Tackling Zero Pronoun Resolution and Non-Zero Coreference Resolution Jointly
Summary
Chinese text often omits pronouns, so a system must recover these hidden "zero pronouns" and link them to their antecedents. It must also cluster explicit mentions of the same entity, which is ordinary (non-zero) coreference. Earlier work handled these tasks separately, often assumed that zero pronoun positions were known, or depended on hand-crafted syntactic features.
Chen et al. propose one end-to-end neural model that solves both tasks together without gold zero pronoun positions. Every gap between tokens is a candidate zero pronoun, and every span is a candidate mention. A gap-masked self-attention encoder places gaps and tokens in the same embedding space. Gaps draw context from neighbouring tokens, while token representations stay unaffected. A two-stage interaction mechanism uses the fact that a gap inside a mention cannot be a zero pronoun, and vice versa. It applies this constraint at both the representation level and the scoring level.
Tested on OntoNotes 5.0 Chinese, the joint model beats strong baselines on both tasks. This shows that the two problems benefit from shared learning.
Key points
- The model builds on the BERT-based coreference system of Joshi et al. (2019), using the official Chinese BERT-base. Maximum span width is reduced from 30 to 20 tokens to save memory and computation.
- Training jointly improves non-zero coreference by at least 0.8% over a mention-only baseline. Adding the coreference objective raises zero pronoun resolution by more than 0.46%, compared with Song et al. (2020) as the prior state of the art.
- In the encoder comparison, gap-masked self-attention performed best, with NZ-F1 of 67.40 and Z-F1 of 33.77. Inserting gaps directly into BERT performed worst, with 58.96 and 27.96.
- The score-level interaction with max aggregation produced the best scores, NZ-F1 67.71 and Z-F1 34.13. Max aggregation generally did better than mean. Not allowing mentions to resolve to zero pronouns as antecedents ("Not to ZP") worked better overall.
- Code is available at github.com/cheniison/e2e-joint-coref.
Cited by
- CorefInst: Leveraging LLMs for Multilingual Coreference Resolution
- Investigating Multilingual Coreference Resolution by Universal Annotations
Cite
@inproceedings{chen2021tackling,
title = {{Tackling Zero Pronoun Resolution and Non-Zero Coreference Resolution Jointly}},
author = {Chen, Shisong and Gu, Binbin and Qu, Jianfeng and Li, Zhixu and Liu, An and Zhao, Lei and Chen, Zhigang},
booktitle = {Proceedings of the 25th Conference on Computational Natural Language Learning},
year = {2021},
doi = {10.18653/v1/2021.conll-1.40},
url = {https://doi.org/10.18653/v1/2021.conll-1.40}
}References
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