Japanese Discourse and the Process of Centering

This paper has three aims: (1) to generalize a computational account of the discourse process called {\sc centering}, (2) to apply this account to discourse processing in Japanese so that it can be used in computational systems for machine translation or language understanding, and (3) to provide some insights on the effect of syntactic factors in Japanese on discourse interpretation. We argue that while discourse interpretation is an inferential process, syntactic cues constrain this process, and demonstrate this argument with respect to the interpretation of {\sc zeros}, unexpressed arguments of the verb, in Japanese. The syntactic cues in Japanese discourse that we investigate are the morphological markers for grammatical {\sc topic}, the postposition {\it wa}, as well as those for grammatical functions such as {\sc subject}, {\em ga}, {\sc object}, {\em o} and {\sc object2}, {\em ni}. In addition, we investigate the role of speaker's {\sc empathy}, which is the viewpoint from which an event is described. This is syntactically indicated through the use of verbal compounding, i.e. the auxiliary use of verbs such as {\it kureta, kita}. Our results are based on a survey of native speakers of their interpretation of short discourses, consisting of minimal pairs, varied by one of the above factors. We demonstrate that these syntactic cues do indeed affect the interpretation of {\sc zeros}, but that having previously been the {\sc topic} and being realized as a {\sc zero} also contributes to the salience of a discourse entity. We propose a discourse rule of {\sc zero topic assignment}, and show that {\sc centering} provides constraints on when a {\sc zero} can be interpreted as the {\sc zero topic}.