On the Acquisition of Lexical Entries: The Perceptual Origin of Thematic Relations

On the Acquisition of Lexical Entries: The Perceptual Origin of Thematic Relations James Pustejovsky Department of Computer Science Brandeis University Waltham, MA 02254 617-736-2709 jamesp~br andeis.csnet-relay Abstract This paper describes a computational model of concept acquisition for natural language. We develop a theory of lexical semantics, the Eztended Aspect Calculus, which together with a ~maxkedness theory" for thematic rela- tions, constrains what a possible word meaning can be. This is based on the supposition that predicates from the perceptual domain axe the primitives for more abstract relations. We then describe an implementation of this model, TULLY, which mirrors the stages of lexical acqui- sition for children. I. Introduction In this paper we describe a computational model of con- cept acquisition for natural language making use of po- sitive-only data, modelled on a theory of lexical seman- tics. This theory, the Eztende~t Aspect Calculus acts to- gether with a maxkedness theory for thematic roles to constrain what a possible word type is, just as a gram- mar defines what a well-formed tree structure is in syntax. We argue that linguistic specific knowledge and learning principles are needed for concept acquisition from positive evidence alone: Furthermore, this model posits a close in- teraction between the predicates of visual perception and the early semantic interpretation of thematic roles as used in linguistic expressions. In fact, we claim that these re- lations act as constraints to the development of predicate hierachies in language acquisition. Finally, we describe TULLY, an implementation of this model in ZETALXSP and discuss its design in the context of machine learning research. There has been little work on the acquisition of thematic relation and case roles, due to the absence of any consensus on their formal properties. In this research we begin to address what a theory of thematic relations might look like, using learnabUity theory as a metric for evaluating the model. We claim that there is an impor- tant relationship between visual or imagistic perception and the development of thematic relations in linguistic us- age for a child. This has been argued recently by Jackend- off (1983, 1985) and was an assumption in the pioneering work of Miller and Johnson-Laird (1976). Here we argue that the conceptual abstraction of thematic information does not develop arbitrarily but along a given, predictable path; namely, a developmental path that starts with tan- gible perceptual predicates (e.g. spatial, causative) to later form the more abstract mental and cognitive predi- cates. In this view thematic relations are actually sets of thematic properties, related by a partial ordering. This effectively establishes a maxkedness theory for thematic roles that a learning system must adhere to in the acqui- sition of lexical entries for a larlguage. We will discuss two computational methods for concept development in natural language: (1) F~ature Relaxation of particular features of the ar- guments to a verb. This is performed by a con- straint propagation method. (2) Thematic Decoupling of semantically incorporated information from the verb. When these two learning techniques are …

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