Sharp Nearby, Fuzzy Far Away: How Neural Language Models Use Context

Urvashi Khandelwal, He He, Peng Qi, Dan Jurafsky

Abstract
We know very little about how neural language models (LM) use prior linguistic context. In this paper, we investigate the role of context in an LSTM LM, through ablation studies. Specifically, we analyze the increase in perplexity when prior context words are shuffled, replaced, or dropped. On two standard datasets, Penn Treebank and WikiText-2, we find that the model is capable of using about 200 tokens of context on average, but sharply distinguishes nearby context (recent 50 tokens) from the distant history. The model is highly sensitive to the order of words within the most recent sentence, but ignores word order in the long-range context (beyond 50 tokens), suggesting the distant past is modeled only as a rough semantic field or topic. We further find that the neural caching model (Grave et al., 2017b) especially helps the LSTM to copy words from within this distant context. Overall, our analysis not only provides a better understanding of how neural LMs use their context, but also sheds light on recent success from cache-based models.
Anthology ID:
P18-1027
Volume:
Proceedings of the 56th Annual Meeting of the Association for Computational Linguistics (Volume 1: Long Papers)
Month:
July
Year:
2018
Address:
Melbourne, Australia
Editors:
Iryna Gurevych, Yusuke Miyao
Venue:
ACL
SIG:
Publisher:
Association for Computational Linguistics
Note:
Pages:
284–294
Language:
URL:
https://aclanthology.org/P18-1027
DOI:
10.18653/v1/P18-1027
Bibkey:
Cite (ACL):
Urvashi Khandelwal, He He, Peng Qi, and Dan Jurafsky. 2018. Sharp Nearby, Fuzzy Far Away: How Neural Language Models Use Context. In Proceedings of the 56th Annual Meeting of the Association for Computational Linguistics (Volume 1: Long Papers), pages 284–294, Melbourne, Australia. Association for Computational Linguistics.
Cite (Informal):
Sharp Nearby, Fuzzy Far Away: How Neural Language Models Use Context (Khandelwal et al., ACL 2018)
Copy Citation:
PDF:
https://preview.aclanthology.org/nschneid-patch-2/P18-1027.pdf
Note:
 P18-1027.Notes.pdf
Poster:
 P18-1027.Poster.pdf
Code
 urvashik/lm-context-analysis
Data
Penn Treebank, WikiText-2
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