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<Paper uid="P87-1022">
  <Title>A CENTERING APPROACH TO PRONOUNS</Title>
  <Section position="2" start_page="0" end_page="155" type="intro">
    <SectionTitle>
1 Introduction
</SectionTitle>
    <Paragraph position="0"> 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 pronominalized 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 \[Gui85\].</Paragraph>
    <Paragraph position="1"> The \[GJW86\] centering model is based on the following assumptions. A discourse segment consists of a sequence of utterances U1 ..... U,~. With each utterance 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 expressions 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 &amp;quot;about&amp;quot;, 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.</Paragraph>
    <Paragraph position="2"> 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 coincides with the preferred center of U,~. Definitions of these transition types appear in figure 1.</Paragraph>
    <Paragraph position="3"> These transitions describe how utterances are linked together in a coherent local segment of discourse. If a speaker has a number of propositions to express, one very simple way to do this coherently is to express all the propositions about a given entity (continuing) before introducing a related entity 1U directly realizes c if U is an utterance (of some phrase, not necessarily a full clause) for which c is the semantic interpretation, and U realizes c if either c is an element of the situation described by the utterance U or c is directly realized by some subpart of U. Realizes is thus a generalization of directly realizes\[G JW86\].</Paragraph>
    <Paragraph position="5"> (retaining) and then shifting the center to this new entity. See figure 2. Retaining may be a way to signal an intention to shift. While we do not claim that speakers really behave in such an orderly fashion, an algorithm that expects this kind of behavior is more successful than those which depend solely on recency or parallelism of grammatical function. The interaction of centering with global focusing mechanisms and with other factors such as intentional structure, semantic selectional restrictions, verb tense and aspect, modality, intonation and pitch accent are topics for further research.</Paragraph>
    <Paragraph position="6"> Note that these transitions are more specific than focus movement as described in \[Sid83\]. The extension we propose makes them more specific still. Note also that the Cb of \[GJW86\] corresponds roughly to Sidner's discourse focus and the Cf to her potential foci.</Paragraph>
    <Paragraph position="7"> The formal system of constraints and rules for centering, as we have interpreted them from \[GJW86\], are as follows. For each \[7, in \[71,..., U,n:</Paragraph>
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