Recurrent Neural Networks: The Powerhouse of Language Modeling
During the spring semester of my junior year in college, I had the opportunity to study abroad in Copenhagen, Denmark. I had never been to Europe before that, so I was incredibly excited to immerse myself into a new culture, meet new people, travel to new places, and, most important, encounter a new language. Now although English is not my native language (Vietnamese is), I have learned and spoken it since early childhood, making it second-nature. Danish, on the other hand, is an incredibly complicated language with a very different sentence and grammatical structure. Before my trip, I tried to learn a bit of Danish using the app Duolingo; however, I only got a hold of simple phrases such as Hello (Hej) and Good Morning (God Morgen).
When I got there, I had to go to the grocery store to buy food. Well, all the labels there were in Danish, and I couldn’t seem to discern them. After a long half hour struggling to find the difference between whole grain and wheat breads, I realized that I had installed Google Translate on my phone not long ago. I took out my phone, opened the app, pointed the camera at the labels… and voila, those Danish words were translated into English instantly. Turns out that Google Translate can translate words from whatever the camera sees, whether it is a street sign, restaurant menu, or even handwritten digits. Needless to say, the app saved me a ton of time while I was studying abroad.
Google Translate is a product developed by the Natural Language Processing Research Group at Google. This group focuses on algorithms that apply at scale across languages and across domains. Their work spans the range of traditional NLP tasks, with general-purpose syntax and semantic algorithms underpinning more specialized systems.
Looking at a broader level, NLP sits at the intersection of computer science, artificial intelligence, and linguistics. The goal is for computers to process or “understand” natural language in order to perform tasks that are useful, such as Sentiment Analysis, Language Translation, and Question Answering. Fully understanding and representing the meaning of language is a very difficulty goal; thus it has been estimated that perfect language understanding is only achieved by AI-complete system. The first step to know about NLP is the concept of language modeling.
Language Modeling is the task of predicting what word comes next. For example, given the sentence “I am writing a …”, the word coming next can be “letter”, “sentence”, “blog post” … More formally, given a sequence of words x(1), x(2), …, x(t), language models compute the probability distribution of the next word x(t+1).
The most fundamental language model is the n-gram model. An n-gram is a chunk of n consecutive words. For example, given the sentence “I am writing a …”, then here are the respective n-grams:
unigrams: “I”, “am”, “writing”, “a”
bigrams: “I am”, “am writing”, “writing a”
trigrams: “I am writing”, “am writing a”
4-grams: “I am writing a”
The basic idea behind n-gram language modeling is to collect statistics about how frequent different n-grams are, and use these to predict next word. However, n-gram language models have the sparsity problem, in which we do not observe enough data in a corpus to model language accurately (especially as n increases).
Instead of the n-gram approach, we can try a window-based neural language model, such as feed-forward neural probabilistic language models and recurrent neural network language models. This approach solves the data sparsity problem by representing words as vectors (word embeddings) and using them as inputs to a neural language model. The parameters are learned as part of the training process. Word embeddings obtained through neural language models exhibit the property whereby semantically close words are likewise close in the induced vector space. Moreover, recurrent neural language model can also capture the contextual information at the sentence-level, corpus-level, and subword-level.
Recurrent Neural Net Language Model
The idea behind RNNs is to make use of sequential information. RNNs are called recurrent because they perform the same task for every element of a sequence, with the output depended on previous computations. Theoretically, RNNs can make use of information in arbitrarily long sequences, but empirically, they are limited to looking back only a few steps. This capability allows RNNs to solve tasks such as unsegmented, connected handwriting recognition or speech recognition.
Let’s try an analogy. Suppose you are watching Avengers: Infinity War (by the way, a phenomenal movie). There are so many superheroes and multiple story plots happening in the movie, which may confuse many viewers who don’t have prior knowledge about the Marvel Cinematic Universe. However, you have the context of what’s going on because you have seen the previous Marvel series in chronological order (Iron Man, Thor, Hulk, Captain America, Guardians of the Galaxy) to be able to relate and connect everything correctly. It means that you remember everything that you have watched to make sense of the chaos happening in Infinity War.
Similarly, RNN remembers everything. In other neural networks, all the inputs are independent of each other. But in RNN, all the inputs are related to each other. Let’s say you have to predict the next word in a given sentence, the relationship among all the previous words helps to predict a better output. In other words, RNN remembers all these relationships while training itself.
RNN remembers what it knows from previous input using a simple loop. This loop takes the information from previous time stamp and adds it to the input of current time stamp. The figure below shows the basic RNN structure. At a particular time step t, X(t) is the input to the network and h(t) is the output of the network. A is the RNN cell which contains neural networks just like a feed-forward net.
This loop structure allows the neural network to take the sequence of the input. If you see the unrolled version below, you will understand it better:
First, RNN takes the X(0) from the sequence of input and then outputs h(0)which together with X(1) is the input for the next step. Next, h(1) from the next step is the input with X(2) for the next step and so on. With this recursive function, RNN keeps remembering the context while training.
If you are a math nerd, many RNNs use the equation below to define the values of their hidden units:
of which h(t) is the hidden state at timestamp t, ∅ is the activation function (either Tanh or Sigmoid), W is the weight matrix for input to hidden layer at time stamp t, X(t) is the input at time stamp t, U is the weight matrix for hidden layer at time t-1 to hidden layer at time t, and h(t-1) is the hidden state at timestamp t.
RNN learns weights U and W through training using back propagation. These weights decide the importance of hidden state of previous timestamp and the importance of the current input. Essentially, they decide how much value from the hidden state and the current input should be used to generate the current input. The activation function ∅ adds non-linearity to RNN, thus simplifying the calculation of gradients for performing back propagation.
RNNs are not perfect. It suffers from a major drawback, known as the vanishing gradient problem, which prevents it from high accuracy. As the context length increases, layers in the unrolled RNN also increase. Consequently, as the network becomes deeper, the gradients flowing back in the back propagation step becomes smaller. As a result, the learning rate becomes really slow and makes it infeasible to expect long-term dependencies of the language. In other words, RNNs experience difficulty in memorizing previous words very far away in the sequence and is only able to make predictions based on the most recent words.
Over the years, researchers have developed more sophisticated types of RNNs to deal with this shortcoming of the standard RNN model. Let’s briefly go over the most important ones:
Bidirectional RNNs are simply composed of 2 RNNs stacking on top of each other. The output is then composed based on the hidden state of both RNNs. The idea is that the output may not only depend on previous elements in the sequence but also on future elements.
Long Short-Term Memory Networks are quite popular these days. They inherit the exact architecture from standard RNNs, with the exception of the hidden state. The memory in LSTMs (called cells) take as input the previous state and the current input. Internally, these cells decide what to keep in and what to eliminate from the memory. Then, they combine the previous state, the current memory, and the input. This process efficiently solves the vanishing gradient problem.
Gated Recurrent Unit Networks extends LSTM with a gating network generating signals that act to control how the present input and previous memory work to update the current activation, and thereby the current network state. Gates are themselves weighted and are selectively updated according to an algorithm.
Neural Turing Machines extend the capabilities of standard RNNs by coupling them to external memory resources, which they can interact with through attention processes. The analogy is that of Alan Turing’s enrichment of finite-state machines by an infinite memory tape.
Fun Examples of Generating Text with RNN Language Model:
Alright, let’s look at some fun examples using Recurrent Neural Net to generate text from the Internet:
Obama-RNN (Machine Generated Political Speeches): Here the author used RNN to generate hypothetical political speeches given by Barrack Obama. Taking in over 4.3 MB / 730,895 words of text written by Obama’s speech writers as input, the model generates multiple versions with a wide range of topics including jobs, war on terrorism, democracy, China… Super hilarious!
Harry Potter (Written by AI): Here the author trained an LSTM Recurrent Neural Network on the first 4 Harry Potter books. Then he asked it to produce a chapter based on what it learned. Check it out. I bet even JK Rowling would be impressed!
Seinfeld Scripts (Computer Version): A cohort of comedy writers fed individual libraries of text (scripts of Seinfeld Season 3) into predictive keyboards for the main characters in the show. The result is a 3-page script with uncanny tone, rhetorical questions, stand-up jargons — matching the rhythms and diction of the show.
Real-World Applications of RNN
The beauty of RNNs lies in their diversity of application. When we are dealing with RNNs, they can deal with various types of input and output. Let’s revisit the Google Translate example in the beginning. It is an instance of Neural Machine Translation, the approach of modeling language translation via one big Recurrent Neural Network. This is similar to language modeling in which the input is a sequence of words in the source language. The output is a sequence of target language.
Standard Neural Machine Translation is an end-to-end neural network where the source sentence is encoded by a RNN called encoder and the target words are predicted using another RNN known as decoder. The RNN Encoder reads a source sentence one symbol at a time, and then summarizes the entire source sentence in its last hidden state. The RNN Decoder uses back-propagation to learn this summary and returns the translated version.
Research Papers about Machine Translation:
A Recursive Recurrent Neural Network for Statistical Machine Translation(Microsoft Research Asia + University of Science & Tech of China)
Joint Language and Translation Modeling with Recurrent Neural Networks(Microsoft Research)
Below are other major Natural Language Processing tasks that RNNs have shown great success in, besides Language Modeling and Machine Translation discussed above:
1 — Sentiment Analysis: A simple example is to classify Twitter tweets into positive and negative sentiments. The input would be a tweet of different lengths, and the output would be a fixed type and size.
Research Paper about Sentiment Analysis:
Benchmarking Multimodal Sentiment Analysis (NTU Singapore + NIT India + University of Sterling UK)
2 — Image Captioning: Together with Convolutional Neural Networks, RNNs have been used in models that can generate descriptions for unlabeled images (think YouTube’s Closed Caption). Given an input of image(s) in need of textual descriptions, the output would be a series or sequence of words. While the input might be of a fixed size, the output can be of varying lengths.
Research Papers about Image Captioning:
Explain Images with Multimodal Recurrent Neural Networks (Baidu Research + UCLA)
Deep Visual-Semantic Alignments for Generating Image Descriptions(Stanford University)
Translating Videos to Natural Language Using Deep Recurrent Neural Networks (UT Austin + U-Mass Lowell + UC Berkeley)
3 — Speech Recognition: An example is that given an input sequence of electronic signals from a EDM doing, we can predict a sequence of phonetic segments together with their probabilities. Think applications such as SoundHound and Shazam.
Research Papers about Speech Recognition:
Sequence Transduction with Recurrent Neural Networks (University of Toronto)
Towards End-to-End Speech Recognition with Recurrent Neural Networks(DeepMind + University of Toronto)
Let’s recap major takeaways from this post:
Language Modeling is a system that predicts the next word. As a benchmark task that helps us measure our progress on understanding language, it is also a sub-component of other Natural Language Processing systems, such as Machine Translation, Text Summarization, Speech Recognition.
Recurrent Neural Networks take sequential input of any length, apply the same weights on each step, and can optionally produce output on each step. Overall, RNNs are a great way to build a Language Model.
Besides, RNNs are useful for much more: Sentence Classification, Part-of-speech Tagging, Question Answering…
By the way, have you seen the recent Google I/O Conference? Basically, Google becomes an AI-first company. One of the most outstanding AI systems that Google introduced is Duplex, a system that can accomplish real-world tasks over the phone. Directed towards completing specific tasks (such as scheduling appointments), Duplex can carry out natural conversations with people on the other end of the call.
Incoming sound is processed through an ASR system. This produces text that is analyzed with context data and other inputs to produce a response text that is read aloud through the TTS system.
This is accomplished thanks to advances in understanding, interacting, timing, and speaking. At the core of Duplex is a RNN designed to cope with these challenges, built using TensorFlow Extended (TFX). To obtain its high precision, Duplex’s RNN is trained on a corpus of anonymized phone conversation data. RNN uses the output of Google’s automatic speech recognition technology, as well as features from the audio, the history of the conversation, the parameters of the conversation and more. Hyper-parameter optimization from TFX is used to further improve the model.
Well, the future of AI conversation has already made its first major breakthrough. And all thanks to the powerhouse of language modeling, recurrent neural network.