Chapter 1
Introduction
Inventors have long dreamed of creating machines that think. This desire dates
back to at least the time of ancient Greece. The mythical figures Pygmalion,
Daedalus, and Hephaestus may all be interpreted as legendary inventors, and
Galatea, Talos, and Pandora may all be regarded as artificial life (Ovid and Martin,
2004; Sparkes, 1996; Tandy, 1997).
When programmable computers were first conceived, people wondered whether
such machines might become intelligent, over a hundred years before one was
built (Lovelace, 1842). Today,
artificial intelligence
(AI) is a thriving field with
many practical applications and active research topics. We look to intelligent
software to automate routine labor, understand speech or images, make diagnoses
in medicine and support basic scientific research.
In the early days of artificial intelligence, the field rapidly tackled and solved
problems that are intellectually difficult for human beings but relatively straight-
forward for computers—problems that can be described by a list of formal, math-
ematical rules. The true challenge to artificial intelligence proved to be solving
the tasks that are easy for people to perform but hard for people to describe
formally—problems that we solve intuitively, that feel automatic, like recognizing
spoken words or faces in images.
This book is about a solution to these more intuitive problems. This solution is
to allow computers to learn from experience and understand the world in terms of
a hierarchy of concepts, with each concept defined through its relation to simpler
concepts. By gathering knowledge from experience, this approach avoids the need
for human operators to formally specify all the knowledge that the computer needs.
The hierarchy of concepts enables the computer to learn complicated concepts by
building them out of simpler ones. If we draw a graph showing how these concepts
1
CHAPTER 1. INTRODUCTION
are built on top of each other, the graph is deep, with many layers. For this reason,
we call this approach to AI deep learning.
Many of the early successes of AI took place in relatively sterile and formal
environments and did not require computers to have much knowledge about
the world. For example, IBM’s Deep Blue chess-playing system defeated world
champion Garry Kasparov in 1997 (Hsu, 2002). Chess is of course a very simple
world, containing only sixty-four locations and thirty-two pieces that can move
in only rigidly circumscribed ways. Devising a successful chess strategy is a
tremendous accomplishment, but the challenge is not due to the difficulty of
describing the set of chess pieces and allowable moves to the computer. Chess
can be completely described by a very brief list of completely formal rules, easily
provided ahead of time by the programmer.
Ironically, abstract and formal tasks that are among the most difficult mental
undertakings for a human being are among the easiest for a computer. Computers
have long been able to defeat even the best human chess player but only recently
have begun matching some of the abilities of average human beings to recognize
objects or speech. A person’s everyday life requires an immense amount of
knowledge about the world. Much of this knowledge is subjective and intuitive,
and therefore difficult to articulate in a formal way. Computers need to capture
this same knowledge in order to behave in an intelligent way. One of the key
challenges in artificial intelligence is how to get this informal knowledge into a
computer.
Several artificial intelligence projects have sought to hard-code knowledge
about the world in formal languages. A computer can reason automatically about
statements in these formal languages using logical inference rules. This is known as
the
knowledge base
approach to artificial intelligence. None of these projects has
led to a major success. One of the most famous such projects is Cyc (Lenat and
Guha, 1989). Cyc is an inference engine and a database of statements in a language
called CycL. These statements are entered by a staff of human supervisors. It is an
unwieldy process. People struggle to devise formal rules with enough complexity
to accurately describe the world. For example, Cyc failed to understand a story
about a person named Fred shaving in the morning (Linde, 1992). Its inference
engine detected an inconsistency in the story: it knew that people do not have
electrical parts, but because Fred was holding an electric razor, it believed the
entity “FredWhileShaving” contained electrical parts. It therefore asked whether
Fred was still a person while he was shaving.
The difficulties faced by systems relying on hard-coded knowledge suggest
that AI systems need the ability to acquire their own knowledge, by extracting
2
CHAPTER 1. INTRODUCTION
patterns from raw data. This capability is known as
machine learning
. The
introduction of machine learning enabled computers to tackle problems involving
knowledge of the real world and make decisions that appear subjective. A simple
machine learning algorithm called
logistic regression
can determine whether to
recommend cesarean delivery (Mor-Yosef et al., 1990). A simple machine learning
algorithm called naive Bayes can separate legitimate e-mail from spam e-mail.
The performance of these simple machine learning algorithms depends heavily
on the
representation
of the data they are given. For example, when logistic
regression is used to recommend cesarean delivery, the AI system does not examine
the patient directly. Instead, the doctor tells the system several pieces of relevant
information, such as the presence or absence of a uterine scar. Each piece of
information included in the representation of the patient is known as a
feature
.
Logistic regression learns how each of these features of the patient correlates with
various outcomes. However, it cannot influence how features are defined in any
way. If logistic regression were given an MRI scan of the patient, rather than
the doctor’s formalized report, it would not be able to make useful predictions.
Individual pixels in an MRI scan have negligible correlation with any complications
that might occur during delivery.
This dependence on representations is a general phenomenon that appears
throughout computer science and even daily life. In computer science, operations
such as searching a collection of data can proceed exponentially faster if the collec-
tion is structured and indexed intelligently. People can easily perform arithmetic
on Arabic numerals but find arithmetic on Roman numerals much more time
consuming. It is not surprising that the choice of representation has an enormous
effect on the performance of machine learning algorithms. For a simple visual
example, see figure 1.1.
Many artificial intelligence tasks can be solved by designing the right set of
features to extract for that task, then providing these features to a simple machine
learning algorithm. For example, a useful feature for speaker identification from
sound is an estimate of the size of the speaker’s vocal tract. This feature gives a
strong clue as to whether the speaker is a man, woman, or child.
For many tasks, however, it is difficult to know what features should be
extracted. For example, suppose that we would like to write a program to detect
cars in photographs. We know that cars have wheels, so we might like to use the
presence of a wheel as a feature. Unfortunately, it is difficult to describe exactly
what a wheel looks like in terms of pixel values. A wheel has a simple geometric
shape, but its image may be complicated by shadows falling on the wheel, the sun
glaring off the metal parts of the wheel, the fender of the car or an object in the
3
CHAPTER 1. INTRODUCTION
Figure 1.1: Example of different representations: suppose we want to separate two
categories of data by drawing a line between them in a scatterplot. In the plot on the left,
we represent some data using Cartesian coordinates, and the task is impossible. In the plot
on the right, we represent the data with polar coordinates and the task becomes simple to
solve with a vertical line. (Figure produced in collaboration with David Warde-Farley.)
foreground obscuring part of the wheel, and so on.
One solution to this problem is to use machine learning to discover not only
the mapping from representation to output but also the representation itself.
This approach is known as
representation learning
. Learned representations
often result in much better performance than can be obtained with hand-designed
representations. They also enable AI systems to rapidly adapt to new tasks, with
minimal human intervention. A representation learning algorithm can discover a
good set of features for a simple task in minutes, or for a complex task in hours to
months. Manually designing features for a complex task requires a great deal of
human time and effort; it can take decades for an entire community of researchers.
The quintessential example of a representation learning algorithm is the
au-
toencoder
. An autoencoder is the combination of an
encoder
function, which
converts the input data into a different representation, and a
decoder
function,
which converts the new representation back into the original format. Autoencoders
are trained to preserve as much information as possible when an input is run
through the encoder and then the decoder, but they are also trained to make the
new representation have various nice properties. Different kinds of autoencoders
aim to achieve different kinds of properties.
When designing features or algorithms for learning features, our goal is usually
to separate the
factors of variation
that explain the observed data. In this
4
CHAPTER 1. INTRODUCTION
context, we use the word “factors” simply to refer to separate sources of influence;
the factors are usually not combined by multiplication. Such factors are often not
quantities that are directly observed. Instead, they may exist as either unobserved
objects or unobserved forces in the physical world that affect observable quantities.
They may also exist as constructs in the human mind that provide useful simplifying
explanations or inferred causes of the observed data. They can be thought of as
concepts or abstractions that help us make sense of the rich variability in the data.
When analyzing a speech recording, the factors of variation include the speaker’s
age, their sex, their accent and the words they are speaking. When analyzing an
image of a car, the factors of variation include the position of the car, its color,
and the angle and brightness of the sun.
A major source of difficulty in many real-world artificial intelligence applications
is that many of the factors of variation influence every single piece of data we are
able to observe. The individual pixels in an image of a red car might be very close
to black at night. The shape of the car’s silhouette depends on the viewing angle.
Most applications require us to disentangle the factors of variation and discard the
ones that we do not care about.
Of course, it can be very difficult to extract such high-level, abstract features
from raw data. Many of these factors of variation, such as a speaker’s accent,
can be identified only using sophisticated, nearly human-level understanding of
the data. When it is nearly as difficult to obtain a representation as to solve the
original problem, representation learning does not, at first glance, seem to help us.
Deep learning
solves this central problem in representation learning by intro-
ducing representations that are expressed in terms of other, simpler representations.
Deep learning enables the computer to build complex concepts out of simpler con-
cepts. Figure 1.2 shows how a deep learning system can represent the concept of
an image of a person by combining simpler concepts, such as corners and contours,
which are in turn defined in terms of edges.
The quintessential example of a deep learning model is the feedforward deep
network, or
multilayer perceptron
(MLP). A multilayer perceptron is just a
mathematical function mapping some set of input values to output values. The
function is formed by composing many simpler functions. We can think of each
application of a different mathematical function as providing a new representation
of the input.
The idea of learning the right representation for the data provides one per-
spective on deep learning. Another perspective on deep learning is that depth
enables the computer to learn a multistep computer program. Each layer of the
representation can be thought of as the state of the computer’s memory after
5
CHAPTER 1. INTRODUCTION
Visible layer
(input pixels)
1st hidden layer
(edges)
2nd hidden layer
(corners and
contours)
3rd hidden layer
(object parts)
CAR PERSON ANIMAL
Output
(object identity)
Figure 1.2: Illustration of a deep learning model. It is difficult for a computer to understand
the meaning of raw sensory input data, such as this image represented as a collection
of pixel values. The function mapping from a set of pixels to an object identity is very
complicated. Learning or evaluating this mapping seems insurmountable if tackled directly.
Deep learning resolves this difficulty by breaking the desired complicated mapping into a
series of nested simple mappings, each described by a different layer of the model. The
input is presented at the
visible layer
, so named because it contains the variables that
we are able to observe. Then a series of
hidden layers
extracts increasingly abstract
features from the image. These layers are called “hidden” because their values are not given
in the data; instead the model must determine which concepts are useful for explaining
the relationships in the observed data. The images here are visualizations of the kind
of feature represented by each hidden unit. Given the pixels, the first layer can easily
identify edges, by comparing the brightness of neighboring pixels. Given the first hidden
layer’s description of the edges, the second hidden layer can easily search for corners and
extended contours, which are recognizable as collections of edges. Given the second hidden
layer’s description of the image in terms of corners and contours, the third hidden layer
can detect entire parts of specific objects, by finding specific collections of contours and
corners. Finally, this description of the image in terms of the object parts it contains can
be used to recognize the objects present in the image. Images reproduced with permission
from Zeiler and Fergus (2014).
6
CHAPTER 1. INTRODUCTION
executing another set of instructions in parallel. Networks with greater depth can
execute more instructions in sequence. Sequential instructions offer great power
because later instructions can refer back to the results of earlier instructions. Ac-
cording to this view of deep learning, not all the information in a layer’s activations
necessarily encodes factors of variation that explain the input. The representation
also stores state information that helps to execute a program that can make sense
of the input. This state information could be analogous to a counter or pointer
in a traditional computer program. It has nothing to do with the content of the
input specifically, but it helps the model to organize its processing.
There are two main ways of measuring the depth of a model. The first view is
based on the number of sequential instructions that must be executed to evaluate
the architecture. We can think of this as the length of the longest path through
a flow chart that describes how to compute each of the model’s outputs given
its inputs. Just as two equivalent computer programs will have different lengths
depending on which language the program is written in, the same function may
be drawn as a flowchart with different depths depending on which functions we
allow to be used as individual steps in the flowchart. Figure 1.3 illustrates how this
choice of language can give two different measurements for the same architecture.
x
1
x
1
σ
w
1
w
1
×
x
2
x
2
w
2
w
2
×
+
Element
Set
+
×
σ
xx
ww
Element
Set
Logistic
Regression
Logistic
Regression
Figure 1.3: Illustration of computational graphs mapping an input to an output where
each node performs an operation. Depth is the length of the longest path from input to
output but depends on the definition of what constitutes a possible computational step.
The computation depicted in these graphs is the output of a logistic regression model,
σ
(
w
T
x
), where
σ
is the logistic sigmoid function. If we use addition, multiplication and
logistic sigmoids as the elements of our computer language, then this model has depth
three. If we view logistic regression as an element itself, then this model has depth one.
7
CHAPTER 1. INTRODUCTION
Another approach, used by deep probabilistic models, regards the depth of a
model as being not the depth of the computational graph but the depth of the
graph describing how concepts are related to each other. In this case, the depth
of the flowchart of the computations needed to compute the representation of
each concept may be much deeper than the graph of the concepts themselves.
This is because the system’s understanding of the simpler concepts can be refined
given information about the more complex concepts. For example, an AI system
observing an image of a face with one eye in shadow may initially see only one
eye. After detecting that a face is present, the system can then infer that a second
eye is probably present as well. In this case, the graph of concepts includes only
two layers—a layer for eyes and a layer for faces—but the graph of computations
includes 2
n
layers if we refine our estimate of each concept given the other
n
times.
Because it is not always clear which of these two views—the depth of the
computational graph, or the depth of the probabilistic modeling graph—is most
relevant, and because different people choose different sets of smallest elements
from which to construct their graphs, there is no single correct value for the
depth of an architecture, just as there is no single correct value for the length of
a computer program. Nor is there a consensus about how much depth a model
requires to qualify as “deep.” However, deep learning can be safely regarded as the
study of models that involve a greater amount of composition of either learned
functions or learned concepts than traditional machine learning does.
To summarize, deep learning, the subject of this book, is an approach to AI.
Specifically, it is a type of machine learning, a technique that enables computer
systems to improve with experience and data. We contend that machine learning
is the only viable approach to building AI systems that can operate in complicated
real-world environments. Deep learning is a particular kind of machine learning
that achieves great power and flexibility by representing the world as a nested
hierarchy of concepts, with each concept defined in relation to simpler concepts, and
more abstract representations computed in terms of less abstract ones. Figure 1.4
illustrates the relationship between these different AI disciplines. Figure 1.5 gives
a high-level schematic of how each works.
1.1 Who Should Read This Book?
This book can be useful for a variety of readers, but we wrote it with two target
audiences in mind. One of these target audiences is university students (under-
graduate or graduate) learning about machine learning, including those who are
beginning a career in deep learning and artificial intelligence research. The other
8
CHAPTER 1. INTRODUCTION
AI
Machine learning
Representation learning
Deep learning
Example:
Knowledge
bases
Example:
Logistic
regression
Example:
Shallow
autoencoders
Example:
MLPs
Figure 1.4: A Venn diagram showing how deep learning is a kind of representation learning,
which is in turn a kind of machine learning, which is used for many but not all approaches
to AI. Each section of the Venn diagram includes an example of an AI technology.
target audience is software engineers who do not have a machine learning or statis-
tics background but want to rapidly acquire one and begin using deep learning in
their product or platform. Deep learning has already proved useful in many soft-
ware disciplines, including computer vision, speech and audio processing, natural
language processing, robotics, bioinformatics and chemistry, video games, search
engines, online advertising and finance.
This book has been organized into three parts to best accommodate a variety
of readers. Part I introduces basic mathematical tools and machine learning
concepts. Part II describes the most established deep learning algorithms, which
are essentially solved technologies. Part III describes more speculative ideas that
are widely believed to be important for future research in deep learning.
9
CHAPTER 1. INTRODUCTION
Input
Hand-
designed
program
Output
Input
Hand-
designed
features
Mapping from
features
Output
Input
Features
Mapping from
features
Output
Input
Simple
features
Mapping from
features
Output
Additional
layers of more
abstract
features
Rule-based
systems
Classic
machine
learning
Representation
learning
Deep
learning
Figure 1.5: Flowcharts showing how the different parts of an AI system relate to each
other within different AI disciplines. Shaded boxes indicate components that are able to
learn from data.
Readers should feel free to skip parts that are not relevant given their interests
or background. Readers familiar with linear algebra, probability, and fundamental
machine learning concepts can skip part I, for example, while those who just want
to implement a working system need not read beyond part II. To help choose which
10
CHAPTER 1. INTRODUCTION
1. Introduction
Part I: Applied Math and Machine Learning Basics
2. Linear Algebra
3. Probability and
Information Theory
4. Numerical
Computation
5. Machine Learning
Basics
Part II: Deep Networks: Modern Practices
6. Deep Feedforward
Networks
7. Regularization 8. Optimization 9. CNNs 10. RNNs
11. Practical
Methodology
12. Applications
Part III: Deep Learning Research
13. Linear Factor
Models
14. Autoencoders
15. Representation
Learning
16. Structured
Probabilistic Models
17. Monte Carlo
Methods
18. Partition
Function
19. Inference
20. Deep Generative
Models
Figure 1.6: The high-level organization of the book. An arrow from one chapter to another
indicates that the former chapter is prerequisite material for understanding the latter.
11
CHAPTER 1. INTRODUCTION
chapters to read, figure 1.6 provides a flowchart showing the high-level organization
of the book.
We do assume that all readers come from a computer science background. We
assume familiarity with programming, a basic understanding of computational
performance issues, complexity theory, introductory level calculus and some of the
terminology of graph theory.
1.2 Historical Trends in Deep Learning
It is easiest to understand deep learning with some historical context. Rather than
providing a detailed history of deep learning, we identify a few key trends:
•
Deep learning has had a long and rich history, but has gone by many names,
reflecting different philosophical viewpoints, and has waxed and waned in
popularity.
•
Deep learning has become more useful as the amount of available training
data has increased.
•
Deep learning models have grown in size over time as computer infrastructure
(both hardware and software) for deep learning has improved.
•
Deep learning has solved increasingly complicated applications with increasing
accuracy over time.
1.2.1 The Many Names and Changing Fortunes of Neural Net-
works
We expect that many readers of this book have heard of deep learning as an exciting
new technology, and are surprised to see a mention of “history” in a book about an
emerging field. In fact, deep learning dates back to the 1940s. Deep learning only
appears to be new, because it was relatively unpopular for several years preceding
its current popularity, and because it has gone through many different names, only
recently being called “deep learning.” The field has been rebranded many times,
reflecting the influence of different researchers and different perspectives.
A comprehensive history of deep learning is beyond the scope of this textbook.
Some basic context, however, is useful for understanding deep learning. Broadly
speaking, there have been three waves of development: deep learning known as
cybernetics
in the 1940s–1960s, deep learning known as
connectionism
in the
12
CHAPTER 1. INTRODUCTION
1940 1950 1960 1970 1980 1990 2000
Year
0.000000
0.000050
0.000100
0.000150
0.000200
0.000250
Frequency of Word or Phrase
cybernetics
(connectionism + neural networks)
Figure 1.7: Two of the three historical waves of artificial neural nets research, as measured
by the frequency of the phrases “cybernetics” and “connectionism” or “neural networks,”
according to Google Books (the third wave is too recent to appear). The first wave
started with cybernetics in the 1940s–1960s, with the development of theories of biological
learning (McCulloch and Pitts, 1943; Hebb, 1949) and implementations of the first models,
such as the perceptron (Rosenblatt, 1958), enabling the training of a single neuron. The
second wave started with the connectionist approach of the 1980–1995 period, with back-
propagation (Rumelhart et al., 1986a) to train a neural network with one or two hidden
layers. The current and third wave, deep learning, started around 2006 (Hinton et al.,
2006; Bengio et al., 2007; Ranzato et al., 2007a) and is just now appearing in book form
as of 2016. The other two waves similarly appeared in book form much later than the
corresponding scientific activity occurred.
1980s–1990s, and the current resurgence under the name deep learning beginning
in 2006. This is quantitatively illustrated in figure 1.7.
Some of the earliest learning algorithms we recognize today were intended to
be computational models of biological learning, that is, models of how learning
happens or could happen in the brain. As a result, one of the names that deep
learning has gone by is
artificial neural networks
(ANNs). The corresponding
perspective on deep learning models is that they are engineered systems inspired
by the biological brain (whether the human brain or the brain of another animal).
While the kinds of neural networks used for machine learning have sometimes
been used to understand brain function (Hinton and Shallice, 1991), they are
generally not designed to be realistic models of biological function. The neural
perspective on deep learning is motivated by two main ideas. One idea is that
the brain provides a proof by example that intelligent behavior is possible, and a
conceptually straightforward path to building intelligence is to reverse engineer the
computational principles behind the brain and duplicate its functionality. Another
13
CHAPTER 1. INTRODUCTION
perspective is that it would be deeply interesting to understand the brain and the
principles that underlie human intelligence, so machine learning models that shed
light on these basic scientific questions are useful apart from their ability to solve
engineering applications.
The modern term “deep learning” goes beyond the neuroscientific perspective
on the current breed of machine learning models. It appeals to a more general
principle of learning multiple levels of composition, which can be applied in machine
learning frameworks that are not necessarily neurally inspired.
The earliest predecessors of modern deep learning were simple linear models
motivated from a neuroscientific perspective. These models were designed to
take a set of
n
input values
x
1
, . . . , x
n
and associate them with an output
y
.
These models would learn a set of weights
w
1
, . . . , w
n
and compute their output
f
(
x, w
) =
x
1
w
1
+
···
+
x
n
w
n
. This first wave of neural networks research was
known as cybernetics, as illustrated in figure 1.7.
The McCulloch-Pitts neuron (McCulloch and Pitts, 1943) was an early model
of brain function. This linear model could recognize two different categories of
inputs by testing whether
f
(
x, w
) is positive or negative. Of course, for the model
to correspond to the desired definition of the categories, the weights needed to be
set correctly. These weights could be set by the human operator. In the 1950s, the
perceptron (Rosenblatt, 1958, 1962) became the first model that could learn the
weights that defined the categories given examples of inputs from each category.
The
adaptive linear element
(ADALINE), which dates from about the same
time, simply returned the value of
f
(
x
) itself to predict a real number (Widrow
and Hoff, 1960) and could also learn to predict these numbers from data.
These simple learning algorithms greatly affected the modern landscape of ma-
chine learning. The training algorithm used to adapt the weights of the ADALINE
was a special case of an algorithm called
stochastic gradient descent
. Slightly
modified versions of the stochastic gradient descent algorithm remain the dominant
training algorithms for deep learning models today.
Models based on the
f
(
x, w
) used by the perceptron and ADALINE are called
linear models
. These models remain some of the most widely used machine
learning models, though in many cases they are trained in different ways than the
original models were trained.
Linear models have many limitations. Most famously, they cannot learn the
XOR function, where
f
([0
,
1]
, w
) = 1 and
f
([1
,
0]
, w
) = 1 but
f
([1
,
1]
, w
) = 0
and
f
([0
,
0]
, w
) = 0. Critics who observed these flaws in linear models caused
a backlash against biologically inspired learning in general (Minsky and Papert,
1969). This was the first major dip in the popularity of neural networks.
14
CHAPTER 1. INTRODUCTION
Today, neuroscience is regarded as an important source of inspiration for deep
learning researchers, but it is no longer the predominant guide for the field.
The main reason for the diminished role of neuroscience in deep learning
research today is that we simply do not have enough information about the brain
to use it as a guide. To obtain a deep understanding of the actual algorithms used
by the brain, we would need to be able to monitor the activity of (at the very
least) thousands of interconnected neurons simultaneously. Because we are not
able to do this, we are far from understanding even some of the most simple and
well-studied parts of the brain (Olshausen and Field, 2005).
Neuroscience has given us a reason to hope that a single deep learning algorithm
can solve many different tasks. Neuroscientists have found that ferrets can learn to
“see” with the auditory processing region of their brain if their brains are rewired
to send visual signals to that area (Von Melchner et al., 2000). This suggests that
much of the mammalian brain might use a single algorithm to solve most of the
different tasks that the brain solves. Before this hypothesis, machine learning
research was more fragmented, with different communities of researchers studying
natural language processing, vision, motion planning and speech recognition. Today,
these application communities are still separate, but it is common for deep learning
research groups to study many or even all these application areas simultaneously.
We are able to draw some rough guidelines from neuroscience. The basic
idea of having many computational units that become intelligent only via their
interactions with each other is inspired by the brain. The neocognitron (Fukushima,
1980) introduced a powerful model architecture for processing images that was
inspired by the structure of the mammalian visual system and later became the
basis for the modern convolutional network (LeCun et al., 1998b), as we will see
in section 9.10. Most neural networks today are based on a model neuron called
the
rectified linear unit
. The original cognitron (Fukushima, 1975) introduced
a more complicated version that was highly inspired by our knowledge of brain
function. The simplified modern version was developed incorporating ideas from
many viewpoints, with Nair and Hinton (2010) and Glorot et al. (2011a) citing
neuroscience as an influence, and Jarrett et al. (2009) citing more engineering-
oriented influences. While neuroscience is an important source of inspiration, it
need not be taken as a rigid guide. We know that actual neurons compute very
different functions than modern rectified linear units, but greater neural realism
has not yet led to an improvement in machine learning performance. Also, while
neuroscience has successfully inspired several neural network architectures, we
do not yet know enough about biological learning for neuroscience to offer much
guidance for the learning algorithms we use to train these architectures.
15
CHAPTER 1. INTRODUCTION
Media accounts often emphasize the similarity of deep learning to the brain.
While it is true that deep learning researchers are more likely to cite the brain as an
influence than researchers working in other machine learning fields, such as kernel
machines or Bayesian statistics, one should not view deep learning as an attempt
to simulate the brain. Modern deep learning draws inspiration from many fields,
especially applied math fundamentals like linear algebra, probability, information
theory, and numerical optimization. While some deep learning researchers cite
neuroscience as an important source of inspiration, others are not concerned with
neuroscience at all.
It is worth noting that the effort to understand how the brain works on
an algorithmic level is alive and well. This endeavor is primarily known as
“computational neuroscience” and is a separate field of study from deep learning.
It is common for researchers to move back and forth between both fields. The
field of deep learning is primarily concerned with how to build computer systems
that are able to successfully solve tasks requiring intelligence, while the field of
computational neuroscience is primarily concerned with building more accurate
models of how the brain actually works.
In the 1980s, the second wave of neural network research emerged in great
part via a movement called
connectionism
, or
parallel distributed process-
ing
(Rumelhart et al., 1986c; McClelland et al., 1995). Connectionism arose in
the context of cognitive science. Cognitive science is an interdisciplinary approach
to understanding the mind, combining multiple different levels of analysis. During
the early 1980s, most cognitive scientists studied models of symbolic reasoning.
Despite their popularity, symbolic models were difficult to explain in terms of
how the brain could actually implement them using neurons. The connectionists
began to study models of cognition that could actually be grounded in neural
implementations (Touretzky and Minton, 1985), reviving many ideas dating back
to the work of psychologist Donald Hebb in the 1940s (Hebb, 1949).
The central idea in connectionism is that a large number of simple computational
units can achieve intelligent behavior when networked together. This insight applies
equally to neurons in biological nervous systems as it does to hidden units in
computational models.
Several key concepts arose during the connectionism movement of the 1980s
that remain central to today’s deep learning.
One of these concepts is that of
distributed representation
(Hinton et al.,
1986). This is the idea that each input to a system should be represented by
many features, and each feature should be involved in the representation of many
possible inputs. For example, suppose we have a vision system that can recognize
16
CHAPTER 1. INTRODUCTION
cars, trucks, and birds, and these objects can each be red, green, or blue. One way
of representing these inputs would be to have a separate neuron or hidden unit
that activates for each of the nine possible combinations: red truck, red car, red
bird, green truck, and so on. This requires nine different neurons, and each neuron
must independently learn the concept of color and object identity. One way to
improve on this situation is to use a distributed representation, with three neurons
describing the color and three neurons describing the object identity. This requires
only six neurons total instead of nine, and the neuron describing redness is able to
learn about redness from images of cars, trucks and birds, not just from images
of one specific category of objects. The concept of distributed representation is
central to this book and is described in greater detail in chapter 15.
Another major accomplishment of the connectionist movement was the suc-
cessful use of back-propagation to train deep neural networks with internal repre-
sentations and the popularization of the back-propagation algorithm (Rumelhart
et al., 1986a; LeCun, 1987). This algorithm has waxed and waned in popularity
but, as of this writing, is the dominant approach to training deep models.
During the 1990s, researchers made important advances in modeling sequences
with neural networks. Hochreiter (1991) and Bengio et al. (1994) identified some of
the fundamental mathematical difficulties in modeling long sequences, described in
section 10.7. Hochreiter and Schmidhuber (1997) introduced the long short-term
memory (LSTM) network to resolve some of these difficulties. Today, the LSTM is
widely used for many sequence modeling tasks, including many natural language
processing tasks at Google.
The second wave of neural networks research lasted until the mid-1990s. Ven-
tures based on neural networks and other AI technologies began to make unrealisti-
cally ambitious claims while seeking investments. When AI research did not fulfill
these unreasonable expectations, investors were disappointed. Simultaneously,
other fields of machine learning made advances. Kernel machines (Boser et al.,
1992; Cortes and Vapnik, 1995; Schölkopf et al., 1999) and graphical models (Jor-
dan, 1998) both achieved good results on many important tasks. These two factors
led to a decline in the popularity of neural networks that lasted until 2007.
During this time, neural networks continued to obtain impressive performance
on some tasks (LeCun et al., 1998b; Bengio et al., 2001). The Canadian Institute
for Advanced Research (CIFAR) helped to keep neural networks research alive
via its Neural Computation and Adaptive Perception (NCAP) research initiative.
This program united machine learning research groups led by Geoffrey Hinton at
University of Toronto, Yoshua Bengio at University of Montreal, and Yann LeCun
at New York University. The multidisciplinary CIFAR NCAP research initiative
17
CHAPTER 1. INTRODUCTION
also included neuroscientists and experts in human and computer vision.
At this point, deep networks were generally believed to be very difficult to
train. We now know that algorithms that have existed since the 1980s work quite
well, but this was not apparent circa 2006. The issue is perhaps simply that these
algorithms were too computationally costly to allow much experimentation with
the hardware available at the time.
The third wave of neural networks research began with a breakthrough in
2006. Geoffrey Hinton showed that a kind of neural network called a deep belief
network could be efficiently trained using a strategy called greedy layer-wise
pretraining (Hinton et al., 2006), which we describe in more detail in section 15.1.
The other CIFAR-affiliated research groups quickly showed that the same strategy
could be used to train many other kinds of deep networks (Bengio et al., 2007;
Ranzato et al., 2007a) and systematically helped to improve generalization on
test examples. This wave of neural networks research popularized the use of the
term “deep learning” to emphasize that researchers were now able to train deeper
neural networks than had been possible before, and to focus attention on the
theoretical importance of depth (Bengio and LeCun, 2007; Delalleau and Bengio,
2011; Pascanu et al., 2014a; Montufar et al., 2014). At this time, deep neural
networks outperformed competing AI systems based on other machine learning
technologies as well as hand-designed functionality. This third wave of popularity
of neural networks continues to the time of this writing, though the focus of deep
learning research has changed dramatically within the time of this wave. The
third wave began with a focus on new unsupervised learning techniques and the
ability of deep models to generalize well from small datasets, but today there is
more interest in much older supervised learning algorithms and the ability of deep
models to leverage large labeled datasets.
1.2.2 Increasing Dataset Sizes
One may wonder why deep learning has only recently become recognized as a crucial
technology even though the first experiments with artificial neural networks were
conducted in the 1950s. Deep learning has been successfully used in commercial
applications since the 1990s but was often regarded as being more of an art than a
technology and something that only an expert could use, until recently. It is true
that some skill is required to get good performance from a deep learning algorithm.
Fortunately, the amount of skill required reduces as the amount of training data
increases. The learning algorithms reaching human performance on complex tasks
today are nearly identical to the learning algorithms that struggled to solve toy
problems in the 1980s, though the models we train with these algorithms have
18
CHAPTER 1. INTRODUCTION
undergone changes that simplify the training of very deep architectures. The most
important new development is that today we can provide these algorithms with
the resources they need to succeed. Figure 1.8 shows how the size of benchmark
datasets has expanded remarkably over time. This trend is driven by the increasing
digitization of society. As more and more of our activities take place on computers,
more and more of what we do is recorded. As our computers are increasingly
networked together, it becomes easier to centralize these records and curate them
into a dataset appropriate for machine learning applications. The age of “Big Data”
1900 1950 1985 2000 2015
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Iris
MNIST
Public SVHN
ImageNet
CIFAR-10
ImageNet10k
ILSVRC 2014
Sports-1M
Rotated T vs. C
T vs. G vs. F
Criminals
Canadian Hansard
WMT
Figure 1.8: Increasing dataset size over time. In the early 1900s, statisticians studied
datasets using hundreds or thousands of manually compiled measurements (Garson, 1900;
Gosset, 1908; Anderson, 1935; Fisher, 1936). In the 1950s through the 1980s, the pioneers
of biologically inspired machine learning often worked with small synthetic datasets, such
as low-resolution bitmaps of letters, that were designed to incur low computational cost and
demonstrate that neural networks were able to learn specific kinds of functions (Widrow
and Hoff, 1960; Rumelhart et al., 1986b). In the 1980s and 1990s, machine learning
became more statistical and began to leverage larger datasets containing tens of thousands
of examples, such as the MNIST dataset (shown in figure 1.9) of scans of handwritten
numbers (LeCun et al., 1998b). In the first decade of the 2000s, more sophisticated
datasets of this same size, such as the CIFAR-10 dataset (Krizhevsky and Hinton, 2009),
continued to be produced. Toward the end of that decade and throughout the first half of
the 2010s, significantly larger datasets, containing hundreds of thousands to tens of millions
of examples, completely changed what was possible with deep learning. These datasets
included the public Street View House Numbers dataset (Netzer et al., 2011), various
versions of the ImageNet dataset (Deng et al., 2009, 2010a; Russakovsky et al., 2014a),
and the Sports-1M dataset (Karpathy et al., 2014). At the top of the graph, we see that
datasets of translated sentences, such as IBM’s dataset constructed from the Canadian
Hansard (Brown et al., 1990) and the WMT 2014 English to French dataset (Schwenk,
2014), are typically far ahead of other dataset sizes.
19
CHAPTER 1. INTRODUCTION
Figure 1.9: Example inputs from the MNIST dataset. The “NIST” stands for National
Institute of Standards and Technology, the agency that originally collected this data.
The “M” stands for “modified,” since the data has been preprocessed for easier use with
machine learning algorithms. The MNIST dataset consists of scans of handwritten digits
and associated labels describing which digit 0–9 is contained in each image. This simple
classification problem is one of the simplest and most widely used tests in deep learning
research. It remains popular despite being quite easy for modern techniques to solve.
Geoffrey Hinton has described it as “the drosophila of machine learning,” meaning that it
enables machine learning researchers to study their algorithms in controlled laboratory
conditions, much as biologists often study fruit flies.
has made machine learning much easier because the key burden of statistical
estimation—generalizing well to new data after observing only a small amount
of data—has been considerably lightened. As of 2016, a rough rule of thumb
is that a supervised deep learning algorithm will generally achieve acceptable
performance with around 5,000 labeled examples per category and will match or
20
CHAPTER 1. INTRODUCTION
exceed human performance when trained with a dataset containing at least 10
million labeled examples. Working successfully with datasets smaller than this is
an important research area, focusing in particular on how we can take advantage
of large quantities of unlabeled examples, with unsupervised or semi-supervised
learning.
1.2.3 Increasing Model Sizes
Another key reason that neural networks are wildly successful today after enjoying
comparatively little success since the 1980s is that we have the computational
resources to run much larger models today. One of the main insights of connection-
ism is that animals become intelligent when many of their neurons work together.
An individual neuron or small collection of neurons is not particularly useful.
Biological neurons are not especially densely connected. As seen in figure 1.10,
our machine learning models have had a number of connections per neuron within
an order of magnitude of even mammalian brains for decades.
In terms of the total number of neurons, neural networks have been astonishingly
small until quite recently, as shown in figure 1.11. Since the introduction of hidden
units, artificial neural networks have doubled in size roughly every 2.4 years. This
growth is driven by faster computers with larger memory and by the availability
of larger datasets. Larger networks are able to achieve higher accuracy on more
complex tasks. This trend looks set to continue for decades. Unless new technologies
enable faster scaling, artificial neural networks will not have the same number
of neurons as the human brain until at least the 2050s. Biological neurons may
represent more complicated functions than current artificial neurons, so biological
neural networks may be even larger than this plot portrays.
In retrospect, it is not particularly surprising that neural networks with fewer
neurons than a leech were unable to solve sophisticated artificial intelligence prob-
lems. Even today’s networks, which we consider quite large from a computational
systems point of view, are smaller than the nervous system of even relatively
primitive vertebrate animals like frogs.
The increase in model size over time, due to the availability of faster CPUs,
the advent of general purpose GPUs (described in section 12.1.2), faster network
connectivity and better software infrastructure for distributed computing, is one of
the most important trends in the history of deep learning. This trend is generally
expected to continue well into the future.
21
CHAPTER 1. INTRODUCTION
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Connections per neuron
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Mouse
Cat
Human
Figure 1.10: Number of connections per neuron over time. Initially, the number of connec-
tions between neurons in artificial neural networks was limited by hardware capabilities.
Today, the number of connections between neurons is mostly a design consideration. Some
artificial neural networks have nearly as many connections per neuron as a cat, and it
is quite common for other neural networks to have as many connections per neuron as
smaller mammals like mice. Even the human brain does not have an exorbitant amount
of connections per neuron. Biological neural network sizes from Wikipedia (2015).
1. Adaptive linear element (Widrow and Hoff, 1960)
2. Neocognitron (Fukushima, 1980)
3. GPU-accelerated convolutional network (Chellapilla et al., 2006)
4. Deep Boltzmann machine (Salakhutdinov and Hinton, 2009a)
5. Unsupervised convolutional network (Jarrett et al., 2009)
6. GPU-accelerated multilayer perceptron (Ciresan et al., 2010)
7. Distributed autoencoder (Le et al., 2012)
8. Multi-GPU convolutional network (Krizhevsky et al., 2012)
9. COTS HPC unsupervised convolutional network (Coates et al., 2013)
10. GoogLeNet (Szegedy et al., 2014a)
1.2.4 Increasing Accuracy, Complexity and Real-World Impact
Since the 1980s, deep learning has consistently improved in its ability to provide
accurate recognition and prediction. Moreover, deep learning has consistently been
applied with success to broader and broader sets of applications.
The earliest deep models were used to recognize individual objects in tightly
cropped, extremely small images (Rumelhart et al., 1986a). Since then there has
been a gradual increase in the size of images neural networks could process. Modern
object recognition networks process rich high-resolution photographs and do not
22
CHAPTER 1. INTRODUCTION
1950 1985 2000 2015 2056
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Human
Figure 1.11: Increasing neural network size over time. Since the introduction of hidden
units, artificial neural networks have doubled in size roughly every 2.4 years. Biological
neural network sizes from Wikipedia (2015).
1. Perceptron (Rosenblatt, 1958, 1962)
2. Adaptive linear element (Widrow and Hoff, 1960)
3. Neocognitron (Fukushima, 1980)
4. Early back-propagation network (Rumelhart et al., 1986b)
5. Recurrent neural network for speech recognition (Robinson and Fallside, 1991)
6. Multilayer perceptron for speech recognition (Bengio et al., 1991)
7. Mean field sigmoid belief network (Saul et al., 1996)
8. LeNet-5 (LeCun et al., 1998b)
9. Echo state network (Jaeger and Haas, 2004)
10. Deep belief network (Hinton et al., 2006)
11. GPU-accelerated convolutional network (Chellapilla et al., 2006)
12. Deep Boltzmann machine (Salakhutdinov and Hinton, 2009a)
13. GPU-accelerated deep belief network (Raina et al., 2009)
14. Unsupervised convolutional network (Jarrett et al., 2009)
15. GPU-accelerated multilayer perceptron (Ciresan et al., 2010)
16. OMP-1 network (Coates and Ng, 2011)
17. Distributed autoencoder (Le et al., 2012)
18. Multi-GPU convolutional network (Krizhevsky et al., 2012)
19. COTS HPC unsupervised convolutional network (Coates et al., 2013)
20. GoogLeNet (Szegedy et al., 2014a)
have a requirement that the photo be cropped near the object to be recognized
(Krizhevsky et al., 2012). Similarly, the earliest networks could recognize only
two kinds of objects (or in some cases, the absence or presence of a single kind of
object), while these modern networks typically recognize at least 1,000 different
categories of objects. The largest contest in object recognition is the ImageNet
23
CHAPTER 1. INTRODUCTION
Large Scale Visual Recognition Challenge (ILSVRC) held each year. A dramatic
moment in the meteoric rise of deep learning came when a convolutional network
won this challenge for the first time and by a wide margin, bringing down the
state-of-the-art top-5 error rate from 26.1 percent to 15.3 percent (Krizhevsky
et al., 2012), meaning that the convolutional network produces a ranked list of
possible categories for each image, and the correct category appeared in the first
five entries of this list for all but 15.3 percent of the test examples. Since then,
these competitions are consistently won by deep convolutional nets, and as of this
writing, advances in deep learning have brought the latest top-5 error rate in this
contest down to 3.6 percent, as shown in figure 1.12.
Deep learning has also had a dramatic impact on speech recognition. After
improving throughout the 1990s, the error rates for speech recognition stagnated
starting in about 2000. The introduction of deep learning (Dahl et al., 2010; Deng
et al., 2010b; Seide et al., 2011; Hinton et al., 2012a) to speech recognition resulted
in a sudden drop in error rates, with some error rates cut in half. We explore this
history in more detail in section 12.3.
Deep networks have also had spectacular successes for pedestrian detection and
image segmentation (Sermanet et al., 2013; Farabet et al., 2013; Couprie et al.,
2013) and yielded superhuman performance in traffic sign classification (Ciresan
et al., 2012).
At the same time that the scale and accuracy of deep networks have increased,
2010 2011 2012 2013 2014 2015
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ILSVRC classification error rate
Figure 1.12: Decreasing error rate over time. Since deep networks reached the scale
necessary to compete in the ImageNet Large Scale Visual Recognition Challenge, they
have consistently won the competition every year, yielding lower and lower error rates
each time. Data from Russakovsky et al. (2014b) and He et al. (2015).
24
CHAPTER 1. INTRODUCTION
so has the complexity of the tasks that they can solve. Goodfellow et al. (2014d)
showed that neural networks could learn to output an entire sequence of characters
transcribed from an image, rather than just identifying a single object. Previously,
it was widely believed that this kind of learning required labeling of the individual
elements of the sequence (Gülçehre and Bengio, 2013). Recurrent neural networks,
such as the LSTM sequence model mentioned above, are now used to model
relationships between sequences and other sequences rather than just fixed inputs.
This sequence-to-sequence learning seems to be on the cusp of revolutionizing
another application: machine translation (Sutskever et al., 2014; Bahdanau et al.,
2015).
This trend of increasing complexity has been pushed to its logical conclusion
with the introduction of neural Turing machines (Graves et al., 2014) that learn
to read from memory cells and write arbitrary content to memory cells. Such
neural networks can learn simple programs from examples of desired behavior. For
example, they can learn to sort lists of numbers given examples of scrambled and
sorted sequences. This self-programming technology is in its infancy, but in the
future it could in principle be applied to nearly any task.
Another crowning achievement of deep learning is its extension to the domain of
reinforcement learning
. In the context of reinforcement learning, an autonomous
agent must learn to perform a task by trial and error, without any guidance from
the human operator. DeepMind demonstrated that a reinforcement learning system
based on deep learning is capable of learning to play Atari video games, reaching
human-level performance on many tasks (Mnih et al., 2015). Deep learning has
also significantly improved the performance of reinforcement learning for robotics
(Finn et al., 2015).
Many of these applications of deep learning are highly profitable. Deep learning
is now used by many top technology companies, including Google, Microsoft,
Facebook, IBM, Baidu, Apple, Adobe, Netflix, NVIDIA, and NEC.
Advances in deep learning have also depended heavily on advances in software
infrastructure. Software libraries such as Theano (Bergstra et al., 2010; Bastien
et al., 2012), PyLearn2 (Goodfellow et al., 2013c), Torch (Collobert et al., 2011b),
DistBelief (Dean et al., 2012), Caffe (Jia, 2013), MXNet (Chen et al., 2015), and
TensorFlow (Abadi et al., 2015) have all supported important research projects or
commercial products.
Deep learning has also made contributions to other sciences. Modern convolu-
tional networks for object recognition provide a model of visual processing that
neuroscientists can study (DiCarlo, 2013). Deep learning also provides useful tools
for processing massive amounts of data and making useful predictions in scientific
25
CHAPTER 1. INTRODUCTION
fields. It has been successfully used to predict how molecules will interact in order
to help pharmaceutical companies design new drugs (Dahl et al., 2014), to search
for subatomic particles (Baldi et al., 2014), and to automatically parse microscope
images used to construct a 3-D map of the human brain (Knowles-Barley et al.,
2014). We expect deep learning to appear in more and more scientific fields in the
future.
In summary, deep learning is an approach to machine learning that has drawn
heavily on our knowledge of the human brain, statistics and applied math as it
developed over the past several decades. In recent years, deep learning has seen
tremendous growth in its popularity and usefulness, largely as the result of more
powerful computers, larger datasets and techniques to train deeper networks. The
years ahead are full of challenges and opportunities to improve deep learning even
further and to bring it to new frontiers.
26
