Foreground and Background Lexicons and Word Sense Disambiguation for Information Extraction

In recent years, lexicon acquisition from machine-readable dictionaries and corpora has been a dynamic field of research. However it has not always been evident how lexical information so acquired can be used, or how it relates to more structured meaning representations. In this paper I look at this issue in relation to one particular NLP task, Information Extraction (hereafter IE), and one subtask for which both lexical and general knowledge are required, Word Sense Disambiguation (WSD). The argument is as follows. For an IE task, the output formalism, that is, the database fields or templates which the system is to fill, specifies the objecttypes and relations that the system is to find out about; the ‘ontology’. An IE task operates in a specific domain. The task requires the key terms of that domain, the ‘foreground lexicon’, to be tightly bound to the ontology. This is a task that calls for human input. For all other vocabulary, the ‘background lexicon’, a far shallower semantics will be sufficient. This shallow semantics can be obtained automatically from sources such as machine-readable dictionaries and domain corpora. The foreground and background lexicons are suited to different kinds of WSD strategies. For the background lexicon, statistical methods for coarsegrained disambiguation are appropriate. For the foreground lexicon, WSD will occur as a by-product of finding a coherent semantic interpretation of an input sentence, in which all arguments are of the appropriate type. Once the foreground/background distinction is developed, there is a good match between what is possible, given the state of the art in WSD and acceptable levels of human input, and what is required, for high-quality IE. The two-tier approach has been adopted by a number of IE systems. The POETIC (Evans et al., 1996) and Sussex MUC-5 (Gaizauskas, Cahill, and Evans, 1994) systems used a hand-crafted foreground lexicon and the Alvey Tools lexicon (Carroll and Grover, 1989) as a background lexicon for syntactic