City Research Online City, University of London Institutional Repository Citation: Van der Molen, M. J., Henry, L. and Van Luit, J. E. H. (2014). Working memory development in children with mild to borderline intellectual disabilities. Journal of Intellectual Disability Research, 58(7), pp. 637-650. doi: doi: 10.1111/jir.12061 This is the unspecified version of the paper. This version of the publication may differ from the final published version. Permanent repository link: https://openaccess.city.ac.uk/id/eprint/3821/ Link to published version: doi: 10.1111/jir.12061 Copyright: City Research Online aims to make research outputs of City, University of London available to a wider audience. Copyright and Moral Rights remain with the author(s) and/or copyright holders. URLs from City Research Online may be freely distributed and linked to. Reuse: Copies of full items can be used for personal research or study, educational, or not-for-profit purposes without prior permission or charge. Provided that the authors, title and full bibliographic details are credited, a hyperlink and/or URL is given for the original metadata page and the content is not changed in any way. City Research Online: http://openaccess.city.ac.uk/ [email protected] Journal of Intellectual Disability Research Working memory development in children with mild to borderline intellectual disabilities Van der Molen, M.J., Henry, L.A & van Luit, J.E.H. WM development in MBID Abstract Background The purpose of the current cross-sectional study was to examine the developmental progression in working memory (WM) between the ages of 9 and 16 years in a large sample of children with mild to borderline intellectual disabilities (MBID). Baddeley’s influential WM model was used as a theoretical framework. Furthermore, the relations between working memory on the one hand, and scholastic skills (arithmetic and reading) on the other were examined. Method One-hundred-and-ninety-seven children with MBID between 9 and 16 years old participated in this study. All children completed several tests measuring short-term memory, working memory, inhibition, arithmetic and single word reading. Results WM, visuo-spatial short-term memory and inhibition continued to develop until around age 15 years. However verbal short-term memory showed no further developmental increases after the age of 10 years. Verbal short-term memory was associated with single word reading, whereas inhibition was associated with arithmetic. Discussion The finding that verbal short-term memory ceases to develop beyond the age of 10 years in children with MBID contrasts with results of studies involving typically developing children, where verbal short-term memory develops until around age 15 years. This relative early developmental plateau might explain why verbal short-term memory is consistently considered weak in children with MBID. Keywords: intellectual disabilities, short-term memory, working memory, inhibition, arithmetic, speeded reading, development 2 WM development in MBID Introduction Although there is a substantial literature on how short-term memory (STM) and working memory (WM) develop with age in typical children (e.g. Alloway et al., 2006; Gathercole et al., 2004a), there is very little work tracking such developmental changes in children with mild to borderline intellectual disabilities (MBID; IQ score 50-85). The purpose of the current cross-sectional study was to examine a large sample of children with MBID from a wide age range to determine how STM and WM develop. We hoped this would further our understanding of why children with MBID perform below their mental age levels in some aspects of STM and WM. We also examined how various aspects of STM and WM are related to important scholastic abilities, adding to the limited research evidence in this area. The starting point for this work was existing cross-sectional research on STM and WM in participants with MBID. Although this work has focused almost exclusively on samples of individuals with MBID with restricted age ranges, findings have shown consistent short-term memory (STM) and working memory (WM) delays compared to chronological age matched typically-developing children (CA control group; Alloway, 2010; Hasselhorn & Maehler, 2007; Henry, 2001; Henry & MacLean, 2002; Schuchardt et al., 2010; Van der Molen et al., 2007; 2009). This is not surprising considering that children with MBID have younger mental ages than CA control children. However, more noteworthy is that delays on some types of STM and WM tasks have been reported in children with MBID compared to younger typically-developing mental age matched children (MA control group; Henry & MacLean, 2002; Henry & Winfield, 2010; Van der Molen et al., 2007; 2009). This suggests that at least some aspects of STM and / or WM are characterised by difficulties that go beyond mental age level in children with MBID. Such marked 3 WM development in MBID memory problems are of particular concern because STM and WM are important factors contributing to multiple scholastic abilities such as reading and arithmetic (Henry & Winfield, 2010; Van der Sluis et al., 2004). In terms of theoretical approaches, one generally accepted view is that the cognitive abilities of people with non-organically (or non-detectable organically) based intellectual disabilities (true for most children with MBID, see Heikura et al., 2005), fall at the lower end of the normal distribution. On this account, cognitive abilities in children with MBID should develop along a broadly comparable trajectory to that of typically-developing children, but at a slower rate. A further prediction of this account is that cognitive abilities in children with MBID are expected to plateau at a lower level than in the typically developing population (Bennet-Gates & Zigler, 1998). The first aim of the present study was, therefore, to explore if and how STM and WM developed in a cross-sectional sample of children with MBID between the age of 9 and 16 years. The theoretical framework for this research was Baddeley’s working memory model (Baddeley, 2007), which is frequently used to explain and explore STM and working memory WM in populations of children with and without developmental disorders, including those with MBID (e.g. Henry, 2012). This model comprises four components: (1) a phonological loop to temporarily hold and maintain verbal information, with an accompanying automatic rehearsal component that prevents information from fading away; (2) a visuo-spatial sketchpad to temporarily hold and maintain visual and spatial information; (3) a central executive which functions as an attentional control system by focusing, directing and switching attention (this system is hypothesised to be involved in a range of ‘executive’ functions relevant for goal- directed behaviour such as executive-loaded working memory, switching and 4 WM development in MBID response inhibition – here we focused on executive-loaded working memory and response inhibition); and (4) an episodic buffer, a multidimensional storage system that binds information from different sources into a coherent experience (Baddeley, 2000). In the current terminology, STM is represented by the phonological loop (verbal STM) and the visuo-spatial sketchpad (visuo-spatial STM) while WM refers to the capacity to simultaneously store and manipulate information over brief periods of time, i.e. on memory tasks with a ‘central executive’ load. Several studies have focussed on the development of STM and WM in typically developing children. The results regarding verbal STM are mixed. Some have reported improvements until around 11 years (Alloway et al., 2006), after which verbal STM performance levels off; whereas others have reported a more extended period of development up to around 15 years (Conklin et al., 2007; Gathercole et al., 2004a). Turning to visuo-spatial STM, a distinction is generally made between visual STM and spatial STM. Research on spatial STM has provided evidence of a linear improvement from 4 to 10 years (Pickering et al., 2001), levelling off at around 15 years (Farrell Pagulayan et al., 2006; Gathercole et al., 2004a; Luciana et al., 2005), although note that one study found that spatial STM stabilized somewhat earlier at around 12 years (Conklin et al., 2007). Visual, or static, STM seems to plateau somewhat earlier than spatial, or dynamic, STM, at around 11 years (Gathercole et al., 2004a). For the more complex skill of WM, which requires executive control to co- ordinate both processing and storage concurrently, somewhat longer developmental trajectories might be predicted. Most studies have reported that WM develops linearly from 4 to 10 years, levelling off at around 15 years (Conklin et al., 2007; Gathercole et al, 2004a; Huizinga et al., 2006; Luciana et al., 2005; McAuley & White, 2011), 5 WM development in MBID but there is some evidence for a more protracted development in WM - up until age 20 (for visual WM; Hamilton et al., 2003) or even age 45 (for visual and verbal WM; Swanson; 1999). In the current data set, we also had the opportunity to look at response inhibition, another function ascribed to the central executive component of Baddeley’s working memory system (Baddeley, 1996). In terms of previous research on typical children, the development of response inhibition may reach its asymptotic level at around age 7 years (Gerstadt et al., 1994) or around late adolescence / beginning young adulthood (Huizinga et al., 2006; McAuley & White, 2011; although in this study, the developmental changes disappeared when corrected for processing speed; Williams et al., 1999). Nevertheless, there is also research showing that inhibition performance may plateau by the age of 10 years (Welsh et al., 1991; Klenberg et al., 2001, Lehto et al., 2003). As such, the developmental time course for improvements in inhibition shown by typical children remains unresolved in the literature. Although findings regarding the development of STM, WM and inhibition in typical children are not entirely consistent, it does seem that STM and WM develop at least until around age 15 years.