1. University of Eastern Finland, Department of Applied Physics, P.O. Box 1627, Kuopio 70211, Finland
2. University of Eastern Finland, Department of Applied Physics, P.O. Box 1627, Kuopio 70211, FinlandbUniversity College London, Department of Computer Science, Gower Street, London WC1E 6BT, United Kingdom
3. Aalto University School of Science, Department of Neuroscience and Biomedical Engineering, P.O. Box 12200, Aalto 00076, FinlanddHelsinki University Central Hospital, HUS Medical Imaging Center, BioMag Laboratory, P.O. Box 340, HUS 00029, Finland
4. University College London, Department of Computer Science, Gower Street, London WC1E 6BT, United Kingdom