Introduction

Since the 1940s many theorists have suggested that information might be stored in the cognitive system in a different way over short time periods than for longer time periods. A number of possible mechanisms have been proposed – special or general-purpose buffers, the continuing but temporary activation of the structures that have just processed an input, and the formation of temporary associations or temporary changes in association strength. Theoretical arguments for such mechanisms have been produced from a range of scientific fields from physiological psychology (e.g., Hebb, 1949), information-processing psychology (Broadbent, 1958), and symbol-processing artificial intelligence (Newell & Simon, 1972) through to connectionist neuroscience (e.g., Crick, 1984; Hinton & Plaut, 1987). Empirical support has come from a narrower set of approaches. Most of the relevant findings have come from experimental psychology (e.g., Waugh & Norman, 1965; Atkinson & Shiffrin, 1968; Glanzer, 1972; Baddeley & Hitch, 1974), but these have been subject to many criticisms (see, e.g., Craik & Lockhart, 1972; Crowder, 1982).

From a neuropsychological perspective, if a short-term memory trace were being carried by certain of the aforementioned mechanisms, a selective deficit to that mechanism would be realized in a selective impairment in behaviour. This is clearly true for the most favoured possibility – damage to a specific short-term memory buffer - given that the buffer was not involved in all cognitive operations. It would also apply for a specific impairment of one of two association-weight-changing mechanisms. For instance, Hinton and Plaut (1987) have simulated a connectionist network using two types of weights – “slow” weights (which change slowly and carry the long-term learning) and “fast” weights (which change rapidly but decay fairly quickly to zero).

Introduction

Immediate memory capacity for lists is usually estimated at around 5–6 words, whereas for sentences, Miller and Self ridge (1950) found that 20-word sentences can be produced with nearly 100% recall, a finding replicated by Craik and Masani (1969). In a more recent study, Butterworth, Campbell and Howard (1986) presented subjects with 40 sentences 15–21 words long for immediate recall. Undergraduate subjects recalled about 25 of them perfectly. Most of the errors were omissions and word substitutions, not word order errors (less than 3% of all errors). There were significant serial position effects, with the first two words and the last word recalled better than the others. “Running memory span” tasks (where subjects are allowed to choose the segment to report) show accurate recall for segments up to and exceeding 20 words; in general, about 88% of the words are recalled (discounting order) irrespective of the segment length (Wingfield & Butterworth, 1984, Experiment 1).

Why is immediate serial recall of sentences better than lists? A number of probable solutions will doubtless spring to mind. Sentences encourage chunking; the meaning of a sentence can be stored in long-term memory (LTM) and thence retrieved in recall; sentences are meaningful; sentences have structure; sentence materials utilize quite different memorial systems; and so on. Various authors have made suggestions along one or other of these lines (e.g., Miller & Selfridge, 1950; Craik, 1971; Shallice, 1979; Butterworth et al., 1986; McCarthy & Warrington, 1987a).

It has frequently been argued that a verbatim record of sentence input is held in some limited-capacity store while the construction of a higher-level representation is carried out.

The final part of this book includes five chapters dealing with the very contentious issue of the putative role of phonological memory in sentence comprehension. A first and general difficulty for the neuropsychological analysis of this question, which is independent of the specific problem at issue, is that claims for a positive role are based on an association and not on a dissociation of deficits (see Shallice, 1988, for discussion). This makes it difficult to determine the precise nature of the putative causal role of immediate memory deficits in the genesis of the comprehension impairment in patients with a specific span defect. Consider, for instance, the observations that short-term memory patients, even though their comprehension is comparatively preserved at the clinical level, typically show some impairment in tasks such as the Token Test and that a classical dissociation (i.e., patients with a defective auditory–verbal span and an entirely preserved sentence comprehension) had not been reported so far (see reviews in Caplan & Waters, chapter 14, and Vallar, Basso, & Bottini, chapter 17). This pattern prima facie suggests a role of phonological memory in speech comprehension. However, an association of neuropsychological symptoms produced by brain damage might arise from the anatomical contiguity of the neural structures involved in immediate retention of verbal material and in speech comprehension. After all, the neurological correlate of auditory–verbal short-term memory deficits is a lesion of a left inferoposterior parietal region, which is a part of the language areas. The hypothesis that short-term phonological memory has a specific role in speech comprehension would need to be corroborated by sets of experiments from which it is possible to rule out a simple interpretation in terms of anatomical proximity.

The behaviour of “short-term memory” patients on short-term memory tasks is of interest because their performance contrasts so strongly with those of normal adults. The approach of contrasting normal adult behaviour with that of other groups can be extended to other populations whose performance differs markedly from the normal adult pattern. This part includes discussions of short-term memory performance in children (Hitch, chapter 9); in the elderly (Craik, Morris, & Gick, chapter 10); in the deaf – or rather the procedure, lipreading, they use (Campbell, chapter 11) and in two types of neurological patients (Howard & Franklin, chapter 12; Kinsbourne & Hicks, chapter 13).

Hitch (chapter 9) reports a number of studies concerning the development of working memory in children of various ages. He suggests that developmental studies may usefully complement neuropsychological research in advancing our understanding of normal cognitive processes, such as short-term memory, since both can be based on a “fractionation” methodology. The neuropsychological fractionation method currently used in patients with acquired brain lesions capitalizes on the presumed more or less complete damage of specific functional component(s), for example, the phonological short-term store, to investigate the functional architecture of aspects of the cognitive system. In the case of normal children the fractionation approach advocated by Hitch assumes that the normal development of cognitive abilities may be characterized by the addition of subsystems, which previously were relatively nonoperative. If this is the case, the study of children of different ages should produce results complementary to those obtained with brain-damaged patients.

In recent years the single-case approach has grown greatly in popularity in neuropsychology. Some workers have even argued that no pretheoretical generalizations across patients can be justified (e.g., Caramazza, 1986), for, they argue, one cannot know that any two patients have functionally equivalent lesions. Yet it is equally argued by people who hold these general positions that our theoretical understanding of the neurological organization of cognitive function is rudimentary, so theoretically driven grouping of patients is to be avoided too (see Ellis, 1987).

This is an unsatisfactory state of affairs. Any science needs a data base that has some depth. A tentative understanding of the range of empirical phenomena that occur in a domain should be available. How robust the phenomena are needs to be roughly known.

A practical way out of the dilemma is to take putative syndromes – patients with a common cluster of difficulties that can plausibly be attributed to a common functional cause – and to present multiple mixed practical-theoretical investigations of a domain by different investigators. Even if the syndrome proves not to be a single functional entity, the studies should provide a solid basis for future research. The overlapping empirical observations on different patients will provide an adequate basis for future theoretical analyses. Competing theoretical perspectives will sharpen the perspective for future empirical investigations. Two pioneer books on the acquired dyslexias – Deep Dyslexia (Coltheart, Patterson, & Marshall, 1980) and Surface Dyslexia (Patterson, Marshall, & Coltheart, 1985) – illustrate the value of the approach. Neither syndrome has remained solidly accepted as a single functional entity, but the value of each book in defining its field is undoubted.

The first part of the book comprises four chapters that discuss two main issues: (a) the possible functional architecture of the system(s) involved in the short-term retention of verbal material (Shallice & Vallar, chapter 1; Baddeley, chapter 2; Friedrich, chapter 3); and (b) some neural correlates (chapter 1; Starr, Barrett Pratt, Michalewski, & Patterson, chapter 4).

Two main approaches are suggested on the functional structure of verbal short-term memory. Shallice and Vallar and Baddeley take the view of verbal short-term memory as a multicomponent system, which includes a number of distinct processing and storage subcomponents. Shallice and Vallar review the neuropsychological and, more briefly, the normal evidence for this approach. They consider a number of specific phenomena, such as auditory–verbal memory span and the recency effect in free and serial recall, noting a convergence in the findings obtained from normal subjects and patients. They note that a number of aspects of the original observations (Warrington & Shallice, 1969) of a selective impairment on span tasks have been replicated in a considerable number of cases, which they review. The evidence from normal subjects supports the position that short-term memory effects from paradigms such as span reflect the operation of a buffer store where information is coded phonologically. The results obtained from the patients are very similar to what would be expected if such a component were severely damaged. On the basis of this type of argument, Shallice and Vallar suggest that the selective deficit of auditory-verbal span may be conceived as a functional syndrome, which may be traced back to the selective impairment of a specific component of verbal short-term memory.