Expectations in Incremental Discourse Processing

Expectations in Incremental Discourse Processing Dan Cristea Faculty of Computer Science University "A.I. Cuza" 16, Berthelot Street 6600 - Iasi, Romania dcristea@infoiasi, ro Bonnie Webber Dept. of Computer L: Information Science University of Pennsylvania 200 South 33rd Street Philadelphia PA 19104-6389 USA bonnie©central, cis. upenn, edu Abstract The way in which discourse features ex- press connections back to the previous dis- course has been described in the literature in terms of adjoining at the right frontier of discourse structure. But this does not allow for discourse features that express ez- pectations about what is to come in the subsequent discourse. After characterizing these expectations and their distribution in text, we show how an approach that makes use of substitution as well as adjoining on a suitably defined right frontier, can be used to both process expectations and constrain discouse processing in general. 1 Introduction Discourse processing subsumes several distinguish- able but interlinked processes. These include refer- ence and ellipsis resolution, inference (e.g., inferen- tial processes associated with focus particles such as, in English, "even" and "only"), and identification of those structures underlying a discourse that are as- sociated with coherence relations between its units. In the course of developing an incremental approach to the latter, we noticed a variety of constructions in discourse that raise expectations about its future structural features. We found that we could rep- resent such expectations by adopting a lexical vari- ant of TAG - LTAG (Schabes, 1990) - and using its substitution operation as a complement to ad- joining. Perhaps more interesting was that these expectations appeared to constrain the subsequent discourse until they were resolved. This we found we could model in terms of constraints on adjoining and substitution with respect to a suitably defined Right Frontier. This short paper focuesses on the phenomenon of these expectations in discourse and their expression in a discourse-level LTAG. We con- elude the paper with some thoughts on incremental discourse processing in light of these expectations. The following examples illustrate the creation of expectations through discourse markers: Example 1 a. On the one hand, John is very generous. b. On the other, he is extremely difficult to find. Example 2 a. On the one hand, John is very generous. b. On the other, suppose you needed some money. c. You'd see that he's very difficult to find. Example 3 a. On the one hand, John is very generous. b. For example, suppose you needed some money. c. You would just have to ask him for it. b. On the other hand, he is very difficult to find. Example 1 illustrates the expectation that, follow- ing a clause marked "on the one hand", the discourse will express a constrasting situation (here marked by "on the other"). Examples 2 and 3 illustrate that such an expectation need not be satisfied im- mediately by the next clause: In Example 2, clause (b) partially resolves the expectation set up in (a), but introduces an expectation that the subsequent discourse will indicate …

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