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<?xml version="1.0" standalone="yes"?> <Paper uid="P86-1031"> <Title>A PROPERTY-SHARING CONSTRAINT IN CENTERING</Title> <Section position="2" start_page="0" end_page="203" type="intro"> <SectionTitle> 1. Introduction </SectionTitle> <Paragraph position="0"> Grosz, Joshi, & Weinstein (1983) postulated that each utterance in discourse concerns a set of entities called the centers, and discussed how certain facts of local discourse connectedness (as opposed to global) can be accounted for IThis work was supported in parts by the Center for the Study of Language and Information at Stanford University and by grants from the National Science Foundation (DCR84-11726) for the Department of Computer and Information Science and from the Alfred P. Sloan Foundation for the Cognitive Science Program at the utterance. The difference seems more conceptual than substantial since what is crucial for providing a referent candidate is the expected Cb order given to the Cf set whether this set contains the Cb or not. Relative merits of each approach should be clarified in the future.</Paragraph> <Paragraph position="1"> original Centering rule (Grosz et. al. 1983): 6 2. The SUBJECT constraint (1) If the Cb of the current utterance is the same as the Cb of the previous utterance, apronoun should be used.</Paragraph> <Paragraph position="2"> (1) is stated as a heuristic in the production of English. It is assumed that an equivalent interpretation heuristic is used by a hearer. Roughly, a pronoun &quot;realizes&quot; the current Cb that continues the previous Cb. 7 In this paper, I will first point out certain facts that the basic Centering rule does not explain, then propose a further constraint that substantiates the basic rule. This is called the &quot;property-sharing&quot; constraint, which requires that two pronominal elements realizing the same Cb in adjacent utterances share a certain common grammatical property. This shared property itself is expressed as a default preference order reflecting the nature of the constraint as a discourse rule. The initial formulation of the constraint only refers to the gratnmatical function SUBJECT. It explains the problem cases for the basic Centering rule in Japanese and English. It also accounts for a subset of what appears to be an effect of structural parallelism in anaphora interpretation. Then I will propose an additional dimension of the shared property called the &quot;speaker identification&quot; property. The revised constraint referring to both dimensions accounts for a group of counterexamples to the initial formulation found in Japanese discourse. It also sheds fight on what is involved in interpreting perception reports in both languages.</Paragraph> <Paragraph position="3"> Before starting the discussion, I would like to comment on the nature of the data used here. I will mostly use constructed discourse sequences where the role played by commonsense inferences or special linguistic devices (such as slzess and intonation) for guiding pronoun interpretations is minimal. All examples in this paper are to be read with fiat intonation with unstressed pronouns.</Paragraph> <Paragraph position="4"> These limitations are in order to identify the grammatically-based default order that gives rise to preferred interpretations in neutral contexts. Note that this default order alone does not determine interpretations of pronominal elements. Rather, its role in the centering framework is to give an ordered fist of referents (centers) so that commonsense inferences can be controlled.</Paragraph> <Paragraph position="5"> Interpretations and acceptability judgements of the examples in this paper result from interviews with a number of native speakers in each language.</Paragraph> <Section position="1" start_page="200" end_page="202" type="sub_section"> <SectionTitle> 2.1. Japanese </SectionTitle> <Paragraph position="0"> In Japanese, the expression primarily used to realize the Cb is the zero pronominal O.e., unexpressed subject or object), a The grammatical features (e.g., gender, number, person) of these unexpressed subjects and objects are not morphologically marked elsewhere in the sentence, which distinguishes them from the so-called &quot;pro-drop&quot;, such as the unexpressed finite clause subject in Italian and Spanish whose grammatical features are morphologically marked on the verb inflection. The basic Centering role in Japanese can be obtained by changing the word pronoun to zero pronominal in (1) (Kameyama 1985).</Paragraph> <Paragraph position="1"> In the following discourse fragment, it is reasonable to assume that Rosa is the Cb of the second utterance: 9 If the Cb-status of an entity is homogeneous, we would expect that the two instances of the Cb above have exactly the same effect, if any, on the subsequent utterance. When an identical third utterance is added to both, however, it becomes clear that the centered individual Rosa is not of an equal status in the two cases: 6Grosz et. al. (in preparation) propose various constraints on this rule, and, among other things, distinguish between the retention and continuation of the Cb. I will use the words retain and continue in non-technical sense in this paper.</Paragraph> <Paragraph position="2"> tAn expression realizes a center rather than denoting it. Realization allows either a value-free or value-loaded interpretation (see Grosz et. al. 1983 for discussion).</Paragraph> <Paragraph position="3"> SZero pronominals are also found in Chinese, Korean, Vietnamese, Thai, etc. I will also call them &quot;zero-subject&quot;, &quot;zero-object&quot;, and so on.</Paragraph> <Paragraph position="4"> 9The following symbols are used for grammatical markers in the gloss: SB (subject), OB (direct object), 02 (indirect/second object), TP (topic), ASN (assertion), CMP (complementizer), Q (question). The symbol * is used for a zero pronominal, and its translation equivalent appears in \[\]. (4) * Yuusyoku ni syootaisi-tano-desu.</Paragraph> <Paragraph position="5"> SB OB supper to invited ASN &quot;\[She\] invited \[her\] to dinner.&quot; after(2): \[strong preference: Rosa invited Mary\] after(3): \[weak preference: Mary invited Rosa\] The extension (4) is a multi-zero-pronominal utterance. The zero-subject and zero-object pronominals receive reverse interpretations depending on whether the utterance follows (2) or (3). Although this fact by itself does not contradict the basic rule (I), it poses a question as to which zero pronominal in (4) realizes its Cb. There are the following two possibilities. If the previous Cb continues to be the current Cb by default, it follows that the choice of the Cb-realizing zero pronominal depends entirely on the preceding discourse context. On the other hand, if some inherent property of a zero pronominal (e.g., subject/object) independently decides which one realizes the Cb, the previous context need not be considered. For instance, if a zero-subject is always more closely associated with the Cb than a zero-object, the discourse sequence (3) to (4) changes the Cb from Rosa to Mary.</Paragraph> <Paragraph position="6"> In the extension (5), Rosa (the previous Cb) is mentioned with a full name while the single zero pronominal picks out a previous non-Cb, Mary. If Rosa is still the Cb here, this utlerance violates the basic Centering rule, so the rule predicts unacceptability, which is indeed the case following the sequence (2). rl The same rule, however, provides no clue for the puzzling acceptability of the same extension following the sequence (3). Moreover, it is possible that Rosa is no longer the Cb in (5), in which case, rule (1) simply does not apply.</Paragraph> <Paragraph position="7"> Examples like these are the basis for the first version of the Centering Constraint: (6) Centering Constraint \[Japanese\] (1st approximation) Two zero pronominals that retain the same Cb in adjacent utterances should share one of the following properties: Io=: indicates the association between a linguistic item (leR-hand side) and a non-linguistic entity (right-hand side).</Paragraph> <Paragraph position="8"> llNote that violating a discourse rule like (1) leads to more difficulty in understanding rather than clear-cut &quot;ungrammaticality&quot;.</Paragraph> <Paragraph position="9"> SUBJECT or nonSUBJECT. 12 (6) says that two zero pronominals supporting the same Cb in adjacent utterances should both be either SUBJECT or nonSUBJECT. In the case of discourse extension (4) above, if the Cb is still Rosa, it should be realized with a zero-subject after the sequence (2) and with a zero-object after (3). This is shown below: (7) 1. \[Cb<SUBJ> = Rosa\] <-(2)-2 2. \[Cb<SUBJ> = Rosa\] <--(4) \[strong preference\] (s) I. \[Cb<OBJ> = Rosa\] <-(3)-2 2. \[Cb<OBJ> = Rosa\] <-(4) \[weak preference\] I aUribute the different degree of preference between (7) and (8) to the difference in canonicality of centering. A Cb continued with zero-subjects as in (7) is more stable, or more canonical, than one continued with zero-objects as in (8), which is but one manifestation of the overall significance of SUBJECT in centering. 13 This leads to the second approximation of the Centering Constraint: (9) Centering Constraint \[Japanese\] (2nd approximation) Two zero pronominals that retain the same Cb in adjacent utterances should share one of the following properties (in descending order of preference): 1) SUBJECT, 2) nonSUBJECT.</Paragraph> <Paragraph position="10"> Constraint (9) predicts that retaining a Cb is good when the two pronominals are both either SUBJECT or nonSUBJECT while it is bad (i.e., leading to complex inferences) when one is SUBJECT and the other is not, This in turn predicts that changing the Cb across adjacent utterances is acceptable when the two pronominals have different properties while it is not when they are of the same property.</Paragraph> <Paragraph position="11"> The difference in acceptability between sequence (2) to (5) (marginal) and sequence (3) to (5) (acceptable) would then follow from this constraint. The former is bad because it changes the Cb with two SUBJECT zero pronominals, as shown in (10). The latter is good because it changes the Cb with different zero pronominals (from OBJECT to SUBJECT), as shown in (11): (I0) 1. \[Cb<SUBJ> = Rosa\] <-(2)-2 2. &quot;2 \[Cb<SUBJ> = Mary\] <--(5) \[marginal\] (11) 1. \[Cb<OBJ> -- Rosa\] <-(3)-2 2. \[Cb<SUBJ>=Mary\] <-(5)\[acceptable\] 12I'm refen'ing to the &quot;surface&quot; grammatical function SUBJECT.</Paragraph> <Paragraph position="12"> 13The importance of SUBJECT in centering is also discussed in Grosz et. al. (in preparation).</Paragraph> <Paragraph position="13"> The acceptability of the Cb-shift shown in (11) above contrasts with the unacceptability of retaining the Cb with these pronominals. The latter in fact appeared in the above example as the nonpreferred reading of sequence (3) to (4), which is shown in (12): (12) 1. \[Cb<OBJ> = Rosa\] 2. ?? \[Cb<SUBJ> = Rosa\] The evaluation of the third utterance parallels the Japanese example. This indicates that the SUBJECT-based constraint stated in (9) for Japanese is applicable to English together with all the analogous consequences discussed above. The constraint is restated below for pronominal expressions in general: (15) Centering Constraint \[general\] (approximation) Two pronominal expressions that retain the same Cb in adjacent utterances should share one of the following properties (in descending order of preference): 1) SUBJECT, 2) nonSUBJECT.</Paragraph> <Paragraph position="14"> The particular kind of pronominal expressions relevant here vary from language to language. Kameyama (1985: Ch.1) hypothesized that it is the pronominal element with the &quot;less phonetic content&quot; for each grammatical function of a language 14 and that it is predictable from the typological perspective on available pronominal forms. For instance, it is the unstressed pronoun in English where pronouns must always be overt, and it is the zero pronominal in Japanese where pronouns with no phonetic 14It is possible that only certain grammatical functions (e.g., SUBJECT, OBJECT, and OBJECT2) are relavant.to the Cb. This will have to be clarified in the future. content exist (for subjects and objects). It is further predicted that morphologically bound pronominal forms (i.e., agreement inflections, clitics, and affixes) rather than full independent pronouns are used for Cb-realization if a language has this option. For instance, this option exists for the finite clause subject in Italian and Spanish in terms of the agreement inflection, and for the t'mite clause subject and object in Warlpiri in terms of clities. The constraint in English is stated below: (16) Centering Constraint \[English\] Two unstressed pronouns that retain the same Cb in adjacent utterances should share one of the following properties (in descending order of preference): 1) SUBJECT, 2) nonSUBJECT.</Paragraph> <Paragraph position="15"> 2.3. Accounting for the effect of parallelism in</Paragraph> </Section> <Section position="2" start_page="202" end_page="203" type="sub_section"> <SectionTitle> Cb-establlshment </SectionTitle> <Paragraph position="0"> The given property-sharing constraint has so far been proposed for pronominal elements that retain the same Cb in adjacent utterances. By its reference to the grammatical property SUBJECT, the constraint indicates that adjacent utterances of the same Cb cohere even better when there is a certain degree of grammatical parallelism.</Paragraph> <Paragraph position="1"> Analogous constraints account for at least two other kinds of parallelism effects on pronoun interpretation in English. They are in the context of what I call the Cb.establishmem, that is, the process in which a previous non-Cb becomes the Cb. The case of Cb-shift is a subease of Cb-establishment.15 Ambiguous multi-pronouns. The interpretation of a multi-pronominal establishes a Cb. An example follows: (17) 1. Max is waiting for Fred.</Paragraph> <Paragraph position="2"> 2. He invited him to dinner.</Paragraph> <Paragraph position="3"> \[preference: Max invited Fred\] first is the utterance that (17) shows that when two pronouns are potentially ambiguous in reference, the preferred interpretation conforms to a property-sharing constraint. That is, there is a higher tendency that the SUBJECT pronoun corefers with the SUBJECT of the previous utterance.</Paragraph> <Paragraph position="4"> It is crucial here that (a) there is more than one pronoun and Co) two (or more) of them are potentially ambiguous (i.e., of the same grammatical features). Otherwise, the process of Cb-establishment need not be constrained by the 15In the present approach, the default &quot;expected Cb&quot; is the (matrix) SUBJECT referent, and the Cb is established in the next utterance with a (matrix) (SUBJECT) pronoun, ff there is one. More factors such as TOPIC (wa-marking) and Ident (see below) are also relevant to the centering in Japanese. These are discussed in the longer paper in preparation.</Paragraph> <Paragraph position="5"> property-sharing, as illustrated in the following examples: (18) \[single pronoun\] 1. Carl is talking to Tom in the Lab.</Paragraph> <Paragraph position="6"> 2. Terry was just looking for him.</Paragraph> <Paragraph position="7"> \[preference: h/m=: Carl\] (19) \[unambiguous two pronouns\] 1. Max is waiting for Susan.</Paragraph> <Paragraph position="8"> 2. She invited him to dinner.</Paragraph> <Paragraph position="9"> (18)-2 has only one pronoun and (19)-2 has two pronouns with different gender. In both cases, the nonSUBJECT pronoun naturally corefers with the previous SUBJECT. The property-sharing constraint becomes relevant only in the case of completely ambiguous multipronouns as in (17). Note that this in turn explains why the property-sharing was first recognized for zero pronominals, which lack gender/number/person distinctions altogether. Explicitly signalled parallelism. The second relevant type of parallelism effect is found in a discourse sequence with explicit linguistic signals for a parallel structure. Examples follow: (20) \[Contrast this with (18)\] 1. Carl is talking to Tom in the Lab.</Paragraph> <Paragraph position="10"> 2. Terry wants to talk to him too.</Paragraph> <Paragraph position="11"> \[preference: h/~: Tom\] (21) \[from Sidner 1979:179\] 1. The green Whitierleaf is most commonly found near the wild rose.</Paragraph> <Paragraph position="12"> 2. The wild violet is found near it too.</Paragraph> <Paragraph position="13"> <it=: wild rose> Parallelisms in (20) and (21) are clearly signalled with (i) the same verbal expressions (talk to and be found near) and (ii) the word too. In such cases, a version of the property-sharing scheme would propose the correct specification of the single pronoun as the first choice. Since the pronouns are nonSUBJECT, they should co-specify with the nonSUBJECT in the first utterance, which are Tom and the wild rose, respectively. 16 Significant here is the fact that (21) was a problem case for Sidner's (1979) focusing-based pronoun interpretation algorithm. She in fact concluded that pronoun interpretation involving structuralparaUelism was a source for anaphora inherently different from focusing: &quot;Focussing cannot account for the detection of parallel structure, not only because the computation of such structure is poorly understood, but also because focussing chooses different defaults for co-specification than those required for paraUelism.&quot;(p.236) If a property-sharing constraint is invoked in interpreting 161&quot;he property of nonSUBJECT may have to he broken up into subclasses (possibly into each grammatical function) when there are more than one nonSUBJECTs in the first utterance.</Paragraph> <Paragraph position="14"> (21)-2, the &quot;wild rose&quot; (nonSUBJECT) overrides the default expected Cb, the &quot;green Whitierleaf' (SUBJECT), as the first-choice referent for the pronoun it (nonSUBJECT). The major advantage of the present property-sharing constraint is its role in combining the effects of both focusing/centering and structural parallelism.</Paragraph> </Section> </Section> class="xml-element"></Paper>