1. Laboratorio de Física Estadística, Centro de Física, IVIC, Aparado 21827, Caracas 1020A, Venezuela
2. Institut für Theoretische Physik, Universität Bremen, Otto-Hahn-Allee 1, D-28359 Bremen, Germany
3. Sony Computer Science Labs, Inc., 3-14-13 Higashigotunda, Shinagawa, Tokyo 141-0022, Japan
4. Department of Neurology, Committees of Neurobiology and Computational Neuroscience, The University of Chicago, MC 2030, 5841 South Maryland Ave., Chicago, IL 60637, USA