Acknowledgement

The authors would like to thank Harri Valpola, Sampsa Laine, Kai Zenger, Heikki Hyötyniemi, and Antti Honkela for fruitful discussions and comments. This research has been funded by the Finnish Centre of Excellence Programme (2000-2005) under the project New Information Processing Principles, and by the IST Programme of the European Community, under the PASCAL Network of Excellence, IST-2002-506778. This publication only reflects the authors' views.