1. Physics of Living Systems, Department of Physics, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
2. Redwood Center for Theoretical Neuroscience, and Department of Physics, University of California at Berkeley, Berkeley, CA 94720, USA
3. Department of Social and Decision Sciences, Carnegie Mellon University, 5000 Forbes Avenue, BP 208, Pittsburgh, PA 15213, USA
4. Santa Fe Institute, 1399 Hyde Park Road, Santa Fe, NM 87501, USA