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<Paper uid="J99-3001">
  <Title>Functional Centering Grounding Referential Coherence in Information Structure</Title>
  <Section position="10" start_page="337" end_page="340" type="concl">
    <SectionTitle>
7. Conclusion
</SectionTitle>
    <Paragraph position="0"> In this paper, we provided a novel account for ordering the forward-looking center list, a major construct of the centering model. The new formulation is entirely based on functional notions, grounded in the information structure of utterances in a discourse.</Paragraph>
    <Paragraph position="1">  Strube and Hahn Functional Centering We motivated our proposal by the constraints that hold for a free-word-order language such as German and derived our results from empirical studies of real-world texts. We also augmented the ordering criteria of the forward-looking center list such that it accounts not only for (pro)nominal anaphora but also for inferables (restricted to the subset of functional anaphora), an issue that, up to now, has only been sketchily dealt with in the centering framework. The extensions we proposed were validated by the empirical analysis of various texts of considerable length selected from different domains and genres. The &amp;quot;evaluation metric&amp;quot; we used refers to a new cost-based model of interpreting the validity of centering data. The distinction between cognitively cheap and expensive transition pairs led us to replace Rule 2 from the original model by a formulation that explicitly incorporates this cost-oriented distinction.</Paragraph>
    <Paragraph position="2"> A resolution module for (pro)nominal anaphora (Strube and Hahn 1995) and one for functional anaphora (Hahn, Markert, and Strube 1996) based on this functional centering model has been implemented as part of PARSETALK, a comprehensive text parser for German (Hahn, Schacht, and Br6ker 1994; Hahn, Neuhaus, and Br6ker 1997) in our group. All these modules are fully operational and integrated within the text-understanding backbone of SYNDIKATE, a large-scale text knowledge acquisition system for the two real-world domains of information technology (Hahn and Schnattinger 1998) and medicine (Hahn, Romacker, and Schulz 1999).</Paragraph>
    <Paragraph position="3"> Despite the progress made so far, many research problems remain open for further consideration in the centering framework. The following list mentions only the most pertinent issues that have come to our attention and complements the list given by Grosz, Joshi, and Weinstein (1995): .</Paragraph>
    <Paragraph position="4"> .</Paragraph>
    <Paragraph position="5"> The centering model is rather agnostic about the intricacies of complex sentences such as relative clauses, subordinate clauses, coordinations, and complex noun phrases. The problem caused by these structures for the centering model is how to decompose a complex sentence into center-updating units and how to process complex utterances consisting of multiple clauses. A first proposal is due to Kameyama (1998) who breaks a complex sentence into a hierarchy of center-updating units. Furthermore, she distinguishes several types of constructions in order to decide which part of the sentence is relevant for the resolution of an intersentential anaphor in the following sentence. Strube (1996b) (with respect to centering) and Suri and McCoy (1994) (with respect to the focus model) describe similar approaches and provide algorithms for the interaction of the resolution of inter- and intrasentential anaphora, but the topic has certainly not been dealt with exhaustively. The problem of complex NPs was pointed out by Walker and Prince (1996). Since the grammatical functions in a sentence may be realized by a complex NP, it is not clear how to rank these phrases in the Cf list. Walker and Prince (1996) propose a &amp;quot;working hypothesis&amp;quot; based on the surface order. Strube (1998) provides a complete specification for dealing with complex sentences, but this approach departs significantly from the centering model.</Paragraph>
    <Paragraph position="6"> It seems that there exist only a few fully operational implementations of centering-based algorithms, since the interaction of the algorithm with global and local ambiguities generated by a sentence parser has not received much attention until now. A first proposal for how to deal with center ambiguity in an incremental text parser has been made by Hahn and Strube (1996).</Paragraph>
    <Paragraph position="7">  Computational Linguistics Volume 25, Number 3 .</Paragraph>
    <Paragraph position="8"> .</Paragraph>
    <Paragraph position="9"> .</Paragraph>
    <Paragraph position="10"> The centering model covers the standard cases of anaphora, i.e., pronominal and nominal anaphora and even functional anaphora based on the proposal we have developed in this article. It does not, however, take into account several &amp;quot;hard&amp;quot; issues such as plural anaphora, generic definite noun phrases, propositional anaphora, and deictic forms (but see Eckert and Strube \[1999\] for a treatment of discourse-deictic anaphora in dialogues within a centering-type framework). These shortcomings might be traced back to the fact that the centering model, up to now, did not consider the role of the (main) verb of the utterance under scrutiny. Other cases, such as VP anaphora (Hardt 1992), temporal anaphora (Kameyama, Passonneau, and Poesio 1993; Hitzeman, Moens, and Grover 1995) have already been examined within the centering model.</Paragraph>
    <Paragraph position="11"> The particular phenomenon of paycheck anaphora is described by Hardt (1996), though he uses only a rather simplified centering model for this work. Other cases are only dealt with in the focusing framework such as propositional anaphora (Dahl and Ball 1990).</Paragraph>
    <Paragraph position="12"> Evaluations of the centering model have so far only been carried out manually. This is clearly no longer rewarding, so appropriate computational support environments have to be provided. What we have in mind is a kind of discourse structure bank and associated workbenches comparable to grammar workbenches and parse treebanks.</Paragraph>
    <Paragraph position="13"> Aone and Bennett (1994), for example, report on a GUI-based Discourse Tagging Tool (DTT) that allows a user to link an anaphor with its antecedent and specify the type of the anaphor (e.g., pronoun, definite NP, etc.). The tagged result can be written out to an SGML-marked file.</Paragraph>
    <Paragraph position="14"> Arguing for the need for discourse taggers, this also implies the development of a discourse structure interlingua (some sort of Discourse Structure Mark-up Language) for describing discourse structures in a common format in order to ease nonproblematic exchange and world-wide distribution of discourse structure data sets. Such an environment would provide excellent conditions for further testing, for example, of our assumption that the information structure constraints we suggest might apply in a universal manner.</Paragraph>
    <Paragraph position="15"> Centering theory, so far, is a model of local coherence in the minimal sense, i.e., it allows only the consideration of immediately adjacent centering structures for establishing proper referential links. In order to extend that theory to the level of global coherence, various steps have to be taken.</Paragraph>
    <Paragraph position="16"> At the referential level, mechanisms have to be introduced to account for reference relationships that extend beyond the immediately preceding utterance. Empirical evidence for such phenomena exists in the literature and we also found the need to have such a mechanism available for longer texts. The extension of functional centering to these phenomena is presented in Hahn and Strube (1997), while Walker (1998) builds upon the centering algorithm described in Brennan, Friedman, and Pollard (1987).</Paragraph>
    <Paragraph position="17"> At the level of discourse pragmatics, a richer notion than mere reference between terms is needed to account for coherence relations such as those aimed at by Rhetorical Structure Theory  Strube and Hahn Functional Centering (Mann and Thompson 1988). In addition, an explicit relation to basic notions from speech act theory is also missing, though it should be considered vital for the global coherence of discourse (Grosz and Sidner 1986). In general, it might become increasingly necessary to integrate very deep forms of reasoning, perhaps even nonmonotonic (Dunin-Keplicz and Lukaszewicz 1986) or abductive inference mechanisms (Nagao 1989), into the anaphora resolution process. This might become a sheer necessity when incrementality of processing receives a higher level of attention in the centering community.</Paragraph>
  </Section>
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