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neural networks

neural networks



neural networks &Artificial Neural Network

neural networks &Artificial Neural Network



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  • Propensity ميل
  • Axon is like محور عصبى Dendrite is like الغصن Synapse is like مشبك
  • The brain basically learns from experience. Neural networks are sometimes called machine learning algorithms, because changing of its connection weights (training) causes the network to learn the solution to a problem . The strength of connection between the neurons is stored as a weight-value for the specific connection. The system learns new knowledge by adjusting these connection weights . The learning ability of a neural network is determined by its architecture and by the algorithmic method chosen for training .
  • Unsupervised learning The hidden neurons must find a way to organize themselves without help from the outside. In this approach, no sample outputs are provided to the network against which it can measure its predictive performance for a given vector of inputs. This is learning by doing .

neural networks neural networks Presentation Transcript

  • 02/13/13
  •  Introduction to Artificial Neural Networks, Artificial and human neurons (Biological Inspiration) The learning process, Supervised and unsupervised learning, Reinforcement learning, Applications Development and Portfolio The McCulloch-Pitts Model of Neuron, A simple network layers, Multilayer networks Perceptron, Back propagation algorithm, Recurrent networks, Associative memory, Self Organizing maps, Support Vector Machine and PCA, Applications to speech, vision and control problems. 02/13/13
  •  Main text books: “Neural Networks: A Comprehensive Foundation”, S. Haykin (very good -theoretical) “Pattern Recognition with Neural Networks”, C. Bishop (very good-more accessible) “Neural Network Design” by Hagan, Demuth and Beale (introductory) Books emphasizing the practical aspects: “Neural Smithing”, Reeds and Marks “Practical Neural Network Recipees in C++”’ T. Masters Seminal Paper: “Parallel Distributed Processing” Rumelhart and McClelland et al. Other: “Neural and Adaptive Systems”, J. Principe, N. Euliano, C. Lefebvre 02/13/13
  •  Review Articles: R. P. Lippman, “An introduction to Computing with Neural Nets”’ IEEE ASP Magazine, 4-22, April 1987. T. Kohonen, “An Introduction to Neural Computing”, Neural Networks, 1, 3- 16, 1988. A. K. Jain, J. Mao, K. Mohuiddin, “Artificial Neural Networks: A Tutorial”’ IEEE Computer, March 1996’ p. 31-44. 02/13/13
  •  Introduction to Artificial Neural Networks, Artificial and human neurons (Biological Inspiration) The learning process, Supervised and unsupervised learning, Reinforcement learning, Applications Development and Portfolio The McCulloch-Pitts Model of Neuron, A simple network layers, Multilayer networks Perceptron, Back propagation algorithm, Recurrent networks, Associative memory, Self Organizing maps, Support Vector Machine and PCA, Applications to speech, vision and control problems. 02/13/13
  • Introduction to Artificial NeuralNetworksPart I:1. Artificial Neural Networks2. Artificial and human neurons (Biological Inspiration)3. Tasks & Applications of ANNsPart II:1. Learning in Biological Systems2. Learning with Artificial Neural Networks 02/13/13
  • Digital Computers Artificial Neural Networks Analyze the problem to be solved  No requirements of an explicit description of the problem. Deductive Reasoning. We apply  Inductive Reasoning. Given input known rules to input data to and output data (training produce output. examples), we construct the rules. Computation is centralized,  Computation is collective, synchronous, and serial. asynchronous, and parallel. Not fault tolerant. One transistor goes and it no longer works.  Fault tolerant and sharing of responsibilities. Static connectivity.  Dynamic connectivity. Applicable if well defined rules with precise input data.  Applicable if rules are unknown or complicated, or if data are noisy or partial. 02/13/13
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  • Artificial Neural Networks (1) Branch of "Artificial Intelligence". It is a system modeled based on the human brain. ANN goes by many names, such as connectionism, parallel distributed processing, neuro-computing, machine learning algorithms, and finally, artificial neural networks. Developing ANNs date back to the early 1940s. It experienced a wide popularity in the late 1980s. This was a result of the discovery of new techniques and developments in PCs. Some ANNs are models of biological neural networks and some are not. ANN is a processing device (An algorithm or Actual hardware) whose design was motivated by the design and functioning of human brain.Inside ANN: ANN’s design is what distinguishes neural networks from other mathematical techniques ANN is a network of many simple processors ("units“ or “neurons”), each unit has a small amount of local memory. The units are connected by unidirectional communication channels ("connections"), which carry numeric (as opposed to symbolic) data. The units operate only on their local data and on the inputs they receive via the connections. 02/13/13
  • Artificial Neural Networks (2)ANNs Operation ANNs normally have great potential for parallelism (multiprocessor-friendly architecture), since the computations of the units are independent of each other. Same like biological neural networks.  Most neural networks have some kind of "training" rule whereby the weights of connections are adjusted on the basis of presented patterns. In other words, neural networks "learn" from examples, just like children…and exhibit some structural capability for generalization. 02/13/13
  • Artificial Neural Networks (3)ANNs are a powerful technique (Black Box) to solve many real world problems. They have the ability to learn from experience in order to improve their performance and to adapt themselves to changes in the environment.In addition, they are able to deal with incomplete information or noisy data and can be very effective especially in situations where it is not possible to define the rules or steps that lead to the solution of a problem.Once trained, the ANN is able to recognize similarities when presented with a new input pattern, resulting in a predicted output pattern. 02/13/13
  • What can a ANN do? Compute a known function Approximate an unknown function Pattern Recognition Signal Processing……. Learn to do any of the above 02/13/13
  • Introduction to Artificial NeuralNetworksPart I:1. Artificial Neural Networks (ANNs)2. Artificial and human neurons (Biological Inspiration)3. Tasks & Applications of ANNsPart II:1. Learning in Biological Systems2. Learning with Artificial Neural Networks 02/13/13
  • Biological Neural Networks (BNN) are much morecomplicated in their elementary structures than the mathematical models we use for ANNsAnimals are able to react adaptively to changes in theirexternal and internal environment, and they use theirnervous system to perform these behaviours.An appropriate model/simulation of the nervous systemshould be able to produce similar responses andbehaviours in artificial systems.The nervous system is build by relatively simple units,the neurons, so copying their behaviour and functionalityshould be the solution! 02/13/13
  •  An artificial neural network (ANN) is a massively parallel distributedANN as a model of brain- processor that has a natural propensity for storing like Computer experimental knowledge and making it available for use. It means that:    Knowledge is acquired by the network Brain through a learning (training)The human brain is still not well process;understood and indeed its behavior  The strength of theis very complex! interconnectionsThere are about 10-11 billion between neurons isneurons in the human cortex each implemented byconnected to , on average, 10000others. In total 60 trillion synapses means of the synaptic weightsof connections. used toThe brain is a highly complex, store the knowledge.nonlinear and parallel computer The learning process is a procedure(information-processing system) of the adapting the weights with a 02/13/13 learning algorithm in order to
  • How our brain  A process of pattern manipulates  recognition and pattern with patterns ? manipulation is based on:Massive parallelism Connectionism Associative Brain computer as an information  Brain computer is a highly distributed memoryor signal processing system, is interconnected neurons system incomposed of a large number of a such a way that the state of one Storage of information in a brain issimple processing elements, called neuron affects the potential of the supposed to be concentrated inneurons. These neurons are large number of other neurons synaptic connections of braininterconnected by numerous direct which are connected according to neural network, or more precisely,links, which are called connection, weights or strength. The key idea in the pattern of these connectionsand cooperate which other to of such principle is the functional and strengths (weights) of theperform a parallel distributed capacity of biological neural nets synaptic connections.processing (PDP) in order to soft a deters mostly not so of a singledesired computation tasks. neuron but of its connections  02/13/13
  • Biological Neuron - The simple “arithmetic computing” element02/13/13
  •  Cell structures  Cellbody  Dendrites  Axon  Synaptic terminals 02/13/13
  • dendrites axon synapsesThe information transmission happens at the synapses, i.eSynaptic connection strengths among neurons are used tostore the acquired knowledge.In a biological system, learning involves adjustments to thesynaptic connections between neurons 02/13/13
  • 1. Soma or body cell - is a large, round central body in which almost all the logical functions of the neuron are realized (i.e. the processing unit).2. The axon (output), is a nerve fibre attached to the soma which can serve as a final output channel of the neuron. An axon is usually highly Synapses branched. Axon from3. The dendrites (inputs)- represent a other highly branching tree of fibers. These neuron long irregularly shaped nerve fibers Soma (processes) are attached to the soma carrying electrical signals to the cell Dendrite Axon from4. Synapses are the point of contact other between the axon of one cell and the Dendrites dendrite of another, regulating a chemical connection whose strength The schematic affects the input to the cell. model of a 02/13/13 biological neuron
  •  Learning from examples  labeled or unlabeled Adaptivity  changing the connection strengths to learn things Non-linearity  the non-linear activation functions are essential Fault tolerance  if one of the neurons or connections is damaged, the whole network still works quite well 02/13/13
  • Introduction to Artificial NeuralNetworksPart I:1. Artificial Neural Networks (ANNs)2. Artificial and human neurons (Biological Inspiration)3. Tasks & Applications of ANNsPart II:1. Learning in Biological Systems2. Learning with Artificial Neural Networks 02/13/13
  •  Classification In marketing: consumer spending pattern classification In defence: radar and sonar image classification In agriculture & fishing: fruit, fish and catch grading In medicine: ultrasound and electrocardiogram image classification, EEGs, medical diagnosis Recognition and Identification In general computing and telecommunications: speech, vision and handwriting recognition In finance: signature verification and bank note verification Assessment In engineering: product inspection monitoring and control In defence: target tracking In security: motion detection, surveillance image analysis and fingerprint matching Forecasting and Prediction In finance: foreign exchange rate and stock market forecasting In agriculture: crop yield forecasting , Deciding the category of potential food items (e.g., edible or non-edible) In marketing: sales forecasting In meteorology: weather prediction 02/13/13
  •  Computer scientists want to find out about the properties of non-symbolic information processing with neural nets and about learning systems in general. Statisticians use neural nets as flexible, nonlinear regression and classification models. Engineers of many kinds exploit the capabilities of neural networks in many areas, such as signal processing and automatic control. Cognitive scientists view neural networks as a possible apparatus to describe models of thinking and consciousness (High-level brain function). Neuro-physiologists use neural networks to describe and explore medium- level brain function (e.g. memory, sensory system, motorics). Physicists use neural networks to model phenomena in statistical mechanics and for a lot of other tasks. Biologists use Neural Networks to interpret nucleotide sequences. Philosophers and some other people may also be interested in Neural Networks for various reasons 02/13/13
  • The spikes travelling along the axon of the pre-synapticneuron trigger the release of neurotransmittersubstances at the synapse.The neurotransmitters cause excitation or inhibition inthe dendrite of the post-synaptic neuron.The integration of the excitatory and inhibitory signalsmay produce spikes in the post-synaptic neuron.The contribution of the signals depends on the strengthof the synaptic connection.• Excitation means positive product between the incoming spike rate and the corresponding synaptic weight;• Inhibition means negative product between the incoming spike rate and the corresponding synaptic weight; 02/13/13
  • OutputInputs An artificial neural network is composed of many artificial neurons that are linked together according to a specific network architecture. The objective of the neural network is to transform the inputs into meaningful outputs. 02/13/13
  • Neurons are arranged in layers. Neurons work by processing information. Theyreceive and provide information in form of spikes.The artificial neuron receives one or more inputs (representing the one or moredendrites),At each neuron, every input has an associated weight which modifies thestrength of each input and sums them together,The sum of each neuron is passed through a function known as anactivation function or transfer function in order to produce an output(representing a biological neurons axon) Inputs Output 02/13/13
  • x1 x2 w1 n Output x3 w2 z = ∑ wi xi ; y = H ( z )Inputs i =1 y .. w3 … . xn-1 wn-1 wn xnEach neuron takes one or more inputs and produces an output. At eachneuron, every input has an associated weight which modifies the strength ofeach input. The neuron simply adds together all the inputs and calculates anoutput to be passed on. 02/13/13
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  • Three elements:1. A set of synapses, or connection link: each of which is characterized by a weight or strength of its own wkj. Specifically, a signal xj at the input synapse ‘j’ connected to neuron ‘k’ is multiplied by the synaptic wkj2. An adder: For summing the input signals, weighted by respective synaptic strengths of the neuron in a linear operation.3. Activation function: For limiting of the amplitude of the output of the neuron to limited range. The activation function is referred to as a Squashing (i.e. limiting) function {interval [0,1], or, alternatively [-1,1]} 02/13/13
  • The bias has the effect of increasing or lowering the netinput of the activation function depending on whether it is+/- yk = Ø(vk) = Ø(uk + bk) = Ø(Σ wkjxj + bk)An artificial neuron:-computes the weighted sum of its input (called its net input)-adds its bias (the effect of applying affine transformation to the output vk)-passes this value through an activation functionWe say that the neuron “fires” (i.e. becomesactive) if its outputs is above zero.This extra free variable (bias) makes the neuronmore powerful. 02/13/13
  •  It defines the output of the neuron given an input or set of inputs. A standard computer chip circuit can be seen as a digital network of activation functions that can be "ON" (1) or "OFF" (0), depending on input, The best activation function is the non-linear function. Linear functions are limited because the output is simply proportional to the input. Three basic types of activation function: 1. Threshold function, 2. Linear function, 3. Sigmoid function. 02/13/13
  • Activation functions (2)McColloch-Pitts Model Threshold Logic Unit (TLU), since 1943 02/13/13
  • Activation functions (3) 02/13/13
  • Activation functions (4)- A fairly simple non-linear function, such as the logistic function.- As the slop parameter approaches infinity the sigmoid function becomes athreshold functionWhere “a” is the slope parameter of the sigmoid function 02/13/13
  •  Early ANN Models: McCulloch-Pitts , Perceptron, ADALINE, Hopfield Network, Current Models: Multilayer feed forward networks (Multilayer perceptrons- Back propagation ) Radial Basis Function networks Self Organizing Networks ... 02/13/13
  •  Feedback is a dynamic system whenever occurs in almost every part of the nervous system, Feedback is giving one or more closed path for transmission of signals around the system, It plays important role in study of special class of neural networks known as Recurrent networks. 02/13/13
  • The system is assumed to be linear and has a forward path (A)and a feedback path (B),The output of the forward channel determines its own outputthrough the feedback channel. 02/13/13
  • E.g. consider A is a fixed weight and B is a unit delay operator z-1 . 02/13/13
  • Then, we may express yk(n) as an infinite weighted summation ofpresent and past samples of the input signal xj(n).Therefore, feedback systems are controlled by weight. 02/13/13
  • Feedback systems are controlledby weight.1. For positive weight, we have stable systems, i,e, convergent output y,2. For negative weight, we have, unstable systems, i.e divergent output y.. (Linear and Exponential) 02/13/13
  • Three different classes of network architectures:1. Single-layer feed forward networks,2. Multilayer feed forward networks,3. Recurrent networks. 02/13/13
  • - Input layer of source nodes that projects directly onto an output layer of neurons.- “Single-layer” referring to the output layer ofcomputation nodes (neuron). 02/13/13
  • It contains one or more hiddenlayers (hidden neurons).“Hidden” refers to the part ofthe neural network is not seendirectly from either input oroutput of the network .The function of hidden neuron isto intervene between input andoutput.By adding one or more hiddenlayers, the network is able toextract higher-order statisticsfrom input 02/13/13
  • It is different from feed forwardneural network in that it has atleast one feedback loop.Recurrent network may consistof single layer of neuron witheach neuron feeding its outputsignal back to the inputs of allthe other neurons. Note: Thereare no self-feedback.Feedback loops have a profoundimpact on learning and overallperformance. 02/13/13
  •  What transfer function should be used? How many inputs does the network need? How many hidden layers does the network need? How many hidden neurons per hidden layer? How many outputs should the network have? There is no standard methodology to determinate these values. Even there is some heuristic points, final values are determinate by a trial and error procedure. 02/13/13
  • Knowledge is referred to the stored information or models usedby a person or machine to interpret, predict and, appropriately,respond to the outside. A good solution depends on a good representation of knowledgeThe main characteristic of knowledge representation hastwo folds:1) What information is actually made explicit?2) How the information is physically encoded forsubsequent use? 02/13/13
  • There are two kinds ofKnowledge: 1) The known world states, orfacts, (prior knowledge), 2) Observations (measurements)of the world, obtained by sensors to These observationsprobe thepool of represent the environment. information, from which examples are used to train the NN 02/13/13
  • These Examples can be labeled or unlabeledIn labeled examples Each example representing an input signal is paired witha corresponding desired response,Labeled examples may be expensive to collect, as theyrequire availability of a “teacher” to provide a desiredresponse for each labeled example.Un labeled examplesUnlabeled examples are usually abundant as there is noneed for supervision. 02/13/13
  • Design of neural network mayproceed as follow:An appropriate architecture for the neural network, withan input layer consisting of source nodes equal in numberto the pixels of an input image.The recognition performance of trained network istested with data not seen before (testing). This phase of the network design called learning 02/13/13
  • There are four rules for knowledge representation:Rule 1:Similar inputs (i.e., patterns) drawn from similarclasses should usually produce similarrepresentation inside the network, and shouldtherefore be classified as belonging to the sameclass. There are plethora (many) of measures for determining the similarity between inputs 02/13/13
  • A commonly used measure of similarity is the Euclidian DistanceLet xi denotes an m -by-1 vector (1) 02/13/13
  • Another measure is the dot product or inner product comGiven a pair of vectors xi a nd xj of the same dimension, theirinner product will be (the projection of vector xi ontovector xj)Please note that: 02/13/13
  • The smaller the Euclidean distance ║x i - xj ║(i.e. the more similarthe vector xi a nd xj are), the larger the inner product xiT xj will be. To formalize this relationship, we normalize the vectors x i and xj to have a unit length, i.e.: Using Eq.(1) to writeThe minimization of the Euclidean distance d (x i , xj ) corresponds to maximization of the inner product (x i , xj )..and, therefore, the similarity between the vectors x i and xj 02/13/13
  • If the vectors x i and xj are stochastic (drown from different population of data)Where C-1 is the inverse of the covariancematrix C. It is supposed that thecovariance matrix is the same for both For a prescribed C, the smaller the distance d is the more similar the vectors xi a nd xj will be 02/13/13
  • Rule 2:Item to be categorized as separate classes should be givenwidely different representation in work.Rule 3:If a particular feature is important, then there should belarge number of neurons involved in the representation ofthat item in the network.Rule 4:Prior information and invariance should be built into thedesign of a neural network when ever they are available,so as to simplify the network design by its not having tolearn them. Rule 4 is particularly important and highly desirable 02/13/13
  • Rule 4 is particularly important and highly desirable because it results in an NN with a Specialized Structure (SS)1) Biological visual and auditory networks are very specialized,2) NN with SS has a smaller number of free parameters available for adjustment than other networks. Then, they need a small training dataset, learns faster and generalize better.3) Rate of information transmission through a specialized network is faster,4) Cost of building a specialized network is minimum, due to small 02/13/13 size.
  • There are currently no well-defined rules for doing this; but wehave some procedure are known to yield useful rules. Inparticular, we may use a combination of two techniques:1. Restricting the network architecture (using local connections)2. Constraining the choice of synaptic weight (using the weightsharing) The latter tech is so important because it leads to reducing significantly free parameters 02/13/13
  • Consider any of the following:1) When an object rotates, the perceived image, by observer, will change as well,2) The utterance of a spoken person may be soft or loud..slower or quicker, A classifier should be invariant to different3) ….. transformation Or A class estimate represented by an output of the classifier MUST not be affected by transformations of the observed signal applied to the classifier inputThere are three technique for rendering classifier-type NNsinvariant to transformations:1. Invariance by structure.2. Invariance by training.3. Invariance by feature space 02/13/13
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  • Learning approach based on modeling adaptation inbiological neural systems Learning = learning by adaptationThe young animal learns that the green fruits are sour,while the yellowish/reddish ones are sweet. Thelearning happens by adapting the fruit pickingbehaviour 02/13/13
  •  From experience: examples / training data Learning happens by changing of the synaptic strengths, Synapses change size and strength with experience (or examples or training data), Strength of connection between the neurons is stored as a weight-value for the specific connection, Learning the solution to a problem = changing the connection weights 02/13/13
  • Hebbian Learning When two connected neurons are firing at the same time, the strength of the synapse between them increases, “Neurons that fire together, wire together” 02/13/13
  • We may categorize the learning process throughNeural Networks function as follows:1. Learning with a teacher, - Supervised Learning2. Learning without a teacher, - Unsupervised Learning - Reinforcement Learning 02/13/13
  • Supervised Learning In supervised learning, both the inputs and the outputs are provided. The network then processes the inputs and compares its resulting outputs against the desired outputs Errors are then calculated, causing the system to adjust the weights which control the network. This process occurs over and over as the weights are continually improved. Supervised learning process constitutes a closed-loop feedback system but unknown environment is outside the loop, 02/13/13
  • Supervised Learning It is based on a labeled training set. (2) The class of each piece of ε Class data in training set is known. ε Class A Class labels are pre- B λ Class determined and provided λ Class B in the training phase. A A ε Class λ Class B 02/13/13
  • BA BA B A 02/13/13
  • A B A B B B A AA B B A 02/13/13
  • Various steps have to be considered:1. Determine the type of training examples,2. Gather a training data set that satisfactory describe the givenproblem,3. After the training process we can test the performance oflearned artificial neural network with the test (validation) data set,4. Test data set consist of data that has not been introduced toartificial neural network while learning. 02/13/13
  •  The learning of input –output mapping is performed through continued interaction with the environment in order to minimize a scalar index of performance.Or A machine learning technique that sets parameters of an artificial neural network, where data is usually not given, but generated by interactions with the environment. 02/13/13
  • Reinforcement learning is built around critic that converts primaryreinforcement signal received from the environment into a higher quality reinforcement signal 02/13/13
  •  No help from the outside, No information available on the desired output, Input: set of patterns P, from n-dimensional space S, but little / no information about their classification, evaluation, interesting features, etc. It must learn these by itself! Learning by doing Tasks: Used to pick out structure in the input  Clustering - Group patterns based on similarity,  Vector Quantization - Fully divide up S into a small set of regions (defined by codebook vectors) that also helps cluster P,  Feature Extraction - Reduce dimensionality of S by removing unimportant features (i.e. those that do not help in clustering P) 02/13/13
  •  Task performed  Task performed Classification Clustering, Pattern Pattern Recognition Recognition NN model Feature Extraction, VQ Preceptron,  NN Model Feed-Forward NN Self Organizing Maps, ART 02/13/13