Lexical Transfer based on bilingual signs: Towards interaction during transfer

Lexical Transfer based on bilingual signs: Towards interaction during transfer • Jun-ich Tsujii Kimikazu Fujita Centre for Computational Linguistics University of Manchester Institute of Science and Technology PO Box 88, Manchester M60 1QD, United Kingdom Email: { tsujii,fujita} @ uk.ac.umist.ccl Abstract The lexical transfer phase is the most crucial step in MT because most of difficult problems are caused by lexical differences between two languages. In order to treat lexical issues systemati- cally in transfer-based MT systems, we introduce the concept of bilingual-sings which are defined by pairs of equivalent monolingual signs. The bil- ingual signs not only relate the local linguistic structures of two languages but also play a central role in connecting the linguistic processes of transla- tion with knowledge based inferences. We also show that they can be effectively used to formulate appropriate questions for disambiguating "transfer ambiguities", which is crucial in interactive MT sys- tems. 1. Introduction 'Lexical Transfer' has always been one of the main sources of problems in Machine Translation (MT)[Melby, 19861[Nirenburg, 1988]. Research in transfer-based MT systems has focussed on discovering an appropriate level of linguistic description for translation, at which we can specify 'translation relations" (or transfer rules) in a simple manner. However, lexical differences between languages have caused problems in this attempt. Besides structural changes caused by lexi- cal Iransfer, selecting appropriate translations of source lexical items has been one of the hardest problems in MT. Because languages have their own ways of reflecting the structure of the world in their lexi- cons, and the process of lexicalization is more or less arbitrary, bilingual knowledge about lexical correspondences is highly dependent on language pairs and individual words. We have to .prepare a framework in which such idiosyncratic bilingual knowledge about lexical items can be systemati- cally accumulated. Our approach in this paper follows the general trend in computational linguistics which emphasizes the role of the lexicon in linguistic theory. In partic- ular, our idea of bilingual signs shares a common intuition with [Beaven, 1988] and [Whitelock, 1988]. As with their proposal, we too specify local structural correspondences between two languages in bilingual lexicons. Unlike former approaches, however, we expli- citly define bilingual signs and use them as predi. cates in logical formulae (bilingual pivot expres- sions). Bilingual signs in our framework not only link the local linguistic structures of two languages where the corresponding two monolingual signs appear, but also, by behaving as logical predicates, they connect linguistic-based processes in MT with inference processes. Complicated structural Changes, which are often required in translation of remote language pairs like English and Japanese, are captured by logical inferences [Tsujii, 1990]. The framework has the following advantages over conventional methods. (i) Reversibility of bilingual dictionaries (lexical transfer rules) (ii) Natural interfaces between knowledge-based (inference) processes and MT (iii) Ease of paraphrasing using different words (see section 6) 2. Bilingual signs as logical predicates and their definition The basic idea of bilingual signs is simple. instead of using predicates corresponding directly to surface words, we …

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