Working memory, reading, and mathematical skills in children with developmental coordination disorder
Introduction
The DSM IV introduced the term developmental coordination disorder (DCD) to identify children who have “a marked impairment in the development of motor coordination…that significantly interferes with academic achievement or activities of daily living” (American Psychiatric Association, 1994, p. 53). DCD is believed to be an immaturity of parts of the cortical control processes that prevents messages from being properly transmitted to the body (e.g., Wilson, Maruff, & Lum, 2003). Observable behaviors in children with DCD include clumsiness, poor posture, confusion about which hand to use, difficulties throwing or catching a ball, reading and writing difficulties, and an inability to hold a pen or pencil properly. Findings from longitudinal studies indicate that children with motor deficits experience difficulties throughout their childhood and adolescence (Hellgren, Gillberg, Gillberg, & Enerskog, 1993). It is not uncommon for this condition to persist into adulthood, resulting not only in perceptual and motor difficulties, but also in socio-emotional struggles (Cousins & Smyth, 2003). Estimated prevalence of DCD in children aged between 5 and 11 years is about 6% (Mandich & Polatajko, 2003), with more males than females being affected.
Visual deficits are also characteristic of children with DCD. In visual tasks that do not include a motor component such as length discrimination, gestalt completion, and visual integration, common failures include inaccuracies in estimating object size (e.g., Lord & Hulme, 1988) and difficulties in locating an object’s position in space (Schoemaker et al., 2001). Visual tasks that do include some motor skills, such as Block Design and Object Assembly subtests from the WISC-III (Wechsler, 1992) are often good discriminators of children with DCD from controls (see Alloway, 2006, for a review of visual and motor deficits in children with DCD).
There is substantial heterogeneity of cognitive profiles in children with DCD. In particular, they can have co-morbid reading disabilities and general learning difficulties (Kaplan et al., 1998, Piek and Dyck, 2004). However, very little work has actually investigated the working memory profiles of this group. In light of extensive evidence of a causal link between impairments of working memory and learning difficulties (e.g., Gathercole et al., 2006, Swanson and Siegel, 2001), it is important to understand the working memory profiles associated with DCD, and to establish how this affects learning.
Working memory is the term used to refer to a system responsible for temporarily storing and manipulating information needed in the execution of complex cognitive tasks, such as learning, reasoning, and comprehension. According to Baddeley’s model (2000), working memory consists of four components (see also Baddeley & Hitch, 1974). The central executive is responsible for the high-level control and coordination of the flow of information through working memory, including the temporary activation of long-term memory. It has also been linked with control processes such as switching, updating, and inhibition (Baddeley, 1996). The central executive is supplemented by two slave systems specialized for storage of information within specific domains. The phonological loop provides temporary storage for linguistic material, and the visuospatial sketchpad stores information that can be represented in terms of visual or spatial structure. The fourth component is the episodic buffer, responsible for integrating information from different components of working memory and long-term memory into unitary episodic representations (Baddeley, 2000). This model of working memory has been supported by evidence from studies of children (e.g., Alloway et al., 2004, Alloway et al., 2006), adult participants, neuropsychological patients (see Baddeley, 1996, Gathercole and Baddeley, 1993, for reviews), as well as neuroimaging investigations (see Vallar & Papagno, 2002, for a review).
The key feature of working memory is its capacity both to store and manipulate information. Working memory functions as a mental workspace that can be flexibly used to support everyday cognitive activities that require both processing and storage such as, for example, mental arithmetic. However, the capacity of working memory is limited, and the imposition of either excess storage or processing demands in the course of an ongoing cognitive activity will lead to catastrophic loss of information from this temporary memory system. In contrast to working memory, short-term memory refers to the capacity of storing units of information, and is typically assessed by serial recall tasks involving arbitrary verbal elements such as digits or words.
The capacities of verbal short-term and working memory vary widely between individuals and independently from one another (e.g., Pickering, Gathercole, & Peaker, 1998). Verbal short-term memory skills are much more weakly associated with general academic and cognitive performance than working memory skills (e.g., Daneman & Merikle, 1996). There is, however, a strong and highly specific link between verbal short-term memory and the learning of the sound patterns of new words in both the native language over the early childhood years, and in second language learning at all ages (e.g., Gathercole et al., 1997, Service and Craik, 1993, Service and Kohonen, 1995). Children with poor verbal short-term memory skills have specific impairments in the process of learning the phonological structures of new vocabulary items, and so acquire new vocabulary items at a much slower rate than other children (for review, see Baddeley, Gathercole, & Papagno, 1998).
Verbal working memory skills are effective predictors of performance in many complex cognitive activities including reading (e.g., Swanson, 1994, De Jong, 1998), mathematics (e.g., Bull and Scerif, 2001, Mayringer and Wimmer, 2000, Siegel and Ryan, 1989), and language comprehension (e.g., Nation et al., 1999, Seigneuric et al., 2000), as well as attainments in National Curriculum assessments of English and mathematics (Alloway et al., 2005, Gathercole et al., 2004). In particular, marked deficits of verbal working memory correspond with the severity of learning difficulty experienced by a child (Alloway et al., 2005, Pickering and Gathercole, 2004). Recent research has also established that poor verbal working memory skills, but not general intelligence or verbal short-term memory, are uniquely linked with both reading and mathematical abilities (Gathercole et al., 2006). This asymmetry of associations provides a strong basis for identifying working memory as a specific and significant contributor to general learning difficulties.
Previous evidence has established that visuospatial short-term memory plays a role in mathematical skills, however findings have not been unanimous. Some researchers suggest that visuospatial memory supports number representation, such as place value and alignment in columns, in arithmetic (D’Amico and Guarnera, 2005, Geary, 1990, McLean and Hitch, 1999). However, other studies have found that visuospatial memory was no longer linked with mathematical ability once reading ability and IQ had been controlled (e.g., Bull, Johnston, & Roy, 1999). One explanation for the contradictory findings is that visuospatial memory is linked with arithmetic rather than general mathematical skills as tested in Bull et al.’s study (1999).
There have been very few studies that have looked at the performance of children with DCD on memory tasks (see Alloway, 2006, Pickering, 2004). One aim of the present study was to investigate a larger cohort of children with DCD in order to gain a more comprehensive understanding of their working memory profile. To this end, a sample of 55 children with DCD was administered standardized tests of memory, performance in literacy and numeracy, and subtests of verbal and performance IQ. Of particular interest was whether there would be a degree of specificity in verbal and visuospatial memory impairments in this cohort.
An important issue is whether deficits of working memory impair learning in children with DCD. There is some evidence that children with DCD tend to perform poorly in literacy (e.g., Dewey et al., 2002, Iversen et al., 2005), but to our knowledge, there are no studies investigating DCD and numeracy. On the basis that verbal working memory skills may be a critical determinant of the extent and severity of learning difficulties in children of low general abilities (e.g., Gathercole et al., 2006), the present study investigated whether there would be differential links between verbal and visuospatial memory impairments and learning in children with DCD.
Section snippets
Participants
There were 55 children (44 boys and 11 girls) from primary schools in the North-East England who participated in the study. They were referred by an occupational therapist who had identified them as experiencing motor difficulties using the DSM IV-R criteria and standardized motor assessments such as the Movement Assessment Battery for Children (M-ABC, Henderson & Sugden, 1992). Participants ranged in age from 5 to 11.4 years (mean 8.8 years, SD 19 months). Parental consent was obtained for
Results
Descriptive statistics for children with DCD on measures of working memory, learning, and IQ subtests are shown in Table 1. The composite scores were calculated by averaging standard scores of all three measures in each memory component. When comparing the children’s performance to the test standardized score of 100, mean scores fell within one standard deviation of the mean (i.e., 15 points from the standardized norm of 100) in measures of the verbal short-term memory, with the exception of
Discussion
The present study provides a detailed investigation of the relation between working memory and learning in children with DCD. The deficits observed in measures of visuospatial short-term and working memory were significantly worse than in the verbal short-term memory ones. This was supported by the greater proportion of individual scores that fell below one standard deviation from the mean (standard scores <85) in visuospatial memory tasks. Literacy and numeracy skills were also poor, with
Acknowledgments
The author thanks Chris Ridley from the Durham Local Educational Authority for her contribution; Susan Gathercole for helpful comments; and Kathryn Temple, Emily Pratt, Joni Holmes, and Anastasia Kourkoulou for assistance with data collection.
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