Discourse Coherence and Shifting Centers in Japanese Texts

In languages such as Japanese, the use of zeros, unexpressed arguments of the verb, in utterances that shift the topic involves a risk that the meaning intended by the speaker may not be transparent to the hearer. However, this potentially undesirable conversational strategy often occurs in the course of naturally-occurring discourse. In this chapter, I report on an empirical study of 250 utterances with zeros in 20 Japanese newspaper articles. Each utterance is analyzed in terms of centering transitions and the form in which centers are realized by referring expressions. I also examine lexical subcategorization information, and tense and aspect in order to test the hypothesis that the speaker expects the hearer to use this information in determining global discourse structure. I explain the occurrence of zeros in retain and rough-shift centering transitions, by claiming that a zero can only be used in these cases when the shift of centers is supported by contextual information such as lexical semantics, tense and aspect, and agreement features. I then propose an algorithm by which centering can incorporate these observations to integrate centering with global discourse structure, and thus enhance its ability for non-local pronoun resolution.