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Title: Recurrent Hierarchical Topic-Guided Neural Language Models

Paper content:
To simultaneously capture syntax and semantics from a text corpus, we propose a new larger-context language model that extracts recurrent hierarchical semantic structure via a dynamic deep topic model to guide natural language generation.
Moving beyond a conventional language model that ignores long-range word dependencies and sentence order, the proposed model captures not only intra-sentence word dependencies, but also temporal transitions between sentences and inter-sentence topic dependences.
For inference, we develop a hybrid of stochastic-gradient MCMC and recurrent autoencoding variational Bayes.
Experimental results on a variety of real-world text corpora demonstrate that the proposed model not only outperforms state-of-the-art larger-context language models, but also learns interpretable recurrent multilayer topics and generates diverse sentences and paragraphs that are syntactically correct and semantically coherent.
Both topic and language models are widely used for text analysis.
Topic models, such as latent Dirichlet allocation (LDA) (Blei et al., 2003; Griffiths & Steyvers, 2004; Hoffman et al., 2013) and its nonparametric Bayesian generalizations (Teh et al., 2006; Zhou & Carin, 2015) , are well suited to extract document-level word concurrence patterns into latent topics from a text corpus.
Their modeling power has been further enhanced by introducing multilayer deep representation (Srivastava et al., 2013; Mnih & Gregor, 2014; Gan et al., 2015; Zhou et al., 2016; Zhao et al., 2018; .
While having semantically meaningful latent representation, they typically treat each document as a bag of words (BoW), ignoring word order (Griffiths et al., 2004; Wallach, 2006) .
Language models have become key components of various natural language processing (NLP) tasks, such as text summarization (Rush et al., 2015; Gehrmann et al., 2018) , speech recognition (Mikolov et al., 2010; Graves et al., 2013) , machine translation (Sutskever et al., 2014; Cho et al., 2014) , and image captioning (Vinyals et al., 2015; Mao et al., 2015; Xu et al., 2015; Gan et al., 2017; Rennie et al., 2017) .
The primary purpose of a language model is to capture the distribution of a word sequence, commonly with a recurrent neural network (RNN) (Mikolov et al., 2011; Graves, 2013) or a Transformer based neural network (Vaswani et al., 2017; Dai et al., 2019; Devlin et al., 2019; Radford et al., 2018; 2019) .
In this paper, we focus on improving RNN-based language models that often have much fewer parameters and are easier to perform end-to-end training.
While RNN-based language models do not ignore word order, they often assume that the sentences of a document are independent to each other.
This simplifies the modeling task to independently assigning probabilities to individual sentences, ignoring their orders and document context (Tian & Cho, 2016) .
Such language models may consequently fail to capture the long-range dependencies and global semantic meaning of a document (Dieng et al., 2017; .
To relax the sentence independence assumption in language modeling, Tian & Cho (2016) propose larger-context language models that model the context of a sentence by representing its preceding sentences as either a single or a sequence of BoW vectors, which are then fed directly into the sentence modeling RNN.
An alternative approach attracting significant recent interest is leveraging topic models to improve RNN-based language models.
Mikolov & Zweig (2012) use pre-trained topic model features as an additional input to the RNN hidden states and/or output.
Dieng et al. (2017) ; Ahn et al. (2017) combine the predicted word distributions, given by both a topic model and a language model, under variational autoencoder (Kingma & Welling, 2013) .
Lau et al. (2017) introduce an attention based convolutional neural network to extract semantic topics, which are used to extend the RNN cell.
learn the global semantic coherence of a document via a neural topic model and use the learned latent topics to build a mixture-of-experts language model.
Wang et al. (2019) further specify a Gaussian mixture model as the prior of the latent code in variational autoencoder, where each mixture component corresponds to a topic.
While clearly improving the performance of the end task, these existing topic-guided methods still have clear limitations.
For example, they only utilize shallow topic models with only a single stochastic hidden layer in their data generation process.
Note several neural topic models use deep neural networks to construct their variational encoders, but still use shallow generative models (decoders) (Miao et al., 2017; Srivastava & Sutton, 2017) .
Another key limitation lies in ignoring the sentence order, as they treat each document as a bag of sentences.
Thus once the topic weight vector learned from the document context is given, the task is often reduced to independently assigning probabilities to individual sentences (Lau et al., 2017; 2019) .
In this paper, as depicted in Fig. 1 , we propose to use recurrent gamma belief network (rGBN) to guide a stacked RNN for language modeling.
We refer to the model as rGBN-RNN, which integrates rGBN , a deep recurrent topic model, and stacked RNN (Graves, 2013; Chung et al., 2017) , a neural language model, into a novel larger-context RNN-based language model.
It simultaneously learns a deep recurrent topic model, extracting document-level multi-layer word concurrence patterns and sequential topic weight vectors for sentences, and an expressive language model, capturing both short-and long-range word sequential dependencies.
For inference, we equip rGBN-RNN (decoder) with a novel variational recurrent inference network (encoder), and train it end-to-end by maximizing the evidence lower bound (ELBO).
Different from the stacked RNN based language model in Chung et al. (2017) , which relies on three types of customized training operations (UPDATE, COPY, FLUSH) to extract multi-scale structures, the language model in rGBN-RNN learns such structures purely under the guidance of the temporally and hierarchically connected stochastic layers of rGBN.
The effectiveness of rGBN-RNN as a new larger-context language model is demonstrated both quantitatively, with perplexity and BLEU scores, and qualitatively, with interpretable latent structures and randomly generated sentences and paragraphs.
Notably, rGBN-RNN can generate a paragraph consisting of a sequence of semantically coherent sentences.
We propose a recurrent gamma belief network (rGBN) guided neural language modeling framework, a novel method to learn a language model and a deep recurrent topic model simultaneously.
For scalable inference, we develop hybrid SG-MCMC and recurrent autoencoding variational inference, allowing efficient end-to-end training.
Experiments results conducted on real world corpora demonstrate that the proposed models outperform a variety of shallow-topic-model-guided neural language models, and effectively generate the sentences from the designated multi-level topics or noise, while inferring interpretable hierarchical latent topic structure of document and hierarchical multiscale structures of sequences.
For future work, we plan to extend the proposed models to specific natural language processing tasks, such as machine translation, image paragraph captioning, and text summarization.
Another promising extension is to replace the stacked-RNN in rGBN-RNN with Transformer, i.e., constructing an rGBN guided Transformer as a new larger-context neural language model.

assistant: text: We introduce a novel larger-context language model to simultaneously captures syntax and semantics, making it capable of generating highly interpretable sentences and paragraphs