Department of Physiology and Biophysics, Imperial College School of Medicine at St. Mary’s, London W2 1PG, United Kingdom; IBM Research Division, T. J. Watson Research Center, Yorktown Heights, NY 10598; Department of Neurology, Columbia University, New York, NY 10032; and Department of Physiology, The Medical School, University of Birmingham, Birmingham B15 2TT, United Kingdom (At 1997-10-28)