br Role of the funding source This work was

Role of the funding source This work was supported by the Swiss National Science Foundation, by the Deutsche Forschungsgemeinschaft (Wi 4059/1-1), the Jacobs Foundation and the Child Research Centre of the Children\'s University Hospital, Zürich. The funding sources were not involved in data collection, analysis, interpretation or the decision to submit the article for publication.
Introduction Working memory (WM) allows for the temporary storage and manipulation of information (Baddeley, 1992). WM capacity, the number of items that can be held in WM at one time, plays an important role in the development of other complex cognitive skills, such as reading ability (Engle, 2002; Cain et al., 2004), math performance (Dumontheil and Klingberg, 2012), social ability (Dennis et al., 2009), as well as in general intelligence (Colom et al., 2007; Engle et al., 1999), overall learning (Gathercole and Alloway, 2004) and academic achievement (Gathercole et al., 2004a; Alloway, 2009). Impaired WM capacity has been linked to a number of neurodevelopmental disorders, such as Attention Deficit Hyperactivity Disorder (ADHD; see Martinussen et al., 2005) and Autism Spectrum Disorder (ASD; Southwick et al., 2011), and various learning (Hwang and Hosokawa, 2007; Wang and Liu, 2007) and language processing difficulties (see Wright and Fergadiotis, 2012). Behavioral studies have documented improvements in WM ability throughout childhood to adulthood (e.g., Conklin et al., 2007; Gathercole et al., 2004b; Huizinga et al., 2006; Zald and Iacono, 1998). Whereas many other executive function components show development typically only up until mid-adolescence, WM continues to show protracted development well into young-adulthood (Huizinga et al., 2006), making it particularly susceptible to developmental disturbances. Structural fatty acid amide hydrolase changes throughout development are associated with refinements in various cognitive functions, including WM (Tamnes et al., 2013). Specifically, changes in structure and function of brain regions involved in WM, such as parietal and frontal regions, occur later than many regions, consistent with the protracted maturation of WM functions (Sowell et al., 1999). Given the key role WM plays in cognitive maturation, it is important to understand and characterize the neural basis of this development. Recent functional neuroimaging studies that have examined the neural underpinnings of WM across development have shown that with increased age, children and adolescents exhibit greater activation in prefrontal (Klingberg et al., 2002; Scherf et al., 2006) and parietal (Nagel et al., 2013; Spencer-Smith et al., 2013; Klingberg et al., 2002; Scherf et al., 2006) regions on visuo-spatial WM tasks. Adults showed similar neural patterns as children and adolescents on these tasks, but with more refined, localized activation (Scherf et al., 2006), and some increased activity in “performance-enhancing” regions, such as the dorsolateral prefrontal cortex (DLPFC; Jolles et al., 2011; Scherf et al., 2006). For example, Scherf et al. (2006) found that children showed limited recruitment of critical WM substrates (DLPFC and parietal regions) during a visuo-spatial WM task, and instead relied mainly on ventromedial prefrontal regions. However, they observed more specialized networks (i.e., DLPFC, ventrolateral prefrontal cortex [VLPFC] and supramarginal gyrus) as adolescents moved into adulthood. This developmental pattern of brain activity has also been characterized as a shift from posterior to anterior activation, with adults showing increased activity in the DLPFC and VLPFC (Kwon et al., 2002; Scherf et al., 2006). Thus, previous literature suggests that the development of higher-level WM function involves a combination of increasing localization within core WM regions and their integration with performance-enhancing regions. Fewer studies have examined the neural basis of verbal WM development, as the majority of studies utilized visuo-spatial or other nonverbal tasks. Verbal WM is particularly important, given its vital role in linguistic processes that are necessary for language and other higher-level cognitive functions (Smith et al., 1998). Brahmbhatt et al. (2008) found similar activation patterns between adolescents and adults during an n-back visual word task, with both groups showing activation in the bilateral fusiform gyrus, anterior cingulate, left precentral gyrus, left superior anterior temporal gyrus, left DLPFC, premotor cortex and left thalamus. Age-related changes were evident in the left parietal lobe, in which adults showed significantly greater activity than adolescents. In addition to the nature of tasks, the pattern of brain activation also depends on the amount of information (i.e., load) that needs to be maintained in WM. Previous literature exploring age-related changes in brain activity associated with verbal WM under conditions of increasing load found that adolescents and adults showed a greater increase in activation across load in left parietal (O’Hare et al., 2008; Thomason et al., 2009), left lateral prefrontal (Thomason et al., 2009) and right cerebellar (O’Hare et al., 2008) regions than children. In contrast, Jolles et al. (2011) did not find age-related load sensitivity in children and adults. Also, previous reports used only up to three levels of difficulty.