RAFT/RAPR and Centering: A Comparison and Discussion of Problems Related to Processing Complex Sentences

Several researchers have noted the local coherence exhibited by discourse (Sidner 1979; Grosz, Joshi, and Weinstein 1983; Carter 1987; etc.). A primary component of this local coherence is the way the local focus of the discourse shifts from one sentence to the next and the way this shifting is marked by linguistic choices made by the writer/speaker. By local focus, we refer to that concept a sentence is most centrally about within the discourse context in which it occurs. This is sometimes called the topic or center. A local focusing framework typically consists of focus-tracking algorithms and algorithms for suggesting referents for pronouns. Such a framework can be used in conjunction with an inferencing mechanism to resolve pronouns (and other anaphora) in a Natural Language Understanding system. The focusing framework suggests a referent for a pronoun, and the inferencing mechanism then confirms or rejects the suggested referent on the basis of semantic factors, i.e., semantics, world knowledge, etc. The focusing framework is useful because it only requires an inferencing mechanism to confirm a co-specification rather than requiring an inferencing to find the referent independently. To date, there have been two major frameworks for tracking the local focus from one sentence to the next and for using focus during pronoun resolution. The first framework, Focusing, was introduced by Sidner (1979). In this squib, we will describe our framework, Revised Algorithms for Focus Tracking and Revised Algorithms for Pronoun Resolution (RAFT/RAPR), which is based on Sidner's work. RAFT/RAPR can be characterized as maintaining two foci for a sentence: the subject focus and the current focus, which very often have distinct contents. RAFT/RAPR maintains a set of data structures, and uses rules (which take grammatical roles into account) for pronoun resolution and computing the foci. Taken together, these rules describe how focus can (and is most likely to) shift from one sentence to the next. Note that focus tracking and pronoun resolution are mutually dependent processes: focus tracking is necessary for pronoun resolution, and pronoun resolution, in turn, affects focus tracking.