1. Dipartimento di Psicologia Generale, Universita degli Studi di Padova, via Venezia 8, I 35131 Padua, Italy
2. Department of Neurophysiology, Max-Planck-Institut fur biologische Kybernetik, Spemannstrasse 38, D 72076 Tübingen, Germany; also The Salk Institute, 10010 North Torrey Pines Road, La Jolla, CA 92037, USA