Lexical Selection in the Process of Language Generation

Lexica] Selection in the Process of Language Generation 3ames Pustejovsky Department of Computer Science Brandeis University Waltham, MA 02254 617-736-2709 j amespQ brandeis.caner -relay Sergei Nirenburg Computer Science Department Carnegie-Mellon University Pittsburgh, PA. 15213 412-268-3823 sergeiQcad.cs.cmu.edu Abstract In this paper we argue that lexical selection plays a more important role in the generation process than has com- monly been assumed. To stress the importance of lexical- semantic input to generation, we explore the distinction and treatment of generating open and closed cla~s lexical items, and suggest an additional classification of the lat- ter into discourse-oriented and proposition-oriented items. Finally, we discuss how lexical selection is influenced by thematic ([oc~) information in the input. I. Introduction There is a consensus among computational linguists that a comprehensive analyzer for natural language must have the capability for robust lexical disambiguation, i.e., its central task is to select appropriate meanings of lexical items in the input and come up with a non contra- dictory, unambiguous representation of both the proposi- tional and the non-propositional meaning of the input text. The task of a natural language generator is, in some sense, the opposite task of rendering an unambiguous meaning in a natural language. The main task here is to to per- form principled selection of a) lexical items and b) the syntactic structure for input constituents, based on lexical semantic, pragmatic and discourse clues available in the input. In this paper we will discuss the problem of lexlcal selection. The problem of selecting lexical items in the pro- cess of natural language generation has not received as much attention as the problems associated with express- ing explicit grammatical knowledge and control. In most of the generation systems, lexical selection could not be a primary concern due to the overwhelming complexity of the generation problem itself. Thus, MUMBLE con- centrates on gr:~mmar-intensive control decisions (McDon- ald and Pustejovsky, 1985a) and some stylistic considera- tions (McDonald and Pustejovsk'y, 1985b); TEXT (McKe- own, 1985) stresses the strategical level of control decisions about the overall textual shape of the generation output. ~KAMP (Appelt, 1985) emphasizes the role that dynamic planning plays in controlling the process of generation, and specifically, of referring expressions; NIGEL (Mann and Matthiessen, 1983) derives its control structures from the choice systems of systemic grammar, concentrating on grammatical knowledge without fully realizing the 'deli- cate' choices between elements of what systemicists call leto's (e.g., HaLiiday, 1961). Thus, the survey in Cummlng (1986) deals predominantly with the grammatical aspects of the lexicon. We discuss here the problem of lexical selection and explore the types of control knowledge that are neces- sary for it. In particular, we propose different control strategies and epistemological foundations for the selec- tion of members of a) open-class and b) closed-class lex- ical items. One of the most important aspects of control knowledge our generator employs for lexical selection is the non-propositional information (including knowledge about focus and discourse cohesion markers). Our generation system incorporates the discourse and textual knowledge provided by TEXT …

Content truncated to 500 words