Libro De Redes De Comunicacion

Objectives

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Introduction
Objectives History Applications Biological Inspiration Further Reading 1-1 1-2 1-5 1-8 1-10

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Objectives
As you read these words you are using a complex biological neural network. You have a highly interconnected set of some 1011 neurons to facilitate your reading, breathing, motion and thinking. Each of your biological neurons, a rich assembly of tissueand chemistry, has the complexity, if not the speed, of a microprocessor. Some of your neural structure was with you at birth. Other parts have been established by experience. Scientists have only just begun to understand how biological neural networks operate. It is generally understood that all biological neural functions, including memory, are stored in the neurons and in the connections betweenthem. Learning is viewed as the establishment of new connections between neurons or the modification of existing connections. This leads to the following question: Although we have only a rudimentary understanding of biological neural networks, is it possible to construct a small set of simple artificial ÒneuronsÓ and perhaps train them to serve a useful function? The answer is Òyes.Ó This book,then, is about artificial neural networks. The neurons that we consider here are not biological. They are extremely simple abstractions of biological neurons, realized as elements in a program or perhaps as circuits made of silicon. Networks of these artificial neurons do not have a fraction of the power of the human brain, but they can be trained to perform useful functions. This book is aboutsuch neurons, the networks that contain them and their training.

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History
The history of artificial neural networks is filled with colorful, creative individuals from many different fields, many of whom struggled for decades to develop concepts that we now take for granted. This history has been documented by various authors. One particularly interesting book isNeurocomputing: Foundations of Research by John Anderson and Edward Rosenfeld. They have collected and edited a set of some 43 papers of special historical interest. Each paper is preceded by an introduction that puts the paper in historical perspective. Histories of some of the main neural network contributors are included at the beginning of various chapters throughout this text and will not be repeatedhere. However, it seems appropriate to give a brief overview, a sample of the major developments. At least two ingredients are necessary for the advancement of a technology: concept and implementation. First, one must have a concept, a way of thinking about a topic, some view of it that gives a clarity not there before. This may involve a simple idea, or it may be more specific and include amathematical description. To illustrate this point, consider the history of the heart. It was thought to be, at various times, the center of the soul or a source of heat. In the 17th century medical practitioners finally began to view the heart as a pump, and they designed experiments to study its pumping action. These experiments revolutionized our view of the circulatory system. Without the pumpconcept, an understanding of the heart was out of grasp. Concepts and their accompanying mathematics are not sufficient for a technology to mature unless there is some way to implement the system. For instance, the mathematics necessary for the reconstruction of images from computer-aided tomography (CAT) scans was known many years before the availability of high-speed computers and efficientalgorithms finally made it practical to implement a useful CAT system. The history of neural networks has progressed through both conceptual innovations and implementation developments. These advancements, however, seem to have occurred in fits and starts rather than by steady evolution. Some of the background work for the field of neural networks occurred in the late 19th and early 20th centuries. This...