Zero Pronouns as Experiencer in Japanese Discourse

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 discourses 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 reporting someone's psychological state are categorized into 1) weakly or 2) strongly bound to the expected point of view. Combination of these principles accounts for some problematic Japanese zero anaphora, which cannot be accounted for by the theories so far proposed. 1 I n t r o d u c t i o n Kuno said in [7] that in Japanese discourses we have to omit as many components in a sentence as possible unless we get any ambiguity. In fact more components are omitted than expected in actual discourses, because we have a rich variety of linguistic apparatuses to be used to communicate unambiguously. From the computational viewpoint identifying the antecedent of zero pronoun, which is the omitted part of sentence, is really needed. For this several theories have been developed. Among them we first review the two most important theories in analyzing zero pronoun'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 centers, Cf. Cf's for one utterance are ranked according to their discourse sahence. In Japanese Cf's are ranked in the following order of preference: TOPIC>EMPATHY>SUBJ>OBJ2>OBJ Now we have the following constraints and rules to calculate the backward looking center, Cb, which is regarded as the most salient discourse entity picked up from Cf's of the previous utterance. Cons t r a in t s and Ru les 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 ordered: Continuing is preferred to Retaining is preferred to Shifting-1 is preferred to Shifting. 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 these constraints and rules. This centering mechanism can account for a very broad range of Japanese zero pronoun anaphora. Almost the same centering mechanism applies to Italian pronominal system successfully [2]. Acr~ DB COLING-92, NANTES, 23-28 AOl3"r 1992 3 2 4 PROC. OF COLING-92, NANTES, AU~. 23-28, 1992 Another important theory is Property sharing theory proposed by Kameyama [4, 5]. Her theory concerns the interaction between Cb and zero pronouns. Consider adjacent uttermaces or a sentence including a subordinated clause. Two zero pronouns appearing distinct utterances or clauses can retain the same Cb if they share one of the following properties: 1) IDENT-SUBJECT, 2) IDENT alone, 3) SUBJECT alone, 4) bott, nonIDENT and non-SUBJECT, 5)non-IDENT only, 6)non-SUBJ only, where the descending order means preference, and IDENT is almost the same as, or very near, the empathy proposed by Kuno [7]. Although these theories cover a fairly large part of Japanese zero anaphora resolution, there still remain the problematic cases which can be resolved by neither of them. In the following sections, we will focus on some of the problematic cases and show how to deal with them. 2 Interac t ions b e t w e e n