Processing Complex Sentences in the Centering Framework

Processing Complex Sentences in the Centering Framework Michael Strube Freiburg University Computational Linguistics Lab Europaplatz 1, D-79085 Freiburg, Germany st rube@ coling, uni- freiburg, de Abstract We extend the centering model for the resolution of intia-sentential anaphora and specify how to handle complex sentences. An empirical eval- uation indicates that the functional information structure guides the search for an antecedent within the sentence. 1 Introduction The centering model (Grosz et al., 1995) focuses on the resolution of inter-sentential anaphora. Since intra-sentential anaphora occur at high rates in real- world texts, the model has to be extended for the res- olution of anaphora at the sentence level. However, the centering framework is not fully specified to han- die complex sentences (Suri & McCoy, 1994). This underspocification corresponds to the lack of a pre- cise definition of the expression utterance, a term al- ways used but intentionally left undefined 1. There- fore, the centering algorithms currently under discus- sion are not able to handle naturally occurring dis- course. Possible strategies for treating sentence-level anaphora within the centering framework are 1. processing sentences linearly one clause at a time (as suggested by Grosz et al. (1995)), 2. preference for sentence-external antecedents which are proposed by the centering mechanism, 3. preference for sentence-internal antecedents which are filtered by the usual binding criteria, 4. a mixed-mode which prefers only a particular set of sentence-internal over sentence-external an- tecedents (e.g. Suri & McCoy (1994)). The question arises as to which strategy fits best for the interaction between the resolution of intra- and inter- sentential anaphora. In my contribution, evidence for a mixed-mode strategy is brought forward, which favors a particular set of sentence-internal antecedents given by functional criteria. 1Cf. the sketchy statements by Brennan et al. (1987, p.155): "[...] U is an utterance (not necessarily a full clause) [...]", and by Grosz et al. (1995, p.209): "U need not to be a full clause." 2 Constraints on Sentential Anaphora Our studies on German texts have revealed that the functional information structure of the sentence, con- sidered in terms of the context-boundedness of dis- course elements, is the major determinant for the rank- ing on the forward-looking-centers (C! (U,)) (Strube & Hahn, 1996). Hence, context-bound discourse ele- ments are generally ranked higher in the C! than any other non-anaphoric element. The functional informa- tion structure has impact not only on the resolution of inter-sentential anaphora, but also on the resolution of intra-sentential anaphora. Hence, the most preferred antecedent of an intra-sentential anaphor is a phrase which is also anaphoric. Consider sentences (1) and (2) and the corresponding centering data in Table 1 (Cb: backward-looking center; the first dement of the pairs denotes the discourse entity, the second element the surface). In sentence (1), a nominal anaphor oc- curs, der T3100SX (a particular notebook). In sentence (2), another nominal anaphor appears, der Rechner (the computer), which is resolved to T3100SX from the previous sentence. In the matrix clause, the pro- noun er (it) co-specifies the already resolved anaphor …

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