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Presentation on Neural Networks. Basics Of Neural Networks. Neural networks refers to a connectionist model that simulates the biophysical information processing occurring in the nervous system.
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Basics Of Neural Networks Neural networks refers to a connectionist model that simulates the biophysical information processing occurring in the nervous system. It can also be defined as an interconnected assembly of simple processing elements ,units or nodes whose functionality is loosely based on the animal neuron. And a cognitive information processing structure based (on models of brain function. In a more formal engineering context a highly parallel dynamical system with the topology of a directed graph that can carry out information processing by means of it's state response to continuous or initial input.
Benefits Of Neural Networks Non-linearity Input-output Mapping Adaptivity Evidential Response Contentional Information Fault Tolerance RESEARCH THOUGHT 1. Neural networks are highly parallel structures which is true because human brain functions in the same way. 2. But apart from being parallel it is has priority based parallelism. 3. Apart from being parallel there is interaction between these parallel processes and in the end one process may dominate while others vanish or survive with much lower priority.
Facts 1. Knowledge is acquired by the network from its environment through a learning process. 2. Interneuron connection strengths known as synaptic weights are used to store the acquired knowledge
LEARNING IN NEURAL NETWORKS LEARNING MAY BE DEFINED AS 1. To learn from environment and improve its performance through learning. 2. Learning is a process by which free parameters of neural networks are adapted through the process of simulation by the environment in which the network is embedded. The type of learning is as follows- a. Neural network is simulated by environment. b. Neural network undergoes change. c. Neural network responds in a way to environment.
Types Of Learning a. Error correction learning b. Memory based learning c. Competitive learning d. Boltzman learning RESEARCH THOUGHT 1. Normally learning process is iterative process in which neural networks consistently learn from environment. 2. Neural networks must try for self eradication of of error by heuristically moving towards the goal state. 3. It means that there should be combination of heuristic knowledge and previous data to obtain the final result.
MEMORY BASED LEARNING Past experiences are explicitly stored in a large memory of correctly classified input-output examples. xi is the input vector di denotes the desired response c1 and c2 are classification examples Retrieving and analyzing the training data by putting into classifications c1 and c2. RESEARCH THOUGHT 1. Since memory based learning is only a classification process it is inaccurate because it does not account long term and short term memory. 2. It should be a layered process where information if filtered from the forward layers to the backward layers. 3. The forward layers are short term memory layers where as back layers are long term memory layers. 4. The neural network must operate by considering all the layers giving short term memory layers more priority than long term memory layers.
Memory Based Learning (Working Example) In memory based learning there is classification of input-output examples {(Xi,Di)}i=1 to N.where Xi is the input vector and Di is the desired response. A working example of memory based learning is car movement. We shall classify all the cases into two parts 1 (car speed up) and 0 (car slow down).The input signals are X1 -> road conditions , X2-> traffic signal, X3-> fuel efficiency , X4-> road ascent. Now when a set of inputs are applied to X1, X2, X3, X4 then the response is either speed up or slow down. As per the memory based learning all these cases can be stored during learning and new cases can then be classified as being of either speeding up or slow down of the car depending on the input conditions.
HEBBIAN LEARNING When an axon of cell a is near enough to cell b and repeatedly takes part in firing it some growth process or metabolic change takes place in one or both cells such that efficiency of a as one of the cells firing b is increased. It means that two neurons on the either side of synapse are activated simultaneously causing the strength of synapse to increase. RESEARCH THOUGHT Hebbian learning should be classifieds into two parts 1. A process in which there is gradual shift toward strengthening of synapse if the input total synaptic weights are below a threshold value. 2. If the synaptic weights inputs are above a threshold value there is a fast shift in a single iteration.
Hebbian Learning (Working Example) Hebbian learning in mathematical terms can be expressed by considering a synaptic weight Wkj of neuron k with pre-synaptic and post-synaptic signals denoted by Xj and Yk. If pre-synaptic and post-synaptic signals are synchronous then there is increase in weight .The adjustment applied to the synaptic weight Wkj at time step n is Δ Wkj(n) = F(Yk(n),Xj(n)) A working example is introduction of traffic signal X1->Red , X2-> Yellow and X3-> Green. We can observe that initial slow down at red signal Y1(n) and initial startup at green signal Y2(n) is slow but with time the response becomes stronger and faster.
COMPETITIVE LEARNING In competitive learning the output neurons of a neural network compete among themselves to become active. In competitive learning a set of neurons behave differently to a given set of inputs.
BOLTZMANN LEARNING The neurons constitute a recurrent structure and they operate in a binary manner since for example they may be in +1 or-1 state. There is flipping of states depending on the input. RESEARCH THOUGHT Blotzmann learning puts the neurons in only two states +1 and -1 whereas actually they should take a number of states depending on the set of inputs previous states.
Boltzman Learning (working Example) Boltzman machine operates on the energy generated when a signal moves from neuron j to neuron k. This process continues till the system reaches thermal equilibrium or the desired state. A working example can be a thermostat which keeps a check on the heat energy being released in various processes in a factory. The Boltzmann system can gradually learn the amount of heat energy released during all processes and then learn to adjust the weights maintaining an optimal temperature. In fact it can automatically guide the temperature maintenance all the time. P (change) = 1/1+exp (-∆E/Ti)
We can easily calculate it as- X1 (Temperature process P1) , X2 (Temperature process P2) , X3 (Temperature process P3) , X4 (Temperature process P4) may cause the final temperature reading Ti which is compared with required temperature Tj. Energy change ∆E can be calculated and error correction applied automatically.
SUPERVISED LEARNING In conceptual terms supervised learning may be thought of as neural network having knowledge of the environment and using that knowledge to formulate the neural network by input-output examples. RESEARCH THOUGHT 1. Supervised learning should be object based in which we try to learn about an object from the environment. 2. It means there is need to first learn about the object properties and then about the object methods. 3. Once the object has been learned neural network may simulate it for a set of inputs
Supervised Learning (Working Example) The aircraft control system can become a good example of supervised learning because the aircraft navigation system faces new environmental conditions all the time. But these conditions are fed to the GPS, Ground support and other devices which teach the system to deal with them. The system can do the error correction learning to stay on course and learn to manage the system. When the system has fully learned to automate itself, it can be put onto a pilot less vehicle for self navigation with minimal outside help.
UNSUPERVISED LEARNING In unsupervised learning there is no external teacher. Rather the process is made for a task-independent measure of quality of representation that the network is required to learn. Various stochastic methods like standard deviation regression are used to obtain useful information from data. RESEARCH THOUGHT Since there is no supervision required and all data is collected and then analyzed it would be useful to first create a broad classification of environment Once the environment has been classified data from the environment can be further classified to make the data collected to be more meaningful.
Unsupervised Learning (Working Example) In case of unsupervised learning there is no error correction support applied. The data is statistically classified into one or more classes. Unsupervised learning can be used in weather forecast system. The data can be collected in the form of variable values T (Temperature Conditions), C (Cloud formations), H (Humidity reading in and around a place), A (Air flow readings). These can then use unsupervised learning to learn to make a correct weather forecast as an output of the neural network.
SINGLE LAYER PERCEPTRON A perceptron is the simplest form of a neural network used for the classification of patterns which are linearly separable. It consists of a single neuron with adjustable synaptic weights. Perceptron convergence theorem tries to do pattern classification with only two classes in case of single layer perceptron. RESEARCH THOUGHT Single layer perceptron should be clocked as being a slower and a faster neuron. Further the weights themselves should be a function of time and should depend on delta t.
Single Layer Perceptron (Working Example) Single layer Perceptron does binary classification and then does error correction as per the learning rule by modification of weights. An example can be a Perceptron that calculates the price of a product. We can consider the input variable with some initial weights X1(Market demand), X2(Input material prices), X3(Past growth) X4(Profit expected). This can be expressed as a linear equation. 1.2354 X1 + 2.3338 X2 + 6.4523 X3 + 1.1 X4 = Price Now single layer perceptron can be made to calculate the exact price after error correction done by comparing the output price with the actual price. Eventually it can predict the correct price.
MULTI LAYER PERCEPTRON A multi layer perceptron consists of a sensory units that constitute an input layer, one or more hidden layers of computation nodes and an output layer of computation nodes. Learning takes place using error-correction learning rule. The function used is a non linear activation function called the activation function. RESEARCH THOUGHT As there are numerous layers in multi layer perceptron they should be classified as- 1. Which layer is faster than others? 2. Which layer has a higher priority? 3. Which layer is responsible for what part of output?
RADIAL BASIS FUNCTION RBF looks at multi layer perceptron from curve fitting point of view. RBF does complex pattern classification Task- A complex pattern classification problem is cast in a high dimension space non-linearity is more likely to be linearly separable than in low dimensional space. Interpolation is the technique used for curve fitting from data movement across the layers.
Radial Basis Function (Working Example) In multi-layer perceptron calculation is done for an approximate function to various layers of the multi-layer perceptron. A function Φ(x) which approximates movement of signal from one layer to another. An example can be application of RBF to study growth of disease in a given population infecting people in a phased manner. For example a disease starts by infecting 10% of the population in 5 cities in the beginning. In the next phase it grows by 5% in 10 more cities and grows to 20% in the first 5 cities. This process can be approximated using RBF and another RBF can calculate the move against the spread of the disease.
SUPPORT VECTOR MACHINES Support vector machine is a linear machine which consists of a decision surface in such a way that the margin of separation between positive and negative cases is maximized. It follows the method of structural risk minimization an induction principle based on the fact that the error rate of a learning machine on test data is bounded by the sum of training error rate and a term that depends on vapnik-chervonenkis dimension. SVM supports the following three types of learning machines 1. Polynomial learning machine. 2. RBF function networks. 3.2-layer perceptrons. RESEARCH THOUGHT SVM can calculate the probability of each point being a part of classification. It can further deduce the results as to validity of each input for a classification.
Support Vector Machines (Working Example) Support Vector machines use the hyper plane equation to separate the examples into two classes +1 or -1. Support vector machines use training data { (Xi,di) }i=1 to N. di = +1 or di = -1 is the desired response from the neural network. It uses equations Wt Xi + b >=0 for di=+1 Wt Xi + b < 0 for di= -1 An example can be a neural net which computes whether a person can be given a loan (+1) or may not be given a loan (-1). The input vector consists of the inputs Xi (Income, past transactions, job type, family) etc. Support vector machine can calculate optimal values of these support vectors and then give the desired response
