1. Supervised Training of Deep Networks
Early layers of MLP do not get trained well
Diffusion of gradient – error attenuates as it propagates to earlier layers
Leads to very slow training
Exacerbated since top couple layers can usually learn any task "pretty well" and thus the error to earlier layers drops quickly as the top layers "mostly" solve the task– lower layers never get the opportunity to use their capacity to improve results, they just do a random feature map
Need a way for early layers to do effective work
Often not enough labeled data available while there may be lots of unlabeled data
Can we use unsupervised/semi-supervised approaches to take advantage of the unlabeled data?
Deep networks tend to have more local minima problems than shallow networks during supervised training

2. Deep Learning Overview
Multiple layers build improved feature space
First layer learns 1st order features (e.g. edges…)
2nd layer learns higher order features (combinations of first layer features, combinations of edges, etc.)
In current models layers often learn in an unsupervised mode and discover general features of the input space – serving multiple tasks related to the unsupervised instances (image recognition, etc.)
Then final layer features are fed into a supervised model
Entire network is often subsequently tuned using supervised training of the entire net, using the initial weightings learned in the unsupervised phase

3. Greedy Layer-Wise Training
Train first layer using data without labels (unsupervised)
Since there are no targets at this level, labels don't help. Could also use the more abundant unlabeled data which is not part of the training set
Then freeze the first layer parameters and start training the second layer using the output of the first layer as the unsupervised input to the second layer
Repeat this for as many layers as desired
This builds our set of robust features
Use the outputs of the final layer as inputs to a supervised layer/model and train the last supervised layer(s) (leave early weights frozen)
Unfreeze all weights and fine tune the full network by training with a supervised approach, given the pre-processed weight settings
If do supervised, we may not get the benefits of building up the incrementally abstracted feature space

4. Deep Net with Greedy Layer Wise Training
Supervised
Learning
ML Model
New Feature Space
Unsupervised
Learning
Can input both new and original spaces
Original Inputs

5. Greedy Layer-Wise Training
Avoids many of the problems of trying to train a deep net in a supervised fashion
Each layer gets full learning focus in its turn since it is the only current "top" layer
Can take advantage of the unlabeled data
When you finally tune the entire network with supervised training the network weights have already been adjusted so that you are in a good error basin and just need fine tuning. This helps with problems of
Ineffective early layer learning
Deep network local minima

6. (Stacked) Auto-Encoders (I)
A type of unsupervised learning which tries to discover generic features of the data
Learn identity function by learning important sub-features (not by just passing through data)
Compression, etc.
Can use just new features in the new training set or concatenate both

7. (Stacked) Auto-Encoders (II)
Once features have been learned
Remove output layer
Train output weights with inputs and features

8. Stacked Auto-Encoders (III)
Stack many auto-encoders in succession and train them using greedy layer-wise training
Drop the decode output layer each time

9. Stacked Auto-Encoders (IV)
Do supervised training on the last layer using final features
Then do supervised training on the entire network to fine- tune all weights
Shows softmax, but could use BP or any other variation

10. Adobe – Deep Learning and Active Learning
Deep Learning Tasks
Usually best when input space is locally structured – spatial or temporal: images, language, etc. vs. arbitrary input features
Images Example: view of vision layer
Each square in the figure shows the input image that maximally activates one of 100 hidden units (auto-encoder training on 10x10 images) – detect edges
Adobe – Deep Learning and Active Learning

11. Convolutional Neural Networks (CNNs) (I)
Networks built specifically for problems with low dimensional (e.g., 2-D) local structure
Character recognition – where neighboring pixels will have high correlations and local features (edges, corners, etc.), while distant pixels (features) are un-correlated
Natural images – have the property of being stationary, meaning that the statistics of one part of the image are the same as any other part
Not so good/lousy for general learning with abstract features having no prescribed ordering

12. CNNs (II) Use three basic ideas: We look at each in turn
Local receptive fields
Shared weights, and
Pooling layers
We look at each in turn

13. Local Receptive Fields
In standard NN, nodes take input from all nodes in the previous layer
In CNNs, nodes receive take input from only a small set of nodes/features that are spatially or temporally close to each other called receptive fields (e.g., 3x3, 5x5), in the previous layer
Local receptive field slides across the entire input image. For each local receptive field, there is a different hidden neuron in the next layer
E.g., 28x28 input image and 5x5
local receptive fields -> next layer
has 24x24 neurons

14. Shared Weights
Instead of each node in the next layer having its own individual weights and bias (e.g., 5x5 weights and 1 bias), all of the hidden nodes share the same weights and bias
E.g., suppose weights and bias are such that hidden neuron picks out, say, a vertical edge in a particular local receptive field. That ability is likely useful at other places in the image: makes sense to apply same feature detector everywhere (translation invariance)
Map from input to hidden layer is a feature map
In practice, there is more than one feature map, each defined by its own set of shared weights and bias

15. Pooling Layers Generally used right after convolutional layers
Applied to each feature map
Each unit in pooling layer summarizes a region (e.g., 2x2) of the feature map (creates a condensed feature map)
Typical: max-pooling (i.e., output maximum value in region)
Checks whether a given feature is found anywhere in a region of the image; exact location not as important as rough location relative to other features; many fewer pooled features/parameters for later layers

16. CNNs (III)
Final layer of connections in the network is a fully-connected layer
Output node computes sigmoid
Convolution and pooling layers often interleaved

17. CNNs (IV) C layers are convolutions, S layers pool/sample
Often starts with fairly raw features at initial input and lets CNN discover improved feature layer for final supervised learner – eg. MLP/BP
CS 678 – Deep Learning

18. CNN Training Trained with BP but with weight tying in each feature map
Randomized initial weights through entire network
Just average the weight updates over the tied weights in feature map layers
Convolution layer
Each feature map has one weight for each input and one bias
Thus a feature map with a 5x5 receptive field would have a total of 26 weights, which are the same coming into each node of the feature map
If a convolution layer had 10 feature maps, then only a total of 260 unique weights to be trained in that layer (much less than an arbitrary deep net layer without sharing)
Pooling/Sub-sampling Layer
All elements of receptive field max’d, averaged, or summed, result multiplied by one trainable weight and a bias added, then squashed for each pooling node
If a layer had 10 pooling feature maps, then 20 unique weights to be trained
While all weights are trained, the structure of the CNN is currently usually hand crafted with trial and error.
Number of total layers, number of receptive fields, size of receptive fields, size of sub-sampling (pooling) fields, which fields of the previous layer to connect to
Typically decrease size of feature maps and increase number of feature maps for later layers

19. Deep Learning Summary Other Approaches:
Deep Belief Networks
LSTM Networks
"Google-Brain"
Sum of Products Nets
Good in structured/Markovian spaces
Problems with strong local spatial/temporal correlation: vision, speech, audio, etc.
Important research question: to what extent can we use Deep Learning in more arbitrary feature spaces?
Potential for significant improvements