer and Information Science at Helsinki University of Technology and funded by the European Commission research project BLISS.
I wish to thank professor Juha Karhunen for supervision and for financial support. I thank Dr. Harri Valpola for essential guidance on the subject. I also wish to thank Antti Honkela and Jaakko Peltonen for creative discussions and technical advice on Matlab, LATEX and Unix.
Finally I wish to thank Anna Hiironen and my family for their encouragement and support.
Otaniemi, December 3, 2001
Tapani Raiko