REDES NEURONALES APRENDIZAJE Supervised Vs No SupervisedDocument Transcript
Supervised versus Unsupervised Learning
Difﬁcult to justify biologically.
Doesn’t ﬁt all situations.
Input Environment only.
Self Organising Neural Networks
The basic design of an Unsupervised Network
What they Learn
Problems with Self Organising Neural Networks
Origins: Rosenblatt’s “spontaneous learning” in per-
Important work by Fukushima, Grossberg, Kohonen,
von der Malsburg, Willshaw
Learn about regularities in the environment
Recognition — familiarity to previous inputs
Classiﬁcation — clustering
Feature Mapping — topographic mappings
Encoding — dimensionality reduction — data com-
What determines what is learnt?
von der Malsburg, C (1973). Self–organisation of orientation sensi-
tive cells in the striate cortex. Kybernetik, 14: 85–100.
Orientation tuned units.
Basic Requirements for Unsupervised Networks
Rumelhart, D. and Zipser, D. (1985). Feature discovery by competi-
tive learning. Cognitive Science, 2: 75–112.
1. Input units or Input lines.
2. Response units.
Number of units.
Units not all the same.
3. Limit the strength of units.
5 2 1 0 1 2000
Input pattern 1 0.5 0.01. Weight normalisation.
4. Allow the units to compete. “winner take all”.
Learning in the Rumelhart and Zipser Network
Winning unit learns.
Weights become more like input patterns
Normalisation by weight redistribution:
¡Û ¼ if loses on stimulus
if wins on stimulus
= 1 (0) if input is (in)active on pattern .
Ò = number of inputs active for pattern
« is the learning constant.
Example of Weight Redistribution
16 inputs; for each stimulus assume 8 inputs are
Assume that for each output unit , weights are ini-
tially normalised: ½ ½ Û ½.
« «Û if wins and is ON
¡Û «Û if wins and is OFF
All weights for wining unit decremented by «Û
Total weight from all lines decremented
ÈÛ ½, loss = total deducted from all weights
on winning unit = «
Each weight on an active line is incremented by ½
gain = total amount of weight added = ¡ ½ « «.
loss = gain, so no net change in weight.
2 classiﬁcation units — binary classiﬁcation
16 input lines
Dipole input (2/16 neighbouring inputs active)
unit 1 unit 2
Also discovers horizontal, diagonal divisions; simi-
lar result in 3-d.
System discovers spatial structure, not in architec-
Geometric representation of Learning
Problems with “Competitive Learning”
How many units?
Normalisation - biological?
Problem of dead units?
1. Leaky learning
2. Conscience mechanism
Not a magic technique. c.f. horizontal/vertical line
task (Rumelhart & Zipser, 1985).
Input space is divided up – units learn about a subset
of the input patterns.
Input space broken into groups of maximum simi-
Two sources of competition:
1. Winner-take-all mechanism
2. Resource limitation (normalisation)
w1 w2 wi wN
x1 x2 xi xN
Simple Hebbian learning:
Linear activation function.
« Û ÜÜ
Ensemble average and slow changing weights:
Û « Û ÜÜ
Û « Û ÜÜ
Û « Û
Û « Û
where is the correlation matrix:
Eigenvectors & Eigenvalues
Vector Ü viewed as a point in N dimensional space
(e.g. Ü = 1,1,1.5 ).
A Matrix as a linear transformation.
Unconstrained Hebbian Learning
Û « Û
Over a large number of patterns the eigenvector with
the largest eigenvalue will be the dominant inﬂu-
ence in weight change.
Weights change fastest in the direction of the eigen-
vector with the largest eigenvalue.
So weights tend to the principle component of the
Solutions to unbounded weights:
Oja type rule – new terms.
Simple weight decay.
Find principal components:
Principal component of data = maximal eigenvector of
the covariance matrix of the data.
Simple Hebbian learning is unstable, weights grow
Oja rule adds weight decay term:
«Ý´Ü ÝÛ µ
Several properties (p202, Hertz et al., 1991)
1. Û tends to 1.
2. Û is maximal eigenvector of .
3. Variance of the output, Ý ¾ , is maximised by Û.
¯ Decorrelate output units (via lateral inhibitory con-
nections) to get other components (Sanger).
Correlation matrices and eigenvectors
Given the simple rule:
Û Û (ignore «)
w can be rewritten in terms of the eigenvectors ( ) of
with eigenvalues :
Û ½ ½ · ¾ ¾ · Ò Ò
Û ´ ½ ½· ¾ ¾ · Ò Ò
But since :
Û ½ ½ ½ · ¾ ¾ ¾ · Ò Ò Ò
So weight derivative grows mostly in direction of eigen-
vector Ñ with largest eigenvalue Ñ
No external teacher needed.
Competition arises from “winner take all” and weight
Discovers principal features of input environment.
Output units have maximal variance.