Towards a Dynamic Version of TAG

This paper proposes a syntactic framework that is appropriate for modeling human language processing. The work moves from the largely held assumption that human language processing is incremental, and aims at exploring the consequences of incrementality when the processing strategy is encoded in the operations of a formal system. This makes a syntactic framework consistent with the Transparency Hypothesis (Berwick and Weinberg, 1984), in that a syntactic theory must reflect the behaviour of the human syntactic processor in its formal operations. Incrementality is a strategy that constrains the processor to analyse the input words from left to right, and to carry out a semantic interpretation of the partial structures (Marslen-Wilson, 1973). A parsimonious version of incrementality is what we call strong incrementality, which restricts incrementality to the case in which the parser maintains a fully connected tree at each state (cf. (Stabler, 1994)). There have been some proposals in the literature that claim that structure building occurs incrementally both in derivation and in parsing. Many aspects of the CCG formalism are grounded on incrementality (Steedman, 2000), and Phillips has provided a large amount of evidence that incrementality in the derivation process solves many problems in the definition of constituency (Phillips, 1998). The incremental nature of the formalism is a major issue in Milward’s proposal of a dynamic dependency grammar (Milward, 1994). In this case, the syntactic formalism is expressed in the terms of a dynamic system, that is a system evolving in time through a number of steps. A dynamic grammar views the syntactic process as a sequence of transitions between adjacent states and while moving from left to right in the input string 1. Thus, it naturally implements a strongly incremental strategy. The state is a partial tree that spans the input string from the beginning to the i-th word, and at each step the parser tries to attach the word into the current partial tree , also called the left context. So, the definition of a formal dynamic grammar involves the definition of the shape of the partial trees and the formal operations that extend these partial trees. Tree Adjoining Grammar (Joshi, Levy and Takahashi, 1975) is a well studied framework, that can realize a range of syntactic theories, provided a few constraints on trees and formal operations are satisfied. The goal of this paper is to devise a dynamic version of TAG, that retains the formal characteristics of the framework in terms of basic machinery, while constraining derivation and parsing to be strongly incremental. Of great interest for implementing incrementality are the wide domain of locality introduced by TAG rules, the adjunction operation, and the linguistic motivation for TAG elementary trees. Some of these considerations are shared by (Frank and Badecker, 2001), as points of strength for TAG in language production. Let us consider them in turn. 1) Wide domain of locality In an empirical study on the Penn Treebank, aimed to discover the amount of non-lexical information which is necessary to implement a fully connected incremental processing strategy, we simulated the sequence of processing steps of an incremental parser on the treebank (Lombardo and Sturt, 2002). Given full connectedness, each new word must be attached to the preceding left context via some syntactic structure, called “connection path”. In the simulation, connection paths often needed to be multiply levelled trees. See, e.g., the structure that connects the word “the” to the left context in: [ [ John] [ [ thinks] [ [ [ the]]]]] However, the form of these connection paths is quite predictable from linguistic observations. The use of multiply levelled elementary trees in TAG is an immediate encoding of this requirement. We also found that the introduction and co-indexation of traces needed to be carefully designed for the incremental setting. In TAG, given the wide domain of locality, filler and trace are both present in the same elementary tree, and then they may be separated through a number of adjunctions. This solution is particularly interesting for incrementality, provided that adjunctions are constrained to occur through the insertion of lexical material only to the right of the word whose elementary tree introduced the filler-trace pair.