Intra-sentential Zero Anaphora Resolution using Subject Sharing Recognition

Proceedings of the 2015 Conference on Empirical Methods in Natural Language Processing, pages 2179–2189, Lisbon, Portugal, 17-21 September 2015. c⃝2015 Association for Computational Linguistics. Intra-sentential Zero Anaphora Resolution using Subject Sharing Recognition Ryu Iida Kentaro Torisawa Chikara Hashimoto Jong-Hoon Oh Julien Kloetzer National Institute of Information and Communications Technology Kyoto 619-0289, Japan {ryu.iida,torisawa,ch,rovellia,julien}@nict.go.jp Abstract In this work, we improve the performance of intra-sentential zero anaphora resolu- tion in Japanese using a novel method of recognizing subject sharing relations. In Japanese, a large portion of intra- sentential zero anaphora can be regarded as subject sharing relations between pred- icates, that is, the subject of some predi- cate is also the unrealized subject of other predicates. We develop an accurate rec- ognizer of subject sharing relations for pairs of predicates in a single sentence, and then construct a subject shared pred- icate network, which is a set of predi- cates that are linked by the subject shar- ing relations recognized by our recognizer. We finally combine our zero anaphora resolution method exploiting the subject shared predicate network and a state-of- the-art ILP-based zero anaphora resolution method. Our combined method achieved a significant improvement over the the ILP- based method alone on intra-sentential zero anaphora resolution in Japanese. To the best of our knowledge, this is the first work to explicitly use an independent sub- ject sharing recognizer in zero anaphora resolution. 1 Introduction In ‘pro-dropped’ languages such as Japanese, Chi- nese and Italian, pronouns are often unrealized in text. For example, the subject of nomu (take) is omitted in example (1). (1) Tomi-wa infuruenza-ni natta-node , Tom-TOP flu-IOBJ had-since punc (ϕi-ga) kusuri-o non-da . hei-SUBJ medicine-OBJ took period Since Tomi had the flu, (hei) took medicine. Such unrealized pronouns are regarded as zero anaphors, which are indicated using ϕ in liter- ature, like ϕi-ga in example (1). refers to its antecedent somewhere.Zero anaphor This phe- nomenon of the reference is called zero anaphora. In Japanese, about 60% of subjects appear as zero anaphors in newspaper articles (Iida et al., 2007b), and thus zero anaphora resolution is an essential task for developing highly accurate machine trans- lation and information extraction systems. In this paper, we propose a novel method of re- solving intra-sentential zero anaphora, in which a subject zero anaphor refers to its antecedent in- side a single sentence. This work does not ad- dress inter-sentential zero anaphora, in which a zero anaphor in a sentence refers to its antecedent in another sentence. The novelty of our method is in the use of subject sharing relations, which are relations between two predicates that share a subject by (zero) anaphora or coreference. For ex- ample, in example (2), there are two subject shar- ing relations for predicate pairs, advance-plan and plan-dispatch, as illustrated in Figure 1. (2) seifui-wa (ϕi-ga) hisaichi-ni governmenti-TOP iti-SUBJ disaster site-IOBJ 50 nin-o hakensuru koto-o (ϕi-ga) 50 people-OBJ dispatch COMP-OBJ iti-SUBJ keikakusi junbisagyo-o susumeru . plan preparation-OBJ advance period The governmenti plans that (iti) will dispatch 50 people to the …

Content truncated to 500 words