The author is really indebted to Prof. Giacomo M. Bisio for fruitful discussions and helpful advice, and to Dr. Luigi Raffo for his stinging comments. This work was supported by ``Programma Nazionale di Ricerca sulle Tecnologie per la Bioelettronica'' of MURST (Italian Ministry of Scientific and Technological Research) and SGS-Thomson Microelectronics; Progetto Strategico Reti Neurali of the National Research Council (CNR) of Italy; Progetto 40% Sistemi Neurali and Architetture VLSI of MURST; and CEC- Esprit BR Project CORMORANT-8503.