1. Department of Neurosurgery, Massachusetts General Hospital, Harvard Medical School, Boston, MA 02114, USA.
2. Institute for Analysis and Scientific Computing, Vienna University of Technology, 1040 Vienna, Austria.
3. Boston VA Healthcare System, Rehabilitation, Research and Development, Boston, MA 02130, USA.
4. Department of Biomedical Engineering, Tufts University, Medford, MA 02155, USA.