Introduction

According to the standard approach to short-term memory (STM), set forth by Baddeley in chapter 2, verbal STM depends on a phonological store of limited capacity. It is not clear, however, how this capacity is to be defined. Thus, if we take span (number of items recalled in correct order) to be our index of STM capacity, it is necessary to deal with the fact that this number does not represent some fixed quantity. Rather, it varies substantially across material type: span for digits (7.98) is greater than span for familiar words (5.86), which is in turn greater than span for nonword materials (2.49) (Brener, 1940). Various manipulations also affect span for words and nonwords differently: Performance on word lists is more resistant to suffix effects (Salter, Springer, & Bolton, 1976), and less affected by such factors as presentation modality and phonemic similarity (Richardson, 1979). Lexicality is evidently a sustaining factor in STM.

There has been little attempt to deal with these lexical influences, however, either from a theoretical perspective or as a matter for empirical study. Far from their being a focus of attention in STM research, experimenters have tended to minimize the effects of lexical variables by relying on digit materials or, when using words as stimuli, by sampling from a restricted item pool. It has sometimes been acknowledged that the information in the phonological store is “postexical,” the implication being that the information represented in this store consists of phonological units filtered through the lexical system (e.g., Richardson, 1979).

Introduction

As a task that requires the integration of temporally distributed input, sentence comprehension clearly involves short-term memory (STM) capacity. Little has been done to define the storage requirements for sentence-processing tasks, although the STM capacities underlying the retention of word list materials have been extensively studied. It is likely, however, since both tasks deal with strings of lexical items, that they have some mnestic requirements in common. This is implicit in the assumption that the phonological store, thought to be the primary vehicle for information storage in spantype tasks (see Baddeley, this volume, chapter 2), contributes to sentence processing as well (e.g., Clark & Clark, 1977). Studies that have demonstrated trade-offs between concurrent comprehension and list memory tasks (Savin & Perchonock, 1965; Wanner & Maratsos, 1978) provide prima facie support for this notion, as do indications that sentential input is held in phonological form prior to the identification of clausal units (Jarvella, 1971; but see Von Eckhardt & Potter, 1985).

Additional evidence for storage capacities common to sentence processing and list retention has come from neuropsychological investigations. Brain-damaged patients with selective STM deficits, as defined by Shallice and Vallar in chapter 1 of this volume, have invariably demonstrated some degree of impairment on tests of sentence comprehension. The difficulties of these patients appear to lie, moreover in structural aspects of sentence processing, which is where access to an information store that represents a linear array of lexical items is likely to be most useful.