1. Department of Neurology and Neurosurgery, Montreal Neurological Institute and McGill University, Montreal, Quebec H3A 2B4, Canada;
2. Department of Experimental Neurophysiology, Istituto Nazionale Neurologico C. Besta, Milan 510, 20133 Italy;
3. Department of Numerical Analysis and Computing Science, Royal Institute of Technology, S-100 44 Stockholm, Sweden; and
4. Department of Psychology, Boston University, Boston, Massachusetts 02215