1. Stroke Research Laboratory, Massachusetts General Hospital, Charlestown, MA 02129.
2. Neurosurgical Service, Harvard Medical School, Charlestown, MA 02129, USA.
3. Cardiovascular Research Center, Department of Medicine, Harvard Medical School, Massachusetts General Hospital, Charlestown, MA 02129, USA.
4. Department of Neurology, Harvard Medical School and Massachusetts General Hospital, Charlestown, MA 02129, USA.