Analysing Dictionary Definitions of Motion Verbs

hi recent years many researchers working in the field of NLP have turned to large text resources such its machine-readable dictionaries (MRDs) and text corpora, in order to cope with the major problem of building a large computational lexicon. Within the Esprit project "Acquilex ''1 the possibility of exploiting existing monoand bi-lingual MRDs of four languages 2. in order to develop a multi-lingual and maximally reusable (by different researchers, for different NLP tasks) lexical knowledge base (LKB) is being explored m~d, indeed, we are finding evidence that important semantic and syntactic information can be semi-automatically extracted from these sources with significant saving in resources, comp,'u-ed to building a lexicon by hand. The task of extracting lexical information from MRDs is a non-trivial one iu that such information is mostly implicit in dictionaries and comple te ly new techn iques and methodologies have to be developed in order to achieve this goal. Furthermore, it is necessary both to take into account the peculiar characteristics of dictionary structures and to make theoretical hypotheses about the kind of information which can be useful for NLP systems and which therefore should be extracted. Indeed, these are two of the major issues being dealt with within Acquilex (another one being the issue regarding the representation of the knowledge extracted). Therefore, our work has been guided by theoretical hypotheses and empirical observations at the same time. First of all we assumed the centrality of the lexicon in the organization of natural languages; then, on the basis of the growing interest of different theoretical frameworks for semantic phenomena and of the fact that contemporary syntactic theories seem to converge on the hypothesis that syntactic structure is, to a large extent, determined by word meaning, we tried to see if it was possible to identify, within our dictionaries, that kind of semantic information on verbs which had been described as determining fundamental syntactic behaviours of the verbs themselves (cf. Levin 1985; Levin & Rappaport 1991). Finally, we tried to follow some indications, provided in works such as those of Pustejovsky (1989) or Boguraev & Pustejovsky (1990), relative to the kinds of lexical data which should be sought within MRDs and other computerized sources in order to be able to deal with problems facing the computational lingnist aiming at building components for NLP. According to Boguraev & Pustejovsky (1990: 39), i.e., tile following information should be individuated within sources of lexical data such as MRDs: argument structure; event structure (Aktionsart); qualia structure (Pns te jovsky (1989)); lexical inheritauce structure. Dictionary definitions are generally structured so that it is possible to identify two fundamental parts within them: the "genus term", which is connected with the entry-word by means of an IS-A (or taxonomical) relation, and the "differentia" part, in which what differenliates the entry from its hypernym is indicated 3 Within Acquilcx we are improving and developing techniques aimed at exploiting both the taxonomical organization of information in dictionaries and the fact that recurrent structures and patterns are found in the differentia; such patterns are being individuated through a pattern-matching procedure (which operates on the output of the syntactic analysis of definitions) in order to associate them with corresponding relations or semantic / conceptual categories (for descriptions of methodologies for extracting taxonomies fiom MRDs, see Calzolari (1984) aud Chodorow, Byrd & fleidorn (1985). For discussion of this way of analysing the differentia, see Calzolari (1991)). In the following, a research being carried out within this project and aimed at developing techniques for extracting different kinds of information related to motion verbs is presented.