Artificial neural network

1. ArtificialArtificial
neural networkneural network
Yük. Blig. Müh. Mustafa Aadel MashjalYük. Blig. Müh. Mustafa Aadel Mashjal
158229001009158229001009

2. Background
- Neural Networks can be :
- BiologicalBiological models
- ArtificialArtificial models
- We wish to produce artificial systems capable of
complex calculation similar to the human brain.

3. describe the transmission ofdescribe the transmission of
information and some main ideasinformation and some main ideas
• The brain is consist of a mass of interconnected
neurons
– each neuron is connected to many other neurons
• Neurons transmit signals to each other
• Whether a signal is sent, depends on the strength of
the bond (synapse) between two neurons

4. How Does the Brain Work ? (1)
NEURON
- It is the cell that performs information processing in the
brain.
- Nervous tissue its consist of neurons, which receive and
transmit impulses

5. Each consists of :
SOMA, DENDRITES, AXON, and SYNAPSE.
How Does the Brain Work ? (2)

6. Brain vs. Digital Computers (1)
- Computers require hundreds of cycles to simulate
a firing of a neuron.
- The brain can fire all the neurons in a single step.
ParallelismParallelism
- Serial computers require billions of cycles to
perform some tasks but the brain takes less than
a second.
e.g. Face Recognition

7. Definition of Neural Network
A Neural Network is a system that consist of
many simple processing elements operating in
parallel which can achieve, store, and use
experimental knowledge.

8. What isWhat is
ArtificialArtificial
NeuralNeural
Network?Network?

9. Neurons vs. Units (1)
- Each element of NN is a node called unit.
- Units are connected by links.
-Each link has a numeric weight.

10. Neurons vs units (2)
• ANNs incorporate the two fundamental components
of biological neural nets:
1. Neurones (nodes)
2. Synapses (weights)

11. 1- A set of connecting links, each link characterized by a weight: W1,
W2, …, Wm
2- An adder function (linear combiner) which computes the weighted
sum of the inputs:
3- Activation function (squashing function) for limiting the amplitude of
the output of the neuron.
∑=
=
m
1
jjxwnetj
j
Structure of a nodeStructure of a node

12. ActivationActivation Functions
- Use different functions to obtain different models.
- 3 most common choices :
1) Step function
2) Sign function
3) Sigmoid function
- An output of 1 represents firing of a neuron down
the axon.

13. Step Function Perceptrons

14. 3 Activation Functions

15. Feed-Forward Neural Network
Architectures
The feed-forward neural network was the first and most simple type of
artificial neural network devised. In this network the information moves in
only one direction—forward: From the input nodes data goes through the
hidden nodes (if any) and to the output nodes. There are no cycles or loops
in the network.
• Two different classes of network architectures
– single-layer feed-forward neurons are organized
– multi-layer feed-forward in acyclic layers
• The architecture of a neural network is linked with the learning algorithm
used to train

16. Single Layer Feed-forward
The simplest kind of neural network is a single-layer perceptron network, which
consists of a single layer of output nodes; the inputs are fed directly to the outputs
via a series of weights. In this way it can be considered the simplest kind of feed-
forward network. The sum of the products of the weights and the inputs is calculated
in each node, and if the value is above some threshold the neuron fires and takes
the activated value; otherwise it takes the deactivated value. Neurons with this kind
of activation function are also called artificial neurons or linear threshold units.

18. This class of networks consists of multiple layers of computational units,
usually interconnected in a feed-forward way. Each neuron in one layer has
directed connections to the neurons of the next layer. In many applications
the units of these networks apply a sigmoid function as an activation function
Multi Layer Feed-forward

19. Supervised Learning Algorithm
• The learning algorithm would fall under this category if the desired
output for the network is also provided with the input while training
the network. By providing the neural network with both an input
and output pair it is possible to calculate an error based on it's
target output and actual output. It can then use that error to make
corrections to the network by updating it's weights.
• Single Layer neural network can be trained by a simple learning
algorithm that is usually called the Delta rule.
• Multi Layer neural network can be trained by a learning algorithm
that is usually called the Bakpropgation.

20. Delta rule
 The delta rule is specialized version of backpropagation's learning rule, for
use with single layer neural networks.
 It calculates the errors between calculated output and sample output data,
and uses this to create a modification to the weights, thus implementing a
form of gradient descent.

t (target) , o (output)
Mu (learning rate) , beta (error) , x (input)
collect old weight with the new modification weight and thus changing the
networks weight

21. Unsupervised Learning Algorithm
In this paradigm the neural network is only given a set of
inputs and it's the neural network's responsibility to find
some kind of pattern within the inputs provided without
any external aid.

22. Hebb Rule
Hebb method is used in the learning of networks that use
Unsupervised Learning , and the weight is modify by the
equation:
Mu (learning rate) , o (output) , x (input)
Then collect the output with the initial weight :

23. Introduction toIntroduction to
BackpropagationBackpropagation
- In 1969 a method for learning in multi-layer network, Backpropagation,
was invented by Bryson and Ho.
- Backpropagation is generalization of the delta rule to multi-layered
feedforward networks.
- Backpropagation is a common method of training artificial neural networks
used in conjunction with an optimization method such as gradient descent
(a first-order repeated optimization algorithm). It calculates the gradient of a
loss function with respect to all the weights in the network, so that the
gradient is fed to the optimization method which in turn uses it to update
the weights, in an attempt to minimize the loss function.

24. Backpropagation LearningBackpropagation Learning
DetailsDetails
• Use gradient descent to minimize the error
– propagate deltas to adjust for errors backward from
outputs to hidden layers to inputs

25. Backpropagation Algorithm – Main
Idea – error in hidden layers
The ideas of the algorithm can be summarized as follows :
1. Computes the error term for the output units using the
observed error.
2. From output layer, repeat
- propagating the error term back to the previous layer
and
- updating the weights between the two layers
until the earliest hidden layer is reached.

26. Forward Propagation of ActivityForward Propagation of Activity
• Step 1: Initialise weights at random, choose a
learning rate η
• Until network is trained:
• For each training example i.e. input pattern and
target output(s):
• Step 2: Do forward pass through net (with fixed
weights) to produce output(s)
– i.e., in Forward Direction, layer by layer:
• Inputs applied
• Multiplied by weights
• Summed
• ‘Squashed’ by sigmoid activation function
• Output passed to each neuron in next layer
– Repeat above until network output(s) produced

27. XOR- exampleXOR- example

28. Step 3. Back-Step 3. Back-
propagation ofpropagation of
errorerror
Backprop output layer

32. bias neuron in input layer
Bias Neurons inBias Neurons in BackpropBackpropagationagation
LearningLearning

33. Least-Mean-Square (LMS)Least-Mean-Square (LMS)
AlgorithmAlgorithm
Least mean squares (LMS) algorithms are a class of adaptive filter
used to simulate a required filter by finding the difference between the
desired and the actual signal. It was invented in 1960 by Stanford
University professor Bernard Widrow .

37. Examples andExamples and
ApplicationsApplications
of ANNof ANN

38. Neural Network in Practice
NNs are used for classification and function approximation
or mapping problems which are:
- Tolerant of some imprecision.
- Have lots of training data available.
- Hard and fast rules cannot easily be applied.

39. NETalk (1987)
• Mapping character strings into phonemes so they
can be pronounced by a computer
• Neural network trained how to pronounce each
letter in a word in a sentence, given the three
letters before and three letters after it in a window
• Output was the correct phoneme
• Results
– 95% accuracy on the training data
– 78% accuracy on the test set

40. Other Examples
• Neurogammon (Tesauro & Sejnowski, 1989)
– Backgammon learning program
• Speech Recognition (Waibel, 1989)
• Character Recognition (LeCun et al., 1989)
• Face Recognition (Mitchell)

41. ALVINNALVINN
• Steer a van down the road
– 2-layer feedforward
• using backpropagation for learning
– Raw input is 480 x 512 pixel image 15x per sec
– Color image preprocessed into 960 input units
– 4 hidden units
– 30 output units, each is a steering direction

42. Neural Network Approaches
ALVINN - Autonomous Land Vehicle In a Neural Network

43. Learning on-the-
fly
• ALVINN learned as the vehicle
traveled
– initially by observing a human
driving
– learns from its own driving by
watching for future corrections
– never saw bad driving
• didn’t know what was
dangerous, NOT correct
• computes alternate views of
the road (rotations, shifts, and
fill-ins) to use as “bad”
examples
– keeps a buffer pool of 200 pretty
old examples to avoid overfitting
to only the most recent images

44. Feed-forward vs. Interactive
Nets
• Feed-forward
– activation propagates in one direction
– We usually focus on this
• Interactive
– activation propagates forward & backwards
– propagation continues until equilibrium is reached in
the network
– We do not discuss these networks here, complex
training. May be unstable.

45. Ways of learning with an ANN
• Add nodes & connections
• Subtract nodes & connections
• Modify connection weights
– current focus
– can simulate first two
• I/O pairs:
– given the inputs, what should the output be?
[“typical” learning problem]

46. More Neural NetworkMore Neural Network
ApplicationsApplications
- May provide a model for massive parallel computation.
- More successful approach of “parallelizing” traditional
serial algorithms.
- Can compute any computable function.
- Can do everything a normal digital computer can do.
- Can do even more under some impractical assumptions.

47. Neural Network Approaches to driving
- Developed in 1993.
- Performs driving with
Neural Networks.
- An intelligent VLSI image
sensor for road following.
- Learns to filter out image
details not relevant to
driving.
Hidden layer
Output units
Input units
•Use special hardware
•ASIC
•FPGA
•analog

48. Neural Network Approaches
Hidden Units Output unitsInput Array

49. Actual Products Available
ex1. Enterprise Miner:ex1. Enterprise Miner:
- Single multi-layered feed-forward neural networks.
- Provides business solutions for data mining.
ex2. Nestor:ex2. Nestor:
- Uses Nestor Learning System (NLS).
- Several multi-layered feed-forward neural networks.
- Intel has made such a chip - NE1000 in VLSI technology.

50. Ex1. Software tool - Enterprise Miner
- Based on SEMMA (Sample, Explore, Modify, Model,
Access) methodology.
- Statistical tools include :
Clustering, decision trees, linear and logistic
regression and neural networks.
- Data preparation tools include :
Outliner detection, variable transformation, random
sampling, and partition of data sets (into training,
testing and validation data sets).

51. Ex 2. Hardware Tool - Nestor
- With low connectivity within each layer.
- Minimized connectivity within each layer results in rapid
training and efficient memory utilization, ideal for VLSI.
- Composed of multiple neural networks, each specializing
in a subset of information about the input patterns.
- Real time operation without the need of special computers
or custom hardware DSP platforms
•Software exists.

52. SummarySummary
- Neural network is a computational model that simulate
some properties of the human brain.
- The connections and nature of units determine the
behavior of a neural network.
- Perceptrons are feed-forward networks that can only
represent linearly separable functions.

53. Summary
- Given enough units, any function can be represented
by Multi-layer feed-forward networks.
- Backpropagation learning works on multi-layer
feed-forward networks.
- Neural Networks are widely used in developing
artificial learning systems.

54. ReferencesReferences
- Russel, S. and P. Norvig (1995). Artificial Intelligence - A
Modern Approach. Upper Saddle River, NJ, Prentice
Hall.
- Sarle, W.S., ed. (1997), Neural Network FAQ, part 1 of 7:
Introduction, periodic posting to the Usenet newsgroup
comp.ai.neural-nets,
URL: ftp://ftp.sas.com/pub/neural/FAQ.html

55. Eddy Li
Eric Wong
Martin Ho
Kitty Wong
Sources