A Centering Approach to Pronouns

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 center- ing approach to modeling attentional structure in dis- course 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 en- tities in the discourse gives rise to the intersentential transitional states of continuing, re~aining and shift- ing. 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 lin- guistic choices made by the discourse participants, e.g. choice of grammatical function, syntactic struc- ture, and type of referring expression (proper noun, definite or indefinite description, reflexive or per- sonal pronoun, etc.). Pronominalization in partic- ular 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 no~ 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 fol- lowing assumptions. A discourse segment consists of a sequence of utterances U1 ..... U,~. With each ut- terance Ua is associated a list of forward.looking cen- ~ers, Cf(U,), consisting of those discourse entities that are directly realized or realized I by linguistic ex- pressions in the utterance. Ranking of an entity on this list corresponds roughly to the likelihood that it will be the primary focus of subsequent discourse; the first entity on this list is the preferred cen~er, Cp(U, O. U,~ actually centers, or is "about", only one entity at a time, the backward-looking cen~er, Cb(U=). The backward center is a confirmation of an entity that has already been introduced into the discourse; more specifically, it must be realized in the immediately preceding utterance, Un-1. There are several distinct types of transitions from one utterance to the next. The typology of transitions is based on two factors: whether or not the center of attention, Cb, is the same from Un-1 to Un, and whether or not this entity co- incides with the preferred center of U,~. Definitions of these transition types appear in figure 1. These transitions describe how utterances are linked together in a coherent local segment of dis- course. If a speaker has a number of propositions …

Content truncated to 500 words