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<Paper uid="J81-2001">
  <Title>Discourse-Oriented Anaphora Resolution in Natural Language Understanding: A Review</Title>
  <Section position="2" start_page="0" end_page="0" type="abstr">
    <SectionTitle>
1. Introduction
</SectionTitle>
    <Paragraph position="0"> To resolve various forms of definite reference u anaphora in particular -- early natural language understanding systems (reviewed in Hirst 1981) typically used a simple kind of history list of concepts previously mentioned in the input, with heuristics for selecting from this list. The history list was usually just a shift register containing the noun phrases from the last sentence or two, and the heuristics would take into account (among other things) selectional restrictions and syntactic constraints on pronominalization. SHRDLU (Winograd 1972) exemplifies this approach. Although able to resolve some types of reference, these systems were not able to handle reference in general, primarily because they did not take into account the effects of discourse structure on reference and pronominalization. This failure motivated work in computational discourse understanding that attempted to exploit discourse structure, especially the relationship between reference and discourse theme, to resolve definite reference. null The present paper 1 is a review of recent work in this area. Five principal approaches are surveyed:  1. Concept activatedness (Kantor) -- an examination of the factors affecting the pronominalizability of a concept; 2. Task-oriented dialogues (Grosz) -- using a priori knowledge of discourse structure to resolve references; 3. Frames as focus (Sidner) -- using discourse cues to choose a frame from a knowledge structure to act as focus; 4. Logical formalism (Webber) -- choosing a predicate calculus-like representation to handle problems such as quantification in reference resolution; 5. Discourse cohesion (Hobbs, Lockman, and others) -- building a focus and resolving  reference by discovering the cohesive ties in a text.</Paragraph>
    <Paragraph position="1"> Some preliminary definitions: By focus we mean the set containing exactly those concepts available for anaphoric or other definite reference at a point in a text, a set which may conveniently be divided into parts for nominal concepts, temporal concepts, verbal concepts and so forth.2 The focus is closely related to, but not necessarily identical to, the theme of a discourse -- what the discourse is about -- and since the latter is also sometimes termed focus, there is some terminological confusion. (See Section 2.6 and Chapter 4 of Hirst 1981 for further discussion of the distinction between theme and focus.) Strictly speaking, we mean by the referent of an anaphor or reference the real-world entity that it specifies, while by antecedent we mean the textual item through which the reference is made. In (1-1): 1 This paper is condensed from a chapter of a longer review of research concerning anaphora and its computational resolution (Hirst 1981).</Paragraph>
    <Paragraph position="2"> 2 In this paper we will be concerned mostly with focus for nominal concepts. Temporal, locative and verbal focus are discussed in Hirst (1981).</Paragraph>
    <Paragraph position="3"> Copyright 1981 by the Association for Computational Linguistics. Permission to copy without fee all or part of this material is granted provided that the copies are not made for direct commercial advantage and the Journal reference and this copyright notice are included on the first page. To copy otherwise, or to republish, requires a fee and/or specific permission.  speaks. 3 the antecedent of she is the text The Queen and the referent is the person who is queen. Generally, however, the two words can be (and are) used interchangeably without confusion.</Paragraph>
  </Section>
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