Deriving Metonymic Coercions from WordNet.

This paper presents a method for deriving metonymic coercions from the knowledge available in WordNet. Two different classes of metonymies are inferred by using (1) lexico-semantic connections between concepts or (2) morphological cues and logical formulae defining lexical concepts. In both cases the derivation of metonymic paths is based on approximations of sortal constraints retrieved from WordNet. This novel method of inferring coercions validates the related knowledge through coreference links. As a result, metonymic coercions are potentially useful for the recognition of coreferring entities in information extraction systems. 1 Problem description The pervasive phenomenon of metonymy raises a problem for the interpretation of real-world texts. Metonymies are figures of speech in which, according to the literature definition from (Lakoff and Johnson, 1990), "one entity is used to refer to another, that is related to it". Characteristic of a metonymic reading of a textual expression is the fact that the satisfaction of sortal constraints guides the coercion to related knowledge. The comprehensive account of the semantics of meaning transfers presented in (Nunberg, 1995) indicates that coercions need to be embedded in a conceptual and lexico-semantic space, ideally provided by a linguistic knowledge base. Nunberg also notes that coercions are licensed by pragmatic circumstances, specifically pertaining to the Gricean principles (Grice, 1975). In this paper, we revisit the notion of metonymy and address the computational aspects of its resolution in the context of the relational semantics provided by the recently released WordNet 1.6 lexical database (www.cogsci.princeton.edu/~wn ). Following the lessons learned from the WordNetbased inference of Gricean implicatures, reported in (Harabagiu et al., 1996), a novel methodology of producing metonymic paths was devised. The coercions combine WordNet relations with st'142 mantic information derived from conceptual definitions. In WordNet (Miller, 1995) synony m words are structured in synsets, underlying a linguistic concept. Every synset is associated with a gloss, representing a textual definition, that can be translated in a logical form following the notation introduced in (Hobbs, 1986-1). This formalism, used in the implementation of TACITUS (Hobbs, 1986-2), accommodates a large variety of discourse inferences and, moreover, provides an elegant manner of localizing ambiguities, as was shown in (Bear and Hobbs, 1988). Conceptual support from linguistic knowledge bases was already considered in the implementation of several metonymy resolution systems (e.g. (Markerr and Hahn, 1997), (Fass, 1991) (Hobbs. 1986-2)), but none of these systems provided with more inferential flexibility than the typical coercion classes formulated by Lakoff (Lakoff and Johnson, 1990). We propose here a metonymy resolution approach that accounts for an open class of coercions. Similarly to Nunberg (Nunberg, 1995) and more recently to Markert and Hahn (Markert and Hahn, 1997), we find metonymy and nominal reference resolution to be two interacting processes; therefore, the proposed computational model validates metonymies through coreference links. 2 Classes of metonymic coercions Stallard proposed in (Stallard, 1993) a distinction between two kinds of metonymy: (1) referential raetonymy, in which the referent of a nominal predicate argument requires coercion and (2) predicative metonymy, featuring the coercion of the predicate usually corresponding to a verbal lexicalization. In his study, Stallard focuses on metonymic inferences required by a specific performative context, characterized by wh-questions and imperatives. His formalization of referential and predicative metonymies is based on the logical form readings of utterances from the DARPA ATIS (Air Travel Information Service) domain (MADCOW. 1992), a question-answering database about commercial air flights, comprising questions of the form: