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<Paper uid="J99-3001">
  <Title>Functional Centering Grounding Referential Coherence in Information Structure</Title>
  <Section position="2" start_page="0" end_page="311" type="abstr">
    <SectionTitle>
1. Introduction
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    <Paragraph position="0"> The problem of establishing referential coherence in discourse can be rephrased as the problem of determining the proper antecedent of a given anaphoric expression in the current or the preceding utterance(s) and the rendering of both as referentially identical (coreferential). This task can be approached in a very principled way by stating general constraints on the grammatical compatibility of the expressions involved (e.g., Haddock 1987; Alshawi 1992). Linguists have devoted a lot of effort to identifying conclusive syntactic and semantic criteria to reach this goal, e.g., for intrasentential anaphora within the binding theory part of the theory of Government and Binding (Chomsky 1981), or for intersentential anaphora within the context of the Discourse Representation Theory (Kamp and Reyle 1993).</Paragraph>
    <Paragraph position="1"> Unfortunately, these frameworks fail to uniquely determine anaphoric antecedents in a variety of cases. As a consequence, referentially ambiguous interpretations have to be dealt with in those cases in which several alternatives fulfill all the required syntactic and semantic constraints. It seems that syntactic and semantic criteria constitute only necessary but by no means sufficient conditions for identifying the valid antecedent among several possible candidates. Hence, one is left with a preferential choice problem that falls outside of the scope of those strict grammaticality constraints relating to the level of syntax or semantics only. Its solution requires considering pat- null * Institute for Research in Cognitive Science, 3401 Walnut Street, Suite 400A, Philadelphia, PA 19104, USA t Computational Linguistics Group, Text Understanding Lab, Werthmannplatz 1, 79085 Freiburg, Germany (~) 1999 Association for Computational Linguistics  Computational Linguistics Volume 25, Number 3 terns of language use and, thus, introduces the level of discourse context and further pragmatic factors as a complementary description level.</Paragraph>
    <Paragraph position="2"> Computational linguists have recognized the need to account for referential ambiguities in discourse and have developed various theories centered around the notion of discourse focus (Grosz 1977; Sidner 1983). In a seminal paper, Grosz and Sidner (1986) wrapped up the results of their research and formulated a model in which three levels of discourse coherence are distinguished--attention, intention, and discourse segment structure. While this paper gives a comprehensive picture of a complex, yet not explicitly spelled-out theory of discourse coherence, the centering model (Grosz, Joshi, and Weinstein, 1983, 1995) marked a major step in clarifying the relationship between attentional states and (local) discourse segment structure. More precisely, the centering model accounts for the interactions between local coherence and preferential choices of referring expressions. It relates differences in coherence (in part) to varying demands on inferences as required by different types of referring expressions, given a particular attentional state of the hearer in a discourse setting (Grosz, Joshi, and Weinstein 1995, 204-205). The claim is made then that the lower the inference load put on the hearer, the more coherent the underlying discourse appears.</Paragraph>
    <Paragraph position="3"> The centering model as formulated by Grosz, Joshi, and Weinstein (1995) refines the structure of &amp;quot;centers&amp;quot; of discourse, which are conceived as the representational device for the attentional state at the local level of discourse. They distinguish two basic types of centers, which can be assigned to each utterance Ui--a single backward-looking center, Cb(Ui), and a partially ordered set of discourse entities, the forward-looking centers, Cf(Ui). The ordering on Cf is relevant for determining the Cb. It can be viewed as a salience ranking that reflects the assumption that the higher the ranking of a discourse entity in Cf, the more likely it will be mentioned again in the immediately following utterance. Thus, given an adequate ordering of the discourse entities in Cf, the costs of computations necessary to establish local coherence are minimized.</Paragraph>
    <Paragraph position="4"> Given that the ordering on the Cf list is crucial for determining the Cb, it is no surprise that there has been much discussion among researchers about the ranking criteria appropriate for different languages. In fact, Walker, Iida, and Cote (1994) hypothesize that the Cf ranking criteria are the only language-dependent factors within the centering model. Though evidence for many additional criteria for the Cf ranking have been brought forward in the literature, to some extent consensus has emerged that grammatical roles play a major role in making ranking decisions (e.g., whether the referential expression appears as the grammatical subject, direct object, or indirect object of an utterance). Our own work on the centering model 1 (Strube and Hahn 1996; Hahn and Strube 1996) brings in evidence from German, a free-word-order language in which grammatical role information is far less predictive of the organization of centers than for fixed-word-order languages such as English. In establishing proper referential relations, we found the functional information structure of the utterances to be much more relevant. By this we mean indicators of whether or not a discourse entity in the current utterance refers to another discourse entity already introduced by previous utterances in the discourse. Borrowing terminology from Prince (1981, 1992), an entity that does refer to another discourse entity already introduced is called discourse-old or hearer-old, while an entity that does not refer to another discourse entity is called discourse-new or hearer-new.</Paragraph>
    <Paragraph position="5">  Strube and Hahn Functional Centering Based on evidence from empirical studies in which we considered German as well as English texts from different domains and genres, we make three contributions to the centering approach. The first, the introduction of functional notions of information structure into the centering model, is purely methodological in nature and concerns the centering approach as a theory of local coherence. The second deals with an empirical issue, in that we demonstrate how a functional model of centering can be successfully applied to the analysis of different forms of anaphoric text phenomena, namely pronominal, nominal, and functional anaphora. Finally, we propose a new evaluation methodology for centering data in terms of a cost-based evaluation approach that can be directly derived from and justified by the cognitive load premises of the centering model.</Paragraph>
    <Paragraph position="6"> At the methodological level, we develop arguments that (at least for some free-word-order languages) grammatical role criteria should be replaced by functional role criteria, since they seem to more adequately account for the ordering of discourse entities in the Cf list. In Section 4, we elaborate on particular information structure criteria underlying such a functional center ordering. We also make a second, more general methodological claim for which we have gathered some preliminary, though still not conclusive evidence. Based on a reevaluation of centering analyses of some challenging language data that can be found in the literature on centering, we will argue that exchanging grammatical for functional criteria might also be a reasonable strategy for fixed-word-order languages. What makes this proposal so attractive is the obvious gain in the generality of the model--given a functional framework, fixed- and free-word-order languages might be accounted for by the same ordering principles.</Paragraph>
    <Paragraph position="7"> The second major contribution of this paper is related to the unified treatment of different text coherence phenomena. It consists of an equally balanced treatment of intersentential (pro)nominal anaphora and inferables (also called functional, bridging, or partial anaphora). The latter phenomenon (cf. the examples in the next section and the in-depth treatment in Hahn, Markert, and Strube \[1996\]) is usually only sketchily dealt with in the centering literature, e.g., by asserting that the entity in question &amp;quot;is realized but not directly realized&amp;quot; (Grosz, Joshi, and Weinstein 1995, 217). Furthermore, the distinction between these two kinds of realization is not part of the centering mechanisms but delegated to the underlying semantic theory. We will develop arguments for how to discern inferable discourse entities and relate them properly to their antecedent at the center level. The ordering constraints we supply account for all of the types of anaphora mentioned above, including (pro)nominal anaphora (Strube and Hahn 1995; Hahn and Strube 1996). This claim will be validated by a substantial body of empirical data in Section 5.</Paragraph>
    <Paragraph position="8"> Our third contribution relates to the way the results of centering-based anaphora resolution are usually evaluated. Basically, we argue that rather than counting resolution rates for anaphora or comparing isolated transition types holding among head positions in the center lists--preferred transition types stand for a high degree of local coherence, while less preferred ones signal that the underlying discourse might lack coherence--one should consider adjacent transition pairs and annotate such pairs with the processing costs they incur. This way, we define a dual theory-internal metric of inference load by distinguishing between &amp;quot;cheap&amp;quot; and &amp;quot;expensive&amp;quot; transition types. Based on this distinction, some transition types receiving bad marks in isolation are ranked &amp;quot;cheap&amp;quot; when they occur in the appropriate context, and vice versa.</Paragraph>
    <Paragraph position="9"> The article is organized as follows: In Section 2, we introduce the different types of anaphora we consider subsequently, viz. pronominal, nominal, and functional anaphora.</Paragraph>
    <Paragraph position="10"> We then turn to the proposed modification of the centering model. After a brief in- null Computational Linguistics Volume 25, Number 3 troduction into what we call the &amp;quot;grammatical&amp;quot; centering model (actually, a recap of Grosz, Joshi, and Weinstein \[1995\]) in Section 3, we turn in Section 4 to our approach, the functional model of centering. In Section 5, we present the methodological framework and the empirical data from two evaluation studies we carried out. In Section 6, we relate our work to alternative approaches dealing with local text coherence. In Section 7, we discuss some remaining unsolved problems.</Paragraph>
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