Zero Pronouns as Experiencer in Japanese Discourse

Zero Pronouns as Experiencer in Japanese Discourse Hiroshi Nakagawa Dept. of Electronics and Computer Engineering, Yokohama National University, 156 Tokiwadai, Hodogaya, Yokohama, 240, Japan e-mail: nakagawa@ naklab.dnj.ynu.ac.jp Abstract The process of finding the antecedent of zero pronoun, that is indispensable to Japanese language understanding, is the topic of this paper. Here we mainly concern with dis- courses comprising two sentences that are in a subordinate relation, especially one of them describes the agent's volitional action and the other describes the reason of the action. We propose basically two new principles: (1) The agent of an action should experience a certain psychological reason, (2) Predicates report- ing someone's psychological state are catego- rized into 1) weakly or 2) strongly bound to the expected point of view. Combination of these principles accounts for some problem- atic Japanese zero anaphora, which cannot be accounted for by the theories so far pro- posed. 1 Introduction Kuno said in [7] that in Japanese discourses we have to omit as many components in a sentence as possible unless we get any ambi- guity. In fact more components are omitted than expected in actual discourses, because we have a rich variety of linguistic appara- tuses to be used to communicate unambigu- ously. From the computational viewpoint identifying the antecedent of zero pronoun, which is the omitted part of sentence, is re- ally needed. For this several theories have been developed. Among them we first review the two most important theories in analyzing zero pro- noun's antecedent in Japanese proposed in 80's. The first one is Centering theory in Japanese proposed by Walker etal [8]. Each utterance of a discourse has a set of discourse entities called forward looking cen- ters, Cf. Cf's for one utterance are ranked according to their discourse sahence. In Japanese Cf's are ranked in the following or- der of preference: TOPIC>EMPATHY>SUBJ>OBJ2>OBJ Now we have the following constraints and rules to calculate the backward looking cen- ter, Cb, which is regarded as the most salient discourse entity picked up from Cf's of the previous utterance. Constraints and Rules For each Ui in a discourse segment UI,...,U,~ 1. There is precisely one Cb. 2. Every dement of Cf(U;_I) must be realized in Ui. Notice that if the antecedent of zero pronoun in U; is known to be one of Cf(U;-1), then that Cf is regarded to be realized in Ui. 3. The center, Cb(Ui) , is the highest ranked element of Cf(U;_I) that is realized in U;. 4. If some element of Cf(Ui_t) is reahzed as a pronoun in U;, then so is Cb(U/). 5. Transition states to decide Cb(Ui) are or- dered: Continuing is preferred to Retaining is preferred to Shifting-1 is preferred to Shift- ing. These states are characterized as follows. Here Cp(U~) means the highest ranked Cf(Ui). Continuing: Cb(U/_I) = Cb(Ui) = Cp(U/) Retaining: Cb(U,_I) = Cb(Ui) # Cp(Ui) Shifting-l: Cb(U,_t) # Cb(U,) = Cp(U,) Shifting: Cb(U;_~) # Cb(Ui) # Cp(Ui) Actually, the antecedent of zero pronoun is determined under satisfying all …

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