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<Paper uid="P98-1044">
  <Title>Veins Theory: A Model of Global Discourse Cohesion and Coherence</Title>
  <Section position="1" start_page="0" end_page="281" type="abstr">
    <SectionTitle>
Abstract
</SectionTitle>
    <Paragraph position="0"> In this paper, we propose a generalization of Centenng Theory (CT) (Grosz, Joshi, Weinstein (1995)) called Veins Theory (VT), which extends the applicability of centering rules from local to global discourse. A key` facet of the theory involves the idenufication of &lt;&lt;veins&gt;&gt; over discourse structure trees such as those defined in RST, which delimit domains of referential accessibility for each unit in a discourse. Once identified, reference chains can be extended across segment boundaries, thus enabling the application of CT over the entire discourse. We describe the processes by which veins are defined over discourse structure trees and how CT can be applied to global discourse by using these chains. We also define a discourse &lt;&lt;smoothness&gt;&gt; index which can be used to compare different discourse structures and interpretations, and show how VT can be used to abstract a span of text in the context of the whole discourse.</Paragraph>
    <Paragraph position="1"> Finally, we validate our theory by analyzing examples from corpora of English, French, and Romanian.</Paragraph>
    <Paragraph position="2"> Introduction As originally postulated, Centering Theory (CT) (Grosz, Joshi, and Weinstein (1995)) accounts for references between adjacent units but is restricted to local reference (i.e., within segment boundaries). Recently, CT-based work has emerged which considers the relation of global discourse structure and anaphora, all of which proposes extensions to centering in order to apply it to global discourse.</Paragraph>
    <Paragraph position="3"> We approach the relationship between global structure and anaphora resolution from a different, but related, perspective. We identify domains of referential accessibility for each discourse unit over discourse structure trees such as those defined in Rhetorical Structure Theory (RST ; Mann and Thompson (1987)) and show how CT can then be applied to global discourse by. using these domains. As such, our approach differs from Walker's (1996), whose account of referentialit~, within the cache memory model does not rely on discourse structure, but rather on cue phrases and matching constraints together  with constraints on the size of the cache imposed to reflect the plausible limits of the attentional span. Our approach is closer to that of Passonneau (1995) and Hahn and Strtibe (1997), who both use a stack-based model of discourse structure based on Grosz and Sidner's (1986) focus spaces. Such a model is equivalent to a dynamic processing model of a tree-like structure reflecting the hierarchical nesting of discourse segments, and thus has significant similarities to discourse structure trees produced by RST (see Moser and Moore (1996)).</Paragraph>
    <Paragraph position="4"> However, using the RST notion of nuclearity, we go beyond previous work by revealing a &amp;quot;hidden&amp;quot; structure in the discourse tree, which we call veins, that enables us to determine the referential accessibility domain for each discourse unit and ultimately to apply CT globally, without extensions to CT or addltional Oata structures.</Paragraph>
    <Paragraph position="5"> In this paper, we describe Veins Theory (VT) by showing how veins are defined over discourse structure trees, and how CT can be applied to global discourse by using them. We use centering transitions (Brennan, Friedman and Pollard (1987)) to define a &lt;&lt;smoothness&gt;&gt; index, which is used to compare different discourse structures and interpretations. Because veins define the domains of referential access for each discourse unit, we further demonstrate how VT may` be potentially used to determine the &lt;&lt;minimal&gt;&gt; parts ota text required to resolve references m a given utterance or, more generally, to understand it out of the context of the entire discourse. Finally, we validate our theory by analyzing examples from corpora of English, French, and Romanian.</Paragraph>
  </Section>
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