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.     

STM capacity for Chinese and English language materials

This paper delineates the theoretical implications of a program of research on short-term memory using Chinese characters as stimuli, and compares the findings with studies of short-term memory that use English language materials. The 14 experiments on which it is principally based, carried out in the People’s Republic of China and in the U.S.A., are reported in detail in Yu et al. (1984), W. Zhanget al. (1984), and G. Zhang and Simon (this issue). One major theoretical product of this research is an experimentally tested model that reconciles the chunking theory of STM capacity with the articulatory loop theory of Baddeley, making good quantitative predictions of capacity compatible with both. Another result, obtained by using homophones as stimuli, is a demonstration that STM is mainly acoustically encoded, but that there are an additional two or three chunks of visually or semantically encoded short-term memory available. Chunks are shown to play the same role in immediate recall and rote learning in both the Chinese language and English language stimuli; and STM capacity, measured in chunks, is essentially the same for materials in both languages.     

Chunks in expert memory: evidence for the magical number four ... or is it two?

This study aims to test the divergent predictions of the chunking theory (Chase & Simon, 1973) and template theory (Gobet & Simon, 1996a, 2000) with respect to the number of chunks held in visual short-term memory and the size of chunks used by experts. We presented game and random chessboards in both a copy and a recall task. In a within-subject design, the stimuli were displayed using two presentation media: (a) physical board and pieces, as in Chase and Simon's (1973) study; and (b) a computer display, as in Gobet and Simon's (1998) study. Results show that, in most cases, no more than three chunks were replaced in the recall task, as predicted by template theory. In addition, with game positions in the computer condition, chess Masters replaced very large chunks (up to 15 pieces), again in line with template theory. Overall, the results suggest that the original chunking theory overestimated short-term memory capacity and underestimated the size of chunks used, in particular with Masters. They also suggest that Cowan's (2001) proposal that STM holds four chunks may be an overestimate.     

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.     

Constant capacity in an immediate serial-recall task: a logical sequel to Miller (1956)

We assessed a hypothesis that working memory capacity should include a constant number of separate mental units, or chunks (cf. Miller, 1956). Because of the practical difficulty of measuring chunks, this hypothesis has not been tested previously, despite wide attention to Miller's article. We used a training procedure to manipulate the strength of associations between pairs of words to be included in an immediate serial-recall task. Although the amount of training on associations clearly increased the availability of two-item chunks and therefore the number of items correct in list recall, the number of total chunks recalled (singletons plus two-word chunks) appeared to remain approximately constant across association strengths, supporting a hypothesis of constant capacity.     

Familiarity increases the number of remembered Pokémon in visual short-term memory

Long-term memory (LTM) can influence many aspects of short-term memory (STM), including increased STM span. However, it is unclear whether LTM enhances the quantitative or qualitative aspect of STM. That is, do we retain a larger number of representations or more precise representations in STM for familiar stimuli than unfamiliar stimuli? This study took advantage of participants’ prior rich multimedia experience with Pokémon, without investing on laboratory training to examine how prior LTM influenced visual STM. In a Pokémon visual STM change detection task, participants remembered more first-generation Pokémon characters that they were more familiar with than recent-generation Pokémon characters that they were less familiar with. No significant difference in memory quality was found when quantitative and qualitative effects of LTM were isolated using receiver operating characteristic (ROC) analyses. Critically, these effects were absent in participants who were unfamiliar with first-generation Pokémon. Furthermore, several alternative interpretations were ruled out, including general video-gaming experience, subjective Pokémon preference, and verbal encoding. Together, these results demonstrated a strong link between prior stimulus familiarity in LTM and visual STM storage capacity.     

Models of verbal working memory capacity: what does it take to make them work?

Theories of working memory (WM) capacity limits will be more useful when we know what aspects of performance are governed by the limits and what aspects are governed by other memory mechanisms. Whereas considerable progress has been made on models of WM capacity limits for visual arrays of separate objects, less progress has been made in understanding verbal materials, especially when words are mentally combined to form multiword units or chunks. Toward a more comprehensive theory of capacity limits, we examined models of forced-choice recognition of words within printed lists, using materials designed to produce multiword chunks in memory (e.g., leather brief case). Several simple models were tested against data from a variety of list lengths and potential chunk sizes, with test conditions that only imperfectly elicited the interword associations. According to the most successful model, participants retained about 3 chunks on average in a capacity-limited region of WM, with some chunks being only subsets of the presented associative information (e.g., leather brief case retained with leather as one chunk and brief case as another). The addition to the model of an activated long-term memory component unlimited in capacity was needed. A fixed-capacity limit appears critical to account for immediate verbal recognition and other forms of WM. We advance a model-based approach that allows capacity to be assessed despite other important processing contributions. Starting with a psychological-process model of WM capacity developed to understand visual arrays, we arrive at a more unified and complete model.     

Human information processing capacity in counting several things simultaneously

Abstract.— In the area of human information processing an important concept is that of a limited capacity central processing system (CPS). Posner (1970) emphasized the distinction between a short-term memory (STM) and an operational memory (OM). To evaluate this distinction, as well as to obtain data on the capacity limit of the CPS, and experiment was carried out, in which 14 subjects had to keep a running count of the number of occurrences of two or more predetermined items in a series of stimuli. Based on the experimental data a register model of the CPS was suggested which emphasized the distinction between the STM and the OM as one of two different memory functions carried out by a common memory system. The model also suggested that the capacity limit of the CPS was around eight items of verbally coded information.     

Item strength affects working memory capacity

Do the processing and online manipulation of stimuli that are less familiar require more working memory (WM) resources? Is it more difficult to solve demanding problems when the symbols involved are less rather than more familiar? We explored these questions with a dual-task paradigm in which subjects had to solve algebra problems of different complexities while simultaneously holding novel symbol–digit associations in WM. The symbols were previously unknown Chinese characters, whose familiarity was manipulated by differential training frequency with a visual search task for nine hour-long sessions over 3 weeks. Subsequently, subjects solved equations that required one or two transformations. Before each trial, two different integers were assigned to two different Chinese characters of the same training frequency. Half of the time, those characters were present as variables in the equation and had to be substituted for the corresponding digits. After attempting to solve the equation, subjects had to recognize which two characters were shown immediately before that trial and to recall the integer associated with each. Solution accuracy and response times were better when the problems required one transformation only; variable substitution was not required; or the Chinese characters were high frequency. The effects of stimulus familiarity increased as the WM demands of the equation increased. Character–digit associations were also recalled less well with low-frequency characters. These results provide strong support that WM capacity depends not only on the number of chunks of information one is attempting to process but also on their strength or familiarity.     

高难度材料的元记忆监测更受记忆广度的制约

以不同难度(有、无意义联系)的两种中文词对为记忆材料,随机选取在校大学生200名,采用经典的回忆-判断-再认(recall-judgement-recognition,RJR)范式,考察短时记忆广度和一般自我效能感对个体在不同难度材料上元记忆监测的影响。结果发现,相比于无意义联系的材料,个体在有意义联系的材料上,JOL、FOK和JOC判断等级更高,线索回忆测验成绩更好。但是,个体在不同材料上的JOL判断和线索回忆受短时记忆广度的调节。不同短时记忆广度的个体在有意义联系材料上的JOL判断等级和线索回忆测验成绩无显著差异,但是在无意义联系材料上,高短时记忆广度个体的JOL判断等级和线索回忆测验成绩均显著高于低短时记忆广度个体。同时,相关分析结果也发现,相比于低短时记忆广度的个体,高短时记忆广度的个体,其线索回忆测验成绩和再认测验成绩与JOL、FOK和JOC判断等级之间更加相关。实验结果表明,作为一种人格变量,一般自我效能感对个体元记忆监测的影响可能并不明显。而短时记忆广度,很大程度上反映了个体的记忆能力,是个体元记忆监测的一个重要影响因素,尤其是对难度较大的材料。     

Chunk limits and length limits in immediate recall: a reconciliation

Whereas some research on immediate recall of verbal lists has suggested that it is limited by the number of chunks that can be recalled (e.g., N. Cowan, Z. Chen, & J. N. Rouder, 2004; E. Tulving & J. E. Patkau, 1962), other research has suggested that it is limited by the length of the material to be recalled (e.g., A. D. Baddeley, N. Thomson, & M. Buchanan, 1975). The authors investigated this question by teaching new paired associations between words to create 2-word chunks. The results suggest that both chunk capacity limits and length limits come into play. For the free recall of 12-word lists, 6 pre-learned pairs could be recalled about as well as 6 pre-exposed singletons, suggesting a chunk limit. However, for the serially ordered recall of 8-word lists, 4 pre-learned pairs could be recalled about as well as 8 pre-exposed singletons, suggesting a length limit. Other conditions yielded intermediate results suggesting that sometimes both limits may operate together.     

语音短时记忆与词汇知识对第二语言词汇学习的影响

选取两种语音熟悉程度不同的非词为实验材料,把语音短时记忆区分为项目短时记忆和序列短时记忆,考察语音短时记忆与词汇知识对汉英双语者第二语言（英语）词汇学习的影响。实验1采用产出性的方式进行学习,结果发现,词汇知识与项目短时记忆对语音熟悉非词的学习起预测作用;词汇知识与序列短时记忆对语音不熟悉非词学习起预测作用。实验2采用接受性的方式进行学习,结果发现,项目短时记忆、序列短时记忆和词汇知识都对语音熟悉非词学习起独立的预测作用;项目短时记忆和序列短时记忆对语音不熟悉非词学习起独立的预测作用。实验结果表明,语音短时记忆和词汇知识都是影响英语词汇学习的重要因素。具体而言,学习语音熟悉的词汇,词汇知识起着更为重要的作用;学习语音不熟悉的词汇,语音短时记忆、特别是序列短时记忆起着更为重要的作用,而且两者作用的大小随着词汇学习方式的不同而发生变化。     

内隐序列学习中转移组块的数量和位置效应

表征质量理论对意识增长持渐进观点,忽视新异刺激对意识的突变式影响;新异刺激理论强调意识突变,忽视新异刺激本身的表征质量增长。本研究采用经典确定性内隐序列学习范式,将转移组块作为新异刺激,操控其数量和位置,探究新异刺激如何通过表征质量来影响内隐学习和意识。结果表明：（1）数量效应显著,即两个转移组块更能促进内隐学习量,说明新异刺激本身需要足够的表征质量才能发挥“意外事件”的作用。（2）在位置效应上,两个转移组块且靠前的设置更能提高受控意识,表明第一个新异刺激必须出现在原序列初级表征质量阶段,才能使被试对新异刺激和原序列进行对比,加之第二个新异刺激的与之呼应,促进原序列意识增加。     

视觉感觉记忆在字音选取作业中的作用

该研究通过四项实验探讨了视觉感觉记忆(VSM)在字音选取作业中的作用。实验材料为48个汉字。实验结果表明:当被试只能利用短时记忆(STM)中的信息进行选取作业时,能选取的汉字数与转换到STM中的汉字数之间无显著性差异;当被试可以同时利用VSM和STM中的信息时,能选取的汉字数要显著多于转换到STM中的汉字数。这表明,人能根据字音特征从VSM信息中选取字符。     

Short-term memory for serial order supports vocabulary development: New evidence from a novel word learning paradigm

Although recent studies suggest a strong association between short-term memory (STM) for serial order and lexical development, the precise mechanisms linking the two domains remain to be determined. This study explored the nature of these mechanisms via a microanalysis of performance on serial order STM and novel word learning tasks. In the experiment, 6- and 7-year-old children were administered tasks maximizing STM for either item or serial order information as well as paired-associate learning tasks involving the learning of novel words, visual symbols, or familiar word pair associations. Learning abilities for novel words were specifically predicted by serial order STM abilities. A measure estimating the precision of serial order coding predicted the rate of correct repetitions and the rate of phoneme migration errors during the novel word learning process. In line with recent theoretical accounts, these results suggest that serial order STM supports vocabulary development via ordered and detailed reactivation of the novel phonological sequences that characterize new words.     

Visual statistical learning produces implicit and explicit knowledge about temporal order information and scene chunks: Evidence from direct and indirect measures

We examined whether visual statistical learning (VSL) produced implicit and/or explicit knowledge about temporal order information and scene chunks, using a rapid serial visual presentation target detection task and a two-alternative forced-choice (2AFC) familiarity test as indirect and direct measures of VSL, respectively. In the familiarization phase, participants viewed a visual stream of natural scenes consisting of four triplets (i.e., three images that always appeared in the same order). In the subsequent target detection task, participants were required to detect target items embedded in a stream of 12 images or 12 words representing each natural scene. In the final 2AFC familiarity test, participants observed two test sequences (statistically related triplets vs. unrelated foils) and decided whether the first or second sequence was more familiar based on the familiarization phase. In both test phases, we included the same (forward) and reverse (backward) order of scenes as presented during the familiarization phase to examine whether the expression of VSL was based on temporal order of scenes or scene chunks. The results of the target detection task showed a learning effect for both temporal order in the forward condition and chunks in the backward condition, irrespective of whether stimuli were images or words; in contrast, we did not observe a learning effect in the backward condition for scene images in the familiarity test. Our findings are compatible with a learning mechanism that has both implicit and explicit components based on temporal order information and scene chunks.     

Concatenating familiar movement sequences: the versatile cognitive processor

Earlier studies demonstrated that practicing a series of key presses in a fixed order yields memory representations (i.e., motor chunks) that can be selected and used for sequence execution as if familiar key pressing sequences are single responses. In order to examine whether these motor chunks are robust in different situations and whether preparation for one sequence may overlap with execution of another one, two experiments were carried out in which participants executed two highly practiced keying sequences in rapid succession in response to two simultaneously presented stimuli. The results confirmed robustness of motor chunks, even when the sequences included only two elements, and showed that preparation (and in particular, selection) of a forthcoming sequence may occur during execution of the earlier sequence. Sequences including only two keys appeared to be slowed more by concurrent preparation than longer sequences. Together these results suggest that the execution of familiar keying sequences is predominantly carried out by a dedicated motor processor, and that the cognitive processor can be allocated to preparing a forthcoming sequence (e.g., during execution of an earlier sequence) or, some times, to selecting individual sequence elements in parallel to the motor processor.     

Competition between memory and perceptual tasks involving physically similar stimuli

Performance on a perceptual recognition task and a concurrently performed short-term memory (STM) task was studied. The stimuli used in the STM task were either visually similar, acoustically similar, or dissimilar to the stimuli in the perceptual task. The results demonstrated that any type of concurrent list memorization has a detrimental effect upon perceptual performance, but this effect is magnified if the stimuli used in the 2 tasks are visually similar. Acoustic similarity had no effect. The results document the fact that distinct tasks interfere, when the tasks compete for a limited total processing capacity, or when the tasks demand the same visual coding program.     

Aging and Visual Short-term Memory: Effects of Object Type and Information Load

ABSTRACT

Previous research has observed that the size of age differences in short-term memory (STM) depends on the type of material to be remembered, but has not identified the mechanism underlying this pattern. The current study focused on visual STM and examined the contribution of information load, as estimated by the rate of visual search, to STM for two types of stimuli – meaningful and abstract objects. Results demonstrated higher information load and lower STM for abstract objects. Age differences were greater for abstract than meaningful objects in visual search, but not in STM. Nevertheless, older adults demonstrated a decreased capacity in visual STM for meaningful objects. Furthermore, in support of Salthouse's processing speed theory, controlling for search rates eliminated all differences in STM related to object type and age. The overall pattern of findings suggests that STM for visual objects is dependent upon processing rate, regardless of age or object type.     

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.