Centering in-the-Large: Computing Referential Discourse Segments

Centering in-the-Large: Computing Referential Discourse Segments Udo Hahn & Michael Strube Computational Linguistics Research Group Freiburg University, Werthmannplatz 1 D-79085 Freiburg, Germany http://www.coling.uni-freiburg.de/ Abstract We specify an algorithm that builds up a hi- erarchy of referential discourse segments from local centering data. The spatial extension and nesting of these discourse segments constrain the reachability of potential antecedents of an anaphoric expression beyond the local level of adjacent center pairs. Thus, the centering model is scaled up to the level of the global referential structure of discourse. An empiri- cal evaluation of the algorithm is supplied. 1 Introduction The centering model (Grosz et al., 1995) has evolved as a major methodology for computational discourse analy- sis. It provides simple, yet powerful data structures, con- straints and rules for the local coherence of discourse. As far as anaphora resolution is concerned, e.g., the model requires to consider those discourse entities as potential antecedents for anaphoric expressions in the current ut- terance Ui, which are available in the forward-looking centers of the immediately preceding utterance Ui- 1. No constraints or rules are formulated, however, that ac- count for anaphoric relationships which spread out over non-adjacent utterances. Hence, it is unclear how dis- course elements which appear in utterances preceding utterance Ui-1 are taken into consideration as potential antecedents for anaphoric expressions in Ui. The extension of the search space for antecedents is by no means a trivial enterprise. A simple linear backward search of all preceding centering structures, e.g., may not only turn out to establish illegal references but also contradicts the cognitive principles underlying the lim- ited attention constraint (Walker, 1996b). The solution we propose starts from the observation that additional constraints on valid antecedents are placed by the global discourse structure previous utterances are embedded in. We want to emphasize from the beginning that our pro- posal considers only the referential properties underlying the global discourse structure. Accordingly, we define the extension of referential discourse segments (over sev- eral utterances) and a hierarchy of referential discourse segments (structuring the entire discourse). 1 The algo- rithmic procedure we propose for creating and manag- ing such segments receives local centering data as input and generates a sort of superimposed index structure by which the reachability of potential antecedents, in par- ticular those prior to the immediately preceding utter- ance, is made explicit. The adequacy of this definition is judged by the effects centered discourse segmentation has on the validity of anaphora resolution (cf. Section 5 for a discussion of evaluation results). 2 Global Discourse Structure There have been only few attempts at dealing with the recognition and incorporation of discourse structure be- yond the level of immediately adjacent utterances within the centering framework. Two recent studies deal with this topic in order to relate attentional and intentional structures on a larger scale of global discourse coher- ence. Passonneau (1996) proposes an algorithm for the generation of referring expressions and Walker (1996a) integrates centering into a cache model of attentional state. Both studies, among …

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