1. Department of Macromolecular Science and Engineering, University of Michigan, Ann Arbor, Michigan.
2. Department of Neuroscience, University of California San Diego, La Jolla, California.
3. Veterans Administration Medical Center, La Jolla, California.
4. Department of Anesthesiology, University of California San Diego, La Jolla, California.
5. Program in Neuroscience, University of California San Diego, La Jolla, California.
6. Department of Mechanical Engineering, University of Michigan, Ann Arbor, Michigan.