neural networks teacher: elena marchiori r4.47 elena@cs.vu.nl assistant: kees jong s2.22...

Neural Networks

Teacher:Elena Marchiori

R4.47elena@cs.vu.nl

Assistant: Kees Jong

S2.22 cjong@cs.vu.nl

Neural Net types

Basics of neural network theory and practice for supervised and unsupervised learning.

Most popular Neural Network models:• architectures• learning algorithms• applications

Neural Networks

• A NN is a machine learning approach inspired by the way in which the brain performs a particular learning task:

– Knowledge about the learning task is given in the form of examples.

– Inter neuron connection strengths (weights) are used to store the acquired information (the training examples).

– During the learning process the weights are modified in order to model the particular learning task correctly on the training examples.

Connectionism

• Connectionist techniques (a.k.a. neural networks) are inspired by the strong interconnectedness of the human brain.

• Neural networks are loosely modeled after the biological processes involved in cognition: 1. Information processing involves many simple elements

called neurons. 2. Signals are transmitted between neurons using

connecting links. 3. Each link has a weight that controls the strength of its

signal. 4. Each neuron applies an activation function to the input

that it receives from other neurons. This function determines its output.

• Links with positive weights are called excitatory links. • Links with negative weights are called inhibitory links.

What is a neural network

• A NN is a machine learning approach inspired by the way in which the brain performs a particular learning task:– Knowledge about the learning task is given in the

form of examples.

– Inter neuron connection strengths (weights) are used to store the acquired information (the training examples).

– During the learning process the weights are modified in order to model the particular learning task correctly on the training examples.

What is a Neural Network

• A neural network is characterized by three things: 1. Its architecture: the pattern of nodes and

connections between them. 2. Its learning algorithm, or training method:

the method for determining the weights of the connections.

3. Its activation function: the function that produces an output based on the input values received by a node.

• Supervised Learning– Recognizing hand-written digits, pattern recognition,

regression.– Labeled examples

(input , desired output)– Neural Network models: perceptron, feed-forward,

radial basis function, support vector machine.• Unsupervised Learning

– Find similar groups of documents in the web, content addressable memory, clustering.

– Unlabeled examples (different realizations of the input alone)

– Neural Network models: self organizing maps, Hopfield networks.

Learning

Network architectures

• Three different classes of network architectures

– single-layer feed-forward neurons are organized– multi-layer feed-forward in acyclic layers– recurrent

• The architecture of a neural network is linked with the learning algorithm used to train

Single Layer Feed-forward

Input layerof

source nodes

Output layerof

neurons

Multi layer feed-forward

Inputlayer

Outputlayer

Hidden Layer

3-4-2 Network

Recurrent Network with hidden neuron(s): unit delay operator z-1 implies dynamic system

z-1

z-1

z-1

Recurrent network

inputhiddenoutput

Neural Network Architectures

The Neuron• The neuron is the basic information processing unit of a NN. It

consists of:

1 A set of synapses or 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

1jj xwu

j

) (u y b

The Neuron

Inputsignal

Synapticweights

Summingfunction

Bias

b

ActivationfunctionLocal

Field

vOutput

y

x1

x2

xm

w2

wm

w1

)(

Bias as extra input

Inputsignal

Synapticweights

Summingfunction

ActivationfunctionLocal

Field

vOutput

y

x1

x2

xm

w2

wm

w1

)(

w0x0 = +1

• Bias is an external parameter of the neuron. Can be modeled by adding an extra input.

bw

xwv j

m

j

j

0

0

Bias of a Neuron

• Bias b has the effect of applying an affine transformation to u

v = u + b• v is the induced field of the neuron

v

u

m

1jj xwu

j

Dimensions of a Neural Network

• Various types of neurons

• Various network architectures

• Various learning algorithms

• Various applications

Face Recognition

90% accurate learning head pose, and recognizing 1-of-20 faces

Handwritten digit recognition

A Multilayer Net for XOR

The XOR problem

A single-layer neural network cannot solve the XOR problem.

Input Output 00 -> 0 01 -> 1 10 -> 1 11 -> 0

To see why this is true, we can try to express the problem as a linear equation: aX + bY = Z

a0 + b0 = 0 a0 + b1 = 1 -> b = 1 a1 + b0 = 1 -> a = 1 a1 + b1 = 0 -> a = -b

But adding a third bit makes it doable.

Input Output 000 -> 0 010 -> 1 100 -> 1 111 -> 0

We can try to express the problem as a linear equation: aX + bY + cZ = W

a0 + b0 + c0 = 0 a0 + b1 + c0 = 1 -> b=1 a1 + b0 + c0 = 1 -> a=1 a1 + b1 + c1 = 0 -> a + b + c = 0 -> 1 + 1 + c = 0 -> c =

-2 So the equation: X + Y - 2Z = W will solve the problem.

Hidden Units

• Hidden units are a layer of nodes that are situated between the input nodes and the output nodes.

• Hidden units allow a network to learn non-linear functions.

• The hidden units allow the net to represent combinations of the input features.

• Given too many hidden units, however, a net will simply memorize the input patterns.

• Given too few hidden units, the network may not be able to represent all of the necessary generalizations.

Backpropagation Nets

• Backpropagation networks are among the most popular and widely used neural networks because they are relatively simple and powerful.

• Backpropagation was one of the first general techniques developed to train multilayer networks, which do not have many of the inherent limitations of the earlier, single-layer neural nets criticized by Minsky and Papert.

• Backpropagation networks use a gradient descent method to minimize the total squared error of the output.

• A backpropagation net is a multilayer, feedforward network that is trained by backpropagating the errors using the generalized delta rule.

Training a backpropagation net

Feedforward training of input patterns Each input node receives a signal, which is broadcast to all of the hidden units. Each hidden unit computes its activation, which is broadcast to all of the output nodes.

Backpropagation of errors Each output node compares its activation with the desired output. Based on this difference, the error is propagated back to all previous nodes.

Adjustment of weights The weights of all links are computed simultaneously based on the errors that were propagated backwards.

Terminology Input vector: X = (x1 , x2 , ..., xn )

Target vector: Y = (y1 , y2 , ..., ym )

Input unit: X i

Hidden unit: Z i

Output unit: Y i

v ij : weight on link from X i to Z j

w ij : weight on link from Z i to Y j

v0j : bias on Z j

w0j : bias on Y j

i : error correction term for output unit Y i

: learning rate

The Feedforward Stage

1. Initialize the weights with small, random values. 2. While the stopping condition is not true

For each training pair (input/output) Each input unit broadcasts its value to all of the

hidden units. Each hidden unit sums its input signals and applies

its activation function to compute its output signal.

Each hidden unit sends its signal to the output units.

Each output unit sums its input signals and applies its

activation function to compute its output signal.

Backpropagation

Adjusting the Weights

1. Each output unit updates its weights and bias: w ij (new) = w ij (old) + w ij

2. Each hidden unit updates its weights and bias: v ij (new) = v ij (old) + v ij

Check stopping conditions. Each training cycle is called an epoch. Typically,

many epochs are needed (often thousands). The weights are updated in each cycle.

The learning rate w ij (new) = j z i + w ij (old)

• The learning rate, ff, controls how big the weight changes are for each iteration.

• Ideally, the learning rate should be infinitesimally small, but then learning is very slow.

• If the learning rate is too high then the system can suffer from severe oscillations.

• You want the learning rate to be as large as possible (for fast learning) without resulting in oscillations. (0.02 is common)

An Example: One Layer

Multi-Layer

The numbers

I1 1

I2 1

W13 .1

W14 -.2

W23 .3

W24 .4

W35 .5

W45 -.4

3 .2

4 -.3

5 .4

Output!Where N= output of node

O= Activation function

= threshold value of node

Backpropagating!

How long should you train the net?

• The goal is to achieve a balance between correct responses for the training patterns and correct responses for new patterns. (That is, a balance between memorization and generalization.)

• If you train the net for too long, then you run the risk of overfitting to the training data.

• In general, the network is trained until it reaches an acceptable error rate (e.g., 95%).

• One approach to avoid overfitting is to break up the data into a training set and a training test set. The weight adjustments are based on the training set. However, at regular intervals the test set is evaluated to see if the error is still decreasing. When the error begins to increase on the test set, training is terminated.