Semantic Representation of Negation Using Focus Detection

Proceedings of the 49th Annual Meeting of the Association for Computational Linguistics, pages 581–589, Portland, Oregon, June 19-24, 2011. c⃝2011 Association for Computational Linguistics Semantic Representation of Negation Using Focus Detection Eduardo Blanco and Dan Moldovan Human Language Technology Research Institute The University of Texas at Dallas Richardson, TX 75080 USA {eduardo,moldovan}@hlt.utdallas.edu Abstract Negation is present in all human languages and it is used to reverse the polarity of part of statements that are otherwise affirmative by default. A negated statement often carries pos- itive implicit meaning, but to pinpoint the pos- itive part from the negative part is rather dif- ficult. This paper aims at thoroughly repre- senting the semantics of negation by revealing implicit positive meaning. The proposed rep- resentation relies on focus of negation detec- tion. For this, new annotation over PropBank and a learning algorithm are proposed. 1 Introduction Understanding the meaning of text is a long term goal in the natural language processing commu- nity. Whereas philosophers and linguists have pro- posed several theories, along with models to rep- resent the meaning of text, the field of computa- tional linguistics is still far from doing this automati- cally. The ambiguity of language, the need to detect implicit knowledge, and the demand for common- sense knowledge and reasoning are a few of the dif- ficulties to overcome. Substantial progress has been made, though, especially on detection of semantic relations, ontologies and reasoning methods. Negation is present in all languages and it is al- ways the case that statements are affirmative by default. Negation is marked and it typically sig- nals something unusual or an exception. It may be present in all units of language, e.g., words (incredible), clauses (He doesn’t have friends). Negation and its correlates (truth values, lying, irony, false or contradictory statements) are exclu- sive characteristics of humans (Horn, 1989; Horn and Kato, 2000). Negation is fairly well-understood in grammars; the valid ways to express a negation are documented. However, there has not been extensive research on detecting it, and more importantly, on representing the semantics of negation. Negation has been largely ignored within the area of semantic relations. At first glance, one would think that interpreting negation could be reduced to finding negative key- words, detect their scope using syntactic analysis and reverse its polarity. Actually, it is more com- plex. Negation plays a remarkable role in text un- derstanding and it poses considerable challenges. Detecting the scope of negation in itself is chal- lenging: All vegetarians do not eat meat means that vegetarians do not eat meat and yet All that glitters is not gold means that it is not the case that all that glitters is gold (so out of all things that glitter, some are gold and some are not). In the former example, the universal quantifier all has scope over the nega- tion; in the latter, the negation has scope over all. In logic, two negatives always cancel each other out. On the other hand, in language this is …

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