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<Paper uid="J95-2003">
  <Title>Centering: A Framework for Modeling the Local Coherence of Discourse</Title>
  <Section position="2" start_page="0" end_page="205" type="abstr">
    <SectionTitle>
1. Introduction
</SectionTitle>
    <Paragraph position="0"> This paper presents an initial attempt to develop a theory that relates focus of attention, choice of referring expression, and perceived coherence of utterances within a discourse segment. The research described here is a further development of several strands of previous research. It fits within a larger effort to provide an overall theory of discourse structure and meaning. In this section we describe the larger research context of this work and then briefly discuss the previous work that led to it.</Paragraph>
    <Paragraph position="1"> Centering fits within the theory of discourse structure developed by Grosz and Sidner (1986). Grosz and Sidner distinguish among three components of discourse structure: a linguistic structure, an intentional structure, and an attentional state. At the level of linguistic structure, discourses divide into constituent discourse segments; an embedding relationship may hold between two segments. The intentional structure comprises intentions and relations among them. The intentions provide the basic rationale for the discourse, and the relations represent the connections among these intentions. Attentional state models the discourse participants' focus of attention at any given point in the discourse. Changes in attentional state depend on the intentional structure and on properties of the utterances in the linguistic structure.</Paragraph>
    <Paragraph position="2"> Each discourse segment exhibits both local coherence--i.e., coherence among the utterances in that segment--and global coherence--i.e., coherence with other segments in the discourse. Corresponding to these two levels of coherence are two components of attentional state; the local level models changes in attentional state within a discourse segment, and the global level models attentional state properties at the intersegmental level.</Paragraph>
    <Paragraph position="3"> Grosz and Sidner argue that global coherence depends on the intentional structure.</Paragraph>
    <Paragraph position="4"> They propose that each discourse has an overall communicative purpose, the discourse purpose (DP); and each discourse segment has an associated intention, its discourse segment purpose (DSP). The DP and DSP are speaker intentions; they are correlates at the discourse level of the intentions Grice argued underlay utterance meaning (Grice 1969). If a discourse is multi-party (e.g., a dialogue), then the DSP for a given segment is an intention of the conversational participant who initiates that segment. Lochbaum (1994) employs collaborative plans (Grosz and Kraus 1993) to model intentional structure, and is thus able to integrate intentions of different participants. Satisfaction of the DSPs contributes to the satisfaction of the DP. Relationships between DSPs provide the basic structural relationships for the discourse; embeddings in the linguistic structure are derived from these relationships. The global coherence of a discourse depends on relationships among its DP and DSPs. Grosz and Sidner model the global-level component of the attentional state with a stack; pushes and pops of focus spaces on the stack depend on intentional relationships.</Paragraph>
    <Paragraph position="5"> This paper is concerned with local coherence and its relationship to attentional state at the local level. Centering is proposed as a model of the local-level component of attentional state. We examine the interactions between local coherence and choices of referring expressions, and argue that differences in coherence correspond in part to the different demands for inference made by different types of referring expressions,  Barbara J. Grosz et al. Centering given a particular attentional state. We describe how the attentional state properties modeled by centering can account for these differences.</Paragraph>
    <Paragraph position="6"> Three pieces of previous research provide the background for this work. Grosz (1977) defined two levels of focusing in discourse: global and immediate. Participants were said to be globally focused on a set of entities relevant to the overall discourse. These entities may either have been explicitly introduced into the discourse or sufficiently closely related to such entities to be considered implicitly in focus (Grosz 1981). In contrast, immediate focusing referred to a more local focusing process--one that relates to identifying the entity that an individual utterance most centrally concerns.</Paragraph>
    <Paragraph position="7"> Sidner (1979) provided a detailed analysis of immediate focusing, including a distinction between the current discourse focus and potential foci. She gave algorithms for tracking immediate focus and rules that stated how the immediate focus could be used to identify the referents of pronouns and demonstrative noun phrases (e.g., &amp;quot;this party, .... that party&amp;quot;).</Paragraph>
    <Paragraph position="8"> Joshi and Kuhn (1979) and Joshi and Weinstein (1981) provided initial results on the connection between changes in immediate focus and the complexity of inferences required to integrate a representation of the meaning of an individual utterance into a representation of the meaning of the discourse of which it was a part. To avoid confusion with previous uses of the term &amp;quot;focus&amp;quot; in linguistics, they introduced the centering terminology. Their notions of &amp;quot;forward-looking&amp;quot; and &amp;quot;backward-looking&amp;quot; centers correspond approximately to Sidner's potential foci and discourse focus.</Paragraph>
    <Paragraph position="9"> In all of this work, focusing, whether global or immediate, was seen to function to limit the inferences required for understanding utterances in a discourse. Grosz and Sidner were concerned with the inferences needed to interpret anaphoric expressions of various sorts (e.g. pronouns, definite descriptions, ellipsis). They used focusing to order candidates; as a result the need for search was greatly reduced and the use of inference could be restricted to determining whether a particular candidate was appropriate given the embedding utterance interpretation. Joshi, Kuhn, and Weinstein were concerned with reducing the inferences required to integrate utterance meaning into discourse meaning. They used centering to determine an almost monadic predicate representation of an utterance in discourse; they then used this representation to reduce the complexity of inference.</Paragraph>
    <Paragraph position="10"> In this paper, we generalize and clarify certain of Sidner's results, but adopt the &amp;quot;centering&amp;quot; terminology. We also abstract from Sidner's focusing algorithm to specify constraints on the centering process. We consider the relationship between coherence and inference load and examine how both interact with attentional state and choices in linguistic expression.</Paragraph>
    <Paragraph position="11"> The remainder of this paper is organized as follows: in Section 2, we briefly describe the phenomena motivating the development of centering that this paper aims to explain. Section 3 provides the basic definitions of centers and related definitions needed to present the theoretical claims of the paper. In Section 4, we state the main properties of the centering framework and the major claims of centering theory. In Section 5, we discuss several factors that affect centering constraints and govern the centering rules given in Section 6. In Section 7, we discuss applications of the rules and their ability to explain several discourse coherence phenomena. In Section 8, we briefly outline the properties of an underlying semantic framework that are required by centering. Finally, in Section 9 we conclude with a brief comparison of centering with the research that preceded it and a summary of research that expands on Grosz, Joshi, and Weinstein (1986). In particular, Section 9 provides references to subsequent investigations of additional factors that control centering and examinations of its crossqinguistic applicability and empirical validity.</Paragraph>
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