1. Max Planck Institute for Human Cognitive and Brain Sciences, Stephanstrasse 1a, 04103 Leipzig, Germany
2. Department of Mathematics, Faculteit der Exacte Wetenschappen, Vrije Universiteit (VU University Amsterdam), De Boelelaan 1081a, 1081 HV Amsterdam, Netherlands and Mathneuro Team, Inria Sophia Antipolis, 2004 Rue des Lucioles, Sophia Antipolis, 06902 Cedex, France