Age-related differences in immediate serial recall: Dissociating chunk formation and capacity

We assessed the contribution of two hypothesized mechanisms to impaired memory performance of older adults in an immediate serial recall task: decreased temporary information storage in a capacity-limited mechanism, such as the focus of attention, and a deficit in binding together different components into cohesive chunks. Using a method in which paired associations between words were taught at varying levels to allow an identification of multiword chunks (Cowan, Chen, & Rouder, 2004), we found that older adults recalled considerably fewer chunks and, on average, smaller chunks than did young adults. Their performance was fairly well simulated by dividing attention in younger adults, unlike what has been found for long-term associative learning. Paired-associate knowledge may be used in an implicit manner in serial recall, given that younger adults under divided attention and older adults use it well despite the relatively small chunk capacities displayed by these groups.     

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.     

Investigating the childhood development of working memory using sentences: New evidence for the growth of chunk capacity

Child development is accompanied by a robust increase in immediate memory. This may be due to either an increase in the number of items (chunks) that can be maintained in working memory or an increase in the size of those chunks. We tested these hypotheses by presenting younger and older children (7 and 12 years of age) and adults with different types of lists of auditory sentences: four short sentences, eight short sentences, four long sentences, and four random word lists, each read with a sentence-like intonation. Young children accessed (recalled words from) fewer clauses than did older children or adults, but no age differences were found in the proportion of words recalled from accessed clauses. We argue that the developmental increase in memory span was due to a growing number of chunks present in working memory with little role of chunk size.     

Working memory capacity for spoken sentences decreases with adult ageing: recall of fewer but not smaller chunks in older adults

Previous studies show that older adults have poorer immediate recall for language but the reason is unknown. Older adults may recall fewer chunks from working memory, or may have difficulty binding words together to form multi-unit chunks. We examined these two hypotheses by presenting four types of spoken sentences for immediate free recall, differing in the number and length of chunks per trial: four short, simple sentences; eight such sentences; four compound sentences, each incorporating two meaningful, short sentences; and four random word lists, each under a sentence-like intonation. Older adults recalled words from (accessed) fewer clauses than young adults, but there was no ageing deficit in the degree of completion of clauses that were accessed. An age-related decline in working memory capacity measured in chunks appears to account for deficits in memory for spoken language.     

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.     

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.     

Core verbal working-memory capacity: The limit in words retained without covert articulation

Verbal working memory may combine phonological and conceptual units. We disentangle their contributions by extending a prior procedure (Chen & Cowan, 2005 Chen, Z. and Cowan, N. 2005. Chunk limits and length limits in immediate recall: A reconciliation. Journal of Experimental Psychology: Learning, Memory, and Cognition, 31: 1235–1249. [Crossref], [PubMed], [Web of Science ®] , [Google Scholar]) in which items recalled from lists of previously seen word singletons and of previously learned word pairs depended on the list length in chunks. Here we show that a constant capacity of about 3 chunks holds across list lengths and list types, provided that covert phonological rehearsal is prevented. What remains is a core verbal working-memory capacity.     

Predicting immediate ordered recall of lists with products of probabilities

Immediate ordered recall performance is often reported in terms of recall of individual items or of lists. Schweickert, Chen and Poirier (Int. J. Psychol. 34 (1999) 447) proposed that the probability of recalling a list approximately equals the product of the conditional probabilities that each item is recalled, given its immediate predecessor is recalled. This Product of Adjacent Conditionals gave a close lower bound to the probability of recalling a list, although it is not exactly right. An alternative is that dependencies are expressed through probabilities conditioned on retrieval of higher level units such as chunks. An example is a key assumption of Anderson and Matessa (Psychol. Rev. 104 (1997) 728). This alternative is rejected here. Overall, the Product of Adjacent Conditionals formula and the Anderson and Matessa theory predicted recall of a list about equally well.     

Ordering and reordering in the auditory and visual modalities

This study investigates the hypothesis that the modality effect (i.e., the often-found recall advantage of the last few items presented in the auditory- rather than visual-input modality) is attributable to a directional auditory trace. Such a directional trace should help performance of forward item-to-item recall, but should hurt performance if mental reordering is required. However, it was found in our Experiment 1 that mental item-to-item reordering of the materials did not affect the modality effect. In Experiment 2, strict control was exercised over the order of recall of chunks of items, as well as over the order of recall within chunks. It was found that the item-to-item reordering of the materials had little effect on the modality effect. However, the larger scale order of recall of the chunks had a large effect on the modality effect. If recall was in a forward order (i.e., first chunk, second chunk, third chunk), there was a dramatic superiority of the auditory materials on the last chunk. If the recall order was backward (third chunk, second chunk, first chunk), the modality effect was inconsequential. These results suggest that the modality effect is related to access to large memorial units rather than to item-to-item associations.     

On the auditory modality superiority effect in serial recall: separating input and output factors

The modality effect in immediate recall refers to superior recall of the last few items within lists presented in spoken as opposed to printed form. The locus of this well-known effect has been unclear. N. Cowan, J. S. Saults, E. M. Elliott, and M. Moreno (2002) introduced a new method to distinguish between the effects of input serial position, output serial position, and the number of items yet to be recalled and found that large modality effects occurred only in conditions in which delay and interference at output (from items already recalled) was high. The authors replicated that finding, even when the response period included output interference acoustically similar to the spoken stimuli to be recalled. However, the authors found that output delay and interference act only by lowering the level of performance to a more sensitive range. The modality effect thus originates during encoding of the list to be recalled, not during output.     

What’s magic about magic numbers? Chunking and data compression in short-term memory

Short term memory is famously limited in capacity to Miller's (1956) magic number 7±2-or, in many more recent studies, about 4±1 "chunks" of information. But the definition of "chunk" in this context has never been clear, referring only to a set of items that are treated collectively as a single unit. We propose a new more quantitatively precise conception of chunk derived from the notion of Kolmogorov complexity and compressibility: a chunk is a unit in a maximally compressed code. We present a series of experiments in which we manipulated the compressibility of stimulus sequences by introducing sequential patterns of variable length. Our subjects' measured digit span (raw short term memory capacity) consistently depended on the length of the pattern after compression, that is, the number of distinct sequences it contained. The true limit appears to be about 3 or 4 distinct chunks, consistent with many modern studies, but also equivalent to about 7 uncompressed items of typical compressibility, consistent with Miller's famous magical number.     

Infants hierarchically organize memory representations

Throughout development, working memory is subject to capacity limits that severely constrain short‐term storage. However, adults can massively expand the total amount of remembered information by grouping items into chunks. Although infants also have been shown to chunk objects in memory, little is known regarding the limits of this ability. In particular, it remains unknown whether infants can create more complex memory hierarchies, binding representations of chunks into still larger chunks in recursive fashion. Here we tested the limits of early chunking, first measuring the number of items infants can bind into a single chunk and the number of chunks infants can maintain concurrently, and then, critically, whether infants can embed chunked representations into larger units. We tested 14‐month‐old infants' memory for hidden objects using a manual search task in which we manipulated memory load (the number of objects infants saw hidden) and the chunking cues provided. We found that infants are limited in the number of items they can chunk and in the number of chunks they can remember. However, we also found that infants can bind representations of chunks into ‘superchunks’. These results suggest that hierarchically organizing information strongly affects working memory, starting in infancy.     

Skilled perception in Go: Deducing memory structures from inter-response times

Experts appear able to handle much larger amounts of specialized information than nonexperts, and handle it without an apparent superior memory capacity. This finding, based on research on chess players with chess information, was replicated on Go players with Go information. Assuming this superiority occurs because the experts process chunks of information through their limited capacities rather than individual elements, the question then becomes one of defining what the chunks are and how they are related. To this end, the technique of partitioning recall and reproduction data into chunks on the basis of inter-response times (IRTs) (introduced in their work on chess by Chase and Simon, 1973) was applied to the reproduction and recall of Go patterns by a Go Master and a Go beginner. Unlike its application in chess, no single IRT was able to produce consistent, veridical chunks for either Go player. Subsequent analysis of the underlying assumptions of the technique showed it to be limited to only those patterns that can be partitioned into a linear set of chunks, not nested chunks, and to situations in which retrieval and overt recall of each chunk is completed before retrieval of the next chunk. In a supplementary task, the Master Go player indicated that the Go patterns were not seen as linear chunks nor as strictly nested hierarchies, but rather as overlapping clusters. IRTs were found to be correlated with this structure, but were not reliable enough to reflect its details.     

Chunking during serial learning by a pigeon: II. Integrity of a chunk on a new list

Can a group of items that a pigeon chunks on one list function as such on a second list? In Experiment 1, the ordinal position of the chunk was held constant across both lists. Following training on a list of colors and achromatic geometric forms (A----B----C----D'----E'), the integrity of the color chunk [A----B----C] was maintained on a list of five colors (A----B----C----F----G) even though the basis for establishing that chunk was eliminated. The integrity of the chunk [A----B----C] was also maintained on a new list of colors and forms (A----B----C----D*----E*). In Experiment 2, the ordinal position of a chunk established on list1 (A----B----C'----D') was changed on list 2. As shown by positive transfer between lists 1 and 2, the integrity of the chunks [A----B] and [C'----D'] was maintained on lists X'----A----B----Y' and X'----C'----D'----Y', respectively. Conversely, the heterogeneous list X'----B----C'----Y' took longer to learn than the original list.     

Semantic binding,not attentional control,generates coherent global structure in children's narrative memory

Encoding and retrieving global narrative structure influences children's narrative recall. The influence of age and attentional/executive resources on binding processes during sentence list recall was examined in 5- to 6- and 8- to 9-year-old children. Older and younger children showed superior recall of lists, and achieved higher scores on two metrics of chunking; access to different sentences (i.e., number of chunks) and sentence completion (i.e., chunk size), when lists were presented within a coherent global structure. Children's list recall and sentence access, but not their sentence completion scores, were affected by a concurrent self-paced attention-demanding task. Children, unlike adults, engage in active storage of verbal information in thematically related sentence lists. The coherence-advantage effect was stable across age groups and insensitive to the secondary task. Overall, findings imply that semantic binding generates stronger memory representations and superior recall for sentences within a story context than for sentence sets that lack global narrative structure.     

Mental imagery and chunks: Empirical and computational findings

To investigate experts' imagery in chess, players were required to recall briefly presented positions in which pieces were placed on the intersections between squares (intersection positions). Position types ranged from game positions to positions in which both the piece distribution and the location were randomized. Simulations were run with the CHREST model (Gobet & Simon, 2000). The simulations assumed that pieces had to be centered back, one by one, to the middle of the squares in the mind's eye before chunks could be recognized. Consistent with CHREST's predictions, chess players (N = 36), ranging from weak amateurs to grandmasters, exhibited much poorer recall for intersection positions than for standard positions (pieces placed on the centers of the squares). For the intersection positions, the skill difference in recall was larger for game positions than for the randomized positions. The participants recalled bishops better than they recalled knights, suggesting that Stroop-like interference impairs recall of the latter. The data supported both the time parameter in CHREST for shifting pieces in the mind's eye (125 msec per piece) and the seriality assumption. In general, the study reinforces the plausibility of CHREST as a model of cognition.     

Continued word associations and free recall

Two experiments on the short-term free recall of 12-word associated and non-associated lists are reported. Degree of association (derived from norms obtained by continuous controlled association) and word frequency were varied. Significant facilitation as a result of the associative manipulations was obtained and clustering of the responses was positively related to this. Clustering was also affected by the method of presentation of the associated words; this occurred more often when they were grouped in presentation than when they were presented randomly arranged among other words in the list. Low frequency associated word lists were generally found to be more efficiently recalled than those of comparable association values but consisting of high frequency words.     

On the relative and absolute strength of a memory trace

Subjects studied either an 8- or 16-word list and later recalled the items while a voice key recorded each response latency. The trials were partitioned by recall total in order to examine the means and distributions of both latencies and interresponse times as a function of recall total. Each analysis was consistent with the view that an item’s absolute strength determineswhether it is recalled whereas an item’s relative strength determineswhen it is recalled. In addition, mean latency was effectively proportional to study list length yet independent of recall total. All of the analyses were consistent with the view that the set of study items is sampled according to a relative-strength rule until all items are found and that a sampled item is recovered into consciousness only when its absolute strength exceeds a fixed threshold.     

Word frequency and the mixed-list paradox in immediate and delayed serial recall

In free recall tasks, when low- and high-frequency items are mixed within the to-be-remembered lists, the usual recall advantage found for high-frequency words is eliminated or reversed. Recently, this mixedlist paradox has also been demonstrated for short-term serial recall (Hulme, Stuart, Brown, & Morin, 2003). Although a number of theoretical interpretations of this mixed-list paradox have been proposed, researchers have also suggested that it could simply be a result of participant-controlled strategies (M. J. Watkins, LeCompte, & Kim, 2000). The present study was designed to assess whether this explanation could be applied to immediate and delayed serial recall. The results showed that high-frequency words were recalled better than low-frequency words in pure lists, but that this effect was eliminated in mixed lists, whether they were given under intentional or incidental learning conditions. This pattern suggests that the mixed-list paradox cannot be explained by participant-controlled strategies.     

Post-iconic visual storage: chunking in the reproduction of briefly displayed visual patterns

Two experiments are reported which investigate the organization of visuospatial information in post-iconic storage. In both experiments, stimuli consisting of 10 disks randomly placed in a four-by-five array were tachistoscopically presented to subjects whose task was to recreate the pattern. In Experiment 2, reproduction was constrained (on a row-by-row basis) while in Experiment 1 it was unconstrained. The results of Experiment 1 showed that subjects recalled in terms of “chunks” of spatially adjacent disks, with most “chunks” consisting of about three of four disks. Within each sequence of 10 responses the probability of correctly recalling a chunk decreased with its serial position but was relatively independent of chunk size per se (for chunks containing seven or less disks). In addition, clear topographical variations in accuracy were found, which tended to covary strongly with order of recall. In Experiment 2, the order of reproduction was prespecified (either top row down to bottom row, or bottom row up to top row) in order to induce chunking by rows. The direction of reproduction was either pre- or post-cued. The results of this study showed that subjects encode the stimulus, wherever possible, in a form which is compatible with the constraints imposed on recall order. The results for the postcued conditions provide strong support for the argument that topographical variations in accuracy are a function of variations in accuracy of encoding, and not simply a function of order of report. The results are discussed in terms of an attentional model. It is proposed that a general “anticipatory schema” (cf. Neisser, Cognition and Reality 1976) presets the distribution of attention in the visual field, preselects a set of coding heuristics, and subsequently interacts with the present stimulus pattern. Spatial discontinuities in the distribution of attention resulting from this interaction are regarded as “defining” chunks of stimulus elements.