Nelson Cowan

The magical number 4 in short-term memory: A reconsideration of mental storage capacity

Miller (1956) summarized evidence that people can remember about seven chunks in short-term memory (STM) tasks. However, that number was meant more as a rough estimate and a rhetorical device than as a real capacity limit. Others have since suggested that there is a more precise capacity limit, but that it is only three to five chunks. The present target article brings together a wide variety of data on capacity limits suggesting that the smaller capacity limit is real. Capacity limits will be useful in analyses of information processing only if the boundary conditions for observing them can be carefully described. Four basic conditions in which chunks can be identified and capacity limits can accordingly be observed are: (1) when information overload limits chunks to individual stimulus items, (2) when other steps are taken specifically to block the recording of stimulus items into larger chunks, (3) in performance discontinuities caused by the capacity limit, and (4) in various indirect effects of the capacity limit. Under these conditions, rehearsal and long-term memory cannot be used to combine stimulus items into chunks of an unknown size; nor can storage mechanisms that are not capacity-limited, such as sensory memory, allow the capacity-limited storage mechanism to be refilled during recall. A single, central capacity limit averaging about four chunks is implicated along with other, noncapacity-limited sources. The pure STM capacity limit expressed in chunks is distinguished from compound STM limits obtained when the number of separately held chunks is unclear. Reasons why pure capacity estimates fall within a narrow range are discussed and a capacity limit for the focus of attention is proposed.

Evolving conceptions of memory storage, selective attention, and their mutual constraints within the human information-processing system.

The purpose of this review is to formulate a revised model of information processing that takes into account recent research on memory storage, selective attention, effortful versus automatic processing, and the mutual constraints that these areas place on one another. One distinctive aspect of the proposed model is the inclusion of two phases of sensory storage in each modality. The first phase extends sensation for several hundred milliseconds, whereas the second phase is a vivid recollection of sensation. The mechanism of at least the longer phase is the activation of features in long-term memory, comparable to the mechanism of non-sensory, short-term storage. Another distinctive aspect of the model is that habituation/dishabituation and central executive processes together are assumed to determine the focus of attention, without the need for either an early or a late attentional filter. Research issues that contribute to a comparison of models are discussed.

A latent variable analysis of working memory capacity, short-term memory capacity, processing speed, and general fluid intelligence

Significant relationships exist between general fluid intelligence and each of the following constructs: short-term memory capacity, working memory capacity (WMC), and processing speed. However, the interrelationship among all four constructs has not been investigated. Multiple measures of each of these constructs were obtained from 120 healthy young adults. Structural equation modeling was then performed to determine which construct served as the best predictor of general fluid intelligence. The results suggest that WMC, but not short-term memory capacity or processing speed, is a good predictor of general fluid intelligence in young adults. Possible mechanisms underlying the link between WMC and general fluid intelligence are discussed. D 2002 Elsevier Science Inc. All rights reserved.

On the Capacity of Attention: Its Estimation and Its Role in Working Memory and Cognitive Aptitudes

Working memory (WM) is the set of mental processes holding limited information in a temporarily accessible state in service of cognition. We provide a theoretical framework to understand the relation between WM and aptitude measures. The WM measures that have yielded high correlations with aptitudes include separate storage-and-processing task components, on the assumption that WM involves both storage and processing. We argue that the critical aspect of successful WM measures is that rehearsal and grouping processes are prevented, allowing a clearer estimate of how many separate chunks of information the focus of attention circumscribes at once. Storage-and-processing tasks correlate with aptitudes, according to this view, largely because the processing task prevents rehearsal and grouping of items to be recalled. In a developmental study, we document that several scope-of-attention measures that do not include a separate processing component, but nevertheless prevent efficient rehearsal or grouping, also correlate well with aptitudes and with storage-and-processing measures. So does digit span in children too young to rehearse.

The cocktail party phenomenon revisited: the importance of working memory capacity.

Wood and Cowan (1995) replicated and extended Moray’s (1959) investigation of thecocktail party phenomenon, which refers to a situation in which one can attend to only part of a noisy environment, yet highly pertinent stimuli such as one’s own name can suddenly capture attention. Both of these previous investigations have shown that approximately 33% of subjects report hearing their own name in an unattended, irrelevant message. Here we show that subjects who detect their name in the irrelevant message have relatively low working-memory capacities, suggesting that they have difficulty blocking out, or inhibiting, distracting information.

The Magical Mystery Four How Is Working Memory Capacity Limited, and Why?

Working memory storage capacity is important because cognitive tasks can be completed only with sufficient ability to hold information as it is processed. The ability to repeat information depends on task demands but can be distinguished from a more constant, underlying mechanism: a central memory store limited to 3 to 5 meaningful items for young adults. I discuss why this central limit is important, how it can be observed, how it differs among individuals, and why it may exist.

Memory prerequisites of mismatch negativity in the auditory event-related potential (ERP)

The mismatch negativity (MMN) is a component of the auditory event-related brain potential that occurs in response to infrequent changes in the physical properties of homogeneous series of sounds, even when subjects are instructed to ignore the auditory channel of stimulation. It has been proposed (e.g., Näätänen, 1990) that the MMN is generated by an automatic process in which a difference between the deviant sound and the previous, standard sound is detected by the brain. However, it is unclear how the form of memory involved is related to the rest of the memory system. The present study indicates that, for an MMN to be elicited in response to a change in tone frequency, the representation of the standard tone must be both (a) well-established as a standard in memory, and (b) in a currently active state. The relation between physiological and psychological aspects of memory representation is discussed.

An assessment of fixed-capacity models of visual working memory

Visual working memory is often modeled as having a fixed number of slots. We test this model by assessing the receiver operating characteristics (ROC) of participants in a visual-working-memory change-detection task. ROC plots yielded straight lines with a slope of 1.0, a tell-tale characteristic of all-or-none mnemonic representations. Formal model assessment yielded evidence highly consistent with a discrete fixed-capacity model of working memory for this task.

The role of verbal output time in the effects of word length on immediate memory

In three experiments, we examined the role of delays within overt verbal responding in causing effects of word length on immediate recall. Although a phonological memory decay mechanism has been implicated by past research on word length effects, the exact basis of the effect remains unclear. The added difficulty of recalling longer words could arise both while subjects attempt to rehearse words silently and while they attempt to repeat words aloud. To examine the latter mechanism, the lengths of words in the first and second halves of lists to be recalled were varied independently, and both forward and backward recall orders were used. Recall of each word was found to be influenced by the total pronunciation time for all items to be recalled prior to that word, although there was an additional advantage for the last item output. The results clarify and generally support the theory of the articulatory loop, and the method permits an improved analysis of immediate memory into decay-based and other factors.

Separating Cognitive Capacity from Knowledge: A New Hypothesis

We propose that working memory and reasoning share related capacity limits. These limits are quantified in terms of the number of items that can be kept active in working memory, and the number of interrelationships between elements that can be kept active in reasoning. The latter defines the complexity of reasoning problems and the processing loads they impose. Principled procedures for measuring, controlling or limiting recoding and other strategies for reducing memory and processing loads have opened up new research opportunities, and yielded orderly quantification of capacity limits in both memory and reasoning. We argue that both types of limit might be based on the limited ability to form and preserve bindings between elements in memory.