1、l 外文原文 Neural Network Introduction1.ObjectivesAs 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 tissue and
2、 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 biolog
3、ical 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 rudimentary und
4、erstanding 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. They are
5、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 about such neu
6、rons, the networks that contain them and their training.2.HistoryThe 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 b
7、y 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 historical pe
8、rspective.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 are necessary
9、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 illustrate this
10、 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 revolutioni
11、zed 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 necessary for the
12、 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 conceptual innovati
13、ons 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 interdisciplinary wo
14、rk 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 modern view of neural n
15、etworks 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 theneural network field.McCulloch and Pitts were fol
16、lowed 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 came in the late 1950s,
17、 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 neural network researc
18、h. 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 algorithm and used it to t
19、rain 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 from the same inherent
20、limitations, which were widely publicized in a book by Marvin Minsky and Seymour Papert MiPa69. Rosenblatt and Widrow wereaware 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 comp
21、lex networks.Many people, influenced by Minsky and Papert, believed that further research on neural networks was a dead end. This, combined with the fact that there were no powerful digital computers on which to experiment,caused many researchers to leave the field. For a decade neural network resea
22、rch was largely suspended. Some important work, however, did continue during the 1970s. In 1972 Teuvo Kohonen Koho72 and James Anderson Ande72 independently and separately developed new neural networks that could act as memories. Stephen Grossberg Gros76 was also very active during this period in th
23、e investigation of self-organizing networks.Interest in neural networks had faltered during the late 1960s because of the lack of new ideas and powerful computers with which to experiment. During the 1980s both of these impediments were overcome, and researchin neural networks increased dramatically
24、. New personal computers andworkstations, which rapidly grew in capability, became widely available. In addition, important new concepts were introduced. Two new concepts were most responsible for the rebirth of neural net works. The first was the use of statistical mechanics to explain the operatio
25、n of a certain class of recurrent network, which could be used as an associative memory. This was described in a seminal paper by physicist John Hopfield Hopf82. The second key development of the 1980s was the backpropagation algo rithm for training multilayer perceptron networks, which was discover
26、ed independently by several different researchers. The most influential publication of the backpropagation algorithm was by David Rumelhart and James McClelland RuMc86. This algorithm was the answer to the criticisms Minsky and Papert had made in the 1960s. (See Chapters 11 and 12 for a development
27、of the backpropagation algorithm.)These new developments reinvigorated the field of neural networks. In the last ten years, thousands of papers have been written, and neural networks have found many applications. The field is buzzing with new theoretical and practical work. As noted below, it is not
28、 clear where all of this will lead US.The brief historical account given above is not intended to identify all of the major contributors, but is simply to give the reader some feel for how knowledge in the neural network field has progressed. As one might note, the progress has not always been slow
29、but sure. There have been periods of dramatic progress and periods when relatively little has been accomplished.Many of the advances in neural networks have had to do with new concepts, such as innovative architectures and training. Just as important has been the availability of powerful new compute
30、rs on which to test these new concepts. Well, so much for the history of neural networks to this date. The real question is, What will happen in the next ten to twenty years? Will neural networks take a permanent place as a mathematical/engineering tool, or will they fade away as have so many promis
31、ing technologies? At present, the answer seems to be that neural networks will not only have their day but will have a permanent place, not as a solution to every problem, but as a tool to be used in appropriate situations. In addition, remember that we still know very little about how the brain wor
32、ks. The most important advances in neural networks almost certainly lie in the future.Although it is difficult to predict the future success of neural networks, the large number and wide variety of applications of this new technology are very encouraging. The next section describes some of these app
33、lications.3.ApplicationsA recent newspaper article described the use of neural networks in literature research by Aston University. It stated that the network can be taught to recognize individual writing styles, and the researchers used it to compare works attributed to Shakespeare and his contempo
34、raries. A popular science television program recently documented the use of neural networks by an Italian research institute to test the purity of olive oil. These examples are indicative of the broad range of applications that can be found for neural networks. The applications are expanding because
35、 neural networks are good at solving problems, not just in engineering, science and mathematics, but m medicine, business, finance and literature as well. Their application to a wide variety of problems in many fields makes them very attractive. Also, faster computers and faster algorithms have made
36、 it possible to use neural networks to solve complex industrial problems that formerly required too much computation.The following note and Table of Neural Network Applications are reproduced here from the Neural Network Toolbox for MATLAB with the permission of the Math Works, Inc.The 1988 DARPA Ne
37、ural Network Study DARP88 lists various neural network applications, beginning with the adaptive channel equalizer in about 1984. This device, which is an outstanding commercial success, is a single-neuron network used in long distance telephone systems to stabilize voice signals. The DARPA report g
38、oes on to list other commercial applications, including a small word recognizer, a process monitor, a sonar classifier and a risk analysis system.Neural networks have been applied in many fields since the DARPA report was written. A list of some applications mentioned in the literature follows.Aeros
39、pace High performance aircraft autopilots, flight path simulations, aircraft control systems, autopilot enhancements, aircraft component simulations, aircraft component fault detectorsAutomotive Automobile automatic guidance systems, warranty activity analyzers Banking Check and other document reade
40、rs, credit application evaluatorsDefenseWeapon steering, target tracking, object discrimination, facial recognition, new kinds of sensors, sonar, radar and image signal processing including data compression, feature extraction and noise suppression, signal/image identificationElectronics Code sequen
41、ce prediction, integrated circuit chip layout, process control, chip failure analysis, machine vision, voice synthesis, nonlinear modelingEntertainment Animation, special effects, market forecastingFinancialReal estate appraisal, loan advisor, mortgage screening, corporate bond rating, credit line u
42、se analysis, portfolio trading program, corporate financial analysis, currency price predictionInsurance Policy application evaluation, product optimizationManufacturing Manufacturing process control, product design and analysis, process and machine diagnosis, real-time particle identification, visu
43、al quality inspection systems, beer testing, welding quality analysis, paper quality prediction, computer chip quality analysis, analysis of grinding operations, chemical product design analysis, machine maintenance analysis, project bidding, planning and management, dynamic modeling of chemical pro
44、cess systemsMedicalBreast cancer cell analysis, EEG and ECG analysis, prosthesis design, optimization of transplant times, hospital expense reduction, hospital quality improvement, emergency room test advisement 0il and GasExplorationRoboticsTrajectory control, forklift robot, manipulator controller
45、s, vision systemsSpeech Speech recognition, speech compression, vowel classification, text to speech synthesisSecurities Market analysis, automatic bond rating, stock trading advisory systemsTelecommunications Image and data compression, automated information services,real-time translation of spoken
46、 language, customer payment processing systemsTransportation Truck brake diagnosis systems, vehicle scheduling, routing systemsConclusionThe number of neural network applications, the money that has been invested in neural network software and hardware, and the depth and breadth of interest in these
47、 devices have been growing rapidly.4.Biological InspirationThe artificial neural networks discussed in this text are only remotely related to their biological counterparts. In this section we will briefly describe those characteristics of brain function that have inspired the development of artifici
48、al neural networks.The brain consists of a large number (approximately 1011) of highly connected elements (approximately 104 connections per element) called neurons. For our purposes these neurons have three principal components: the dendrites, the cell body and the axon. The dendrites are tree-like
49、 receptive networks of nerve fibers that carry electrical signals into the cell body. The cell body effectively sums and thresholds these incoming signals. The axon is a single long fiber that carries the signal from the cell body out to other neurons. The point of contact between an axon of one cell and a dendrit
陕公网安备 61072602000132号 违法和不良信息举报：0916-4228922