1、外文原文与译文 外文原文 Neural NetworkIntroduction 1.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 ti
2、ssue and 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 a
3、ll biological neural functions, including memory, are stored in the neurons and in the connections between them. 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 rudim
4、entary understanding of biological neural networks, is it possible to construct a small set of simple artificial “neurons” and perhaps train them to serve a useful function? The answer is “yes.”This book, then, is about artificial neural networks. The neurons that we consider here are not biological
5、. 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 abo
6、ut such neurons, the networks that contain them and their training. 2.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
7、 documented by various authors. One particularly interesting book is Neurocomputing: 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
8、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 repeated here. However, it seems appropriate to give a brief overview, a sample of the major developments. At least two ingredients
9、 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 clarity not there before. This may involve a simple idea, or it may be more specific and include a mathematical description. To
10、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 experime
11、nts revolutionized our view of the circulatory system. Without the pump concept, 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 n
12、ecessary for the reconstruction of images from computer-aided topography (CAT) scans was known many years before the availability of high-speed computers and efficient algorithms finally made it practical to implement a useful CAT system. The history of neural networks has progressed through both co
13、nceptual 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 consisted primarily of in
14、terdisciplinary work in physics, psychology and neurophysiology by such scientists as Hermann von Helmholtz, Ernst Much and Ivan Pavlov. This early work emphasized general theories of learning, vision, conditioning, etc.,and did not include specific mathematical models of neuron operation. The moder
15、n view of neural networks began in the 1940s with the work of Warren McCulloch and Walter Pitts McPi43, who showed that networks of artificial neurons could, in principle, compute any arithmetic or logical function. Their work is often acknowledged as the origin of the neural network field. McCulloc
16、h and Pitts were followed by Donald Hebb Hebb49, who proposed that classical conditioning (as discovered by Pavlov) is present because of the properties of individual neurons. He proposed a mechanism for learning in biological neurons. The first practical application of artificial neural networks ca
17、me in the late 1950s, with the invention of the perception network and associated learning rule by Frank Rosenblatt Rose58. Rosenblatt and his colleagues built a perception network and demonstrated its ability to perform pattern recognition. This early success generated a great deal of interest in n
18、eural network research. Unfortunately, it was later shown that the basic perception network could solve only a limited class of problems. (See Chapter 4 for more on Rosenblatt and the perception learning rule.) At about the same time, Bernard Widrow and Ted Hoff WiHo60 introduced a new learning algo
19、rithm and used it to train adaptive linear neural networks, which were similar in structure and capability to Rosenblatts perception. The Widrow Hoff learning rule is still in use today. (See Chapter 10 for more on Widrow-Hoff learning.) Unfortunately, both Rosenblatts and Widrows networks suffered
20、from the same inherent limitations, which were widely publicized in a book by Marvin Minsky and Seymour Papert MiPa69. Rosenblatt and Widrow were aware of these limitations and proposed new networks that would overcome them. However, they were not able to successfully modify their learning algorithms to train the more complex networks. Many people, influenced by Minsky and Papert, believed that further research on
