PhD Public Defense
Breaking the Curse of Dimensionality
in Deep Neural Networks by
Learning Invariant Representations
Candidate: Leonardo Petrini
Thesis Advisor: Prof. Matthieu Wyart
Physics of Complex Systems Lab
November 17, 2023
Making Predictions from Labelled Data
(supervised learning)
Example: predict person height based on age.
age
height
Making Predictions from Data
(supervised learning)
Example: predict person height based on age.
- Tune prediction to fit the data (training);
- Use the prediction on new test data;
age
height
train point
prediction
test point
true value
predicted value
Supervised learning aims for accurate predictions on test data
Image Classification
Modern supervised learning can handle more complex tasks as recognizing images.
model
"cat"
"dog"
What's in the black box?
model
Opening the Black Box
Solving supervised learning tasks with deep neural networks.
depth
neurons
?
input
output
model
CAT
DOG
Training Neural Networks
CAT
DOG
Training Neural Networks
CAT
DOG
Training Neural Networks
CAT
DOG
Training Neural Networks
CAT
DOG
Training Neural Networks
repeat for thousands of images
Training Neural Networks
Testing on unseen images
CAT
DOG
- It learned the task
- Performance: fraction of correct predictions on test images, can be close to 100%!!
new image
Surprising as this is a complicated task for computers to solve!
An Image is a Table of Numbers
81
81
81
81
81
81
81
81
81
81
81
81
74
72
70
70
65
62
32
34
35
35
43
44
22
22
22
21
21
21
21
21
21
21
21
21
An Image is a Table of Numbers
pixel
- The computer just sees a table of numbers, task looks harder now!
- How do we compare these tables?
0 100
One number can be represented on a line
(one dimension)
81
still a dog?
Images as d−dimensional vectors
- Represent images as points in space:
- The computer just sees a table of numbers, task looks harder now!
- How do we compare these tables?
Images as d−dimensional vectors
- Represent images as points in space:
0 100
Two numbers on a square
(two dimensions)
(81, 81)
100
- Images can be made of a million pixels.
- How do we represent one million dim space?
- The computer just sees a table of numbers, task looks harder now!
- How do we compare these tables?
Images as d−dimensional vectors
0 100
Three numbers on a cube
(three dimensions)
(81, 81, 74)
100
- Represent images as points in space:
- We cannot represent high-dimensional spaces;
- Still, we can study some of their properties.
The Curse of Dimensionality
as the dimensionality increases, images are further and further apart
- When testing generalization, the new image could be very far away from all the ones in the training set!
- For each new test point to be at distance ϵ from a training point we need a number of training points P exponential in the dimension!!
- To be learnable, real data must be highly structured!
- How to characterize this structure?
Understanding a Visual Scene
- We want to identify properties that give structure to the space of images
- How do we humans solve an image classification task?
Step 1: Locate the object
usually pixels at the border do not matter for recognizing the object
Step 2: hierarchically recognize edges → parts → full object
lines → textures → paws, eyes etc. → head etc. → dog
Understanding a Visual Scene
- We want to identify properties that give structure to the space of images
- How do we humans solve an image classification task?
Understanding a Visual Scene
- How do we humans solve an image classification task?
- New image, and now?
Still a dog, though many things changed
- it moved
- legs etc. in different
relative positions - ears realized differently
Bottomline: many irrelevant details for solving the task
Invariances give structure to real data
- Irrelevant details are variations in the input that do not change the label: we call them invariances
pixels here can be of different colors without affecting the class
position does not affect the class
Bottomline: many irrelevant details for solving the task
- Do neural networks understand these invariances?
- How to test it?
"Breaking the Curse of Dimensionality in Deep Neural Networks
by Learning Invariant Representations"
Thesis Title and Outline
- Irrelevant pixels → linear invariance;
- Irrelevance of exact feature positions → deformation invariance;
- Hierarchical structure → synonymic invariance;
Linear Invariance
- This first invariance can be modeled as: x∼N(0,1) f∗(x)=g(x∥)
- In our work, we show that simple neural networks are able to learn this invariance
label:
input:
- Weights align in the relevant direction
- We can predict how many datapoints are needed to solve this kind of tasks.
Neural networks perform well as they can learn linear invariance
- In real data, features only occupy a small
portion of the image frame.
- Consequently, the frame can be deformed, and
relevant features be moved, without altering the content (deformation invariance).
- Hypothesis: neural networks performance related to how good they are at understanding this property.
Bruna and Mallat '13, Mallat '16
Deformation Invariance
Can we test this hypothesis?
Measuring deformation invariance
Invariance measure: relative stability
(normalized such that is =1 if no diffeo stability)
we introduced a model to generate deformations of controlled magnitude
Correlation between Deformation Invariance and Performance?
- Rf∼1 at initialization for all arch.
- Modern neural networks learn deformation invariance
more invariant
initialization: Rf∼1
more performant
Suggest that understanding deformation invariance is crucial for solving image classification
Learning Hierarchical Tasks with Deep Neural Networks
...
dog
face
paws
eyes
nose
mouth
ear
edges
Hard to formally characterize the hierarchical structure in real tasks like images or text:
Physicist approach:
Introduce a simplified model of data
Simple Model of Hierarchical Data
classes:
high-level
features:
etc...
low-level
features:
etc...
synonyms
- Invariance of this model is with respect to exchanges of synonyms
Simple Model of Hierarchical Data
- Having defined the rules we can sample datapoints
- Crucial to make the task non-trivial: low-level features are shared
- Crucial to make the task solvable: there exist correlations between low-level features and labels (gain info about class by observing low level features)
start from class:
intermediate
representations
inputs
Results on Hierarchical Data
- Deep neural networks are able to solve hierarchical tasks without incurring in the curse of dimensionality (sample complexity is poly(d))
- They do that by learning the hierarchical structure of the task: recognize synonyms
- They recognize synonyms by exploiting input-output correlations
- This allows us to predict the number of training points
needed to learn depending on the hierarchical structure
(num classes, num levels, num features etc...)
synonyms
original
rescaled x−axis
number of training points
Conclusions
- How does deep learning solve high-dimensional tasks like image classification?
- We identified properties that give structure to real data:
Invariance to:
- Irrelevant pixels
- Exact features position
- Exchange of synonyms
"dog"
DL model
- We have shown that deep neural networks are able to perform well as they can learn these invariances
Work done in collaboration with:
Matthieu Wyart, Francesco Cagnetta, Mario Geiger, Alessandro Favero, Umberto Tomasini, Jonas Paccolat, Eric Vanden-Eijden, Kevin Tyloo
Thank you PCSL for these nice years :)
Thank you!
Grazie!
Merci !
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