1 Mammalian Locomotor Laboratory, Department of Neuroscience, Karolinska Institutet, Retziusväg 8, 17177 Stockholm, Sweden. Electronic address: Ole.Kiehn@ki.se.
2 Department of Neuroscience, Grossman Center for the Statistics of Mind, David Mahoney Center for Brain and Behavior Research, Kavli Institute for Brain Science, Columbia University Medical Center, New York, NY 10032, United States. Electronic address: mc3502@cumc.columbia.edu.