textTOvec: DEEP CONTEXTUALIZED NEURAL AUTOREGRESSIVE TOPIC MODELS OF LANGUAGE WITH DISTRIBUTED COMPOSITIONAL PRIOR
Authors: Pankaj Gupta, Yatin Chaudhary, Florian Buettner, Hinrich Schuetze
Keywords: neural topic model, natural language processing, text representation, language modeling, information retrieval, deep learning
TL;DR: Unified neural model of topic and language modeling to introduce language structure  in topic models for contextualized topic vectors 
Abstract: We address two challenges of probabilistic topic modelling in order to better estimate
the probability of a word in a given context, i.e., P(wordjcontext) : (1) No
Language Structure in Context: Probabilistic topic models ignore word order by
summarizing a given context as a “bag-of-word” and consequently the semantics
of words in the context is lost. In this work, we incorporate language structure
by combining a neural autoregressive topic model (TM) with a LSTM based language
model (LSTM-LM) in a single probabilistic framework. The LSTM-LM
learns a vector-space representation of each word by accounting for word order
in local collocation patterns, while the TM simultaneously learns a latent representation
from the entire document. In addition, the LSTM-LM models complex
characteristics of language (e.g., syntax and semantics), while the TM discovers
the underlying thematic structure in a collection of documents. We unite two complementary
paradigms of learning the meaning of word occurrences by combining
a topic model and a language model in a unified probabilistic framework, named
as ctx-DocNADE. (2) Limited Context and/or Smaller training corpus of documents:
In settings with a small number of word occurrences (i.e., lack of context)
in short text or data sparsity in a corpus of few documents, the application of TMs
is challenging. We address this challenge by incorporating external knowledge
into neural autoregressive topic models via a language modelling approach: we
use word embeddings as input of a LSTM-LM with the aim to improve the wordtopic
mapping on a smaller and/or short-text corpus. The proposed DocNADE
extension is named as ctx-DocNADEe.

We present novel neural autoregressive topic model variants coupled with neural
language models and embeddings priors that consistently outperform state-of-theart
generative topic models in terms of generalization (perplexity), interpretability
(topic coherence) and applicability (retrieval and classification) over 6 long-text
and 8 short-text datasets from diverse domains.