Anaphora Resolution of Japanese Zero Pronouns with Deictic Reference

This paper proposes a method to resolve the reference of deictic Japanese zero pronouns which can be implemented in a practical machine translation system. This method focuses on semantic and pragmatic constraints such as semantic constraints on cases, modal expressions, verbal semantic attributes and conjunctions to determine the deictic reference of Japanese zero pronouns. This method is highly effective because the volume of knowledge that must be prepared beforehand is not very large and its precision of resolution is good. This method was implemented in the Japanese-to-English machine translation system, ALT-J/E. According to a window test for 175 zero pronouns with deictic referent in a sentence set for the evaluation of Japanese-to-English machine translation systems, all of zero pronouns could be resolved consistently and correctly. 1 Introduction In all natural language, elements that can be easily deduced by the reader are frequently omitted from expressions in texts (Kuno, 1978). This phenomenon causes considerable problems in natural language processing systems. For example in a machine translation system, the system needs to recognize that elements which are not present in the source language, may become mandatory elements in the target language. In particular, the subject and object are often omitted in Japanese; whereas they are often mandatory in English. Thus, in Japanese-to-English machine translation systems, it is necessary to identify case elements omitted from the original Japanese (these are referred to as "zero pronouns") for their translation into English expressions. Several methods have been proposed with re-When considering the application of these methods to a practical machine translation system for which the translation target area can not be limited , it is not possible to apply them directly, both because their precision of resolution is low as they only use limited information, and because the volume of knowledge that must be prepared betbre-hand is so large. The zero pronouns that must be resolved by a machine translation system carl be classitied into 3 types; (a) zero pronouns with antecedents within the same sentence (intrasenteutial), (b) zero pronouns with antecedents elsewhere in the text (in-tersentential) and (c) zero pronouns with deictic reference (extrasentential). Regarding type (b), Nakaiwa and Ikehara (1992) proposed a method to determine the intersentential antecedents using verbal semantic attributes. The rules used in this method are independent of the field of the source text. Therefore, anaphora resolution may be conducted with a relatively small volume of knowledge, making the proposed …