Resolving Zero Anaphora in Japanese

The paper presents a computational theory for resolving Japanese zero anaphora, based on the notion of discourse segment. We see that the discourse segment reduces the domain of antecedents for zero anaphora and thus leads to their efficient resolution. Also we make crucial use of functional notions such as empathy hierarchy and minimal semantics thesis to resolve reference for zero anaphora [Kuno, 1987]. Our al)proach differs from the Centering analysis [Walker et al., 1990] in that the resolution works by matching one empathy hierarchy against another, which makes it possible to deal with discourses with no explicit topic and those with cataphora [Halliday and Hassan, 1990]. The theory is formalized through the definite clause grammar(DCG) formalism [Pereira and Warren, 1980],[Gazdar and Mellish, 1989; Longacre, 1979]. Finally, we show that graphology i.e., quotation mark, spacing, has an important effect on the interpretation of zero anaphora in Japanese discourse. 1 I n t r o d u c t i o n Over the past years, schemes like Focusing and Centering have dominated computational approaches to resolving anaphora [Sidner, 1983; Walker et al., 1990]. Their success derives from the utility they have in identifying salient discourse entities such as topic and thereby locating the antecedent for an anaphor. But they all suffer from the problem of directionality; they process the text (the list of sentences) from left to right, picking out focus along the way and see if a anaphor corefers with a focus already encountered. With the one-way processing, forward-looking pronouns (cataphora) are not possible to resolve. Since Japanese has great tolerance with forward reference, a proper theory of zero pronouns should meet the problem of directionality. In what follows, we discuss some points about discourse segment and zero pronoun in Japanese. We begin by introducing the idea of discourse segment. Consider the pair: (1) Taro-go sara-wo dasi, Hanako nora plate ace prepare-and -go 02 ryori -wo morituketa. nora food acc arranged Taro prepared the plates, Hanako arranged food on them. (2) Taro -ga sara<~> -wo dasi, Hanako -wa top 01 02 ryori-wo morituketa. Taro prepared the plates, Hanako arranged food. Here, 02 represents a suppressed expression. It acts as an indirect object of the verb moritsuketa. 1 1 and 2 are morphologically identical except that 1 has ga (nominative marker) where 2 has wa (topic marker). But they differ widely in meaning:l implies that Hanako arranged food on the plates that Taro prepared, the reading 2 does not imply; in 2, 1Here and throughout, we intend the term 01 to represent a zero pronoun for the subject, 02 for the indirect object, and 03 for the direct object.