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In this paper we present a formalization of the centering approach to modeling attentional structure in discourse and use it as the basis for an algorithm to track discourse context and bind pronouns. As described in [GJW86], the process of centering attention on entities in the discourse gives rise to the intersentential transitional states of continuing, re~aining and shifting. We propose an extension to these states which handles some additional cases of multiple ambiguous pronouns. The algorithm has been implemented in an HPSG natural language system which serves as the interface to a database query application. . | A CENTERING APPROACH TO PRONOUNS Susan E. Brennan Marilyn w. Friedman Carl J. Pollard Hewlett-Packard Laboratories 1501 Page Mill Road Palo Alto CA 94304 USA Abstract In this paper we present a formalization of the centering approach to modeling attentional structure in discourse and use it as the basis for an algorithm to track discourse context and bind pronouns. As described in GJW86 the process of centering attention on entities in the discourse gives rise to the intersentential transitional states of continuing retaining and shifting. We propose an extension to these states which handles some additional cases of multiple ambiguous pronouns. The algorithm has been implemented in an HPSG natural language system which serves as the interface to a database query application. 1 Introduction In the approach to discourse structure developed in Sid83 and GJW86 a discourse exhibits both global and local coherence. On this view a key element of local coherence is centering a system of rules and constraints that govern the relationship between what the discourse is about and some of the linguistic choices made by the discourse participants e.g. choice of grammatical function syntactic structure and type of referring expression proper noun definite or indefinite description reflexive or personal pronoun etc. . Pronominalization in particular serves to focus attention on what is being talked about inappropriate use or failure to use pronouns causes communication to be less fluent. For instance it takes longer for hearers to process a pronominal-ized noun phrase that is not in focus than one that is while it takes longer to process a non-pronominalized noun phrase that is in focus than one that is not Gui85 . The GJW86 centering model is based on the following assumptions. A discourse segment consists of a sequence of utterances U1 . Um- With each utterance Un is associated a list of forward-looking centers Cf Un consisting of those discourse entities that are directly .
