Automatic Acquisition of the Lexical Semantics of Verbs From Sentence Frames

AUTOMATIC ACQUISITION OF THE LEXICAL SEMANTICS OF VERBS FROM SENTENCE FRAMES* Mort Webster and Mitch Marcus Department of Computer and Information Science University of Pennsylvania 200 S. 33rd Street Philadelphia, PA 19104 ABSTRACT This paper presents a computational model of verb acquisition which uses what we will call the princi- ple of structured overeommitment to eliminate the need for negative evidence. The learner escapes from the need to be told that certain possibili- ties cannot occur (i.e., are "ungrammatical") by one simple expedient: It assumes that all proper- ties it has observed are either obligatory or for- bidden until it sees otherwise, at which point it decides that what it thought was either obliga- tory or forbidden is merely optional. This model is built upon a classification of verbs based upon a simple three-valued set of features which repre- sents key aspects of a verb's syntactic structure, its predicate/argument structure, and the map- ping between them. 1 INTRODUCTION The problem of how language is learned is per- haps the most difficult puzzle in language under- standing. It is necessary to understand learning in order to understand how people use and organize language. To build truly robust natural language systems, we must ultimately understand how to enable our systems to learn new forms themselves. Consider the problem of learning new lexical items in context. To take a specific example, how is it that a child can learn the difference between the verbs look and see (inspired by Landau and Gleitman(1985) )? They clearly have similar core meanings, namely ~perceive by sight". One ini- tially attractive and widely-held hypothesis is that *This work was partially supported by the DARPA grant N00014-85-K0018, and Alto grant DAA29-84-9- 0027. The authors also wish to thank Beth Levin and the anonymotm reviewers of this paper for many helpful com- ments. We ~ b~efit~l greatly from disctumion of issues of verb acquisition in children with Lila Gleitman. word meaning is learned directly by observation of the surrounding non-linguistic context. While this hypothesis ultimately only begs the question, it also runs into immediate substantive difficulties here, since there is usually looking going on at the same time as seeing and vice versa. But how can one learn that these verbs differ in that look is an active verb and see is stative? This difference, although difficult to observe in the environment, is clearly marked in the different syntactic frames the two verbs are found in. For example, see, be- ing a stative perception verb, can take a sentence complement: (1) John saw that Mary was reading. while look cannot: (2) * John looked that Mary was reading. Also look can be used in an imperative, (3) Look at the ball! while it sounds a bit strange to command someone to see, (4) ? See the ball! (Examples like "look Jane, see Spot run!" notwithstanding.) This difference reflects the fact that one can command someone to direct their eyes (look) but not to mentally perceive what someone else perceives …

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