1. J. L. Raymond is in the Department of Physiology and the W. M. Keck Foundation Center for Integrative Neuroscience, University of California, San Francisco, CA 94143, USA.
2. S. G. Lisberger is in the Department of Physiology, the W. M. Keck Foundation Center for Integrative Neuroscience, and the Sloan Center for Theoretical Neurobiology, University of California, San Francisco, CA 94143, USA.
3. M. D. Mauk is in the Department of Neurobiology and Anatomy, University of Texas Medical School, Houston, TX 77030, USA.