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Title: Unsupervised Distillation of Syntactic Information from Contextualized Word Representations

Paper content:
Contextualized word representations, such as ELMo and BERT, were shown to perform well on a various of semantic and structural (syntactic) task.
In this work, we tackle the task of unsupervised disentanglement between semantics and structure in neural language representations: we aim to learn a transformation of the contextualized vectors, that discards the lexical semantics, but keeps the structural information.
To this end, we automatically generate groups of sentences which are structurally similar but semantically different, and use metric-learning approach to learn a transformation that emphasizes the structural component that is encoded in the vectors.
We demonstrate that our transformation clusters vectors in space by structural properties, rather than by lexical semantics.
Finally, we demonstrate the utility of our distilled representations by showing that they outperform the original contextualized representations in few-shot parsing setting.
Human language 1 is a complex system, involving an intricate interplay between meaning (semantics) and structural rules between words and phrases (syntax).
Self-supervised neural sequence models for text trained with a language modeling objective, such as ELMo (Peters et al., 2018) , BERT (Devlin et al., 2019) , and RoBERTA (Liu et al., 2019b) , were shown to produce representations that excel in recovering both structure-related information (Gulordava et al., 2018; van Schijndel & Linzen; Wilcox et al., 2018; Goldberg, 2019) as well as in semantic information (Yang et al., 2019; Joshi et al., 2019) .
In this work, we study the problem of disentangling structure from semantics in neural language representations: we aim to extract representations that capture the structural function of words and sentences, but which are not sensitive to their content.
For example, consider the sentences:
We aim to learn a function from contextualized word representations to a space that exposes these similarities.
Crucially, we aim to do this in an unsupervised manner: we do not want to inform the process of the kind of structural information we want to obtain.
We do this by learning a transformation that attempts to remove the lexical-semantic information in a sentence, while trying to preserve structural properties.
Disentangling syntax from lexical semantics in word representations is a desired property for several reasons.
From a purely scientific perspective, once disentanglement is achieved, one can better control for confounding factors and analyze the knowledge the model acquires, e.g. attributing the predictions of the model to one factor of variation while controlling for the other.
In addition to explaining model predictions, such disentanglement can be useful for the comparison of the representations the model acquires to linguistic knowledge.
From a more practical perspective, disentanglement can be a first step toward controlled generation/paraphrasing that considers only aspects of the structure, akin to the style-transfer works in computer vision, i.e., rewriting a sentence while preserving its structural properties while ignoring its meaning, or vice-versa.
It can also inform search-based application in which one can search for "similar" texts while controlling various aspects of the desired similarity.
To achieve this goal, we begin with the intuition that the structural component in the representation (capturing the form) should remain the same regardless of the lexical semantics of the sentence (the meaning).
Rather than beginning with a parsed corpus, we automatically generate a large number of structurally-similar sentences, without presupposing their formal structure ( §3.1).
This allows us to pose the disentanglement problem as a metric-learning problem: we aim to learn a transformation of the contextualized representation, which is invariant to changes in the lexical semantics within each group of structurally-similar sentences ( §3.3).
We demonstrate the structural properties captured by the resulting representations in several experiments ( §4), among them automatic identification of structurally-similar words and few-shot parsing.
In this work, we propose an unsupervised method for the distillation of structural information from neural contextualized word representations.
We used a process of sequential BERT-based substitu- Figure 4 : Results of the few shot parsing setup tion to create a large number of sentences which are structurally similar, but semantically different.
By controlling for one aspect -structure -while changing the other -lexical choice, we learn a metric (via triplet loss) under which pairs of words that come from structurally-similar sentences are close in space.
We demonstrated that the representations acquired by this method share structural properties with their neighbors in space, and show that with a minimal supervision, those representations outperform ELMo in the task of few-shots parsing.
The method presented here is a first step towards a better disentanglement between various kinds of information that is represented in neural sequence models.
The method used to create the structurally equivalent sentences can be useful by its own for other goals, such as augmenting parse-tree banks (which are often scarce and require large resources to annotate).
In a future work, we aim to extend this method to allow for a more soft alignment between structurally-equivalent sentences.
Table 4 : Results in the closest-word queries, before and after the application of the syntactic transformation.
"Basline" refers to unmodified vectors derived from BERT, and "Transformed" refers to the vectors after the learned syntactic transformation f .
"Difficult" refers to evaluation on the subset of POS tags which are most structurally diverse.

assistant: text: We distill language models representations for syntax by unsupervised metric learning