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Neural Networks Chapter 2

Neural Networks Chapter 2. Joost N. Kok Universiteit Leiden. Hopfield Networks. Network of McCulloch-Pitts neurons Output is 1 iff and is -1 otherwise . Hopfield Networks. Hopfield Networks. Hopfield Networks. Hopfield Networks.

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Neural Networks Chapter 2

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  1. Neural NetworksChapter 2 Joost N. Kok Universiteit Leiden

  2. Hopfield Networks • Network of McCulloch-Pitts neurons • Output is 1 iff and is -1 otherwise

  3. Hopfield Networks

  4. Hopfield Networks

  5. Hopfield Networks

  6. Hopfield Networks • Associative Memory Problem:Store a set of patterns in such a way that when presented with a new pattern, the network responds by producing whichever of the stored patterns most closely resembles the new pattern.

  7. Hopfield Networks • Resembles = Hamming distance • Configuration space = all possible states of the network • Stored patterns should be attractors • Basins of attractors

  8. Hopfield Networks • N neurons • Two states: -1 (silent) and 1 (firing) • Fully connected • Symmetric Weights • Thresholds

  9. Hopfield Networks w13 w16 w57 -1 +1

  10. Hopfield Networks • State: • Weights: • Dynamics:

  11. Hopfield Networks • Hebb’s learning rule: • Make connection stronger if neurons have the same state • Make connection weaker if the neurons have a different state

  12. Hopfield Networks neuron 1 synapse neuron 2

  13. Hopfield Networks • Weight betweenneuron i and neuron j is given by

  14. Hopfield Networks • Opposite patterns give the same weights • This implies that they are also stable points of the network • Capacity of Hopfield Networks is limited: 0.14 N

  15. Hopfield Networks • Hopfield defines the energy of a network: E = - ½ ijSiSjwij + i Siqi • If we pick unit i and the firing rule does not change its Si, it will not change E. • If we pick unit i and the firing rule does change its Si, it will decrease E.

  16. Hopfield Networks • Energy function: • Alternative Form: • Updates:

  17. Hopfield Networks

  18. Hopfield Networks • Extension: use stochastic fire rule • Si := +1 with probability g(hi) • Si := -1 with probability 1-g(hi)

  19. 1 g(x) = 1 + e – xb Hopfield Networks • Nonlinear function: b g(x) b 0 x

  20. Hopfield Networks • Replace the binary threshold units by binary stochastic units. • Defineb = 1/T • Use “temperature” T to make it easier to cross energy barriers. • Start at high temperature where its easy to cross energy barriers. • Reduce slowly to low temperature where good states are much more probable than bad ones. A B C

  21. Hopfield Networks • Kick the network our of spurious local minima • Equilibrium: becomes time independent

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