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 intended to model the local component of attentional state. The paper examines interactions between local coherence and choice of referring expressions; it argues that differences in coherence correspond in part to the inference demands made by different types of referring expressions, given a particular attentional state. It demonstrates that the attentional state properties modeled by centering can account for these differences. PrefaceOur 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 different types of referring expressions. Centering was proposed as a model that accounted for this phenomenon. We argued that the coherence of discourse was affected by the compatibility between centering properties of an utterance and choice of referring expression. Subsequently, we revised and expanded the ideas presented therein. We defined 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) has led to a number of papers by others on this topic and has been extensively cited, but has never been published. 1We have been urged to publish the more detailed description of the centering framework and theory proposed in Grosz, Joshi, and Weinstein (1986) so that an official 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 Grosz, Joshi, and Weinstein (1986). It has become ever more clear that it would be useful to have a "definitive" statement of

Centering theory, developed within computotionol linguistics, provides an account of ways in which patterns of interutteronce reference con promote the local coherence of discourse. It states thot each utterance in o coherent discourse segment contains a single semantic entity-the backword-looking center-that provides a link to the previous utteronce, ond an ordered set of entities-the forward-looking centers-that offer potential links to the next utterance. We report five reading-time experiments thot test predictions of this theory with respect to the conditions under which it is preferable to realize (refer to) on entity using a pronoun rother thon o repeated definite description or name. The experiments show that there is a single backward-looking center thot is preferentiolly realized as a pronoun, ond thot the backward-looking center is typically reolized as the grammoticol subiea of the utterance. They also provide evidence that there is a set of forward-looking centers that is ranked in terms of prominence, and that a key factor in determining prominence-surface-initial position--does not affect determination of the bockword-looking center. This provides evidence for the dissociation of the coherence processes of looking bockward and looking forward.A central issue in language processing is determining the way in which utterances fit together to form a coherent discourse. Utterances that seem equivalent in isolation, because their main propositional content is identical, may have radically different effects on discourse coherence. When an utterance is put into a discourse, the particular way in which it realizes a given propositional content determines the extent to which it coheres with the previous discourse. In this article, we examine the ways in which the structure and coherence of a discourse are influenced by the manner in which its utterances

The original fo~ulation of SharedPlans by B. Grosz and C. Sidner (1990) was developed to provide a model of collaborative planning in which it was not necessary for one agent to have intentions-to toward an act of a different agent. Unlike other contemporaneous approaches (J.R. Searle, 1990), this formulation provided for two agents to coordinate their activities without introducing any notion of irreducible joint intentions. However, it only treated activities that directly decomposed into single-agent actions, did not address the need for agents to commit to their joint activity, and did not adequately deal with agents having only partial knowledge of the way in which to perform an action. This paper provides a revised and expanded version of SharedPlans that addresses these shortcomings. It also reformulates Pollack's (1990) definition of individual plans to handle cases in which a single agent has only partial knowledge; this reformulation meshes with the definition of SharedPlans. The new definitions also allow for contracting out certain actions. The formalization that results has the features required by Bratrnan's (1992) account of shared cooperative activity and is more general than alternative accounts (H. Levesque et al., 1990; E. Sonenberg et al., 1992).

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