What do estimates of working memory capacity tell us

We sketch our recent research leading toward a revision of the concept of working memory capacity. This includes a rationale for believing that working memory capacity is a fundamental ability, and for estimating working memory based on the number of independent chunks of information recalled. We provide evidence that the often-noted relation between working memory and both individual and developmental differences in intellectual aptitudes does not depend on using a dual task to measure working memory, even though that has become a standard practice in the field. Basic intellectual aptitudes can be studied more cleanly using working-memory tasks that impede mnemonic strategies, so that a core capacity must come into play. Last, we offer evidence that this core capacity may be attention serving as a storage device. Cowan et al., Page 2 What Do Estimates of Working Memory Capacity Tell Us? Working memory can be viewed as the collection of mental processes that preserve a limited amount of information in an especially accessible form, long enough for it to be of use in ongoing cognitive tasks. By almost all accounts, working memory is indeed a collection of processes (e.g., for diverse theoretical descriptions of working memory, see the chapters of Miyake & Shah 1999). In our laboratory, we have been investigating "flavors" of working memory that we believe to be closely related to human conscious awareness, namely the primary memory described as the trailing edge of consciousness by James (1890). This chapter serves as a sort of tour of the laboratory's current endeavors, made more or less interactive by the inclusion of a considerable amount of data from recently-published studies, presented here in tabular form so it can be explored and questioned We examine research that makes four related points in progression: (1) that working memory is an important concept that amounts to more than some notion of general processing efficiency; (2) that the often-discussed relation between working memory and intellectual maturation and aptitude does not depend on using a dual task to examine working memory, but instead depends only on using a task that impedes mnemonic strategies such as covert verbal rehearsal; (3) that working memory performance depends on the notion of capacity, expressed in terms of chunks; and (4) that the core working-memory capacity limit is related to the scope of attention. Premise 1: Working Memory Is Not Just General Processing Efficiency There is a tradition in the research literature suggesting that developmental differences in working memory and cognitive aptitudes throughout the life span result from a developmental Cowan et al., Page 3 increase, and then decrease, in the speed of processing as neural efficiency improves with maturation in childhood and later declines with aging (e.g., Kail & Salthouse 1994). The theoretical hypothesis is that individuals with a better working memory are able to reactivate information more quickly before it can decay from working memory, and therefore that the speed of processing may be the key factor distinguishing between people with better or poorer working memory. The basis of this claim was a wealth of evidence that processing speed and the accuracy of performance are highly correlated across individuals. Cowan, Wood, Wood, Keller, Nugent, and Keller (1998) documented a strong correlation between two measures of processing efficiency and performance on a digit span task. The digit span task is one of the oldest measures of the short-term retention processes that form a key component of working memory, and is included in standard tests of intelligence. In this task, a series of digits between 1 and 9, with each digit used no more than once per list, is presented on each trial. The list length starts very short and increases every few trials, with the exact number of trials per list length differing from one procedure to another. If the participant is incorrect on every trial at a given length, the test stops, and therefore the duration of the test is adjusted to the abilities of the participant. Some measure of the length of list that can be repeated is taken as an individual's digit span. One measure of efficiency examined by Cowan et al. (1998) was the rate at which digits were pronounced in lists of a particular length within the span task. The durations of every word in the spoken responses, and the durations of silent pauses between words in these responses, were painstakingly measured using a computer. An oscillographic display of the response was measured with auditory guidance. The notion was that individuals who process information faster would repeat more of this information before it decays. Another measure of efficiency Cowan et al., Page 4 was one devised by Baddeley, Thomson, and Buchanan (1975). In this type of measure, a small number of items is to be repeated aloud as quickly as possible, either with a set short enough to be memorized and repeated over and over or with a set that is read rather than remembered. This type of measure was thought to estimate how quickly the participant is capable of covertly rehearsing a longer list of words drawn from the same set, in a span task. Cowan et al. (1998) found that both the speed of retrieval and the speed of rehearsal were significantly related to span. A statistical method (structural equation modeling) was used to characterize the relations between measures. It was found, contrary to the expectations of a single type of efficiency across tasks, that retrieval and rehearsal speed were unrelated to each other. However, they both were related to digit span, to an extent described in Table 1. These were both strong, separate relations and together they accounted for 87% of the age-related variance in digit span (and 60% of its total variance). Table 1 At the time, this finding provided one of the strongest arguments in favor of some sort of efficiency theory of working memory, although it required that one stipulate the presence of more than one efficiency parameter per individual (because retrieval and rehearsal rates did not correlate with one another). However, it always bears repeating that correlations do not amount to evidence of causation, even with the use of structural equation modeling. Recently, Cowan et al. (in press) showed that at least the retrieval rate factor is not causally related to digit span. Ordinarily, children who are 8 to 9 years old have a smaller digit span than adults and retrieve items (pronounce them in the response) much more slowly. In two different ways within separate experiments, Cowan et al. were able to get the children in an experimental group to recall items more quickly. In Experiment 1, this was accomplished by speeding up the Cowan et al., Page 5 stimuli. In Experiment 2, one group of children was simply instructed to speak more quickly in their responses at a certain point (Phase 2 in Table 2), and the children were able to do so. In fact, these children sped up their responses to be faster than adults usually speak when recalling lists of an equivalent length. Yet, as illustrated in Table 2, speeding up did not affect span at all. The quickly-speaking children still were just as far behind the adults in digit span. This study emphasizes that the correlation between efficiency or speed and span does not reflect a forward direction of causation. Perhaps, instead, the most comfortable speed of responding and memory capacity are separate factors that increase during childhood. By analogy, the head and the arms both grow larger during childhood, but one instance of growth does not cause the other; stretching out the arms as far as they can go does not affect the size of the head. Table 2 Premise 2: The Relation Between Working Memory and Intellectual Maturation and Aptitude Does Not Depend on Using a Dual Task to Examine Working Memory The traditional view of working memory (Baddeley & Hitch 1974; Baddeley 1986; Baddeley & Logie 1999) and its development (Gathercole & Hitch 1993; Hitch, Towse, & Hutton 2001) is one that incorporates storage and processing mechanisms that are assumed to be distinct , both neurologically and behaviorally. From that viewpoint, it stands to reason that, to test the capabilities of working memory, the test should tax or engage both storage and processing. That has been the logical underlying the type of test that has become prevalent in the literature to examine individual and developmental differences in working memory. In the reading and listening span tests (Daneman & Carpenter 1980), a sentence is presented and some comprehension is required. The participant is also to remember the last word in the sentence or, in a variation of this procedure, is to remember a separate word. After two or more such Cowan et al., Page 6 sentences, all of the final words are to be repeated. In the counting span test (Case, Kurland, & Goldberg 1982), the participant is to count arrays of dots while remembering the sums, and then to repeat all of the sums at the end of the trial. In the operation span test (Turner & Engle 1989), the participant is to complete multiple arithmetic problems, remembering either the answers to the problems or a separate word presented after each problem. The number of sentences, arrays, or problems that can be processed along with successful serial repetition of the items following the processes (indicating that these items were successfully stored) is taken as the working memory span. In sum, the structure of a trial in these studies is: Process 1, Store Item 1, Process 2, Store Item 2.Process n, Store Item n; Recall of Items The finding has been that this storage-and-processing type of test generally yields correlations with aptitudes that are considerably stronger and more consistent than the correlations between simple span tasks and aptitudes, and that are general across the processing domains (Conway, Kane