1. F. C. Donders Centre for Cognitive Neuroimaging, Radboud University Nijmegen, 6525 EK Nijmegen, Netherlands.
2. Department of Biophysics, Radboud University Nijmegen, 6525 EZ Nijmegen, Netherlands.
3. Center for Sensory-Motor Interaction (SMI), Aalborg University, Fredrik Bajers Vej 7 D-3, 9220 Aalborg, Denmark.