The hyperonym problem revisited: Conceptual and lexical hierarchies in language generation

When a lexical item is selected in the language production process, it needs to be explained why none of its superordinates gets selected instead, since their applicability conditions are fulfilled all the same. This question has received much attention in cognitive modelling and not as much in other branches of NLG. This paper describes the various approaches taken, discusses the reasons why they are so different, and argues that production models using symbolic representations should make a distinction between conceptual and lexical hierarchies, which can be organized along fixed levels as studied in (some branches of) lexical semantics. 1 I n t r o d u c t i o n Representations used in language processing owe much to the tradition of 'semantic networks', which nowadays have been successfully formalized and organized especially around one particular kind of link between nodes: the ISAlink, which connects entities to subordinate entities. This link is, by definition, the root of the so-called 'hyperonym 1 problem': When a speaker utters a word, she presumably needs to retrieve a lemma from her mental lexicon, and the 'applicability conditions" of the lemma automatically render the lemma's hyperonyms also applicable, thus raising the question how the choice among a set of more or less specific words is made. In this paper, I briefly review approaches to the hyperonym problem in psycholinguistics, natural language generation, and lexical semantics. In doing that, I will refer to different branches of NLG according to their roots I Alternat ively called 'hypernym' in many publications: 'hyperonym" seems preferable, as the Greek root is 'hyper" (super) + ' onoma ' (name). . . . . ~ . . . . . . . . . . . . . . . . . . . . . . . . . • . . . . . . . : . . . . . . . . ~ . : : . . . . . . and main motivations. Generally acknowledged are the two poles of 'cognition-inspired' and 'engineering-inspired' language production: Cognition-inspired work (CI-NLG, for short) seeks to build models that replicate performance data and explain phenomena of human language production with the help of psychological experiments; engineering-inspired work (EINLG) seeks to build programs that provide linguistic output to some particular computer application. These goals are extremely different, and it seems that the gap between the respective methodologies will persist for quite some time. In between the two, however, I would situate a third category, which may be called 'linguistics-inspired'. For this branch, here abbreviated as LI-NLG, the primary motivation is neither in modelling human performance nor in efficiently performing a technical application; rather, LI-NLG seeks production models that replicate 'competence data', i.e. that account for observed linguistic regularities, without con> miting to statements about the human production p~vcess. Arguing that progress hinges on a better understanding of the structure of the mental vocabulary, which includes a clear picture of the nature of the ISA-link, I will sketch a framework of distinct (but related) conceptual and lexical hierarchies, which offers possibilities to account for at least some of the phenomena to be discussed. 2 T h e h y p e r o n y m p r o b l e m Following tile psycholinguistics literature, the hyperonym problem is regarded as all aspect of lemrna retrieval. Roelofs [1996, p. 308] describes a 'lemma' as a representation of the meaning and the syntactic properties of a word, and the task of lemma retrieval as a crucial step in the