The present study examined the effects of increasing load

The present study examined the effects of increasing load, with six difficulty levels in a verbal WM task, and how load-dependent change in pten pathway function differed in children and adults. Prior developmental visual verbal WM studies that included load as a manipulation used complex tasks that required maintenance and reordering (Jolles et al., 2011) or changed stimuli appearance and size (O’Hare et al., 2008; Thomason et al., 2009) as difficulty level increased. In a typical n-back task, participants view a series of stimuli and indicate whether the currently presented stimulus matches one presented ‘n’ (e.g., 0, 1, 2 or 3) trials prior. As difficulty level increases, the number of interfering stimuli between the target and relevant stimulus increases, requiring the utilization of different mental strategies at each level (e.g., 0-back: recognition; 1-back: maintenance; 2-back: maintenance and monitoring). These manipulations increase both memory load and executive function demands (i.e., strategy needed to complete the task) in a non-linear fashion from each level to the next, making function-specific alterations difficult to quantify and relate with specific brain regions. To avoid these confounds, we used a 1-back letter matching task (LMT) which manipulated memory load while keeping executive function uniform across the difficulty levels, allowing us to investigate directly the impact of cognitive load on verbal WM. The executive demands (i.e., procedural strategies for solving the task) were constant across all levels of the LMT; what varied with each level was the number of items (letters) that had to be remembered. A visuo-spatial analogue of LMT has been used successfully to explore WM in functional neuroimaging studies of adults (Arsalidou et al., 2013) and children with and without ASD (Vogan et al., 2014). Observations from these studies point to a linear pattern of WM function across load. Our task can capture neural correlates associated with this linear pattern of activation with cognitive load, and our objective is to determine whether patterns of activation in WM processing change across development. Understanding the effect of age on brain regions implicated in WM and WM capacity can provide insight into the developmental trajectory of verbal WM networks, and enhance our ability to determine optimal timing for interventions in paediatric populations with WM difficulties. Given previous literature and findings from our recent work using the visuo-spatial version of LMT, we expected frontal and parietal cortical areas associated with WM would be under-recruited in children compared to adults, and these differences would increase with increasing cognitive load. Further, neural activation in both groups would be left-hemisphere dominant, given the verbal nature of the task (e.g., Brahmbhatt et al., 2008), and this localized pattern would be less evident in children who are more likely to demonstrate diffuse activation (Scherf et al., 2006).
Methods
Results
Discussion Although children exhibited task-related activity in many of the same brain regions as adults, they failed to exhibit the same degree of increasing activation across cognitive load as adults in multiple frontal and parietal cortical regions, consistent with previous studies (Thomason et al., 2009; O’Hare et al., 2008). These areas included the bilateral superior pariety gyrus extending to the precuneus, bilateral inferior frontal gyrus, left middle frontal gyrus and right cerebellum. Thus, although the number of brain regions showing load-dependent activation did not change across age, the extent to which participants relied on these areas in response to increasing cognitive load changed between childhood and adulthood. Increased recruitment of frontal and parietal areas during working memory tasks has been shown to underlie improvements in working memory and cognitive control over the course of development (see Bunge and Wright, 2007, for a review). The DLPFC (i.e., middle frontal gyrus) is referred to as a core “performance-enhancing” region believed to play a critical role in holding information ‘online’ (Powell and Voeller, 2004) and mediating strategic organization and data compression processes (Rypma et al., 2002; Bor et al., 2003). This is consistent with the dorsolateral prefrontal cortex being sensitive to increasing cognitive demand in ours and other studies (Scherf et al., 2006; Thomason et al., 2009). The inferior frontal gyrus has also been identified as an area involved in higher cognitive monitoring (e.g. choosing, comparing, judging and retrieving), as well as language processing, specifically in the left hemisphere (see Liakakis et al., 2011 for review; Strand et al., 2008). Findings from our study suggest that adults are increasingly recruiting these regions, more so than children, to adjust for increasing task demand.