1. The Philip Bard Laboratories of Neurophysiology, Department of Neuroscience, The Johns Hopkins University School of Medicine, Baltimore, MD 21205.
2. Department of Psychology, McGill University, Montreal, Quebec, Canada H3A 1B1.
3. Division of Neurobiology, St. Joseph's Hospital and Medical Center, Barrow Neurological Institute, Phoenix, AZ 85013.
4. Department of Neuroscience and Department of Biomedical Engineering, The Johns Hopkins University School of Medicine, Baltimore, MD 21205.