Memory Capacity and Sentence Processing

MEMORY CAPACITY AND SENTENCE PROCESSING Edward Gibson Department of Philosophy, Carnegie Mellon University Pittsburgh, PA 15213-3890 gibson@cs.cmu.edu ABSTRACT The limited capacity of working memory is intrinsic to human sentence processing, and therefore must be addressed by any theory of human sentence processing. This paper gives a theory of garden-path effects and pro- cessing overload that is based on simple as- sumptions about human short term memory capacity. 1 INTRODUCTION The limited capacity of working memory is intrinsic to human sentence processing, and therefore must be addressed by any theory of human sentence process- ing. I assume that the amount of short term memory that is necessary at any stage in the parsing process is determined by the syntactic, semantic and pragmatic properties of the structure(s) that have been built up to that point in the parse. A sentence becomes unaccept- able for processing reasons if the combination of these properties produces too great a load for the working memory capacity (cf. Frazier 1985): (1) n E Aixi > K i=1 where: K is the maximum allowable processing load (in processing load units or PLUs), xl is the number of PLUs associated with prop- erty i, n is the number of properties, Ai is the number of times property i appears in the structure in question. Furthermore, the assumptions described above pro- vide a simple mechanism for the explanation of com- mon psycholinguistic phenomena such as garden-path effects and preferred readings for ambiguous sentences. Following Fodor (1983), I assume that the language processor is an automatic device that uses a greedy al- gorithm: only the best of the set of all compatible rep- resentations for an input string are locally maintained from word to word. One way to make this idea explicit is to assume that restrictions on memory allow at most one representation for an input string at any time (see, for example, Frazier and Fodor 1978; Frazier 1979; Marcus 1980; Berwick and Weinberg 1984; Pritchett 1988). This hypothesis, commonly called the serial 39 hypothesis, is easily compatible with the above view of processing load calculation: given a choice between two different representations for the same input string, simply choose the representation that is associated with the lower processing load. The serial hypothesis is just one way of placing local memory restrictions on the parsing model, however. In this paper I will present an alternative formulation of local memory restrictions within a parallel framework. There is a longstanding debate in the psycholinguis- tic literature as to whether or not more than one rep- resentation for an input can be maintained in parallel (see, for example, Kurtzman (1985) or Gorrell (1987) for a history of the debate). It turns out that the par- aUel view appears to handle some kinds of data more directly than the serial view, keeping in mind that the data are often controversial. For example, it is difficult to explain in a serial model why relative processing load increases as ambiguous input is encountered (see, for example, …

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