Download Lecture 9

Bioinspired Computing
Lecture 9
Artificial Neural Networks:
Feed-Forward ANNs
Netta Cohen
In Lecture 6…
We introduced artificial neurons, and saw that they can
perform some logical operations that can be used to solve
limited classification problems. We also implemented our
first learning algorithm for an artificial neuron.
Today
• We will build on the single neuron’s simplicity to achieve
immense richness at the network level.
• We will examine the simplest architecture of feed-forward
neural networks and generalise the delta-learning rule to
these multi-layer networks.
• We look at simple applications and
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MP neuron reminder
x1
..
..
xn
w1

output
wn
MP neuron (aka single layer perceptron)
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The Multi-Layer Network
output
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Activation Flows Forward
Hidden
Input Units
Output Units
Units
Error Propagates Backwards
0
input
Step thresholds
(which only allow “on”
or “off” responses)
are replaced by
smooth sigmoidal
activation functions
that are more
informative… 4
Backprop Walk-Through…
Which of the
weights should be
changed, and by
how much?
Activation Flows Forward




? 
Units
Error Propagates Backwards



Target
OutputOutput
Units
Input Units
Hidden
We need to know
which weights
contributed most to
the error…
Once the error is
propagated back,
we can solve the
credit assignment
problem…
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Some definitions & notation…
• Binary inputs enter the network through an input layer.
For training purposes, inputs are taken from a training set,
for which the desired outputs are known.
• Neurons (nodes) are arranged in hidden layers & transmit
their outputs forward to the output layer.
• At each output node, the error:
 = desired output - actual output.
Let desired output = d &
Let actual output = o so  =d-o.
Hidden

Units
j
k
z
• Label hidden layers, e.g. a,…i,j,k,…;
label output layer z. Denote weight
on a node in layer k by wjk
• After each training epoch, adjust
weights by wjk.
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Backpropagation in detail
• Initialise weights.
• Pick rate parameter r.
• Until performance is satisfactory:
- Pick an input from the training set
- Run the input through the network & calculate the output.
- For each output node, compare actual output with
desired output to find any discrepancies.
- Back-propagate the error to the last hidden layer of
neurons (say layer k) as follows:
k 
w
kz
z
oz (1  oz )  z
and repeat for each
node in layer k.
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Backpropagation in detail (cont.)
- Continue to back-propagate error, one layer at a time
- Given the errors, compute the corresponding weight
changes. For instance, for a node in layer j:
wi  j  
r oi o j (1  o j ) j
- Repeat for different inputs,
while summing over weight changes for each node.
- Update the network.
• Halting criterion: typically training stops when a stable
minimum is reached in the weight changes, or else when
the errors reach an acceptable value (say under 0.1).
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Backprop Walk-Through
(take 2)
Activation Flows Forward

Input Units





Units
j

k
Target
OutputOutput
Units
Hidden
? 
z =dz - oz
k 
w
kz
oz (1  oz )  z
j k
ok (1  ok )  k
z
j 
w
k
w j k  
r o j ok (1  ok ) k
Iterate until trained...
Error Propagates Backwards
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Hidden neurons for curve-fitting
output
hidden
bias
bias
input
bias
Each hidden unit can be used to represent some feature in our
model of the data. Hidden units can be added for additional
features. With enough units (and layers), a nnet can fit arbitrarily
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complex shapes and curves.
after http://users.rsise.anu.edu.au/~nici/teach/NNcourse/
How do we train a network?
How to choose the learning rate?
One solution: adaptive learning rate The longer we train, the more fine tuned
the training, and the slower the rate.
How often to update the weights?
“batch” learning: the entire training set is run through before
updating weights.
“online” learning: weights updated with every input sample.
Faster convergence possible, but not guaranteed.
(Note: Randomise input order for each epoch of training).
How to avoid under- and overfitting? Under- and Over-fitting
might occur when the network
size or configuration does not
match the complexity of the
problem at hand.
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How do we train a network (cont.)
Growing and Pruning:
growing algorithm:
• Start with only one hidden unit
• If training results in too large an error, add another hidden node.
• Continue training & growing the network, until no more
improvement is achieved.
Pruning:
start with a large network & successively remove nodes until
an optimal architecture is found. Neurons are assessed for
their relative weight in the net & least significant units are
removed. Examples of pruning techniques: “optimal brain
damage” and “optimal brain surgeon” reflect difficulty in
identifying least significant units.
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How do we train a network (cont.)
Weight decay: Extraneous curvature often accompanies
overfitting. Areas with large curvature typically require
large weights. Penalising large weights can smooth out
the fit. Thus, weight decay helps avoid over-fitting.
Training with noise: Add a small random number to each
input, so each epoch will look a little different, and the
neural net will not gain by overfitting.
Validation sets: A good way to know when to stop training
the net (i.e. before overfitting) is by splitting the data into a
training set and a validation set. Every once in a while, test
the network on the validation set. Do not alter the weights!
Once the network performs well on both training and
validation sets, stop.
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Pros and Cons
Feed-forward ANNs can overcome the problem of
linear separability: Given enough hidden neurons, a
feed-forward network can perform any discrimination
over its inputs.
After a period of training, ANNs can automatically
generalise what they learn to new input patterns that
they have not yet been exposed to.
ANNs are able to tolerate noisy inputs, or faults in
their architecture, because each neuron contributes
to a parallel distributed process. When neurons fail,
or inputs are partially corrupted, ANNs degrade
gracefully.
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Pros and Cons (cont.)
However, unlike the single-unit, the learning algorithm
is not guaranteed to find the best set of weight. It may
gets stuck at a sub-optimal configuration.
Backprop is a form of supervised learning: a “teacher”
with all the correct answer must be present, and many
examples must be given.
Also, unlike Hebbian learning, there is no evidence
that backprop takes place in the brain.
Feed-forward ANNs are powerful but not entirely
natural pattern recognition & categorisation devices…
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NETtalk
An early success for feed-forward ANNs
In 1987 Sejnowski & Rosenberg built a large three-layer
perceptron that learned to pronounce English words.
The net was presented with seven consecutive
characters (e.g., “_a_cat_”) simultaneously as input.
NETtalk learned to pronounce the phoneme associated
with the central letter (“c” in this example)
NETtalk achieved a 90% success rate during training.
When tested on a set of novel inputs that it had not
seen during training, NETtalk’s performance remained
steady at 80%-87%.
How did NETtalk work?
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The NETtalk Network
teacher
target output
/k/
26 output units
80 hidden units
7 groups of
29 input units
_
a
(after Hinton, 1989)
_
c
target letter
a
t
_
7 letters of
text input
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NETtalk’s Learning
Initially (with random weights) NETtalk babbled
incoherently when presented with English input.
As back-propagation gradually altered the weights the
target phoneme was produced more and more often.
As NETtalk learned pronunciation (e.g., the “a” sound in
cat), it generalised this knowledge to other similar inputs:
• Sometimes this generalisation is useful
– producing the same sound when it saw the “a” in bat
• Sometimes it is inappropriate
– producing the same sound when it saw the “a” in
mate
After repeated training, NETtalk refined its generalisation,
learning to use the context surrounding a letter to correctly
influence how the letter was pronounced.
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NETtalk’s Behaviour
After learning, NETtalk’s “knowledge” of pronunciation
behaves very much like our own in some respects:
• NETtalk can generalise its knowledge to new inputs
• NETtalk can cope with internal noise & corrupted inputs
• When NETtalk fails, its performance degrades gracefully
NETtalk achieves these useful abilities automatically.
In contrast, a programmer would have to work very hard
to equip a standard database with them.
• NETtalk’s knowledge is robust and flexible
• A database is fragile and brittle
What does NETtalk’s knowledge look like?
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NETtalk’s Hidden Unit Subspaces
Each hidden neuron in the net
is used to detect a different
feature of the input.
These features were then used
to divide up the input space into
useful regions.
By detecting which regions an
input falls within, the net can tell
whether it should return 1 or
return 0.
NETtalk uses the same trick.
It uses the hidden units to
detect 79 different features…
In other words, its weights
divide its input space into 79
regions
There are 79 regions because
there are 79 English letter-tophoneme relationships.
Examining the weights allows
us to cluster these
features/regions, grouping
similar ones together…
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NETtalk’s Knowledge
How sophisticated is NETtalk’s knowledge?
Does NETtalk possess the concept of a “vowel” or a “\k\”?
No – NETtalk can only use its knowledge in a fixed and
limited way.
Philosophers have imagined that concepts resemble
Prolog propositions.
• They are distinct, general-purpose, logical, and symbolic.
They represent facts in the same way that English
sentences do and can enter into any kind of reasoning or
logic. They are part of the language of thought…
In contrast, NETtalk’s knowledge is more like a skill:
• Muddled together, special purpose, not logical or symbolic.
More on this distinction later…
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A Vision Application
Hubert & Wiesel’s work on cat retinae has inspired a class of
ANNs that are used for sophisticated image analysis.
Neurons in the retina are arranged in large arrays, and each
has its own associated receptive field. How does this
arrangement enable pattern formation? One key component
is edge detection based on lateral inhibition.
An ANN can do the same thing:
A pattern of light falls across an array of neurons
that each inhibit their right-hand neighbour.
Only neurons along the left-hand dark-light
boundary escape inhibition.
Lateral inhibition such as this is characteristic of
natural retinal networks.
Now let the receptive fields of different neurons be coarse grained, with
large overlaps between adjacent neurons. What advantage is gained?
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Distributed Representations
In the examples today, nnets learn to represent the
information in a training set by distributing it across a set
of simple connected neuron-like units.
Some useful properties of distributed representations:
• they are robust to noise
• they degrade gracefully
• they are content addressable
• they allow automatic completion or repair of input
patterns
• they allow automatic generalisation from input patterns
• they allow automatic clustering of input patterns
In many ways, this form of information processing
resembles that carried out by real nervous systems. 23
Distributed Representations
However, distributed representations are quite hard for us
to understand, visualise or build by hand.
To aid our understanding we have developed various
ideas:
• the partitioning of the input space
• the clustering of the input data
• the formation of feature detectors
• the characterisation of hidden unit subspaces
• etc.
To build distributed representations automatically, ANNs
resort to learning algorithms such as backprop.
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Problems
ANNs often depart from biological reality:
Supervision: Real brains cannot rely on a supervisor
to teach them, nor are they free to self-organise.
Training vs. Testing: This distinction is an artificial one.
Temporality: Real brains are continuously engaged with
their environment, not exposed to a series of
disconnected “trials”.
Architecture: Real neurons and the networks that they
form are far more complicated than the artificial neurons
and simple connectivity that we have discussed so far.
Does this matter? If ANNs are just biologically inspired
tools, no, but if they are to model mind or life-like
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systems, the answer is maybe.
Problems Problems
Fodor & Pylyshyn raise a second, deeper problem,
objecting to the fact that, unlike classical AI systems,
distributed representations have no combinatorial
syntactic structure.
Cognition requires a language of thought. Languages
are structured syntactically. If ANNs cannot support
syntactic representations, they cannot support cognition.
F&P’s critique is perhaps not a mortal blow, but is a
severe challenge to the naive ANN researcher…
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Next Lecture on this topic…
• More neural networks…
• More learning algorithms...
• More distributed representations...
• How neural networks deal with temporality.
Reading
• Follow the links in today’s slides.
• In particular, much of today was based on
http://www.idsia.ch/NNcourse/intro.html
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