A Cross-Lingual ILP Solution to Zero Anaphora Resolution

Proceedings of the 49th Annual Meeting of the Association for Computational Linguistics, pages 804–813, Portland, Oregon, June 19-24, 2011. c⃝2011 Association for Computational Linguistics A Cross-Lingual ILP Solution to Zero Anaphora Resolution Ryu Iida Tokyo Institute of Technology 2-12-1, ˆOokayama, Meguro, Tokyo 152-8552, Japan ryu-i@cl.cs.titech.ac.jp Massimo Poesio Universit`a di Trento, Center for Mind / Brain Sciences University of Essex, Language and Computation Group massimo.poesio@unitn.it Abstract We present an ILP-based model of zero anaphora detection and resolution that builds on the joint determination of anaphoricity and coreference model proposed by Denis and Baldridge (2007), but revises it and extends it into a three-way ILP problem also incorporat- ing subject detection. We show that this new model outperforms several baselines and com- peting models, as well as a direct translation of the Denis / Baldridge model, for both Italian and Japanese zero anaphora. We incorporate our model in complete anaphoric resolvers for both Italian and Japanese, showing that our approach leads to improved performance also when not used in isolation, provided that sep- arate classifiers are used for zeros and for ex- plicitly realized anaphors. 1 Introduction In so-called ‘pro-drop’ languages such as Japanese and many romance languages including Italian, phonetic realization is not required for anaphoric references in contexts in which in English non- contrastive pronouns are used: e.g., the subjects of Italian and Japanese translations of buy in (1b) and (1c) are not explicitly realized. We call these non- realized mandatory arguments zero anaphors. (1) a. [EN] [John]i went to visit some friends. On the way, [he]i bought some wine. b. [IT] [Giovanni]i and`o a far visita a degli am- ici. Per via, φi compr`o del vino. c. [JA] [John]i-wa yujin-o houmon-sita. Tochu-de φi wain-o ka-tta. The felicitousness of zero anaphoric reference depends on the referred entity being sufficiently salient, hence this type of data–particularly in Japanese and Italian–played a key role in early work in coreference resolution, e.g., in the devel- opment of Centering (Kameyama, 1985; Walker et al., 1994; Di Eugenio, 1998). This research high- lighted both commonalities and differences between the phenomenon in such languages. Zero anaphora resolution has remained a very active area of study for researchers working on Japanese, because of the prevalence of zeros in such languages1 (Seki et al., 2002; Isozaki and Hirao, 2003; Iida et al., 2007a; Taira et al., 2008; Imamura et al., 2009; Sasano et al., 2009; Taira et al., 2010). But now the availabil- ity of corpora annotated to study anaphora, includ- ing zero anaphora, in languages such as Italian (e.g., Rodriguez et al. (2010)), and their use in competi- tions such as SEMEVAL 2010 Task 1 on Multilin- gual Coreference (Recasens et al., 2010), is lead- ing to a renewed interest in zero anaphora resolu- tion, particularly at the light of the mediocre results obtained on zero anaphors by most systems partici- pating in SEMEVAL. Resolving zero anaphora requires the simulta- neous decision that one of the arguments of a verb is phonetically unrealized (and which argu- ment …

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