A Critical Review of Recurrent Neural Networks for Sequence Learning

A Critical Review of Recurrent Neural Networks for Sequence Learning by Zachary C. Lipton.


Countless learning tasks require awareness of time. Image captioning, speech synthesis, and video game playing all require that a model generate sequences of outputs. In other domains, such as time series prediction, video analysis, and music information retrieval, a model must learn from sequences of inputs. Significantly more interactive tasks, such as natural language translation, engaging in dialogue, and robotic control, often demand both.

Recurrent neural networks (RNNs) are a powerful family of connectionist models that capture time dynamics via cycles in the graph. Unlike feedforward neural networks, recurrent networks can process examples one at a time, retaining a state, or memory, that reflects an arbitrarily long context window. While these networks have long been difficult to train and often contain millions of parameters, recent advances in network architectures, optimization techniques, and parallel computation have enabled large-scale learning with recurrent nets.

Over the past few years, systems based on state of the art long short-term memory (LSTM) and bidirectional recurrent neural network (BRNN) architectures have demonstrated record-setting performance on tasks as varied as image captioning, language translation, and handwriting recognition. In this review of the literature we synthesize the body of research that over the past three decades has yielded and reduced to practice these powerful models. When appropriate, we reconcile conflicting notation and nomenclature. Our goal is to provide a mostly self-contained explication of state of the art systems, together with a historical perspective and ample references to the primary research.

Lipton begins with an all too common lament:

The literature on recurrent neural networks can seem impenetrable to the uninitiated. Shorter papers assume familiarity with a large body of background literature. Diagrams are frequently underspecified, failing to indicate which edges span time steps and which don’t. Worse, jargon abounds while notation is frequently inconsistent across papers or overloaded within papers. Readers are frequently in the unenviable position of having to synthesize conflicting information across many papers in order to understand but one. For example, in many papers subscripts index both nodes and time steps. In others, h simultaneously stands for link functions and a layer of hidden nodes. The variable t simultaneously stands for both time indices and targets, sometimes in the same equation. Many terrific breakthrough papers have appeared recently, but clear reviews of recurrent neural network literature are rare.

Unfortunately, Lipton gives no pointers to where the variant practices occur, leaving the reader forewarned but not forearmed.

Still, this is a survey paper with seventy-three (73) references over thirty-three (33) pages, so I assume you will encounter various notation practices if you follow the references and current literature.

Capturing variations in notation, along with where they have been seen, won’t win the Turing Award but may improve the CS field overall.

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