Centering : A Framework for Modellingthe Local Coherence of Discourse

This paper concerns relationships among focus of attention, choice of referring expression, and perceived coherence of utterances within a discourse segment. It presents a framework and initial theory of centering which are intended to model the local component of attentional state. The paper examines interactions between local coherence and choice of referring expressions; it argues that di erences in coherence correspond in part to the inference demands made by di erent types of referring expressions given a particular attentional state. It demonstrates that the attentional state properties modelled by centering can account for these di erences. 3 1 Preface Our original paper (Grosz, Joshi, and Weinstein, 1983) on centering claimed that certain entities mentioned in an utterance were more central than others and that this property imposed constraints on a speaker's use of di erent types of referring expressions. Centering was proposed as a model that accounted for this phenomenon. We argued that the coherence of discourse was a ected by the compatibility between centering properties of an utterance and choice of referring expression. Subsequently, we revised and expanded the ideas presented therein. We de ned various centering constructs and proposed two centering rules in terms of these constructs. A draft manuscript describing this elaborated centering framework and presenting some initial theoretical claims has been in wide circulation since 1986. This draft (Grosz, Joshi, and Weinstein 1986, hereafter, gjw86) has led to a number of papers by others on this topic and has been extensively cited, but has never been published.1 We have been urged to publish the more detailed description of the centering framework and theory proposed in gjw86 so that an o cial version would be archivally available. The task of completing and revising this draft became more daunting as time passed and more and more papers appeared on centering. Many of these papers proposed extensions to or revisions of the theory and attempted to answer questions posed in gjw86. It has become ever more clear that it would be useful to have a \de nitive" statement of the original motivations for centering, the basic de nitions underlying the centering framework, and the original theoretical claims. This paper attempts to meet that need. To accomplish this goal, we have chosen to remove descriptions of many open research questions posed in gjw86 as well as solutions that were only partially developed. We have also greatly shortened the discussion of criteria for and constraints on a possible semantic theory as a foundation for this work. 2 Introduction 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 ts within a larger e ort to provide an overall theory of discourse structure and meaning. In this section we describe the larger research context of this work and then brie y discuss the previous work that led to it. Centering ts within the theory of discourse structure developed by Grosz and Sidner (1986), henceforth, G&S. G&S 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 1Early drafts of gjw86 were in circulation from 1983. Some citations to other work have dates between 1983 and 1986. This work utilized these earlier drafts. 4 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. 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. G&S argue that global coherence depends on the intentional structure. 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 DSPS 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 (Lochbaum, 1994) employs collaborative plans (Grosz and Kraus, 1993) to model intentional structure and is thus able to integrate intentions of di erent 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. G&S 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. 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 di erences in coherence correspond in part to the di erent demands for inference made by di erent types of referring expressions, given a particular attentional state. We describe how the attentional state properties modelled by centering can account for these di erences. Three pieces of previous research provide the background for this work. Grosz (1977) de ned 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 be su ciently 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. 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. \this party," \that party"). Joshi and Kuhn (1979) and Joshi and Weinstein (1981) provided initial results on the con5 nection 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 \focus" in linguistics, they introduced the centering terminology. Their notions of \forward-looking" and \backward-looking" centers correspond approximately to Sidner's potential foci and discourse focus. 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, de nite 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 use this representation to reduce the complexity of inference. In this paper, we generalize and clarify certain of Sidner's results, but adopt the \centering" 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. The remainder of this paper is organized as follows: In Section 3, we brie y describe the phenomena motivating the development of centering that this paper aims to explain. Section 4 provides the basic de nitions of centers and related de nitions needed to present the theoretical claims of the paper. In Section 5, we state the main properties of the centering framework and the major claims of centering theory. In Section 6, we discuss several factors that a ect centering constraints and that govern the centering rules given in Section 7. In Section 8, we discuss applications of the rules and their ability to explain several discourse coherence phenomena. In Section 9, we brie y outline the properties of an underlying semantic framework that are required by centering. Finally, in Section 10 we conc