Individual differences in working memory.

Working memory can be defined as the ability to hold in mind information in the face of potentially interfering distraction in order to guide behavior. The experimental manipulation of working memory tasks has shed considerable light on the probable structure of the human working memory system, and, to a lesser extent, the specific processes captured by working memory paradigms. However, individual differences research has also had a crucial role to play in the development of theories of working memory. In particular, correlational approaches have been particularly informative in three areas of working memory research, each of which is reviewed here. These are, first, the importance of working memory measures as correlates of high-level cognitive skills such as reading, mathematics, reasoning, and fluid intelligence; second, the extent to which human working memory relies on domain-general or domain-specific component subsystems, and third, the precise reasons why working memory measures do relate to other important indices of human cognitive functioning. The findings from each of these areas suggest that working memory depends on a combination of domain-specific representational systems and domain-general processing and control systems, and that working memory measures capture individuals' ability to combine maintenance and processing demands in a manner that limits information loss from forgetting or distraction.

Short-term memory and working memory as indices of children's cognitive skills.

In the current literature, empirical and conceptual distinctions have been drawn between a more or less passive short-term memory (STM) system and a more dynamic working memory (WM) system. Distinct tasks have been developed to measure their capacity and research has generally shown that, for adults, WM, and not STM, is a reliable predictor of general cognitive ability. However, the locus of the differences between the tasks has received little attention. We present data from children concerning measures of matrices reasoning ability, reading, and numerical skill along with forward and backward order serial recall of WM, STM, and STM with articulatory suppression tasks. As indices of children's cognitive skills, STM and WM are shown to be rather similar in terms of memory per se. Neither the opportunity for rehearsal nor task complexity provides satisfactory explanations for differences between memory tests.

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.

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.

On random generation and the central executive of working memory.

Four experiments explore participants' attempts to generate random sequences. Experiment 1 showed that oral random number generation is strongly affected by both response speed and response set size, in contrast to a random key-pressing task in Expt 2. Expt 3 confirmed differences between output modalities in the set size effect, and Expt 4 indicated that an important source of difficulty in producing random numbers orally lies in the requirement to represent candidate choices. Across experiments, data show a strong tendency on the part of participants to suppress response repetitions, an effect which decays over intervening responses. Whilst consistent with the possibility that random generation taps some executive functions, findings suggest the need to expand current models of attentional control in working memory to account for distinct constraints.

Linguistic influences on children's number concepts: methodological and theoretical considerations.

From observations of how children match numerals to number tokens, previous research has suggested that cognitive representations of numbers vary with the linguistic demarcation of numerals. It is argued here that this paradigm does not always support the idea that language affects number concepts and that children's performance is shaped by other constraints. In Experiment 1, increasing the salience of multiple unit tokens significantly altered the distribution of response strategies in 6 1/2- to 7 1/2-year-old children. In Experiments 2 and 3, the importance of instructions in biasing responses is demonstrated, as 5 1/2- to 7 1/2-year-old children's use of multiunit tokens increased when given an example of their potential use. It appears that the task reflects in part children's interpretations and misinterpretations of task demands. Implications for the role of language in acquiring base-10 number concepts are discussed.

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.

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.

Temporal generalizations in humans: Three further studies.

Three experiments investigated human performance on variants of the temporal generalization procedure developed by Wearden (1992) from an experiment with animals by Church and Gibbon (1982). All experiments used short tone durations as stimuli to be judged, and chronometric counting was not spontaneously used by subjects. Experiment 1 studied the effects of extended exposure to a temporal generalization procedure, with a 400 ms standard duration. With increasing exposure, subjects' tendency to produce judgemental asymmetries (i.e. confusing a 500 ms stimulus more with the standard than a 300 ms one, a standard result in temporal generalization with human subjects) decreased, and this was modelled theoretically by assuming that subjects came to adopt stricter response criteria for durations longer than the standard than those which were shorter. Experiment 2 used standard durations which were at the end of a stimulus set ranging from 200 to 800 ms. Orderly generalization functions were obtained in these cases, and data were well-fitted by a standard model of temporal generalization. Experiment 3 used an 'episodic' variant of temporal generalization, involving the comparison of the durations of 2 stimuli present on a trial. Once again judgemental asymmetries were found, but standard models of temporal generalization failed to fit data from some cases. Overall, both experimental effects and theoretical treatments from temporal generalization studies were robust in the face of procedural variations.