Backpropagation

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# Backpropagation - PowerPoint PPT Presentation

Backpropagation. Linear separability constraint. 3. 1. 0. w 1. w 2. 1. 2. 0. 1. What if we add an extra layer between input and output?. 5. w 5. w 6. 3. 4. w 2. w 3. w 1. w 4. 1. 2. Same as a linear network without any hidden layer!. What if we use thresholded units?. 5.

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## PowerPoint Slideshow about 'Backpropagation' - shay-farmer

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Presentation Transcript

### Backpropagation

3

1

0

w1

w2

1

2

0

1

5

w5

w6

3

4

w2

w3

w1

w4

1

2

Same as a linear network without

any hidden layer!

5

w5

w6

If netj > thresh, aj = 1

Else aj = 0

3

4

w2

w3

w1

w4

1

2

0

1

1

1

0

0

1

0

1

0

5

If netj > 9.9, aj = 1

Else aj = 0

10

-10

0

1

3

4

10

10

5

5

Unit 3

1

2

Unit 4

• …and solutions can be viewed as “re-representing” the inputs, so as to make the mapping to the output unit learnable.
• BUT, how can we learn the correct weights instead of just setting them by hand?

But what if:

Simple delta rule:

…What function should we use for aj?

1.00

0.90

0.80

0.70

0.60

0.50

Change in activation

0.40

Activation

0.30

0.20

0.10

0.00

-10

-5

0

5

10

Net input

5

w5

w6

3

4

w2

w3

w1

w4

1

2

5

6

Targets

For outputs delta computed directly based on error.

Delta is stored at each unit and also used directly to adjust each incoming weight.

5

6

Output

3

4

For hidden units, there are no targets; “error” signal

is instead the sum of the output unit deltas. These are used to compute deltas for the hidden units, which are again stored with unit and used to directly change incoming weights.

Hidden

1

2

Deltas, and hence error signal at output, can propagate backward through network through many layers until it reaches the input.

Input

Sum-squared error:

5

w5

w6

3

4

w2

w3

w1

w4

1

2

Cross-entropy error:

5

w5

w6

3

4

w2

w3

w1

w4

1

2

1

0

0

1

3

w1

w2

1

2

2