2. WHAT ARE NEURAL
NETWORKS?
• In machine learning and cognitive
science, artificial neural networks (ANNs)
are a family of models inspired by biological
neural networks (the central nervous
systems of animals, in particular the brain)
and are used to estimate or approximate
functions that can depend on a large
number of inputs and are generally
unknown. The brain consists of a densely
interconnected set of nerve cells, or basic
information-processing units, called neurons.
3. Neural Networks in the Brain
Biological Neural Network
In human brain a neuron is a information processing unit which
receives several input signals from the environment, computes a new
activation level and sends an output signal to the other neurons or
body muscles through output links.
4. • Neural networks exhibit plasticity.
• In response to the stimulation
pattern, neurons demonstrate long-
term changes in the strength of their
connections.
• Neurons also can form new
connections with other neurons. Even
entire collections of neurons may
sometimes migrate from one place
to another.
• These mechanisms form the basis for
learning in the brain.
5. MACHINE LEARNING
• In general, machine learning involves
adaptive mechanisms that enable
computers to learn from experience,
learn by example and learn by analogy.
• Learning capabilities can improve the
performance of an intelligent system
over time.
• Machine learning mechanisms form the
basis for adaptive systems.
6. WHY MACHINE LEARNING?
• The techniques that we’ve seen before relies on expert knowledge to set the rules.
Once the rules are set, the decision making is automated.
• What happens if the rules become obsolete or new information is gathered?
• The change, then needs to happen at a very basic level and needs to be done
manually.
• ANNs attempt to automate the process.
• The objective is to come up with a model to predict a set of outputs Y <y1, y2,…, yn>
from a given set of inputs X <x1, x2,…, xm> given training dataset with records of the
form (X, Y).
• The result must be a function f(X) that approximates Y for values of X not in the
dataset.
7. THE PERCEPTRON
As like as human brain, in ANN the PERCEPTRON is the simplest form of a
neural network. It consists of a single “neuron” which computes an output
function by assigning weights to each of the links to the n parameters.
8. How does the Perceptron learn from experience?
Weights(w1,w2….) are assigned to inputs of
perceptron initially in the range [-0.5,0.5] and
then updated to obtain the output consistent
with the training examples. Thus weights
Are updated and summation of these weights
is calculated in linear combiner at each
training level.
10. UPDATE RULES
• Error function for pth training example:
• e(p) = Yd(p) – Y(p); where p = 1, 2, 3, . . .
• Update:
• wi(p+1) = wi(p) + α × xi(p) × e(p);
• where α is the learning rate, a positive constant less than unity
• Only works on Linearly Separable data.
13. BACKPROPAGATION
• The indices i, j and k here refer to neurons in the input, hidden
and output layers, respectively.
• Input signals, x1, x2, . . . , xn, are propagated through the network
from left to right, and error signals, e1, e2, . . .,el, from right to left.
The symbol wij denotes the weight for the connection between
neuron i in the input layer and neuron j in the hidden layer, and
the symbol wjk the weight between neuron j in the hidden layer
and neuron k in the output layer.
The error signal at the output of neuron
k at iteration p is defined by.
Update rule
16. IMPROVEMENTS
A multilayer network, in general, learns much faster when
the sigmoidal activation function is represented by a
hyperbolic tangent:
We also can accelerate training by including a momentum
term in the earlier expression for delta, according to the
observations made in Watrous (1987) and Jacobs (1988),
the inclusion of momentum in the back-propagation
algorithm has a stabilising effect on training:
17. LINKS
• Intro
• Forward Propagation [until 3:45]
• Gradient Descent [Cost, Curse of Dimensionality not covered]
• Backpropagation
18. SELF-ORGANISING MAPS
(LEARNING WITHOUT A ‘TEACHER’)
• Hebb’s Law:
• If two neurons on either side of a connection are activated synchronously,
then the weight of that connection is increased.
• If two neurons on either side of a connection are activated asynchronously,
then the weight of that connection is decreased.
Forgetting Factor (To allow for the
possibility of decreasing connection
strength):
19. HOW TO CHOOSE THE FORGETTING
FACTOR
• Forgetting factor specifies the weight decay in a single learning cycle. It usually falls in
the interval between 0 and 1.
• If the forgetting factor is 0, the neural network is capable only of strengthening its
synaptic weights, and as a result, these weights grow towards infinity. On the other hand,
if the forgetting factor is close to 1, the network remembers very little of what it learns.
• Therefore, a rather small forgetting factor should be chosen, typically between 0.01 and
0.1, to allow only a little ‘forgetting’ while limiting the weight growth.
