Working memory in children with reading disabilities

Abstract

This study investigated associations between working memory (measured by complex memory tasks) and both reading and mathematics abilities, as well as the possible mediating factors of fluid intelligence, verbal abilities, short-term memory (STM), and phonological awareness, in a sample of 46 6- to 11-year-olds with reading disabilities. As a whole, the sample was characterized by deficits in complex memory and visuospatial STM and by low IQ scores; language, phonological STM, and phonological awareness abilities fell in the low average range. Severity of reading difficulties within the sample was significantly associated with complex memory, language, and phonological awareness abilities, whereas poor mathematics abilities were linked with complex memory, phonological STM, and phonological awareness scores. These findings suggest that working memory skills indexed by complex memory tasks represent an important constraint on the acquisition of skill and knowledge in reading and mathematics. Possible mechanisms for the contribution of working memory to learning, and the implications for educational practice, are considered.

Introduction

The purpose of this study was to investigate the extent to which impairments of working memory contribute to the severity of the learning difficulties experienced by children with reading disabilities. Although close links between memory function and many aspects of learning and academic achievement in unselected samples of children are well established, the degree to which working memory function constrains learning progress in children with learning disabilities is less well understood. The study focused in particular on the extent to which impairments of working memory contribute to the problems in both reading and mathematics commonly experienced by children with learning disabilities and on whether any associations that are found could be mediated by other aspects of cognitive function.

Immediate memory involves several related subsystems of memory. The capacity to store material over short periods of time in situations that do not impose other competing cognitive demands is typically referred to as short-term memory (STM). Findings from experimental, developmental, and neuropsychological studies indicate that STM is fractionated into at least two domain-specific components that are specialized for the retention of phonological and visuospatial material (for reviews, see Gathercole, 1999, Vallar and Papagno, 2002). In the influential working memory model of Baddeley and Hitch (1974), developed subsequently by Baddeley, 1986, Baddeley, 2000, these components correspond to different slave systems. The phonological loop retains material in a phonological code that is highly susceptible to time-based decay, and the visuospatial sketchpad has limited capacities to represent information in terms of its visual and spatial characteristics. The phonological loop is assessed using methods such as the recall of digit or word sequences, and visuospatial sketchpad functioning is typically measured by tasks involving the recall or recognition of visual patterns or sequences of movement.

Working memory is related to, but distinguishable from, STM. The term is widely used to refer to the capacity to store information while engaging in other cognitively demanding activities, and it is most commonly assessed using complex memory paradigms that impose demands for both temporary storage and significant processing activity with selected task components varied across domains. An example of a complex memory task is listening span, where participants are asked to make a meaning-based judgment about each of a series of spoken sentences and then to remember the last word of each sentence in sequence (e.g., Daneman & Carpenter, 1980). Another task is counting span, where participants are asked to count target items in successive arrays and then to recall in sequence the tallies of the arrays (Case, Kurland, & Goldberg, 1982). Despite disparate processing demands, scores on the two tasks are highly correlated (e.g., Gathercole, Pickering, Ambridge, & Wearing, 2004) and are also linked with performance on memory updating tasks that are believed to tap working memory (Jarvis and Gathercole, in press, Miyake et al., 2000).

Most theoretical accounts of immediate memory incorporate a distinction between the storage-only capacities of STM and the broader and more flexible nature of working memory. In addition to the domain-specific storage systems of the phonological loop and the visuospatial sketchpad, the Baddeley and Hitch (1974) model includes the central executive, responsible for a range of functions such as retrieval of information from long-term memory, regulation of information within working memory, attentional control of both encoding and retrieval strategies, and task shifting (Baddeley, 1986, Baddeley, 1996). Proponents of the working memory model have suggested that the storage demands of complex memory tasks depend on appropriate subsystems, with processing demands supported principally by the central executive (Baddeley and Logie, 1999, Cocchini et al., 2002). Thus, complex memory span, such as listening span and counting span, appears to tap both the central executive and the phonological loop (Lobley, Gathercole, & Baddeley, 2005), whereas analogous visuospatial complex memory tasks (Jarvis and Gathercole, 2003, Shah and Miyake, 1996) may draw on the resources of the central executive and the visuospatial sketchpad. There is a substantial domain-general component to such working memory tasks (e.g., Bayliss et al., 2003, Kane et al., 2004, Swanson and Sachse-Lee, 2001) that has been interpreted as reflecting central executive function.

Another influential conceptualization of working memory is of a limited resource that can be flexibly allocated to support either processing or storage (e.g., Daneman and Carpenter, 1980, Just and Carpenter, 1992). According to one model in this theoretical tradition, developmental increases in complex memory performance reflect improvements in processing speed and efficiency that release additional resources to support storage (Case et al., 1982). Other theorists have proposed that working memory consists of activated long-term memory representations and that STM is the subset of working memory that falls within the focus of attention (Cowan, 2001, Engle et al., 1999).

Because the current research is not concerned specifically with distinctions between models, the theoretically neutral terms phonological and visuospatial STM are used to refer to storage-only assessments of the respective informational domains, and complex memory tasks are interpreted as tapping working memory. The primary focus is on the extent to which complex memory performance is associated with the scholastic abilities of reading disabilities, characterized by marked difficulties in mastering skills such as word recognition, spelling, and reading comprehension. Links between complex memory scores and reading ability are well established, with scores predicting reading achievement independently of measures of phonological STM (e.g., Swanson, 2003, Swanson and Howell, 2001). Current evidence suggests that although phonological STM is significantly associated with reading achievement over the early years of reading instruction, its role is as part of a general phonological awareness construct related to reading development rather than representing a causal factor per se (Wagner and Muse, in press, Wagner et al., 1997). It is also well established that children with reading disabilities show significant and marked decrements on working memory tasks relative to typically developing individuals (Siegel and Ryan, 1989, Swanson, 1994, Swanson, 1999, Swanson et al., 1996).

Mathematical difficulties commonly accompany reading disabilities (Swanson & Saez, 2003) and are also characterized by deficits in working memory. Associations between complex memory performance and mathematical abilities vary across age and level of expertise, probably due to the changes in procedures and strategies that characterize mathematical development. For example, addition commences with simple counting strategies, success at which contributes to the gradual acquisition of arithmetic facts. More complex addition computations require memory-based problem solving involving either direct retrieval of facts or problem decomposition, leading to eventual automatic retrieval of facts (Geary, 2004). Working memory appears to play an important role at the earliest stage of counting; children with low scores on complex memory tasks are more likely to use primitive finger-based counting strategies than are those with high scores, possibly due to the relatively low working memory demands of the activities (Geary, Hoard, Byrd-Craven, & DeSoto, 2004). In addition, low complex memory scores have been found to be strongly and specifically associated with both poor computational skills (Wilson & Swanson, 2001) and difficulties in solving mathematical problems expressed in everyday language (Swanson & Sachse-Lee, 2001).

A key issue is how deficits of working memory impair reading and mathematical abilities. One explanation is that poor working memory capacities compromise the crucial process, for both mathematics and reading, of maintaining recently retrieved knowledge and integrating this with recent inputs (Swanson & Beebe-Frankenberger, 2004). A related suggestion is that learning activities in which children must engage in literacy and mathematics classes often impose heavy demands on working memory, resulting in frequent task failures in children with poor working memory function. As a result, the normal incremental process of acquiring knowledge and skills in these domains is impaired (Gathercole, 2004). In a more specific account of the association between working memory and mathematical abilities, Geary and colleagues (2004) proposed that poor working memory capacity impairs the process of acquiring mathematical facts that arises from successful counting strategies.

The participants in the current study were children who were identified by their schools as having reading difficulties of sufficient severity to warrant remedial support and who scored at least 1 standard deviation below the mean on a standardized measure of reading ability administered as part of this study that included subtests of word recognition, spelling, and reading comprehension (Wechsler, 1993). These selection criteria were less restrictive than those in the majority of studies in this field, which typically exclude children with low performance IQ or low scores on other measures of nonverbal ability measures. Although working memory deficits in children with learning difficulties have been found to persist even after measures of performance IQ have been taken into account (Swanson & Sachse-Lee, 2001), the inclusion in such studies of only children with scores in the normal range limits sensitivity to this potentially confounding factor. Selecting children purely on the basis of their reading disabilities, as we did in the current study, provides a much stronger test of this hypothesis.

Three further factors that could potentially mediate the link between complex memory performance and scholastic attainments were also investigated. It has been argued that the key factor underlying individual differences on working memory tests is general verbal ability (Nation et al., 1999, Stothard and Hulme, 1992). There is already some evidence that, in fact, complex memory performance is dissociable from verbal ability more generally (Cain et al., 2004, Siegel, 1988). However, it was considered important to test whether the two factors could be distinguished in the current sample of children with learning disabilities. If complex memory performance taps general verbal abilities, potential associations between complex memory and abilities in mathematics and literacy should be eliminated when variation in language and verbal IQ scores is taken into account.

Another factor that could contribute to the association between working memory and learning achievements is phonological STM. Scores on standard measures of STM, such as digit span and complex memory measures, are moderately associated with one another (e.g., Gathercole and Pickering, 2000, Gathercole et al., 2004), possibly due to the role played by phonological STM in supporting the storage component of the complex memory measures (Baddeley and Logie, 1999, Lobley et al., 2005). The extent to which STM and complex span measures are independently associated with learning achievements in this sample would establish whether possible associations are mediated by the contribution of STM abilities rather than working memory more generally.

The final mediating factor investigated in this study was phonological awareness. Phonological awareness skills, as tapped by tasks requiring the manipulation of phonological structure, are highly associated with both reading ability (e.g., Bradley and Bryant, 1985, Brady and Shankweiler, 1991, Catts et al., 2002, Stanovich and Siegel, 1994, Wagner and Torgesen, 1987, Wolf and Bowers, 1999) and mathematical skills (e.g., Geary et al., 1999, Rourke and Conway, 1997). It has been argued that both phonological awareness and STM measures reflect a common phonological processing substrate (Bowey, 1996, Metsala, 1999). On the basis of the significant verbal storage component of working memory tasks, this account could also be extended to encompass verbal working memory. To test whether possible associations between working memory and learning abilities are mediated by phonological processing skills more generally, standardized assessments of phonological awareness abilities (Frederickson, Frith, & Reason, 1997) were also included in the current study.

Finally, it was predicted in this study that working memory places general constraints, rather than specific constraints, on reading and mathematics abilities, so that associations between complex memory measures and reading should be abolished when differences in mathematical abilities are taken into account and vice versa. This prediction was made on the basis of our recent findings that children classified by their schools as having problems in both reading and mathematics had depressed performance on complex memory tasks, whereas individuals with difficulties restricted to reading did not (Pickering & Gathercole, 2004). Impairments of working memory deficits, therefore, appear to be associated with learning disabilities that extend beyond reading.