The discourse functions of Italian subjects: a centering approach

The discourse functions of Italian subjects: a centering approach Barbara Di Eugenio Computational Linguistics Carnegie Mellon University Pittsburgh, PA, 15213 USA dieugeni(c~.andrew.cmu.edu Abstract This paper examines the discourse fluic- tions that different types of subjects perform in Italian within the centering framework (Grosz el; al., 1995). I build on my previous work (Di gugenio, 1990) that accounted for the alternation of null and strong prottouns in subject position. I extend my previous atialysis in several wws: for examI)le, I refine the notion of CONTINUE altd discuss the centering functions of full NPs. 1 Introduction hiterpreting referential expressions is importatlt for any large coverage NL system; while such sys- tems do exist for Italian, e.g. (Stock et al., 1993; Lombardo and Lesmo, 1994), to my knowledge not mudi attention has been devoted to the inter- pretation of Italian referential expressions. Some exceptions are (Samek-Lodovici and Strapparava, 1990), that discusses interpretation of referential expressions within dialogues to access a videodisc on Italimi art; (Not and Zancanaro, 1995), that adopts a systemic gra,nmar approadi (Halliday, 1976); and (Di Eugenio, 1990), which uses center- ing theory (Grosz et al., 1995) to account for the alternation of null and strong subjects. hi this paper, I build on and expand (Di Eu- genio, 1990) in several ways. First, I reanalyzc the hypotheses I proposed earlier with respect to a corl)uS of naturally occurring data} I show that those hypotheses are basically supported, iThe examples in (Di Euge.nio, 199{)) were constructed. and that when they aren't an elegant explmia- tion can be found by looking at a two member sequence of ce,ttering tra,tsitions rather than at just ()tie transition. Second, I extend my previous analysis by also discussing the centering functions of full NPs in subject position, and some occur- rences of pronotuis tuiaccounted for by centering. 2 Centering theory Centering the.ory (Grosz et al., 1986; Brennan ct al., 1987; Grosz et al., 1995) models local coher- e.nee in discourse: it keet)s track of how local focus varies from one utterance to the next. (]entering postulates that: ~ * Each utterance Un has associated with it a set of discourse entities, the FOH.WAI{I)-LOO|<IN(~ CEN- TERS or Cfs. The Cf list is ranked according to discourse salience. - The I{ACKWARD-LOOKIN(] CENTEI{~ or Cb~ ix the t member of the Cf list that lJ,~ most centrally con- cerns~ attd that links U~ to the previous discourse. * Finally, the PI%EFERI{.ED CENTEI{, or Cp: is the highest rauked member of the Of list. The Cp rep- resents a prediction about die Cb of the following utterance. Transitions between two adjacent utterances U,>_I and U,~ can be characterized as a function of looking backward whether Cb(Un) is the same as Cb(Un 1) - attd of looking forward whether Cb(Ur~) is the same as Cp(U,~). Table 1 illustrates the Pour transitions that are detined according to diese constraints. (Brennan et al., 1987) proposes a default ordering on transitions which correlates with discourse coherence: CONTINUE is preferred to RETAIN is prelbrred to SMOOTH-SHIFT …

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