Combining Lexical Resources for Contextual Synonym Expansion

International Conference RANLP 2009 - Borovets, Bulgaria, pages 404–410 Combining Lexical Resources for Contextual Synonym Expansion Ravi Sinha and Rada Mihalcea University of North Texas ravisinha@my.unt.edu, rada@cs.unt.edu Abstract In this paper, we experiment with the task of contextual synonym expansion, and com- pare the benefits of combining multiple lexical resources using both unsupervised and super- vised approaches. Overall, the results obtained through the combination of several resources ex- ceed the current state-of-the-art when selecting the best synonym for a given target word, and place second when selecting the top ten syn- onyms, thus demonstrating the usefulness of the approach. Keywords lexical semantics, synonym expansion, lexical substitution 1 Introduction Word meanings are central to the semantic interpre- tation of texts. The understanding of the meaning of words is important for a large number of nat- ural language processing applications, including in- formation retrieval [11, 10, 19], machine translation [4, 3], knowledge acquisition [7], text simplification, question answering [1], cross-language information re- trieval [18, 5]. In this paper, we experiment with contextual syn- onym expansion as a way to represent word meanings in context. We combine the benefits of multiple lexical resources in order to define flexible word meanings that can be adapted to the context at hand. The task, also referred to as lexical substitution, has been officially introduced during Semeval-2007 [16], where partici- pating systems were asked to provide lists of synonyms that were appropriate for selected target words in a given context. Although it may sound simple at first, the task is remarkably difficult, as evidenced by the accuracies reported by the participating systems in Semeval-2007. In the experiments reported in this paper, we fo- cus on the usefulness of different lexical resources – used individually or in tandem – for the purpose of contextual synonym expansion. We experiment with several resources to determine which ones provide the best synonyms for a given word in context. 2 Synonym expansion in con- text Contextual synonym expansion, also known as lexical substitution [16], is the task of replacing a certain word in a given context with another, suitable word. See for example the four sentences from table 1, drawn from the development data from the Semeval-2007 lexical substitution task. In the first sentence, for instance, assuming we choose bright as the target word, a suit- able substitute could be brilliant, which would both maintain the meaning of the target word and at the same time fit the context. Sentence Target Synonym The sun was bright. bright brilliant He was bright and independent. bright intelligent His feature film debut won awards. film movie The market is tight right now. tight pressured Table 1: Examples of synonym expansion in context We perform contextual synonym expansion in two steps: candidate synonym collection, followed by context-based synonym fitness scoring. Candidate synonym collection refers to the task of collecting a set of potential synonym candidates for a given target word, starting with various resources. Note that this step does not account for the meaning of the …

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