<?xml version="1.0" standalone="yes"?>
<Paper uid="P97-1072">
  <Title>Towards resolution of bridging descriptions</Title>
  <Section position="3" start_page="0" end_page="522" type="intro">
    <SectionTitle>
1 Background
</SectionTitle>
    <Paragraph position="0"> As part of our research on definite description (DD) interpretation, we asked 3 subjects to classify the uses of DDs in a corpus using a taxonomy related to the proposals of (Hawkins, 1978) (Prince, 1981) and (Prince, 1992). Of the 1040 DDs in our corpus, 312 (30%) were identified as anaphoric (same head), 492 (47%) as larger situation/unfamiliar (Prince's discourse new), and 204 (20%) as bridging references, defined as uses of DDs whose antecedents-coreferential or not--have a different head noun; the remaining were classified as idioms or were cases for which the subjects expressed doubt--see (Poesio and Vieira, 1997) for a description of the experiments.</Paragraph>
    <Paragraph position="1"> In previous work we implemented a system capable of interpreting DDs in a parsed corpus (Vieira and Poesio, 1997). Our implementation employed fairly simple techniques; we concentrated on anaphoric (same head) descriptions (resolved by matching the head nouns of DDs with those of their antecedents) and larger situation/unfamiliar descriptions (identified by certain syntactic structures, as suggested in (Hawkins, 1978)). In this paper we describe our subsequent work on bridging DDs, which involve more complex forms of common-sense reasoning.</Paragraph>
    <Paragraph position="2"> 2 Bridging descriptions: a corpus study Linguistic and computational theories of bridging references acknowledge two main problems in their resolution: first, to find their antecedents (ANCHORS) and second, to find the relations (LINKS) holding between the descriptions and their anchors (Clark, 1977; Sidner, 1979; Heim, 1982; Carter, 1987; Fraurud, 1990; Chinchor and Sundheim, 1995; Strand, 1997). A speaker is licensed in using a bridging DD when he/she can assume that the common-sense knowledge required to identify the relation is shared by the listener (Hawkins, 1978; Clark and Marshall, 1981; Prince, 1981). This reliance on shared knowledge means that, in general, a system could only resolve bridging references when supplied with an adequate lexicon; the best results have been obtained by restricting the domain and feeding the system with specific knowledge (Carter, 1987). We used the publicly available lexical database Word-Net (WN) (Miller, 1993) as an approximation of a knowledge basis containing generic information.</Paragraph>
    <Paragraph position="3"> Bridging DDs and WordNet As a first experiment, we used WN to automatically find the anchor of a bridging DD, among the NPs contained in the previous five sentences. The system reports a semantic link between the DD and the NP if one of the following is true: * The NP and the DD are synonyms of each other, as in the suit -- the lawsuit.</Paragraph>
    <Paragraph position="4"> * The NP and the DD are in direct hyponymy relation with each other, for instance, dollar -- the currency.</Paragraph>
    <Paragraph position="5"> * There is a direct or indirect meronymy (partof relation) between the NP and the DD. Indirect meronymy holds when a concept inherits parts from its hypernyms, like car inherits the part wheel from its hypernym wheeled_vehicle.</Paragraph>
    <Paragraph position="6"> * Due to WN's idiosyncratic encoding, it is often  necessary to look for a semantic relation between sisters, i.e. hyponyms of the same hypernym, such as home -- the house.</Paragraph>
    <Paragraph position="7"> An automatic search for a semantic relation in 5481 possible anchor/DD pairs (relative to 204 bridging DDs) found a total of 240 relations, distributed over 107 cases of DDs. There were 54 correct resolutions (distributed over 34 DDs) and 186 false positives.</Paragraph>
    <Paragraph position="8"> Types of bridging definite descriptions A closer analysis revealed one reason for the poor results: anchors and descriptions are often linked by other means than direct lexico-semantic relations. According to different anchor/link types and their processing requirements, we observed six major classes of bridging DDs in our corpus: Synonymy/Hyponymy/Meronymy These DDs are in a semantic relation with their anchors that might be encoded in WN. Examples are: a) Synonymy: new album -- the record, three bills -the legislation; b) Hypernymy-Hyponymy: rice -the plant, the television show -- the program; c) Meronymy: plants -- the pollen, the house -- the chimney.</Paragraph>
    <Paragraph position="9"> Names Definite descriptions may be anchored to proper names, as in: Mrs. Park -- the housewife and Pinkerton's Inc -- the company.</Paragraph>
    <Paragraph position="10"> Events There are cases where the anchor of a bridging DD is not an NP but a VP or a sentence. Examples are: ...individual investors contend. -- They make the argument in letters...; Kadane Oil Co. is currently drilling two wells... -- The activity ... Compound Nouns This class of DDs requires considering not only the head nouns of a DD and its anchor for its resolution but also the premodifiers. Examples include: stock market crash -- the markets, and discount packages -- the discounts.</Paragraph>
    <Paragraph position="11"> Discourse Topic There are some cases of DDs which are anchored to an implicit discourse topic rather than to some specific NP or VP. For instance, the industry (the topic being oil companies) and the first half (the topic being a concert).</Paragraph>
    <Paragraph position="12"> Inference One other class of bridging DDs includes cases based on a relation of reason, cause, consequence, or set-members between an anchor (previous NP) and the DD (as in Republicans/Democratics -the two sides, and last week's earthquake -- the suffering people are going through).</Paragraph>
    <Paragraph position="13"> The relative importance of these classes in our corpus is shown in Table 1. These results explain in part the poor results obtained in our first experiment: only 19% of the cases of bridging DDs fall into the category which we might expect WN to handle.</Paragraph>
  </Section>
class="xml-element"></Paper>