Ch4: Backpropagation (BP) Werbos -> Ponker -> Rummelhart -> McClelland 。BP Architecture: Characteristics: Multilayer, feedforward, fully connected

。 Potential problems being solved by BP 1. Data translation, e.g., data compression 2. Best guess , e.g., pattern recognition, classification Example: Character recognition application a) Traditional method: Translate a 7 × 5 image to 2–byte ASCII code

Lookup table Suffer from: a. Noise, distortion, incomplete b. Time consuming

b) Recent method: Recognition-by-components Traditional approach Neural approach

4.1. BP Neural Network During training, self-organization of nodes on the intermediate layers s.t. different nodes recognize different features or their relationships. Noisy and incomplete patterns can thus be handled.

4.1.2. BP NN Learning Given examples: where Find an approximation of through learning

。 Propagate – adapt learning cycle

4.2. Generalized Delta Rule (GDR) Consider input vector Hidden layer: Net input to the jth hidden unit hidden layer jth hidden unit ith input unit bias term with jth unit Output of the jth hidden unit transfer function

Output layer: 。Update of output layer weights The error at a single output unit k, The error to be minimized where M: # output units The descent direction The learning rule where: learning rate

This error surface in the weight space is much more complex than the hyperparaboloid of the Adline.

。 Determine L: # hidden units

The weights on the output layer are updated as 。 Consider Two forms for the output functions i) Linear ii) Sigmoid or

For linear function, (A) For sigmoid function, Let (A)

。Example 1: Quadratic neurons for output nodes Compare with Net input to output node: the jth input value Output function: sigmoid Determine the weight update equations for w and v.

◎ Updates of hidden-layer weights Difficulty: Unknown outputs of the hidden-layer units Idea: Relate error E to the output of the hidden layer

: function of

Consider sigmoid output function

BPN Summary

※ The known error (or loss) on the output layer are propagated back to a hidden layer of interest to determine the weight changes on that layer

4.3. Practical Considerations 。 Principles of determining netwrok size: i) Use as few nodes as possible. If the NN fails to converge to a solution, it may need more nodes. ii) Prune the hidden nodes whose weights change very little during training 。 Principles of choosing training data i) Cover the entire domain (representative) ii) Use as many data as possible (capacity) iii) Adding noise to the input vectors (generalization)

。 Parameters: i) Initialize weights with small random values ii) Learning rate η decreases with # iterations η small slow; η large perturbation iii) Momentum technique -- Adding a fraction of the preview change, while tends to keep the weight changes going in the same direction to the weight change, iv) Perturbation – Repeat training using multiple initial weights.

4.4. Applications Dimensionality reduction: A BPN can be trained to map a set of patterns from an n-D space to an m-D space (m < n).

Data compression － video images The hidden layer represents the compressed form of the data. The output layer represents the reconstructed form of the data. Each image vector will be used as both the input and the target output.

‧ Size: NTSC: National Television Standard Code 525 × 640 = 336000 #pixels/image ‧ Strategy: Divide images into blocks, e.g., 8 × 8 = 64 pixels,  64－output layer, 16－hidden layer, 64－input layer, #nodes = 144

◎ Paint quality inspection Reflects a laser beam off the painted panel and onto a screen Poor paint: Reflected laser beam diffused ripples, orange peel, lacks shine Good paint: Relatively smooth and bright luster Closely uniform throughout its image

。 Idea

The output was to be a numerical score (1(best) -- 20(worst))