A Constraint-Based Approach to Linguistic Performance

This paper investigates linguistic performance, from the viewpoint that the information processing in cognitive systems should be designed in terms of constraints rather than procedures in order to deal with partiality of information. In this perspective, the same grammar, the same belief and the same processing architecture should underly both sentence comprehension and production. A basic model of sentence processing, for both comprehension and production, is derived along this llne of reasoning. This model is demonstrated to account for diverse linguistic phenomena apparently unrelated to each other, lending empirica! support to the constraint paradigm. 1 I n t r o d u c t i o n All the cognitive agents, with limited capacity for information processing, face partiality of information: Information relevant to their activities is only partially accessible, and also the distribution pattern of the accessible information is too diverse to predict. In sentence comprehension, for example, the phonological or morphological information may or may not be partially missing due to some noise, the semantic information may or may not be abundant because of familiality or ignorance on the topics, and so forth. Thus the information distribution is very diverse and rather orthogonal to the underlying information structure consisting of the modules of morphological, syntactic, pragmatic, and other constraints. This diversity of information distribution gives rise to a very complex, non-modular flow of information in cognitive processes, as information flows from places possessing information to places lacking information. In order to deal with this complexity, a cognitive system must be designed to include two different logical layers: '(1) Information represented in terms of constraints, *The work reported here started as the author's doctoral research at Tokyo University, and has developed filrther at Electrotechnical Laboratory and ICOT. The author's current affiliation is ICOT. ltis thanks go to Prof. YAMADA Itisao, who was the supervisor of the doctoral program, and too many other people to enu,nerate here. by abstracting away information flow. (2) A general processing mechanism to convey information across constraints, from places possessing information to places lacking it. >'on-modular flow of information may be captured on the basis of modular design of cognitive architecture, only by separating the representation of underlying information (as (1)) and flow of information (as (2)) fl'om each other. Procedural approaches break down under partiality of information, because procedures stipulate, and hence restrict, information flow. If one. be it human or nature, were to implement such diverse information flow by procedural programming, the entire system would quickly become too complex to keep track of, failing to maintain the modularity of the system. This is what has always happened, for example, in the development of natural language processing systems. The rest of the paper exemplifies the efficacy of the constraint paradig,n with regard to natural language. We wil! first discuss a general picture of language faculty immediately obtained fl'om the constraint-based view, and then derive a model of sentence processing neutral between comprehension and production. This model will be shown to fit several linguistic phenomena. Due to the generality of the perspective, the phenomena discussed below encompass apparently unrelated aspects of natural language. 2 L a n g u a g e a n d C o n s t r a i n t From tile constraint-based perspective immediately follows a hypothesis tI'lat the same constraints (i.e., lexical, syntactic, semantic, pragmatic, and whatever), corresponding to (1), and the same processing architecture, corresponding to (2), should underly both sentence comprehension and production. Other authors have expressed less radical stances. For instance, Kay [11] adopts two different grammars for parsing and generation. Our hypothesis is also stronger than Shieber's [16]; Although he proposes to share not only one grammar but also one processing architecture between the two tasks, this 'common' architecture is, unlike ours, parameterized so as to adapt itself to parsing and generation in accordance with different parameter settings.