A Hierarchical Account of Referential Accessibility

A Hierarchical Account of Referential Accessibility Nancy IDE Department of Computer Science Vassar College Poughkeepsie, New York 12604-0520 USA ide@cs.vassar.edu Dan CRISTEA Department of Computer Science University “Al. I. Cuza” Iasi, Romania dcristea@infoiasi.ro Abstract In this paper, we outline a theory of referential accessibility called Veins Theory (VT). We show how VT addresses the problem of "left satellites", currently a problem for stack-based models, and show that VT can be used to significantly reduce the search space for antecedents. We also show that VT provides a better model for determining domains of referential accessibility, and discuss how VT can be used to address various issues of structural ambiguity. Introduction In this paper, we outline a theory of referential accessibility called Veins Theory (VT). We compare VT to stack-based models based on Grosz and Sidner's (1986) focus spaces, and show how VT addresses the problem of "left satellites", i.e., subordinate discourse segments that appear prior to their nuclei (dominating segments) in the linear text. Left-satellites pose a problem for stack-based models, which remove subordinate segments from the stack before pushing a nuclear or dominating segment, thus rendering them inaccessible. The percentage of such cases is typically small, which may account for the fact that their treatment has been largely overlooked in the literature, but the phenomenon nonetheless persists in most texts. We also show how VT can be used to address various issues of 1structural ambiguity. Veins Theory Veins Theory (VT) extends and formalizes the relation between discourse structure and reference proposed by Fox (1987). VT identifies “veins” over discourse structure trees that are built according to the requirements put forth in Rhetorical Structure Theory (RST) (Mann and Thompson, 1987). RST structures are represented as binary trees, with no loss of information. Veins are computed based on the RST-specific distinction between nuclei and satellites; therefore, RST relations labeling nodes in the tree are ignored. Terminal nodes in the tree represent discourse units and non- terminal nodes represent discourse relations. The fundamental intuition underlying VT is that the distinction between nuclei and satellites constrains the range of referents to which anaphors can be resolved; in other words, the nucleus-satellite distinction induces a domain of referential accessibility (DRA) for each referential expression. More precisely, for each anaphor a in a discourse unit u, VT hypothesizes that a can be resolved by examining referential expressions that were used in a subset of the discourse units that precede u; this subset is called the DRA of u. For any elementary unit u in a text, the corresponding DRA is computed automatically from the text's RST tree in two steps: 1. Heads for each node are computed bottom- up over the rhetorical representation tree. Heads of elementary discourse units are the units themselves. Heads of internal nodes, i.e., discourse spans, are computed by taking the union of the heads of the immediate child nodes that are nuclei. For example, for the text in Figure 1,1 with the rhetorical structure shown in Figure 2,2 the head of …

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