Research in the field of neural networks has been attracting increasing attention in recent years. Since 1943, when Warren McCulloch and Walter Pitts presented the first model of artificial neurons, new and more sophisticated proposals have been made from decade to decade. Mathematical analysis has solved some of the mysteries posed by the new models but has left many questions open for future investigations. Needless to say, the study of neurons, their interconnections, and their role as the brain´s elementary building blocks is one of the most dynamic and important research fields in modern biology. We can illustrate the relevance of this endeavor by pointing out that between 1901 and 1991 approximately ten percent of the Nobel Prizes for Physiology and Medicine were awarded to scientists who contributed to the understanding of the brain. It is not an exaggeration to say that we have learned more about the nervous system in the last fifty years than ever before.