Functional Centering

Functional Centering Michael Strube & Udo Hahn Freiburg University (~[} Computational Linguistics Lab Europaplatz 1, D-79085 Freiburg, Germany {strube, hahn}@coling, uni-freiburg, de Abstract Based on empirical evidence from a free word order language (German) we propose a fundamental revision of the principles guid- ing the ordering of discourse entities in the forward-looking centers within the center- ing model. We claim that grammatical role criteria should be replaced by indicators of the functional information structure of the utterances, i.e., the distinction between context-bound and unbound discourse ele- ments. This claim is backed up by an empir- ical evaluation of functional centering. 1 Introduction The centering model has evolved as a methodology for the description and explanation of the local coherence of discourse (Grosz et al., 1983; 1995), with focus on pronominal and nominal anaphora. Though several cross-linguistic studies have been carded out (cf. the enumeration in Grosz et al. (1995)), an almost canon- ical scheme for the ordering on the forward-looking centers has emerged, one that reflects well-known reg- ularities of fixed word order languages such as En- glish. With the exception of Walker et al. (1990; 1994) for Japanese, Turan (1995) for Turkish, Ram- bow (1993) for German and Cote (1996) for English, only grammatical roles are considered and the (par- tial) ordering in Table 11 is taken for granted. I subject > dir-object > indir-object I > complement(s) > adjunct(s) Table 1: Grammatical Role Based Ranking on the C! ~Table 1 contains the most explicit ordering of grammat- ical roles we are aware of and has been taken from Bren- nan et al. (1987). Often, the distinction between comple- ments and adjuncts is collapsed into the category "others" (c.f., e.g., Grosz et al. (1995)). Our work on the resolution of anaphora (Strube & Hahn, 1995; Hahn & Strube, 1996) and textual el- lipsis (Hahn et al., 1996), however, is based on Ger- man, a free word order language, in which grammat- ical role information is far less predictive for the or- ganization of centers. Rather, for establishing proper referential relations, the functional information struc- ture of the utterances becomes crucial (different per- spectives on functional analysis are brought forward in Dane~ (1974b) and Dahl (1974)). We share the no- tion of functional information structure as developed by Dane~ (1974a). He distinguishes between two cru- cial dichotomies, viz. given information vs. new infor- mation (constituting the information structure of ut- terances) on the one hand, and theme vs. rheme on the other (constituting the thematic structure of utter- ances; cf. Halliday & Hasan (1976, pp.325-6)). Dane~ refers to a definition given by Halliday (1967) to avoid the confusion likely to arise in the use of these terms: "[...] while given means what you were talking about (or what I was talking about before), theme means what I am talking about (now) [...]" Halliday (1967, p.212). Dane~ concludes that the distinction between given information and theme is justified, while the dis- tinction between new information and rheme is not. Thus, we …

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