What limits children's working memory span? Theoretical accounts and applications for scholastic development.

Explanations of working memory span in children were studied in a longitudinal follow-up of J. N. Towse, G. J. Hitch, and U. Hutton (1998). Reading span and operation span were lower when within-task retention intervals were lengthened. For each task, variation in span between test waves and age cohorts was systematically related to changes in processing speed. The two spans explained substantial shared variance in both reading and arithmetic scores, with some evidence for domain specificity. Combined span scores predicted unique variance in scholastic attainment over a 1-year interval. The authors concluded that working memory span is constrained by rapid loss of active codes and is not simply a measure of capacity for resource sharing. Working memory is also implicated in scholastic development.

Development of working memory: should the Pascual-Leone and the Baddeley and Hitch models be merged?

The data presented by Kemps, De Rammelaere, and Desmet (2000, this issue) appear to have some aspects that fit most readily into our own model (Baddeley & Hitch, 1974), while others appear to support that of Pascual-Leone (1970). We accept that our initial model said little about development and was better able to account for relatively simple memory-based tasks than more complex cognitive activities. More recent elaborations of the model are, however, able to throw new light on the processes underlying cognitive development, offering a better account than that provided by existing neo-Piagetian interpretations. Meanwhile, the addition of a fourth component to the model, namely the episodic buffer, offers a way of dealing with more complex cognitive activities. Given the major differences between our own model and that of Pascual-Leone in basic assumptions, and in theoretical style, we suggest that any attempt to combine the two would be premature.

Separate roles for executive and phonological components of working memory in mental arithmetic.

A dual-task methodology was used to investigate the roles played by executive and phonological aspects of working memory in mental arithmetic. Experiment 1 showed that suppression of articulation impaired the ability to add a pair of briefly presented three-digit numbers. Suppression had no effect when the need to store temporarily was minimized by making the numbers visible throughout calculation. Experiment 2 showed that disrupting executive processes by requiring concurrent performance of a Trails task impaired the ability to add numbers that remained permanently visible. Performance on the Trails task deteriorated as the number of carry operations in the addition increased. Experiment 3 showed that this decline in Trails performance was not simply due to the extra time taken by carrying. These and other features of the results suggest that the carrying component of mental arithmetic places substantial demands on executive processes, whereas the need to retain problem information is met by the phonological loop. The results are consistent with an interpretation of executive processes according to which there is a limit on the capacity to inhibit strongly primed routine operations.

On the interpretation of working memory span in adults.

Experimental research into children's working memory span has shown that retention duration contributes substantially to span performance, while processing efficiency need not be related to concurrent memory load (Towse, Hitch, & Hutton, 1998). These findings have been used to argue for a model of working memory span that emphasizes time-based forgetting rather than the popular resource-sharing or tradeoff framework. The present paper considers whether adults perform working memory span tasks in a qualitatively different way. Data from reading span and operation span tasks show that adults' performance can be distinguished from that of children, but also that a task-switching model of working memory span can explain some important aspects of performance.

Mutual inhibition in collaborative recall: evidence for a retrieval-based account.

In Experiment 1 participants gave 3 successive free recalls of items learned either individually or in pairwise collaboration. The first and third recalls were performed individually, the second alone or in collaboration. Collaborative recall led to an inhibitory effect after individual learning but not after collaborative learning, in which partners had similar retrieval strategies. Consistent with a retrieval locus for collaborative inhibition, non-recalled items reappeared in subsequent individual recall. Experiment 2 showed that collaborative inhibition was eliminated when a separate retrieval cue was given for each item. Experiments 2 and 3 also showed that when participants learned items in the same order, their retrieval strategies were more similar and they showed less collaborative inhibition. It is concluded that mutual interference in collaborative recall is due to the mutual disruption of individual retrieval strategies.

Working memory impairments in children with specific arithmetic learning difficulties.

Working memory impairments in children with difficulties in arithmetic have previously been investigated using questionable selection techniques and control groups, leading to problems concluding where deficits may occur. The present study attempted to overcome these criticisms by assessing 9-year-old children with difficulties specific to arithmetic, as indicated by normal reading, and comparing them with both age-matched and ability-matched controls. A battery of 10 tasks was used to assess different aspects of working memory, including subtypes of executive function. Relative to age-matched controls, children with poor arithmetic had normal phonological working memory but were impaired on spatial working memory and some aspects of executive processing. Compared to ability-matched controls, they were impaired only on one task designed to assess executive processes for holding and manipulating information in long-term memory. These deficits in executive and spatial aspects of working memory seem likely to be important factors in poor arithmetical attainment.

Cognitive effort and recollective experience in recognition memory.

The difficulty of the cognitive operations required to process study items was manipulated in two experiments investigating recollective experience. In subsequent recognition tests, subjects indicated whether their recognition judgements for items processed in these tasks were based on recollection ("remember" responses) or on familiarity ("know" responses). In Experiment 1 target items were presented in the context of a category decision task. It was found that remember responses increased with the difficulty of the category decision. For positive instances, remember responses were greater for items of low instance frequency than for items of high instance frequency, while for negative instances remember responses were greater for items from similar categories than for items from dissimilar categories. These effects were not present in know responses. In Experiment 2, remember responses were more frequent when study items had been presented in the form of anagrams to be solved than when they had been presented in the form of words to be read aloud. The incidence of know responses was not affected by the format in which study items were presented. Source judgements were also more accurate when recognition was based on recollection. It is argued that the type of conscious awareness experienced during recognition is determined by the knowledge activated by items presented in the recognition test, which in turn is determined by the nature of the operations engaged at encoding.

Magnitude comparisons by children with specific language impairments: evidence of unimpaired symbolic processing.

A size judgement task was used to investigate number processing skills in children with specific language impairments (SLI). Previous work with unimpaired adults and children has shown that when comparing the size of written numbers and other ordinal stimuli, there is a symbolic distance effect (SDE) such that decision time decreases with the size distance between items. This study examined the ability of children to judge stimulus pairs which were varied to contrast the processing of symbolic material against direct perceptual judgement and to test processing of numeric versus non-numeric material. Children with SLI were compared with a control group matched on verbal comprehension level. The children with SLI responded faster than the control subjects. The SLI and control groups showed similar SDE and a similar pattern of response across materials. No indication was found in the SLI data of any selective deficit in processing symbolic information. Findings are discussed in relation to theories of numeracy acquisition which acknowledge the importance of non-verbal representation of number meanings.

Phonological short-term memory and new word learning in children.

Sixty-five 5-year old children participated in 4 experimental tasks of word learning that varied systematically in the amounts of phonological and nonphonological learning required. Measures of the children's performances on 2 measures of phonological memory (digit span and nonword repetition), vocabulary knowledge, and nonverbal ability were also obtained. Learning of the sound structures of new words was significantly, and to some degree independently, associated with aspects of both phonological memory skill and vocabulary knowledge. Learning of pairs of familiar words was linked with current vocabulary knowledge, although not with phonological memory scores. The findings suggest that both existing lexical knowledge and phonological short-term memory play significant roles in the long-term learning of the sounds of new words. The study also provides evidence of both shared and distinct processes contributing to nonword repetition and digit span tasks.

Working memory and children's mental addition.

Two experiments investigated the extent to which children's mental arithmetic is constrained by working memory rather than their arithmetical competence. A span procedure was used to measure the limit on English- and German-speaking children's ability to add together pairs of multidigit numbers. The children's ages ranged from 7 years 7 months to 11 years 5 months. Spans for mental addition were higher when the numbers to be added were visible throughout calculation than when they were not, consistent with a working memory constraint. Variation in addition span with children's age and with difficulty of the arithmetical operations approximated to a linear function of the speed of adding integers. A similar speed/span relationship has previously been observed for counting span, an artificial task designed to load working memory by combining separate processing and storage subtasks. We conclude that the natural task of mental addition, which combines processing and storage as intrinsic components, reflects working memory in a similar way. Results were remarkably similar both between cultures and across age groups, consistent with the notion of working memory as a general-purpose resource with dynamics that are indifferent to the detailed nature of operations.

The representation of nonstructural information in visual memory: evidence from image combination.

Two experiments investigated the differential representation of the figure and ground of a picture in visual short-term and long-term memory. It is known (Hitch, Brandimonte, & Walker, 1995) that subjects find it more difficult to combine mental images of two separately presented pictures in order to identify a novel form when the two pictures are incongruent in color (i.e., when a black-on-white line drawing has to be combined with a white-on-black drawing). In the present experiments, the figures were depicted in solid form to allow color congruity to be varied independently for figure and ground. Results showed a clear impairment in image combination when the to-be-combined figures were incongruent in color (black-on-gray and white-on-gray) but not when their grounds were incongruently colored (gray-on-black and gray-on-white). In this way, image combination was seen to be supported by a representation of the object depicted in the picture rather than by a literal representation of the picture itself (i.e., a pictorial code). In line with previous findings, the same representation was seen to support image combination based on short-term memory (Experiment 1) and long-term memory (Experiment 2), provided that in the latter case verbal recoding was precluded. When verbal recoding was allowed, image combination based on long-term memory was insensitive to color congruity, implying the involvement of a more abstract structural representation.

Performance demands in the selection of objects for counting.

This paper considers the visual processes in object counting among children. Experiment 1 presented identical objects to 7- and 8-year-old children and found that spatially random configurations were counted more quickly than linear arrays, illustrating the difficulty of isolating objects grouped together in rows. However, the younger children were more prone to miscounting these random arrays than rows. The study also established a spatial proximity effect, with a dense arrangement of items being difficult to count. Experiment 2 revealed that this proximity effect can be removed by differentiating objects by color, providing further evidence that object counting involves overcoming Gestalt grouping forces and arguing against fine-motor control as a limiting factor in counting.

Two types of representation in visual memory: evidence from the effects of stimulus contrast on image combination.

An imagery task was used to explore the distinction between visual memory representations that preserve surface features and more abstract descriptions. Subjects were shown two line drawings and were then asked to combine a visual image of each drawing in order to identify a novel figure. The main experimental manipulation concerned the contrast in which each drawing was shown, which was either normal or reversed. Experiment 1 showed that image combination was more efficient when the contrasts of the two drawings were congruent with each other, but only when the imagery task was performed under conditions emphasizing short-term memory (STM). There was no effect of congruity when one of the images was generated from long-term memory (LTM). These results are consistent with the idea that the surface characteristics of a stimulus are preserved in visual STM, but a more abstract description is stored in visual LTM. In Experiment 2 we explored the influence of verbal recoding on performance of the imagery task by requiring subjects to suppress articulation. Under LTM conditions, performance of the imagery task was improved by suppression and became sensitive to contrast congruity. Under STM conditions, imagery was unaffected by suppression. Overall, these results support the distinction between surface and abstract descriptions in visual memory. However, they suggest that this distinction does not map onto that between STM and LTM in any simple way. It is suggested that short-term visual memory maintains surface descriptions and long-term visual memory preserves both surface and abstract descriptions. Verbal coding of visual stimuli appears to encourage the use of abstract visual descriptions.

Is there a relationship between task demand and storage space in tests of working memory capacity?

This paper considers working memory capacity, critically examining the hypothesis that counting span (the ability to count arrays of objects and store count totals) reflects a trade-off in resources available for processing and short-term storage. Previous evidence interpreted as favouring this hypothesis has confounded task difficulty with counting time. Experiment 1 validated a manipulation of the attentional demands of counting in which target objects were differentiated from non-targets by either a single feature (colour) or a feature conjunction (a combination of line orientations). The results confirmed that the two presentations involved qualitatively different attentional loads. Experiment 2 used these displays to compare counting span for children aged 6 to 11, both with and without an adjustment of target numerosity to control for differences in processing time. At all ages, span was lower when counting took longer, but there was no difference between feature and conjunction arrays once counting time was accounted for. These results argue against a resource trade-off interpretation of counting span. Rather, they support a hypothesis of resource-switching among children, implying that counting span acts as a measure of time-based forgetting.

Collaboration in recall: do pairs of people cross-cue each other to produce new memories?

When people collaborate over their recall of a shared experience, it might be expected that they could "cross-cue" each other so as to produce new memories not available to either member of the pair on their own. In a previous series of experiments (Meudell et al., 1992), we found that pairs of people always recalled more than one person, but we failed to show that social interaction facilitated performance so as to produce such "emergent" new memories. However, a phenomenon akin to cross-cuing was employed by Tulving and Pearlstone (1966) in their classic study of the availability and accessibility of memories; accordingly, in this study, we repeated Tulving and Pearlstone's work directly in a social context. So as to assess whether new memories emerged in collaborating pairs, a sequential design was employed. People learned categorized lists of words, and then all the subjects recalled the items strictly on their own. Subjects then recalled again in pairs (collaboratively) or once more on their own. The results showed that even when the opportunity for cross-cuing was directly manipulated through the provision of categorized lists, no additional new memories emerged in the collaborating groups. Possible mechanisms for the results are considered.

Do young children rehearse? An individual-differences analysis.

The issue of whether young children rehearse in auditory memory tasks was investigated across a series of three studies comparing individual differences in articulation rates and memory spans. Applying the principles of the working-memory model, children with faster rates of speaking should have superior memory spans if they engage in rehearsal. Two of the experiments, with 4-year-old children, failed to establish any significant association between articulation rate and memory span, although both the memory span and articulation rate procedures were found to be highly reliable in this age group. A third experiment confirmed that, as expected, articulation rates and memory spans were significantly associated with one another in adult subjects. The results indicate that, contrary to recent theories of children's short-term-memory development, 4-year-old children do not engage in subvocal rehearsal during auditory memory span tasks.

The prevalence of specific arithmetic difficulties and specific reading difficulties in 9- to 10-year-old boys and girls.

Data from an epidemiological sample (n = 1206) of British schoolchildren were used to estimate the proportions of 9- to 10-year-olds with specific arithmetic difficulties (SAD), combined with arithmetic-and-reading difficulties (ARD), and specific reading difficulties (SRD). Children in the sample contributed scores on separate tests of arithmetic, reading and nonverbal intelligence. Using a cutting-score approach, which took into account performance on all three tests, a small group of children with SAD (1.3%) were distinguished from larger groups with ARD (2.3%) and SRD (3.9%). Contrary to some previous reports, there were equal numbers of males and females within each of the two groups with arithmetic difficulties but a preponderance of males over females amongst the group with specific reading difficulties.

The effect of visual similarity on short-term memory for spatial location: implications for the capacity of visual short-term memory.

Evidence from the recency effect suggests that visual short-term memory is limited to preserving information about a single pattern. Three experiments explored the capacity of visual short-term memory using a task which involved presenting a series of patterns in a random spatio-temporal sequence and probing memory for the spatial location of one of them. Experiment 1 used sequences of quasi-random block patterns which were either visually similar or dissimilar. Serial position curves showed a single-item recency effect. However, there was also a deleterious effect of visual similarity for the earlier patterns, indicating that visual short-term memory was supporting their retention. Experiment 2 showed that performance was unaffected by articulatory suppression, confirming that phonological coding is not an important factor in the localisation task. Experiment 3 switched to letter stimuli and, because of the known tendency for this type of stimulus to be phonologically recoded, required articulatory suppression. Letters were shown in a customised form in sequences which were either visually similar or visually dissimilar. The results replicated experiment 1 in showing restricted recency combined with an effect of visual similarity extending across earlier list items. When the same letters were shown in standard form, the recency effect remained but the similarity effect disappeared, ruling out possible interpretations in terms of semantic as well as phonological coding. It is concluded that the recency effect gives a misleading indication of the capacity of visual short-term memory. The visual similarity effect suggests that it is capable of supporting the retention of several patterns.

Verbal recoding of visual stimuli impairs mental image transformations.

Two experiments were carried out to test the hypothesis that verbal recoding of visual stimuli in short-term memory influences long-term memory encoding and impairs subsequent mental image operations. Easy and difficult-to-name stimuli were used. When rotated 90 degrees counterclockwise, each stimulus revealed a new pattern consisting of two capital letters joined together. In both experiments, subjects first learned a short series of stimuli and were then asked to rotate mental images of the stimuli in order to detect the hidden letters. In Experiment 1, articulatory suppression was used to prevent subjects from subvocal rehearsal when learning the stimuli, whereas in Experiment 2, verbal labels were presented with each stimulus during learning to encourage a reliance on the verbal code. As predicted, performance in the imagery task was significantly improved by suppression when the stimuli were easy to name (Experiment 1) but was severely disrupted by labeling when the stimuli were difficult to name (Experiment 2). We concluded that verbal recoding of stimuli in short-term memory during learning disrupts the ability to generate veridical mental images from long-term memory.

Manipulation of visual mental images in children and adults.

Recently, there has been a debate on whether visual patterns can be transformed and reinterpreted in mental imagery. In the present study, task demands and age of subjects were manipulated to see whether children and adults were able to discover a novel visual pattern after transforming a mental image. Two tasks, called combination and subtraction, were devised. They consist of either compounding or taking away parts of images to discover a new construal. Results indicate that not only adults, but also children aged 6 and 10, are able to transform a mental image so as to yield another image with a different interpretation. Task demands had a greater effect on children than adults, consistent with the suggestion that the ease of manipulating mental images is a function of the efficiency of control processes.