The development of increasingly sophisticated cognitive skills relies on the maturation of control processes for orienting attention and allocating resources for task relevant information 
. Such control processes are important for virtually every complex cognitive task, and there is growing evidence that they rely on functional interactions between multiple brain regions 
. Despite the critical role of control processes in cognitive development, little is known about the maturation of functional brain systems underlying control mechanisms in the developing brain. Here we use a novel neurocognitive network approach with multimodal imaging to investigate the maturation of functional brain systems underlying control processes that support problem solving skills in young children.
Based on experimental studies across a wide range of cognitive domains, a number of cortical areas within the frontal lobe, including the anterior cingulate cortex (ACC), ventrolateral prefrontal cortex (VLPFC), dorsolateral prefrontal cortex (DLPFC) and the fronto-insular cortex (FIC) have emerged as putative sites for implementing different aspects of control 
. Yet, even in adults, how these brain regions interact and implement control is poorly understood. This is especially surprising because, almost by definition, control processes should involve multiple interacting nodes of a network. A key challenge in untangling the potentially complex hierarchy of frontal control mechanisms is identifying patterns of their interconnectivity and how causal interactions emerge during performance of a cognitively demanding task. To date, however, there have been few systematic investigations of network interactions underlying control processes in adults and almost nothing is known about how these processes mature with development.
In this study we use a theoretically motivated approach to this problem based on neurocognitive network models derived from studies of intrinsic brain connectivity. Studies in adults have shown that the human brain is intrinsically organized into distinct functional networks 
. Remarkably, intrinsic functional connectivity analysis has identified two distinct neurocognitive networks which are particularly important for implementing dynamic control across a wide range of cognitive tasks: a ‘salience network’ (SN) 
, anchored in the FIC and dorsal ACC, and a dorsal fronto-parietal ‘central executive network’ (CEN) anchored in the DLPFC and the supramarginal gyrus within the posterior parietal cortex (PPC) 
. In adults the FIC node of the SN has been shown to play a major role in attentional capture, task-switching and generation of control signals that facilitate access to working memory resources necessary for a wide range of cognitive tasks 
. The FIC consists of at least two cytoarchtectonically distinct regions – the VLPFC and the anterior insula (AI). While the VLPFC has been the focus of many investigations of control 
, there is growing evidence to suggest that the AI, by virtue of its tight coupling with the ACC, plays a critical and distinctive role 
. Notably, analysis of dynamic causal interactions has suggested that the AI initiates control signals which engage the ACC, DLPFC and PPC while disengaging the default mode network during cognitively challenging tasks 
. In this study we use a neurocognitive network model based on the SN and CEN for investigating fundamental mechanisms mediating the development of dynamic control processes during cognition.
Over the past decade, several studies have examined developmental changes in the recruitment of brain areas belonging to these networks using cognitive tasks ranging from response inhibition, attention, and memory, to decision-making, reasoning and problem solving 
. Both increased and decreased recruitment of insula-cingulate and fronto-parietal systems have been reported over the course of development 
. Although developmental neuroimaging studies have provided evidence for immature task-related activation in the VLPFC, AI, ACC, and DLPFC 
, nothing is currently known about the maturation of dynamic interactions between these brain regions. Based on previous studies which have pointed to developmental changes in activation of areas that overlap with the SN and CEN we hypothesized that a neurocognitive network model would help clarify and significantly enhance our understanding of the mechanisms by which control processes mature in children.
A systematic network approach has the potential for providing insights into general development mechanisms mediating dynamic control processes during cognition. However, in both adults and children, the differential role and primacy of control signals has been difficult to disentangle, partly because these areas are typically coactivated during a wide range of cognitive tasks 
. More specifically, it has been difficult to disambiguate the contributions of multiple overlapping frontal lobe regions using task-based functional magnetic resonance imaging (fMRI). Critically, the SN and CEN are often co-activated during cognitive tasks in children and adults, and isolating focal responses in a consistent manner from task-based fMRI activations is not straightforward. This is especially true in developmental studies since children tend to show more diffuse activations in the prefrontal cortex, making it difficult to disambiguate regional functional cortical responses 
. To address this issue in a principled manner, we used multimodal imaging combining resting-state fMRI, cognitive task fMRI and DTI to examine developmental changes in dynamic interactions between the SN and CEN during cognition, and the underlying structural connectivity. Resting-state fMRI (rsfMRI) data were acquired and used to characterize the SN and CEN and to identify their five major nodes (SN: AI, VLPFC, ACC and CEN: DLPFC, PPC). We demarcated SN and CEN, and their nodes using analysis of rsfMRI data. An arithmetic problem solving task was used to investigate dynamic interactions between the SN and CEN during cognition. The arithmetic task used is easily understood and performed with high levels of accuracy by most 7–9 year old children, and several previous imaging studies have shown that it consistently activates all major nodes of the SN and CEN in both children and adults 
. DTI, performed in the same group of children and adults, was used to examine whether maturation of functional interactions between specific brain regions was related to the maturation of white matter pathways that link them. We predicted that the AI node of the SN would be a hub mediating dynamic causal interactions in adults but not in children. We further predicted that, compared to adults, children would have weaker dynamic causal interactions between the SN and CEN, and that weaker causal interactions would contribute significantly to reduced levels of activation as well as lower levels of task performance in children. Linking functional and structural connectivity measures, we predicted that immature causal interactions in children would be reflected in weaker integrity and density of white matter pathways linking key nodes of the SN and CEN. Together, these findings would provide novel information on temporal hierarchy of among prefrontal and parietal regions implicated in control processes 
and for immature fronto-parietal causal control signals in children.