Working memory and learning in children with developmental coordination disorder and specific language impairment.(Report)

The extent to which deficits in specific cognitive mechanisms may underlie developmental disorders is a matter of considerable interest in the fields of cognition and cognitive development. The functioning of the working memory system has been implicated in groups of children with marked learning difficulties, such as learning disabilities (e.g., Alloway et al., 2005; Swanson & Saez, 2003), reading impairments (Gathercole, Alloway, Willis & Adams, 2006), and specific language impairment (SLI; e.g., Archibald & Gathercole, 2006a). To date, however, no studies have provided a comparison of working memory profiles in children with different developmental pathologies, and it was the purpose of the present study to do so. The comparison groups represented children with impairments predominantly affecting either language or motor skills: SLI, an impairment disproportionately affecting language skills, and developmental coordination disorder (DCD; also known as motor dyspraxia), characterized by difficulties with motor coordination. The extent to which deficits in subcomponents of working memory may differentiate these groups was explored by measuring skills systematically across the verbal and visuospatial domains.

SLI in children is an unexpected failure to develop language at the usual rate, despite normal general intellectual abilities, sensory functions, and environmental exposure to language. It is a relatively common developmental condition, estimated to occur in approximately 7% of kindergarten children (Tomblin et al., 1997), and is more prevalent in male than female children (e.g., Choudhury & Benasich, 2003; Flax et al., 2003). Affected children have the greatest problems in learning word forms and the grammatical structure of language, with the acquisition of semantics and pragmatics relatively spared (Leonard, 1998). Although the difficulties disproportionately affect the verbal domain, deficits in more general skills such as processing speed (Miller, Kail, Leonard, & Tomblin, 2001) and hypothesis testing (e.g., Ellis Weismer, 1991; Nelson, Kamhi & Apel, 1987) have been reported. In addition, it is widely acknowledged that individuals with SLI commonly experience learning difficulties of a comparable magnitude across all scholastic domains, including mathematics (Arvedson, 2002; Donlan & Gourlay, 1999; Fazio, 1996) and literacy (Bishop & Adams, 1990; Catts, Fey, Tomblin, & Zhang, 2002; Flax et al., 2003).

DCD is characterized by marked motor impairment that affects functioning in daily activities in the absence of intellectual or neurological dysfunction (American Psychiatric Association, 1994). 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. The estimated prevalence of DCD in children ages 5 to 11 years is about 6% (Mandich & Polatajko, 2003), with more male than female children being affected. As with SLI, substantial heterogeneity exists in the cognitive profiles of children with DCD, with some researchers suggesting that comorbidity is so widespread in DCD as to be the norm rather than the exception (Kaplan, Wilson, Dewey, & Crawford, 1998; Piek & Dyck, 2004; Wilson, 2005). The common occurrence of concomitant language impairments with DCD (e.g., Hill, 2001; Visser, 2003) has led to the suggestion that linguistic difficulties may underlie some of the learning problems experienced by children with DCD (Visser, 2003).

Although these developmental disorders are distinguished by the domain of principal deficit, they are both associated with significant learning difficulties. A key contributing factor in learning during development is working memory abilities (e.g., Alloway, Gathercole, Adams, et al., 2005; Gathercole, et al., 2006; Swanson & Sachse-Lee, 2001). Working memory is the capacity to store and manipulate information over brief periods of time (Baddeley & Hitch, 1974; Just & Carpenter, 1992). Extensive research over the past three decades has established that working memory is not a single store but a memory system composed of separable interacting components. Functioning in concert, these components provide a kind of flexible mental workspace that can be used to maintain and transform information in the course of demanding cognitive activities.

There are several theoretical accounts of working memory. In the present study, we used Baddeley and Hitch's (1974) model of working memory (see also Baddeley, 2000), which is widely used in both developmental and adult samples (e.g., Alloway, Gathercole, Willis & Adams, 2004; Baddeley, 1996). According to this model, there are separable components for the temporary storage of verbal and visuospatial information: the phonological loop and the visuospatial sketchpad, respectively. A centralized component is responsible for coordinating the flow of information between these storage systems and the temporary activation of long-term memory. The central executive also functions as a mental workspace involved in the temporary storage and manipulation of information needed in the execution of complex cognitive tasks, such as learning, reasoning, and comprehension.

In recent years, standard methods of assessing both verbal and visuospatial aspects of working memory have been developed and validated. Two tests, the Working Memory Test Battery for Children (WMTB-C; Pickering & Gathercole, 2001) and the Automated Working Memory Assessment (AWMA; Alloway, 2007a), provide multiple measures of subcomponents of working memory standardized for children ages 4 to 11 years. Tasks tapping the temporary storage of the phonological loop and visuospatial sketchpad require the serial retention of either phonological information such as digits (e.g., Conrad & Hull, 1964) or visuospatial material such as visual patterns (e.g., Smyth & Scholey, 1996). Working memory is typically assessed using complex memory span paradigms that impose demands for both temporary storage and significant processing activity with selected task components varied across domains. An example of a verbal complex span task is reading span, in which a participant is asked to make a meaning-based judgment about each of a series of sentences and then remember the last word of each sentence in sequence (Daneman & Carpenter, 1980). A corresponding visuospatial task is spatial span, in which a participant is asked to judge the orientation of a set of letters and then remember the sequence of degrees of rotation of the letters (Shah & Miyake, 1996).

Working memory skills can explain individual differences in learning. Poor performance on working memory measures is characteristic of children failing to progress normally in the areas of reading (e.g., De Jong, 1998; Swanson, 1994), mathematics (e.g., Bull & Scerif, 2001; Mayringer & Wimmer, 2000; Passolunghi & Siegel, 2001; Siegel & Ryan, 1989), and language comprehension (e.g., Nation, Adams, Bowyer-Crain, & Snowling, 1999; Seigneuric, Ehrlich, Oakhill, & Yuill, 2000). There are also links between performance on tasks measuring phonological loop capacity of working memory and vocabulary acquisition (for a review, see Baddeley, Gathercole, & Papagno, 1998). Associations have been reported as well between measures of the visuospatial sketchpad component of working memory and mental arithmetic (Lee & Kang, 2002; McLean & Hitch, 1999).