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<Paper uid="W99-0102">
  <Title>e O O O O O O O O 0 O O @ O O 0 O O O O O O O O O O O 0 O O O @ O O O O @ O O O Approachesto Japanese zero pronouns: Centering and relevance</Title>
  <Section position="2" start_page="0" end_page="12" type="intro">
    <SectionTitle>
1. Introduction
</SectionTitle>
    <Paragraph position="0"> When a system has to assign a referent to a referring expression, it is almost always the case that there is more than one candidate referents and one has to resort to some Way of eliminating the wrong candidates in order to choose the right one.</Paragraph>
    <Paragraph position="1"> There are two main ways of doing this.</Paragraph>
    <Paragraph position="2"> First, candidate referents can be ranked on the basis of accessibility (Erlm &amp; Gundel 1987; Fretheim and Gundal 1996; C~mbacher and Hargreaves 1988, Grosz et al. 1995; Sidner 1983a, b; Walker et al. 1998). Alternatively, they can be checked against the accessibility of contextual assumptions (F'mcher-Kiefex 1993; Kintsch 1988, Magliano et al. 1993; McKoon &amp; Ratcliff 1992; Sanford &amp; Garrod 1981; Sharkey &amp; Sharkey 1987; Singer 1993, 1995).</Paragraph>
    <Paragraph position="3"> In either case, once a candidate is singled out, the acceptability of the referent needs to be tested against some pragmatic criteria.</Paragraph>
    <Paragraph position="4"> Currently, we seem to have at least three distinct criteria available: i. Truth-based criterion- i.e. whether the overall, interpretation is likely to be factually plausible (Clark 1977; Clark &amp;</Paragraph>
    <Paragraph position="6"> the overall interpretation is likely to be coherent (Asher &amp; Lascaries 1993; Grosz et al. 1995, 1998; Hobbs 1979; Lascarides &amp; Asher 1993; Sanders et aL 1992; Walker et aL 1994, 1998); Hi. Relevance-based criterion - i.e. whether the overall interpretation is likely to be optimally relevant (Matsui 1993, 1995, 1998; Wilson 1992; Wilson &amp; Matsui 1998).</Paragraph>
    <Paragraph position="7">  o In this paper, I will focus on one version of coherence-based criterion, namely, centering theory. One of the goals of centering theory is to sort out the various mechanisms used to maintain discourse coherence, and the use of various referring expressions is regarded as one such mechanism. Among the various hypotheses put forward by centering theory, what concerns us most is the following: that 'each utterance \[except the initial utterance\] in a coherent discourse segment contains a single semantic entity - the backward-looking center \[or Cb\] - that provides a link to the previous utterance, and an ordered set of entities - the forward-looking centers \[or Cf\] - that offer potential links to the next utterance'(Gordon et al. 1993:311). There are two rules to provide constraints on choosing centers, which are shown in (1): ( l)Rule 1: If any element of Cf (Un) is realised by a pronoun in Un+l then the Cb (Un+l) must he realised by a pronoun also.</Paragraph>
    <Paragraph position="8"> Rule 2: Sequences of continuation are preferred over sequences of retaining; and sequences of retaining are to be preferred over sequences of shifting.</Paragraph>
    <Paragraph position="9"> The first rule states that the most highly ranked element of the forward-looking center of a previous utterance is the backward-looking center of the current utterance, and must be realised by a pronoun if any element of the Cf of the previous utterance is realised by a pronoun in the current utterance. The following example from Gordon et ai.(1993). shown here in (2). illustrates this nde: (2)\[l\]Susan gave Betsy a pet hamster.</Paragraph>
    <Paragraph position="10"> Cf={Susan, Betsy, hamster I ) \[2\]She reminded her such hamsters were quite shy..</Paragraph>
    <Paragraph position="11"> Cb=Susa,; Cf={St~m. Betsy. hamsters} \[3\]She asked Betsy whether she liked the gift.</Paragraph>
    <Paragraph position="12"> Cb~usan; Cf=-\[Susan. Betsy, gift=hamster I } \[3 &amp;quot;\]Susan asked her whether she liked the gift.</Paragraph>
    <Paragraph position="13"> Here, the first utterance has no Cb because it is  the initial sen'tence of a discourse. Its Cf includes the referents off'Susan' and 'Betsy' and the semantic interpretation of 'a hamster', ranked in that order. The second utterance has Susan as the Cb and a Cf with Susan as its most highly ranked element. The third utterance preserves the Cb and prominent Cf from the previous utterance, therefore it pronominalises the Cb successfully. By contrast, utterance \[3 1, in which Susan is realised by a name and Betsy is realised by a pronoun, leads to stylistic infelicity. According to Gordon et al., this is due to violation of Rule 1 mentioned above.</Paragraph>
    <Paragraph position="14"> The ranking of forward-looking centers is generally based on the discourse salience of each candidate entity. According to Grosz et aL (1995), although an ultimate criteria for deciding the ranking has not been worked out yet, there are evidences to support the idea that grammatical role such as SUBJECT, OBJECT, etc., can affect the Cf ranking. Thus, I will simply assume here the following preference in ranking forward-looking center shown in (3), as suggested by Grosz et al. (ibid.):</Paragraph>
    <Paragraph position="16"> The highest ranked member of the set of forward-looking centers is called the 'preferred center; or Up&amp;quot; As mentioned above, Cp is regarded as the most likely candidate for Cb in the following utterance.</Paragraph>
    <Paragraph position="17"> Mother important claim made by centering theory is that discourse segments are more coherent if they share the same Cb. On the basis of this idea, different degrees of coherence are proposed. For example, Walker et al. adopt the following 4 types of transition between discourse segments, each corresponding to different degree of coherence, using the notion of Cb and Cp;.</Paragraph>
    <Paragraph position="18"> namely. 'continue. 'retain; 'smooth-shift&amp;quot; and &amp;quot;rough-shift'. These are shown in (4). When two utterances, say Ui-I and U share the  same Cb, and the same entity is also the highest-ranked Cf, i.e. Cp, in Ui-l, the transition from Ui-I to Ui is called 'continue: When Ui-1 and Ui share the same Cb, but the</Paragraph>
    <Paragraph position="20"> same entity is not the highest-ranked Cf in Ui, the transition is called 'retain '. When Ui-1 and Ui do not share the same Cb, there are two possibilities: if Cb in Ui is the same as Cp in Ui, the transition is 'smooth-shift'; if Cb in Ui is not the same as Cp in Ui, it is 'rough-shift '.</Paragraph>
    <Paragraph position="21"> It is claimed that when the hearer has to choose one from several possible interpretations, the one based on the most coherent transition should be chosen. The 4 transition states are ordered in the following way according to their preference:</Paragraph>
    <Paragraph position="23"> The second of the two rules in (1) is about this ordered preference of transition states.' 2. Walker et al. (1994) and Japanese zero pronoun resolution Walker et ai. (1994) propose the following ranking order of forward-looking centers to deal with Japanese:</Paragraph>
    <Paragraph position="25"/>
  </Section>
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