Neural-Networks, images and examples details

Ahmad Aljebaly
Artificial Neural Networks

Agenda
 History of Artificial Neural Networks
 What is an Artificial Neural Networks?
 How it works?
 Learning
 Learning paradigms
 Supervised learning
 Unsupervised learning
 Reinforcement learning
 Applications areas
 Advantages and Disadvantages

History of the Artificial Neural Networks
 history of the ANNs stems from the 1940s, the decade of the first electronic
computer.
 However, the first important step took place in 1957 when Rosenblatt
introduced the first concrete neural model, the perceptron. Rosenblatt also
took part in constructing the first successful neurocomputer, the Mark I
Perceptron. After this, the development of ANNs has proceeded as
described in Figure.

 Rosenblatt's original perceptron model contained only one layer. From this,
a multi-layered model was derived in 1960. At first, the use of the multi-
layer perceptron (MLP) was complicated by the lack of a appropriate
learning algorithm.
 In 1974, Werbos came to introduce a so-called backpropagation algorithm
for the three-layered perceptron network.

 in 1986, The application area of the MLP networks remained rather limited
until the breakthrough when a general back propagation algorithm for a
multi-layered perceptron was introduced by Rummelhart and Mclelland.
 in 1982, Hopfield brought out his idea of a neural network. Unlike the
neurons in MLP, the Hopfield network consists of only one layer whose
neurons are fully connected with each other.

 Since then, new versions of the Hopfield network have been developed.
The Boltzmann machine has been influenced by both the Hopfield network
and the MLP.

 in 1988, Radial Basis Function (RBF) networks were first introduced by
Broomhead & Lowe. Although the basic idea of RBF was developed 30
years ago under the name method of potential function, the work by
Broomhead & Lowe opened a new frontier in the neural network
community.

 in 1982, A totally unique kind of network model is the Self-Organizing
Map (SOM) introduced by Kohonen. SOM is a certain kind of topological
map which organizes itself based on the input patterns that it is trained with.
The SOM originated from the LVQ (Learning Vector Quantization)
network the underlying idea of which was also Kohonen's in 1972.

History of Artificial Neural Networks
Since then, research on artificial neural networks has
remained active, leading to many new network types, as
well as hybrid algorithms and hardware for neural
information processing.

Artificial Neural Network
 An artificial neural network consists of a pool of simple
processing units which communicate by sending signals to
each other over a large number of weighted connections.

Artificial Neural Network
 A set of major aspects of a parallel distributed model include:
 a set of processing units (cells).
 a state of activation for every unit, which equivalent to the output of the
unit.
 connections between the units. Generally each connection is defined by a
weight.
 a propagation rule, which determines the effective input of a unit from its
external inputs.
 an activation function, which determines the new level of activation based
on the effective input and the current activation.
 an external input for each unit.
 a method for information gathering (the learning rule).
 an environment within which the system must operate, providing input
signals and _ if necessary _ error signals.

Computers vs. Neural Networks
“Standard” Computers Neural Networks
 one CPU highly parallel processing
fast processing units slow processing units
reliable units unreliable units
static infrastructure dynamic infrastructure

Why Artificial Neural Networks?
There are two basic reasons why we are interested in
building artificial neural networks (ANNs):
• Technical viewpoint: Some problems such as
character recognition or the prediction of future
states of a system require massively parallel and
adaptive processing.
• Biological viewpoint: ANNs can be used to
replicate and simulate components of the human
(or animal) brain, thereby giving us insight into
natural information processing.

• The “building blocks” of neural networks are the
neurons.
• In technical systems, we also refer to them as units or nodes.
• Basically, each neuron
 receives input from many other neurons.
 changes its internal state (activation) based on the current
input.
 sends one output signal to many other neurons, possibly
including its input neurons (recurrent network).

• Information is transmitted as a series of electric
impulses, so-called spikes.
• The frequency and phase of these spikes encodes the
information.
• In biological systems, one neuron can be connected to as
many as 10,000 other neurons.
• Usually, a neuron receives its information from other
neurons in a confined area, its so-called receptive field.

How do ANNs work?
 An artificial neural network (ANN) is either a hardware
implementation or a computer program which strives to
simulate the information processing capabilities of its biological
exemplar. ANNs are typically composed of a great number of
interconnected artificial neurons. The artificial neurons are
simplified models of their biological counterparts.
 ANN is a technique for solving problems by constructing software
that works like our brains.

How do our brains work?
 The Brain is A massively parallel information processing system.
 Our brains are a huge network of processing elements. A typical brain contains a
network of 10 billion neurons.

 A processing element
Dendrites: Input
Cell body: Processor
Synaptic: Link
Axon: Output

A neuron is connected to other neurons through about 10,000
synapses

A neuron receives input from other neurons. Inputs are combined.

Once input exceeds a critical level, the neuron discharges a spike ‐
an electrical pulse that travels from the body, down the axon, to
the next neuron(s)

The axon endings almost touch the dendrites or cell body of the
next neuron.

Transmission of an electrical signal from one neuron to the next is
effected by neurotransmitters.

Neurotransmitters are chemicals which are released from the first neuron
and which bind to the
Second.

This link is called a synapse. The strength of the signal that
reaches the next neuron depends on factors such as the amount of
neurotransmitter available.

How do ANNs work?
An artificial neuron is an imitation of a human neuron

How do ANNs work?
• Now, let us have a look at the model of an artificial neuron.

How do ANNs work?
Output
x1
x2
xm
∑
y
Processing
Input
∑= X1+X2 + ….+Xm =y
. . . . . . . . . . .
.

How do ANNs work?
Not all inputs are equal
Output
x1
x2
xm
∑
y
Processing
Input
∑= X1w1+X2w2 + ….+Xmwm
=y
w1
w2
wm
weights
. . . . . . . . . . .
.
. . . .
.

How do ANNs work?
The signal is not passed down to the
next neuron verbatim
Transfer Function
(Activation Function)
Output
x1
x2
xm
∑
y
Processing
Input
w1
w2
wm
weights
. . . . . . . . . . .
.
f(vk)
. . . .
.

The output is a function of the input, that is
affected by the weights, and the transfer
functions

Three types of layers: Input, Hidden, and
Output

 An ANN can:
1. compute any computable function, by the appropriate
selection of the network topology and weights values.
2. learn from experience!
 Specifically, by trial and error
‐ ‐

Learning by trial and error
‐ ‐
Continuous process of:
Trial:
Processing an input to produce an output (In terms of ANN:
Compute the output function of a given input)
Evaluate:
Evaluating this output by comparing the actual output with
the expected output.
Adjust:
Adjust the weights.

Example: XOR
Hidden
Layer, with
three
neurons
Output
Layer, with
one neuron
Input Layer,
with two
neurons

How it works?
Hidden
Layer, with
three
neurons
Output
Layer, with
one neuron
Input Layer,
with two
neurons

How it works?
 Set initial values of the weights randomly.
 Input: truth table of the XOR
 Do
 Read input (e.g. 0, and 0)
 Compute an output (e.g. 0.60543)
 Compare it to the expected output. (Diff= 0.60543)
 Modify the weights accordingly.
 Loop until a condition is met
 Condition: certain number of iterations
 Condition: error threshold

Design Issues
 Initial weights (small random values [ 1,1])
∈ ‐
 Transfer function (How the inputs and the weights are
combined to produce output?)
 Error estimation
 Weights adjusting
 Number of neurons
 Data representation
 Size of training set

Transfer Functions
 Linear: The output is proportional to the total
weighted input.
 Threshold: The output is set at one of two values,
depending on whether the total weighted input is
greater than or less than some threshold value.
 Non linear:
‐ The output varies continuously but not
linearly as the input changes.

Error Estimation
 The root mean square error (RMSE) is a
frequently-used measure of the differences between
values predicted by a model or an estimator and the
values actually observed from the thing being
modeled or estimated

Weights Adjusting
 After each iteration, weights should be adjusted to
minimize the error.
– All possible weights
– Back propagation

Back Propagation
 Back-propagation is an example of supervised learning is
used at each layer to minimize the error between the
layer’s response and the actual data
 The error at each hidden layer is an average of the
evaluated error
 Hidden layer networks are trained this way

Back Propagation
 N is a neuron.
 Nw is one of N’s inputs weights
 Nout is N’s output.
 Nw = Nw +Δ Nw
 Δ Nw = Nout * (1‐ Nout)* NErrorFactor
 NErrorFactor = NExpectedOutput – NActualOutput
 This works only for the last layer, as we can know
the actual output, and the expected output.

Number of neurons
 Many neurons:
 Higher accuracy
 Slower
 Risk of over fitting
‐
 Memorizing, rather than understanding
 The network will be useless with new problems.
 Few neurons:
 Lower accuracy
 Inability to learn at all
 Optimal number.

Data representation
 Usually input/output data needs pre processing
‐
 Pictures
 Pixel intensity
 Text:
 A pattern

Size of training set
 No one fits all formula
‐ ‐
 Over fitting can occur if a “good” training set is not
chosen
 What constitutes a “good” training set?
 Samples must represent the general population.
 Samples must contain members of each class.
 Samples in each class must contain a wide range of
variations or noise effect.
 The size of the training set is related to the number of
hidden neurons

Learning Paradigms
Supervised learning
Unsupervised learning
Reinforcement learning

Supervised learning
 This is what we have seen so far!
 A network is fed with a set of training samples (inputs and
corresponding output), and it uses these samples to learn
the general relationship between the inputs and the outputs.
 This relationship is represented by the values of the weights
of the trained network.

Unsupervised learning
 No desired output is associated with the training data!
 Faster than supervised learning
 Used to find out structures within data:
 Clustering
 Compression

Reinforcement learning
 Like supervised learning, but:
 Weights adjusting is not directly related to the error
value.
 The error value is used to randomly, shuffle weights!
 Relatively slow learning due to ‘randomness’.

Applications Areas
 Function approximation
 including time series prediction and modeling.
 Classification
 including patterns and sequences recognition, novelty
detection and sequential decision making.
 (radar systems, face identification, handwritten text recognition)
 Data processing
 including filtering, clustering blinds source separation and
compression.
 (data mining, e-mail Spam filtering)

Advantages / Disadvantages
 Advantages
 Adapt to unknown situations
 Powerful, it can model complex functions.
 Ease of use, learns by example, and very little user
domain specific expertise needed
‐
 Disadvantages
 Forgets
 Not exact
 Large complexity of the network structure

Conclusion
 Artificial Neural Networks are an imitation of the biological
neural networks, but much simpler ones.
 The computing would have a lot to gain from neural networks.
Their ability to learn by example makes them very flexible and
powerful furthermore there is need to device an algorithm in
order to perform a specific task.

Conclusion
 Neural networks also contributes to area of research such a
neurology and psychology. They are regularly used to model
parts of living organizations and to investigate the internal
mechanisms of the brain.
 Many factors affect the performance of ANNs, such as the
transfer functions, size of training sample, network topology,
weights adjusting algorithm, …

References
 Craig Heller, and David Sadava, Life: The Science of Biology, fifth edition,
Sinauer Associates, INC, USA, 1998.
 Introduction to Artificial Neural Networks, Nicolas Galoppo von Borries
 Tom M. Mitchell, Machine Learning, WCB McGraw-Hill, Boston, 1997.

Q. How does each neuron work in ANNS?
What is back propagation?
 A neuron: receives input from many other neurons;
 changes its internal state (activation) based on the
current input;
 sends one output signal to many other neurons, possibly
including its input neurons (ANN is recurrent network).
 Back-propagation is a type of supervised learning, used at each
layer to minimize the error between the layer’s response and
the actual data.

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