Reaping the Benefits of Interactive Syntax and Semantics

Semantic feedback is an important source of information that a parser could use to deal with local ambiguities in syntax. However, it is difficult to devise a systematic communication mechanism for interactive syntax and semantics. In this article, I propose a variant of left-corner parsing to define the points at which syntax and semantics should interact, an account of grammatical relations and thematic roles to define the content of the communication, and a conflict resolution strategy based on independent preferences from syntax and semantics. The resulting interactive model has been implemented in a program called COMPERE and shown to account for a wide variety of psycholinguistic data on structural and lexical ambiguities. I N T R O D U C T I O N The focus of investigation in language processing research has moved away from the issue of semantic feedback to syntactic processing primarily due to the difficulty of gett ing the communicat ion between syntax and semantics to work in a clean and systematic way. However, it is unquestionable that semantics does in fact provide useful information which when fed back to syntax could help eliminate many an alternative syntactic structure. In this article, I address three issues in the communication mechanism between syntax and semantics and provide a complete and promising solution to the problem of interactive syntactic and semantic processing. Since natural languages are replete with ambiguities at all levels, it appears intuitively that a processor with incremental interaction between the levels of syntax and semantics which makes the best and immediate use of both syntactic and semantic information to eliminate many alternatives would win over either a syntax-first or a semantics-first mechanism. In order to devise such an interactive mechanism, one has to address three impor tant issues in the communicat ion: (a) When to communicate: at what points should syntax and semantics interact, (b) What to communicate: what and how *The author would like to thank his advisor Dr. Kurt Eiselt and his colleague Justin Peterson for their support and valuable comments on this work. much information should they exchange, and (c) How to agree: how to resolve any conflicting preferences between syntax and semantics. In this article, I propose (a) a part icular variant of left-corner parsing tha t I call Head-Signaled Left Corner Parsing (HSLC) to define the points where syntax and semantics should interact, (b) an account of grammat ica l relations based on thematic roles as a medium for communicat ion, and (c) a simple strategy based on syntactic and semantic preferences for resolving conflicts in the communication. These solutions were mot ivated from an analysis of a large body of psycholinguistic da ta and account for a greater variety of experimental observations on how humans deal with structural and lexical ambiguities than previous models (Eiselt et al, 1993). While it also appears that the proposed interaction with semantics could make improvements to the efficiency of the parser in dealing with real texts, such a conclusion can only be drawn after an empirical evaluation. W H E N TO C O M M U N I C A T E Syntax and semantics should interact only at those times when one can provide some information to the other to help reduce the number of choices being considered. Only when the parser has analyzed a unit that carries some part of the meaning of the sentence (such as a content word) can semantics provide useful feedback perhaps using selectional preferences for fillers of thematic roles. We need to design a parsing strategy that communicates with semantics precisely at such points. While pure bo t tom-up parsing turns out to be too circumspect for this purpose, pure top-down parsing is too eager since it makes its commitments too early for semantics to have a say. A combination strategy called Left Corner (LC) parsing is a good middle ground making expectations for required constituents from the leftmost unit of a phrase but waiting to see the left corner before commit t ing to a bigger syntactic unit (E.g., Abney and Johnson, 1991). In LC parsing, the leftmost child (the left corner) of a phrase is analyzed bot tom-up, the phrase is projected upward from the leftmost child, and other children of the phrase are projected top-down from the phrase.