Artificial neural network by arpit_sharma

CONTENTS
• INTRODUCTION
• BIOLOGICAL NEURON MODEL
• ARTIFICIAL NEURON MODEL
• ARTIFICIAL NEURAL NETWORK
• NEURAL NETWORKARCHITECTURE
• LEARNING
• BACKPROPAGATIONALGORITHM
• APPLICATIONS
• ADVANTAGES
• CONCLUSION

INTRODUCTION
• “Neural“ is an adjective for neuron, and “network” denotes a graph like
structure.
• Artificial Neural Networks are also referred to as “neural nets” , “artificial
neural systems”, “parallel distributed processing systems”, “connectionist
systems”.
• For a computing systems to be called by these pretty names, it is necessary
for the system to have a labeled directed graph structure where nodes
performs some simple computations.
• “Directed Graph” consists of set of “nodes”(vertices) and a set of
“connections”(edges/links/arcs) connecting pair of nodes.
• A graph is said to be “labeled graph” if each connection is associated with a
label to identify some property of the connection

Fig 1: AND gategraph
This graph cannot be considered a neural
network since the connections between the
nodes are fixed and appear to play no other
role than carrying the inputs to the node that
computed their conjunction.
Fig 2: AND gatenetwork
The graph structure which connects the
weights modifiable using a learning
algorithm, qualifies the computing
system to be called an artificial neural
networks.
x2ϵ{0,1}
x1 x2
x1ϵ{0,1}
o = x1 AND x2
multiplier
(x1w1)
(x2w2)
o = x1 AND x2
x1
x2
w1
w2
•The field of neural network was pioneered by BERNARDWIDROW of Stanford Universityin
1950’s.
CONTD…

BIOLOGICAL NEURON MODEL
Four parts of a typical nerve cell : -
• DENDRITES: Accepts the inputs
• SOMA : Process the inputs
• AXON :Turns the processed inputs into
outputs.
• SYNAPSES :Theelectrochemical
contact between the
neurons.

ARTIFICIAL NEURON MODEL
• Inputs to the network are represented by the
mathematical symbol, xn
• Each of these inputs are multiplied by a
connection weight , wn
sum = w1 x1 + ……+wnxn
• These products are simply summed, fed
through the transfer function, f( ) to generate
a result and then output.
w1
w2
xn
x2
x1
wn
f(w1 x1 + ……+wnxn)

TERMINOLOGY
Biological Terminology Artificial Neural Network Terminology
Neuron Node/Unit/Cell/Neurode
Synapse Connection/Edge/Link
Synaptic Efficiency Connection Strength/Weight
Firing frequency Node output

ARTIFICIAL NEURAL NETWORK
• Artificial Neural Network (ANNs) are programs designed to solve
any problem by trying to mimic the structure and the function of our
nervous system.
• Neural networks are based on simulated neurons, Which are joined
together in a variety of ways to form networks.
• Neural network resembles the human brain in the following two
ways: -
* A neural network acquires knowledge throughlearning.
*A neural network’s knowledge is stored within the
interconnection strengths known as synaptic weight.

ARTIFICIAL NEURAL NETWORK MODEL
output layer
connections
Input layer
Hidden layers
Neural network
Including
connections
(called weights)
between neuron
Comp
are
Actual
output
Desired
output
Input
output
Figure showing adjust of neuralnetworkFig 1 : artificial neural network model
CONTD…

NEURAL NETWORK ARCHITECTURES
These are networks in which nodes are
partitioned into subsets called layers, with
no connections from layer j to k if j > k.
Inputnode
Inputnode
output node
output node
Hidden node
Layer3Layer0
(Input layer) (Outputlayer)
Layer 1 Layer2
Hidden Layer
Fig: fully connected network
The neural network in which every node is
connected to every other nodes, and
these connections may be either
excitatory (positive weights), inhibitory
(negative weights), or irrelevant (almost
zero weights).
fig: layered network

This is the subclass of the layered
networks in which there is no intra-layer
connections. In other words, a
connection may exist between any node
in layer i and any node in layer j for i < j,
but a connection is not allowed for i=j.
This is a subclass of acyclic
networks in which a connection is
allowed from a node in layer i only
to nodes in layer i+1
Layer0
(Input layer)
Layer3
(Outputlayer)
Layer 1 Layer2
Hidden Layer
Layer 1 Layer2
Layer3Layer0
(Input layer) (Outputlayer)
Hidden Layer
fig : Feedforward networkFig : Acyclicnetwork
CONTD…

Many problems are best solved using neural networks whose
architecture consists of several modules, with sparse
interconnections between them. Modules can be organized in
several different ways as Hierarchial organization,Successive
refinement, Input modularity
Fig : Modular neural network
CONTD…

LEARNING
• Neurons in an animal’s brain are “hard wired”. It is equally obvious that
animals, especially higher order animals, learn as they grow.
• How does this learning occur?
• What are possible mathematical models of learning?
• In artificial neural networks, learning refers to the method of
modifying the weights of connections between the nodes of a
specified network.
• The learning ability of a neural network is determined by its
architecture and by the algorithmic method chosen for training.

• This is learning by doing.
• In this approach no sample
outputs are provided to the
network against which it can
measure its predictive
performance for a given vectorof
inputs.
• One common form of
unsupervised learning is
clustering where we try to
categorize data in different
clusters by their similarity.
UNSUPERVISED LEARNING
• A teacher is available to indicate
whether a system is performing
correctly, or to indicate the amount of
error in system performance. Here a
teacher is a set of training data.
• The training data consist of pairs of
input and desired output values that
are traditionally represented in data
vectors.
• Supervised learning can also be
referred as classification, where we
have a wide range of classifiers,
(Multilayer perceptron, k nearest
neighbor..etc)
SUPERVISED LEARNING
CONTD…

THE BACKPROPAGATION ALGORITHM
• The backpropagation algorithm (Rumelhart and McClelland, 1986) is
used in layered feed-forwardArtificial NeuralNetworks.
• Back propagation is a multi-layer feed forward, supervised learning
network based on gradient descent learning rule.
• we provide the algorithm with examples of the inputs and outputs we
want the network to compute, and then the error (difference
between actual and expected results) is calculated.
• The idea of the backpropagation algorithm is to reduce this error,
until the Artificial Neural Network learns the training data.

• The activation function of the artificial neurons in ANNs
implementing the backpropagation algorithm is a
weighted sum (the sum of the inputs xi multiplied by their
respective weights wji)
• The most common output function is the sigmoidal
function:
• Since the error is the difference between the actual and
the desired output, the error depends on the weights, and
we need to adjust the weights in order to minimize the
error.We can define the error function for the output of
each neuron:
Inputs, x
Weights,v weights,w
output
Fig: Basic Block of
Back propagation neural network

CONTD…
• The backpropagation algorithm now calculates how the error depends on the
output, inputs, and weights.
the adjustment of each weight (Δwji ) will be the negative of a constant eta (η)
multiplied by the dependance of the “wji” previous weight on the error of the network.
• First, we need to calculate how much the error depends on the output
• Next, how much the output depends on the activation, which in turn depends on the
weights
• And so, the adjustment to each weight will be

Weights,v weights,w
CONTD…
• If we want to adjust vik, the weights (let’s call them vik ) of a previous
layer, we need first to calculate how the error depends not on the
weight, but in the input from the previous layer i.e. replacing w by x
as shown in below equation.
Inputs, x
output
where
• and

ADVANTAGES
• It involves human like thinking.
• They handle noisy or missing data.
• They can work with large number of variables or parameters.
• They provide general solutions with good predictive accuracy.
• System has got property of continuous learning.
• They deal with the non-linearity in the world in which we live.

Artificial neural network by arpit_sharma

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