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.     

Verbal reasoning and working memory

A series of experiments was carried out to explore the hypothesis that a central working memory (WM) system is utilized during verbal reasoning. WM was provisionally defined in terms of two major features of short-term memory (STM): its limited storage capacity and its use of speech-coded information. The reasoning task required subjects to match (verify) as rapidly as possible a sentence of varying grammatical form with a symbolic referent. Experiments I and II studied the effect of storing an additional STM load on sentence verification latencies. As many as six items could be correctly recalled with no slowing of verification speed. Experiment III used a different procedure in which the STM items had to be articulated aloud during verification. In this case six-item STM loads slowed verification speed considerably, and did so more for the more difficult sentences. Only a small non-significant slowing of verification speed was obtained when redundant messages were articulated. Experiment IV showed that latencies were also increased by introducing phonemic similarity into the verification task. Generally the results were not fully consistent with the hypothesis of WM as a limited capacity store called upon by the verification task. Instead they supported the view that the WM is a general executive system with a limited capacity for information processing. It was proposed that the articulatory system, used in rehearsal and concerned with speech-coding, is a “peripheral” of the more central WM executive and plays a relatively minor role in verbal reasoning.     

An information processing analysis of mental multiplication

The specific model of central or conscious processing system (CPS) is presented. It consists of two part: (1) a number of information carrying registers, and (2) a functional unit controlling the processes performed on the contents of the registers. The capacity limit is explained as a combination of storage capacity and processing capacity. The capacity limit was estimated by an experiment using mental multiplication performed according to a particular algorithm. The results suggested that the capacity was around eight to nine registers when the algorithm was strictly followed; this limit could, however, be exceeded because some problems automatically were coded and remembered by rules rather then by rehearsal. The results further suggested that information processing, i.e. the multiplication, had to take place on a time-sharing basis in competition with rehearsal, thus suggesting the existence of higher-order control system.     

Verbal and visual recall span curves between 1 ms and 1 min

Summary The recall spans for verbal and visual information were represented, respectively, in terms of the number of digits and the number of dots in a 5×5 matrix. They were measured independently by the method of limits as functions of presentation times which ranged from 1 ms to 64 s. The curves were very similar: each had two plateaus, one at 10 to 100 ms and one at 2 to 8 s presentation levels. These plateaus suggest two kinds of capacity limitation on information processing. The first limitation is concerned with the transfer of information from iconic memory to short-term memory (STM); the second indicates the saturation of the limited capacity short-term store. Short-term and long-term mechanisms are involved not only in verbal memory but also in visual memory, since the second plateau followed by an increasing portion appears in the visual span curve as well as in the verbal one. Finally, the distinction between verbal and visual STM is discussed, based on a) the selective interference effects of distracting tasks upon the retention of verbal and visual materials, and b) the weak correlation between verbal and visual spans.     

La mémoire verbale à court terme : un simple produit des interactions entre systèmes langagiers, attentionnels et de traitement de l’ordre sériel ?

This literature review examines the links between verbal short-term memory (STM) and language processing, by trying to determine to what extent STM is a distinct system relative to the language system. In order to answer this question, we will distinguish between “item” and “serial order” information, which is a critical distinction for the study of STM. We will develop a theoretical proposal considering that STM results from the interaction of multiple processes and systems: temporary activation of the language system for the storage of “item” information; the intervention of a specialized system for the processing of serial order and the intervention of an attentional modulator.     

Short-term memory limitations in children: Capacity or processing deficits?

This paper evaluates the assertion that short-term memory (STM) capacity increases with age. Initially an analysis is made of the STM system in terms of its parameters and control processes. No evidence was found that can suggest conclusively that either the capacity or the rate of information loss from STM varies with age. On the other hand, substantial evidence exists to show that the processing strategies used by adults are unavailable or deficient in children. Furthermore, considerable differences in the contents and complexity of the long-term memory (LTM) knowledge base (semantic and recognition networks can produce grossly different STM performance between age groups. The second half of this paper reviews three STM-related paradigms-memory span, serial probed recall, and recognition under limited exposure-that have consistently shown performance deficits in children. These deficits are explained in terms of the lack of proper control processes (or processing strategies), as well as an impoverished LTM knowledge base rather than a limitation in STM capacity.     

Neural dynamics of attention switching and temporal-order information in short-term memory

Reeves and Sperling (1986) have developed an experimental paradigm and a model to explain how attention switching influences the storage of temporal-order information in short-term memory (STM), or working memory. The present article suggests that attention switching influences initial storage of items in STM, but that competitive interactions among the STM representations of stored items control the further evolution of temporal-order information as new items are processed. The laws governing these competitive interactions, called the long-term memory (LTM) invariance principle and the STM normalization rule, were originally derived from postulates that ensure that STM is updated in a way that enables temporally stable list learning in LTM to occur. Despite these adaptive constraints, and often because of them, temporal-order information is not always stored veridically. Both feedforward and feedback STM processes, with different invariant properties, are identified in the storage of temporal-order information.     

Impact of auditory selective attention on verbal short-term memory and vocabulary development

This study investigated the role of auditory selective attention capacities as a possible mediator of the well-established association between verbal short-term memory (STM) and vocabulary development. A total of 47 6- and 7-year-olds were administered verbal immediate serial recall and auditory attention tasks. Both task types probed processing of item and serial order information because recent studies have shown this distinction to be critical when exploring relations between STM and lexical development. Multiple regression and variance partitioning analyses highlighted two variables as determinants of vocabulary development: (a) a serial order processing variable shared by STM order recall and a selective attention task for sequence information and (b) an attentional variable shared by selective attention measures targeting item or sequence information. The current study highlights the need for integrative STM models, accounting for conjoined influences of attentional capacities and serial order processing capacities on STM performance and the establishment of the lexical language network.     

视空间定位的短时记忆的超广度研究

采用视空间定位的短时记忆任务研究记忆负载的增加对空间位置信息和时间顺序信息的记忆成绩的影响。结果表明,在记忆负载较低时,位置记忆和顺序记忆的成绩相同;当记忆负载逐渐增大时,位置记忆的成绩逐渐上升,而顺序记忆的成绩先上升后下降,两者出现显著性的差异。在这个过程中,短时记忆的加工发生相应的变化,体现了短时记忆加工资源的有限性。     

信息加工效率与阅读的关系研究

对23名大学生进行知觉广度、记忆广度和阅读理解的多项测验,探求知觉、记忆加工效率与不同语言阅读成绩和阅读眼动特性之间的关系。结果发现：低水平的视觉符号方向信号知觉广度与阅读不存在明显的关系,从眼跳幅度指标上反映出右侧知觉广度大的被试对第二语言加工难度更为敏感。短时记忆和工作记忆能力对阅读的影响主要反映在眼跳幅度方面,与单个注视点获取的信息量有关,但与课文理解水平的关系不明显。短时记忆能力强的被试在眼跳幅度上以及第二语言言语工作记忆弱的被试在注视时间上均出现语言差异效应。     

Out with the old? The role of selective attention in retaining targets in partial report

In the partial-report task, subjects are asked to report only a portion of the items presented. Selective attention chooses which objects to represent in short-term memory (STM) on the basis of their relevance. Because STM is limited in capacity, one must sometimes choose which objects are removed from memory in light of new relevant information. We tested the hypothesis that the choices among newly presented information and old information in STM involve the same process—that both are acts of selective attention. We tested this hypothesis using a two-display partial-report procedure. In this procedure, subjects had to select and retain relevant letters (targets) from two sequentially presented displays. If selection in perception and retention in STM are the same process, then irrelevant letters (distractors) in the second display, which demanded attention because of their similarity to the targets, should have decreased target report from the first display. This effect was not obtained in any of four experiments. Thus, choosing objects to keep in STM is not the same process as choosing new objects to bring into STM.     

Working memory deficits in developmental dyscalculia: The importance of serial order

Although a number of studies suggests a link between working memory (WM) storage capacity of short-term memory and calculation abilities, the nature of verbal WM deficits in children with developmental dyscalculia (DD) remains poorly understood. We explored verbal WM capacity in DD by focusing on the distinction between memory for item information (the items to be retained) and memory for order information (the order of the items within a list). We hypothesized that WM for order could be specifically related to impaired numerical abilities given that recent studies suggest close interactions between the representation of order information in WM and ordinal numerical processing. We investigated item and order WM abilities as well as basic numerical processing abilities in 16 children with DD (age: 8–11 years) and 16 typically developing children matched on age, IQ, and reading abilities. The DD group performed significantly poorer than controls in the order WM condition but not in the item WM condition. In addition, the DD group performed significantly slower than the control group on a numerical order judgment task. The present results show significantly reduced serial order WM abilities in DD coupled with less efficient numerical ordinal processing abilities, reflecting more general difficulties in explicit processing of ordinal information.     

Type-specific proactive interference in patients with semantic and phonological STM deficits

Prior neuropsychological evidence suggests that semantic and phonological components of short-term memory (STM) are functionally and neurologically distinct. The current paper examines proactive interference (PI) from semantic and phonological information in two STM-impaired patients, DS (semantic STM deficit) and AK (phonological STM deficit). In Experiment 1 probe recognition tasks with open and closed sets of stimuli were used. Phonological PI was assessed using nonword items, and semantic and phonological PI was assessed using words. In Experiment 2 phonological and semantic PI was elicited by an item recognition probe test with stimuli that bore phonological and semantic relations to the probes. The data suggested heightened phonological PI for the semantic STM patient, and exaggerated effects of semantic PI in the phonological STM case. The findings are consistent with an account of extremely rapid decay of activated type-specific representations in cases of severely impaired phonological and semantic STM.     

Release from Pl in running memory: What does this tell us about developmental STM?

Several theories consider the development of short-term memory (STM) in children to reflect the development of processing operations. This article reports an experiment that tested some of these theories, using a release from proactive interference (PI) procedure in conjunction with a running memory task, to track down age-related improvements in the processing of various kinds of information. Although evidence for the development of PI release with age appeared for two kinds of information, namely class of item and acoustic similarity, there was no evidence whatsoever for the existence of a relationship between STM performance and PI release. It was concluded that the development of STM on the one hand, and of the capacity to process item or acoustic information on the other, proceed independently of each other. The discussion deals with the apparent conflict between this conclusion and the theoretical positions.     

Estimating working memory capacity for lists of nonverbal sounds

Working memory (WM) capacity limit has been extensively studied in the domains of visual and verbal stimuli. Previous studies have suggested a fixed WM capacity of typically about three or four items, on the basis of the number of items in working memory reaching a plateau after several items as the set size increases. However, the fixed WM capacity estimate appears to rely on categorical information in the stimulus set (Olsson & Poom Proceedings of the National Academy of Sciences 102:8776-8780, 2005). We designed a series of experiments to investigate nonverbal auditory WM capacity and its dependence on categorical information. Experiments 1 and 2 used simple tones and revealed capacity limit of up to two tones following a 6-s retention interval. Importantly, performance was significantly higher at set sizes 2, 3, and 4 when the frequency difference between target and test tones was relatively large. In Experiment 3, we added categorical information to the simple tones, and the effect of tone change magnitude decreased. Maximal capacity for each individual was just over three sounds, in the range of typical visual procedures. We propose that two types of information, categorical and detailed acoustic information, are kept in WM and that categorical information is critical for high WM performance.     

视觉工作记忆的同类别存储优势

概念规律如记忆项间的类别关系如何影响视觉工作记忆容量是一个有争议的问题。针对该问题, 学界存在两种预测截然不同的假说：(1)混合类别优势假说, (2)同类别优势假说。综述文献发现, 该类研究均采用带有细节特征的真实客体作为实验材料, 因此前人研究中发现的混合类别优势效应或同类别优势效应中必然混有低水平知觉特征的影响。故本研究采用去除细节信息的动物剪影作为记忆材料来排除上述因素的影响, 旨在厘清上述两种假设, 并采用对侧延迟活动作为神经指标, 来进一步探讨概念规律影响工作记忆容量的内在机制。两个行为实验发现, 不论记忆项同时呈现还是序列呈现, 均存在同类别记忆优势效应。脑电实验结果发现相比记忆不同类别客体, 记忆同等数量的同类别客体诱发的对侧延迟活动的幅值更小。上述结果一致表明, 视觉工作记忆可借助概念的方式将同类别客体加以组织, 从而有效扩大视觉工作记忆容量, 支持了同类别优势假说。     

Sensory and conceptual representations in memory: Motor images that cannot be imaged

Summary The paper argues for a distinction between sensory-and conceptual-information storage in the human information-processing system. Conceptual information is characterized as meaningful and symbolic, while sensory information may exist in modality-bound form. Furthermore, it is assumed that sensory information does not contribute to conscious remembering and can be used only in data-driven process reptitions, which can be accompanied by a kind of vague or intuitive feeling. Accordingly, pure top-down and willingly controlled processing, such as free recall, should not have any access to sensory data. Empirical results from different research areas and from two experiments conducted by the authors are presented in this article to support these theoretical distinctions. The experiments were designed to separate a sensory-motor and a conceptual component in memory for two-digit numbers and two-letter items, when parts of the numbers or items were imaged or drawn on a tablet. The results of free recall and recognition are discussed in a theoretical framework which distinguishes sensory and conceptual information in memory.     

A Bayesian hierarchical model for the measurement of working memory capacity

Working memory is the memory system that allows for conscious storage and manipulation of information. The capacity of working memory is extremely limited. Measurements of this limit, and what affects it, are critical to understanding working memory. Cowan (2001) and Pashler (1988) suggested applying multinomial tree models to data from change detection paradigms in order to estimate working memory capacity. Both Pashler and Cowan suggested simple formulas for estimating capacity with these models. However, in many cases, these simple formulas are inadequate, and may lead to inefficient or biased estimation of working memory capacity. I propose a Bayesian hierarchical alternative to the Pashler and Cowan formulas, and show that the hierarchical model outperforms the traditional formulas. The models are easy to use and appropriate for a wide range of experimental designs. An easy-to-use graphical user interface for fitting the hierarchical model to data is available.     

Running memory span: A comparison of behavioral capacity limits with those of an attractor neural network

We studied a computational model of short term memory capacity that performs a simulated running memory span task using Hebbian learning and rapid decay of connection strengths to keep recent items active for later recall. This model demonstrates recall performance similar to humans performing the same task, with a capacity limit of approximately three items and a prominent recency effect. The model also shows that this capacity depends on decay to release the model from accumulating interference. Model findings are compared with data from two behavioral experiments that used varying task demands to tax memory capacity limits. Following additional theoretical predictions from the computational model, behavioral data support that when task demands require attention to be spread too thin to keep items available for later recall, capacity limits suffer. These findings are important both for understanding the mechanisms underlying short term memory capacity, and also to memory researchers interested in the role of attention in capacity limitations.     

视觉感觉记忆信息在模式识别中的作用

该研究设计了新的实验方法进行了两项全部报告实验和两项抽样模式识别实验,考察视觉感觉记忆(visual sensorymemory,VSM)信息在模式识别中的作用。结果发现,在限定利用短时记忆(shor-term memory,STM)信息的抽样模式识别作业中,识别出的模式与转换到短时记忆的模式在数量上无显著性差别,而在短时记忆和感觉记忆信息都可能利用的识别作业中,虽然转换到短时记忆的模式数量并未增加,但识别出的模式比转换到短时记忆的模式数量增加了1倍(P<0.001)。这表明,未能转换到短时记忆的感觉记忆信息可被直接用于模式识别。